ArticlePDF Available

Seasonal and diurnal variation of geomagnetic activity: Russell-McPherron effect during different IMF polarity and/or extreme solar wind conditions

Authors:

Abstract and Figures

The Russell-McPherron (R-M) effect is one of the most prevailing hypotheses accounting for semiannual variation of geomagnetic activity. To validate the R-M effect and investigate the difference of geomagnetic activity variation under different interplanetary magnetic field (IMF) polarity and during extreme solar wind conditions (interplanetary shock), we have analyzed 42 years interplanetary magnetic field and geomagnetic indices data and 1270 SSC (storm sudden commencement) events from the year 1968 to 2010 by defining the R-M effect with positive/negative IMF polarity (IMF away/toward the Sun). The results obtained in this study have shown that the response of geomagnetic activity to the R-M effect with positive/negative IMF polarity are rather profound: the geomagnetic activity is much more intense around fall equinox when the direction of IMF is away the Sun, while much more intense around spring equinox when the direction of IMF is toward the Sun. The seasonal and diurnal variation of geomagnetic activity after SSCs can be attributed to both R-M effect and the equinoctial hypothesis; the R-M effect explains most part of variance of southward IMF, while the equinoctial hypothesis explains similar variance of ring current injection and geomagnetic indices as the R-M effect. However, the R-M effect with positive/negative IMF polarity explains the difference between SSCs with positive/negative IMF By accurately, while the equinoctial hypothesis cannot explain such difference at the spring and fall equinoxes. Thus, the R-M effect with positive/negative IMF polarity is more reasonable to explain seasonal and diurnal variation of geomagnetic activity under extreme solar wind conditions.
Content may be subject to copyright.
Seasonal and diurnal variation of geomagnetic activity:
Russell-McPherron effect during different IMF polarity
and/or extreme solar wind conditions
H. Zhao
1,2
and Q.-G. Zong
1,3
Received 23 April 2012; revised 1 October 2012; accepted 2 October 2012; published 29 November 2012.
[1]The Russell-McPherron (R-M) effect is one of the most prevailing hypotheses
accounting for semiannual variation of geomagnetic activity. To validate the R-M effect
and investigate the difference of geomagnetic activity variation under different
interplanetary magnetic field (IMF) polarity and during extreme solar wind conditions
(interplanetary shock), we have analyzed 42 years interplanetary magnetic field and
geomagnetic indices data and 1270 SSC (storm sudden commencement) events from the
year 1968 to 2010 by defining the R-M effect with positive/negative IMF polarity
(IMF away/toward the Sun). The results obtained in this study have shown that the
response of geomagnetic activity to the R-M effect with positive/negative IMF polarity are
rather profound: the geomagnetic activity is much more intense around fall equinox when
the direction of IMF is away the Sun, while much more intense around spring equinox
when the direction of IMF is toward the Sun. The seasonal and diurnal variation of
geomagnetic activity after SSCs can be attributed to both R-M effect and the equinoctial
hypothesis; the R-M effect explains most part of variance of southward IMF, while the
equinoctial hypothesis explains similar variance of ring current injection and geomagnetic
indices as the R-M effect. However, the R-M effect with positive/negative IMF polarity
explains the difference between SSCs with positive/negative IMF B
y
accurately, while the
equinoctial hypothesis cannot explain such difference at the spring and fall equinoxes.
Thus, the R-M effect with positive/negative IMF polarity is more reasonable to explain
seasonal and diurnal variation of geomagnetic activity under extreme solar wind
conditions.
Citation: Zhao, H., and Q.-G. Zong (2012), Seasonal and diurnal variation of geomagnetic activity: Russell-McPherron effect
during different IMF polarity and/or extreme solar wind conditions, J. Geophys. Res.,117, A11222, doi:10.1029/2012JA017845.
1. Introduction
[2] The semiannual variation in geomagnetic activity has
been recognized for a long period of time [Cortie, 1912],
which shows the maximum appears around equinoxes while
the minimum appears around solstices, e.g., geomagnetic
storm annual distribution [Echer et al., 2011]. Over the
decades, several explanations for this variation have been
put forward, such as the axial hypothesis, the equinoctial
hypothesis and the Russell-McPherron effect [Cortie, 1912;
Bartels, 1932; McIntosh, 1959; Svalgaard, 1977; Russell
and McPherron, 1973].
[3] The axial hypothesis takes the varying heliographic
latitude of the earth into consideration; the equinoctial
hypothesis is based on the angle between Earth-Sun line and
the dipole axis of the Earth; the R-M effect holds that the
angle between Z axis in geocentric solar magnetospheric
(GSM) coordinate system and Y axis in geocentric solar
equatorial (GSEQ) coordinate system plays an important
role. Figure 1a shows the semiannual and diurnal variation
of the angle qbetween the Z axis in GSM coordinate system
and the Y axis in GSEQ coordinate system, that is, the
controlling parameter of the R-M effect. According to the
R-M effect, the probability of southward IMF increases when
the angle q, which is smaller than 90 degrees, decreases, so
that the dayside reconnection can be more efficient and more
energy can be conveyed into the magnetosphere. Figure 1b
shows the semiannual and diurnal variation of the angle y
between Earth-Sun line and the dipole axis of the Earth, that
is, the crucial parameter of the equinoctial hypothesis.
1
Institute of Space Physics and Applied Technology, Peking University,
Beijing, China.
2
Laboratory for Atmospheric and Space Physics, Department of
Aerospace Engineering Sciences, University of Colorado Boulder, Boulder,
Colorado, USA.
3
Center for Atmospheric Research, University of Massachusetts Lowell,
Lowell, Massachusetts, USA.
Corresponding author: Q.-G. Zong, Institute of Space Physics
and Applied Technology, Peking University, Beijing 100871, China.
(qgzong@gmail.com)
©2012. American Geophysical Union. All Rights Reserved.
0148-0227/12/2012JA017845
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, A11222, doi:10.1029/2012JA017845, 2012
A11222 1of15
Crooker and Siscoe [1986] suggest the field configuration in
the Chapman-Ferraro current plane may change and prevent
the energy transfer as the yangle changes; whereas Russell
et al. [2003] demonstrate that the tilt of dipole axis controls
the size of dayside reconnection region and thus the recon-
nection rate and geomagnetic activity.
[4] Nowadays, the R-M effect is one of the most prevailing
hypotheses. Orlando et al. [1993] investigated the connec-
tion between semiannual variation of the geomagnetic
activity and interplanetary magnetic field from 1965 to 1987
and verified the accuracy of R-M effect which suggests the
geomagnetic activity is modulated by southward IMF B
z
.
Siscoe and Crooker [1996] analyzed diurnal variation of Dst
index for 13 years and found the R-M effect predicts such an
diurnal oscillation. Also, OBrien and McPherron [2002]
investigated the dynamics of Dst index and demonstrated
the R-M effect is indeed valid.
[5] However, there are also many studies on the semiannual
variation which argued that the R-M effect fails to explain
the diurnal variation of the geomagnetic activity and can
only explain a small part of seasonal variation of the geo-
magnetic activity. Cliver et al. [2000] argued that the R-M
effect can only predict part of semiannual variation, while
the equinoctial hypothesis, which is based on the variation of
the angle ybetween the Earth-Sun line and the dipole axis
of the Earth, accounts for a large part of semiannual varia-
tion of geomagnetic activity. Cliver et al. [2001] investigated
the semiannual variation of Dst index for about 40 years
and found the equinoctial hypothesis dominates the storm
component of the variation of Dst index and the R-M effect
predicts little variance. The similar conclusion has been
derived from the analysis of aa index, too [Cliver et al., 2002].
Moreover, Svalgaard et al. [2002] analyzed the largest geo-
magnetic storms from 1868 to 1998 and indicated the most
difference of occurrence frequency between equinoxes and
solstices arises from an equinoctial effect. Li et al. [2001] used
the models of MeV electron at geostationary orbit and the Dst
index to examine the cause of semiannual variation and found
the equinoctial hypothesis contributes the largest part to
semiannual variation of Dst index and MeV electrons in the
inner magnetosphere. Furthermore, Mursula [2011] have
studied seasonal variation of substorms and geomagnetic
index Ap between 1993 and 2008 and showed that semian-
nual variation is mainly due to the artifact of annual maximum
alternating from spring to fall; however, Svalgaard [2011]
disprove this conclusion by showing the well-established
UT variation of geomagnetic activity and lack of organized
annual variations of the solar driver, and confirm that the
semiannual variation is not overestimated nor an artifact.
[6] On the other hand, the contribution of solar wind,
especially during extreme solar wind conditions, like inter-
planetary shocks, to seasonal and diurnal variation of geo-
magnetic activity has been rare studied although it may play
significant role.
[7] SSCs indicate the arrival of the interplanetary
discontinuities/shocks [Gonzalez et al., 1994]. A SSC is a
sudden increase in the H component of geomagnetic field
preceding a geomagnetic storm. It differs from a sudden
impulse (SI), which is physically the same phenomenon
but without following a geomagnetic storm [Siscoe et al.,
1968; Joselyn and Tsurutani,1990;Araki, 1994; Echer
et al., 2005]. MHD discontinuities have four types: rota-
tional discontinuities, tangential discontinuities, contact dis-
continuities, and shocks. Shocks also have three different
types, that is, fast, intermediate and slow. Either shocks or
tangential discontinuities which have different densities across
them can cause SIs or SSCs. Interplanetary shocks have a
great impact on the Earths magnetosphere. Fast shocks,
which are most likely to cause a SSC or SI, can also lead
to particle energization [Zong et al., 2009], dayside aurora,
creation of new radiation belts, and substorms [Colburn and
Sonett,1966;Tsurutani et al., 2011]. Through the study of
SSCs and shocks between 1978 and 1980, Smith et al.
[1986] found that 8090% of SSCs were associated with
interplanetary shocks. Wang et al. [2006] observed 278 SSCs
from January 1995 to December 2004 and found 225 of
them were associated with interplanetary shocks, that is, the
Figure 1. The Russell-McPherron effect and the equinoctial hypothesis. (a) Seasonal and diurnal variation
of qangle between Z axis in GSM coordinate system and Y axis in GSEQ coordinate system. (b) Seasonal
and diurnal variation of yangle between Earth-Sun line and the dipole axis of the Earth.
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
2of15
probability that a SSC is associated with a interplanetary
shock is 0.75.
[8] Interplanetary shocks can cause intense geomagnetic
storms. Jurac et al. [2002] found, 40% of forward shocks
with shock normals perpendicular to the IMF cause intense
storms(Dst < 100 nT), while 1015% of shocks without
normals perpendicular to the IMF lead to intense storms.
Also, Echer and Gonzalez [2004] reported 57% of inter-
planetary shocks are followed by moderate and intense
geomagnetic storms(Dst 50 nT). Interplanetary shocks
can also trigger substorms [Zhou and Tsurutani, 2001;
Tsurutani and Zhou, 2003]. The upstream IMF of shocks
strongly affect auroral responses: southward IMF can lead to
substorm expansion phase triggerings, nearly zero IMF leads
to pseudobreakup events, and northward IMF for quiescent
events. Yue et al. [2010] investigated 106 interplanetary
shocks during 19972007 and found that IMF B
z
keep south-
ward or northward before the shock arrival and turns out to be
more negative or positive after the arrival of the shock.
[9] In this study, we have examined the validity and
precision of the R-M effect and interplanetary shock related
R-M effect by analyzing a large amount of the data of
magnetic field and geomagnetic indices from 1968 to 2010
and 1270 SSC (storm sudden commencement) events under
different IMF polarity. We showed that the IMF polarity is
one of the most important parameters when investigating
seasonal and diurnal variation of geomagnetic activity.
Geomagnetic activity are rather strong at spring and fall
equinoxes with different IMF orientations: the geomagnetic
activity is much more intense around fall equinox when the
direction of IMF is away the Sun, while much more intense
around spring equinox when the direction of IMF is
toward the Sun, which is identical with the R-M effect
under different IMF polarity. This feature also exists before
and after SSCs.
2. Seasonal and Diurnal Variation
of Geomagnetic Activity: The R-M Effect
With Positive/Negative IMF Polarity
[10] Since the R-M effect was first put forward in 1973,
it becomes the most prevailing hypothesis accounting for
the semiannual variation of geomagnetic activity. The R-M
effect explains the semiannual variation of geomagnetic
activity by the varying probability of IMF southward com-
ponent in GSM coordinate system, which is caused by the
varying orientation of the GSM coordinate system relative
to GSEQ coordinate system. The IMF southward component
is widely believed to be the controlling factor of geomagnetic
activity. The Earth rotation axis tilts 23.5from Z axis in
geocentric solar ecliptic (GSE) coordinate system, causes
the seasonal variation of the projection of IMF onto Z axis
in GSM, and thus causes the seasonal variation of the prob-
ability of southward IMF in GSM. The dipole axis inclines
11.5from the rotation axis and leads to the diurnal varia-
tion of the southward IMF projection.
[11] There are three main assumptions in the R-M effect:
the IMF is always along the Parker spiral direction and its
magnitude is constant; the IMF is either away or toward the
Sun with equal possibility; and the northward IMF has no
effect on the geomagnetic activity. Since different directions
of IMF can have different effects on the magnetosphere,
we study the R-M effect under different IMF polarity, that is,
away the Sun (positive polarity, IMF B
y
> 0 in GSE coor-
dinate system), we define it as the R-M effect with positive
IMF polarity, and toward the Sun (negative polarity, IMF
B
y
< 0), defined as the R-M effect with negative IMF
polarity.
[12] Figure 2 shows the R-M effect with positive/negative
IMF polarity, namely, the contour plot of IMF B
z
southward
component in GSM coordinate system under the situation
that IMF is away/toward the Sun, assuming IMF B
y
of 1 g
in GSEQ coordinate system, and the schematic diagram of
the R-M effect under different IMF polarity.
[13] At spring equinox, the Earth rotation axis is pointed
23.5away from Z axis toward Y axis in GSE coordinate
system, so that the IMF toward the Sun makes a negative
projection onto the direction of Z axis of GSM coordinate
system, which can increase the efficiency of dayside recon-
nection and enhance the geomagnetic activity, and the IMF
away the Sun makes a positive projection onto the direction
of Z axis, which has no effect on the geomagnetic activity
according to assumptions in the R-M effect. At fall equinox,
the angle from the Earth rotation axis to Y axis in GSE
coordinate is 113.5, so that the IMF away the Sun pro-
jects onto the direction of Z axis in GSM coordinate system
with a negative projection, and the IMF toward the Sun
projects onto the Z axis with a positive projection. Figure 2
shows that when IMF is away the Sun, corresponding to
the R-M effect with positive IMF polarity, the maximum of
possibility of southward B
z
appears around fall equinox, and
when IMF is toward the Sun, corresponding to the R-M
effect with negative IMF polarity, the maximum appears
around spring equinox.
[14] To confirm the prospective R-M effect with positive/
negative IMF polarity, we have used the data set with 1 hour
resolution from Omniweb (http://omniweb.gsfc.nasa.gov/)
from 1968 to 2010 to analyze the contours of vB
s
, defined as
vB
z
in GSM coordinate system when IMF is southward,
and defined to be zero when IMF is northward, in a 24 24
lattice of day of year and UT hour, under different polarity of
IMF. Here we divide UT and day of year to 24 bins
respectively, with a bin of 1 hour 15.25 days, and calcu-
late the probability of vB
s
>1mV/mwithin each bin under
different IMF polarity.
[15] Figure 3 shows the contour plots of the probability of
vB
s
>1mV/mwith IMF B
y
positive/negative. The patterns of
variation showed in Figure 3 are generally identical with the
R-M effect with positive/negative IMF polarity, as the
maximum appears at about UT 23:00 at spring equinox
when IMF B
y
< 0, as well as the maximum appears around
UT 10:00 at fall equinox when IMF B
y
> 0. The correlation
coefficient between the probability of vB
s
>1mV/munder
positive IMF B
y
and IMF B
z
southward component in
GSM indicated by the R-M effect with positive IMF polarity
(as Figure 2 shows) is 0.91, while the coefficient between the
probability of vB
s
>1mV/munder negative IMF B
y
and IMF
B
z
indicated by the R-M effect with negative IMF polarity is
0.90. The results indicate the R-M effect with positive/
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
3of15
negative IMF polarity can predict the seasonal and diurnal
variation of vB
s
with positive/negative IMF B
y
precisely.
[16] In order to investigate the seasonal and diurnal varia-
tion of variation of ring current injection, we derive the ring
current injection function Q from the Burton from the Burton
equations [Burton et al., 1975]:
dDst*
dt ¼QtðÞDst*
tð1Þ
Dst* ¼Dst b
ffiffiffi
P
pþcð2Þ
where Q is assumed as a function of vB
s
,Dst* is the
pressure-corrected Dst index, tis the decay time of ring
current, Pis the solar wind dynamic pressure, and bis the
pressure-correction coefficient. OBrien and McPherron
[2000] gave Q(vB
s
)=4.4(VB
s
E
c
) when VB
s
>E
c
,
Q= 0 when VBs <E
c
. Here E
c
is the ring current injection
cutoff and E
c
= 0.49 mV/m;t= 2.40e
9.74/(4.69+VBs)
, and
b = 7.26.
[17] Then, we divide day of year and UT into a 24 24
lattice and plot contours of the probability of Q<10 nT/hwith
positive/negative IMF B
y
. The contours are shown in Figure 4.
It is evident that the pattern of variation of ring current
injection with positive/negative IMF B
y
fits well with R-M
effect with positive/negative IMF polarity, with maximum at
spring equinox when IMF B
y
< 0 and at fall equinox when
IMF B
y
> 0. The correlation coefficients of situations during
positive and negative B
y
with the R-M effect with positive/
negative IMF polarity are 0.69 and 0.72 respectively.
[18] On the other hand, to examine whether the R-M effect
with positive/negative IMF polarity predicts the geomag-
netic activity at low as well as high latitudes accurately, we
perform the contour plots of variation of Dst index and AE
index with different orientations of IMF B
y
and B
z
in a
24 24 lattice, too.
Figure 2. The schematic diagram of the R-M effect with positive/negative IMF polarity. At spring equinox,
IMF toward the Sun projects onto Z axis of GSM coordinate system and converts into an effective southward
component; at fall equinox, IMF away the Sun projects onto Z axis with a southward projection. The R-M
effect with positive/negative IMF polarity shows the semiannual variation of IMF B
z
southward component
in GSM coordinate system due to IMF B
y
in GSEQ coordinate system of 1 g.
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
4of15
[19] The hourly Dst index, calculated from the magnetic
field disturbances measured by midlatitude geomagnetic
stations, is a widely accepted indicator of ring current
intensity. AE index describes the global auroral electrojet
activity so that it can be used as a proxy of geomagnetic
activity at high latitude [Rostoker, 1972]. Figures 5 and 6
show the seasonal and diurnal variation of Dst and AE
indices under different situations of IMF B
y
and B
z
,
respectively.
[20] From both figures we can find the pattern of semi-
annual variation with positive IMF B
y
is consistent with the
R-M effect with positive IMF polarity, which shows the
maximum at fall equinox, while the pattern with negative
IMF B
y
fits well with the R-M effect with negative IMF
polarity, as the maximum appears at spring equinox. The
diurnal variation of Dst index, on the other hand, is not as
clear as the seasonal variation, but Figures 5a and 5b still
reveal some UT variation pattern which is consistent with
the R-M effect with positive/negative IMF polarity.
[21] Also, compared contours of Dst and AE indices with
southward/northward IMF B
z
, we can find that the average
value of geomagnetic indices is much higher when IMF B
z
is
southward than northward, which indicates much more
intense geomagnetic activity resulting from more efficient
dayside reconnection.
[22] Furthermore, by comparing Figures 5a and 5b, we can
see that the average value of Dst index is much larger with
positive IMF B
y
, which indicates that the R-M effect with
positive IMF polarity can further enhance the geomagnetic
activity. On the other hand, Figure 3 doesnt show much
difference in IMF B
s
under positive/negative IMF polarity,
which suggests the enhanced geomagnetic activity under
positive IMF B
y
is more likely caused by the enhanced
dayside reconnection rate.
[23] The correlation coefficients of Figures 5a, 5b, 5c, and
5d with corresponding R-M effect are 0.68, 0.66, 0.67
and 0.72 respectively, and for Figures 6a, 6b, 6c, and 6d,
the correlation coefficients are 0.59, 0.58, 0.72 and 0.68
Figure 3. The seasonal and diurnal variation of probability of vB
s
>1mV/mwith positive/negative IMF
B
y
and southward IMF B
z
, calculated in a 24 24 lattice and smoothed by a 3 3 average.
Figure 4. The seasonal and diurnal variation of probability of Q<10 nT/hwith positive/negative IMF
B
y
and southward IMF B
z
, calculated as Figure 3.
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
5of15
respectively. The correlation coefficients are summarized in
Table 1.
3. Seasonal and Diurnal Variation
of Geomagnetic Activity Under Extreme
Solar Wind Conditions
3.1. Seasonal and Diurnal Variation of Geomagnetic
Activity Before and After SSC Events With Different
Orientation of IMF B
z
[24] To examine seasonal and diurnal variation of geo-
magnetic activity before and after SSCs, we use vB
s
from
OMNI database to demonstrate the R-M effect is present
before and after SSCs. We divide UT and the day of year into
12 bins respectively, with each bin of 2 hours 30.5 days,
and calculate the probability of vB
s
>1mV/mbefore and
after SSCs respectively in each bin. Here we use the SSC list
from IAGA Bulletin and analyze 1270 SSCs between 1968
and 2010.
[25] According to Zhang et al. [2008], the duration of a
CME preceding shock sheath is generally shorter than
24 hours, so we choose a time period of 12 hours before and
after each SSC event respectively to perform contour plots
before and after SSCs. In order to examine the effect of the
orientation of IMF B
z
on the R-M effect, we perform the
contour plots under southward and northward IMF B
z
.
[26] Figure 7 shows the seasonal and diurnal variation of
the probability of vB
s
>1mV/mbefore and after SSCs
respectively under southward IMF B
z
.Its worth mentioning
that the scales of two panelscolor bars are different. From
Figure 7, we find that the pattern of vB
s
variation after SSCs
is quite similar with the R-M effect. It shows higher proba-
bility of southward IMF B
z
at the equinoxes and lower at the
solstices, as well as higher probability at midnight around
spring equinox while at noon around autumn equinox.
Before SSCs, the probability of vB
s
>1mV/mis much
smaller than after SSCs, however, the pattern doesnt well fit
the R-M effect. The correlation coefficient between the
probability of vB
s
>1mV/mbefore/after SSCs and the
angle q, the controlling parameter of the R-M effect, is
0.11/0.60.
[27] To compare with the R-M effect, we also calculate
the correlation coefficient between the probability of
vB
s
>1mV/mand the angle y, which is the dominant
parameter in the equinoctial hypothesis. The correlation
coefficient before/after SSCs is 0.02/0.39 respectively.
Figure 5. The seasonal and diurnal variation of Dst index with positive/negative IMF B
y
and southward/
northward IMF B
z
, calculated as Figure 3.
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
6of15
It indicates that the R-M effect can explain the probability of
southward IMF B
z
after SSCs more precisely, while the
equinoctial hypothesis can hardly explain the semiannual
variation of vB
s
clearly.
[28] We also calculate the probability of Q<10 nT/hin
a1212 lattice. The results are showed in Figure 8.
Figure 8 shows the probability of Q<10 nT/hbefore and
after SSCs under southward IMF, which reveals prominent
semiannual variation as higher at the equinoxes and lower
at the solstices. The correlation coefficient of the probability
of Q<10 nT/hbefore/after SSCs and the angle qis 0.30/
0.57, while for the angle ythe correlation coefficient is
0.15/0.62. The correlation coefficients show that both qand
ycan explain part of seasonal and diurnal variations of
large ring current injection. And also, before SSCs both
hypotheses explain less variations of ring current injections
than after SSCs.
[29] Moreover, in order to show the seasonal and diurnal
variation of geomagnetic activity under extreme solar wind
conditions, we perform the contour plots of Dst index
before and after SSCs in a 12 12 lattice, and compare the
contour plots with the R-M effect as well as the equinoctial
hypothesis to examine whether these two hypotheses
account for variation of Dst index before and after SSCs.
Figure 6. The seasonal and diurnal variation of AE index with positive/negative IMF B
y
and southward/
northward IMF B
z
, calculated as Figure 3.
Table 1. Summarization of Correlation Coefficients of Contours During Positive/Negative IMF B
y
and Southward/Northward B
z
With Corresponding R-M Effect With Positive/Negative IMF Polarity
B
y
>0,B
z
<0 B
y
<0,B
z
<0 B
y
>0,B
z
>0 B
y
<0,B
z
>0
and R-M Effect (+IMF) and R-M Effect (IMF) and R-M Effect (+IMF) and R-M Effect (IMF)
vB
s
0.91 0.90
Q0.69 0.72
Dst index 0.68 0.66 0.67 0.72
AE index 0.59 0.58 0.72 0.68
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
7of15
[30] Figure 9 shows the seasonal and diurnal variation of
Dst index before and after SSCs under southward and
northward IMF B
z
conditions, respectively. From Figure 9,
we find that the semiannual variation with equinoxes maxi-
mum is evident, but the diurnal variation cannot be well
explained by both hypotheses. The correlation coefficient of
Dst variation before/after SSCs under southward IMF con-
ditions (Figures 9a and 9b) with the qangle is 0.42/0.51; as
to the yangle the correlation coefficient is 0.13/0.52;
under northward IMF conditions (Figures 9c and 9d), the
correlation coefficient of Dst variation before/after SSCs
with the qis 0.27/0.26, while for yis 0.01/0.48.
[31] It shows that both hypotheses predict part of the
variation of Dst index after SSCs; however, as for the situ-
ation before SSCs, both hypotheses, especially the equi-
noctial hypothesis, cannot predict the variation of Dst index.
Compared with different IMF orientations, we find that with
southward IMF B
z
, the absolute value of Dst index is much
higher than another two cases, which indicates more energy
transfer and larger geomagnetic storms.
[32] Moreover, by distinguishing different orientations of
IMF B
y
, we can derive higher correlation coefficients
between the R-M effect with positive/negative IMF polarity
and variation of geomagnetic activity, which can be seen in
the following section. The correlation coefficients of each
situation with the angle qand yare summarized in Table 2.
3.2. Seasonal and Diurnal Variation of Geomagnetic
Activity Before and After SSCs With Positive/Negative
IMF B
y
[33] The R-M effect explains semiannual variation of
geomagnetic activity using the changing IMF southward
component, which is caused by the changing of GSM
coordinate system relative to GSEQ coordinate system. To
confirm the R-M effect is indeed present before and after
SSC events and has a great influence on the geomagnetic
activity, we perform the contour plots under different IMF
B
y
orientations, that is, positive and negative. The contour
plots of semiannual variation of the probability of
vB
s
>1mV/m, the probability of Q<10 nT/hand the Dst
Figure 7. Seasonal and diurnal variation of the probability of VB
s
>1mV/mbefore and after SSCs under
southward IMF B
z
.
Figure 8. Seasonal and diurnal variation of the probability of Q<10 nT/hbefore and after SSCs under
southward IMF B
z
.
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
8of15
index before/after SSCs with different IMF B
y
are showed in
Figures 10, 11, and 12. From these figures, we can find that
as to the situation that IMF B
y
is negative, the geomagnetic
activity is more intense at spring equinox and relatively
weak at fall equinox; when IMF B
y
is positive, the geo-
magnetic activity around fall equinox is much more intense
and less intense at the spring equinox, which is consistent
with the R-M effect with positive/negative IMF polarity.
[34] Meanwhile, we perform contour plots of AE index
before/after SSCs under different IMF B
y
conditions, as
shown in Figure 13c, 13d, 13e, and 13f. As a comparison,
we also perform contour plots of AE index before/after SSCs
under all circumstances (Figures 13a and 13b). The variation
of AE index before and after SSCs dont show clear feature
of semiannual variation and UT variation as the R-M effect
or the equinoctial hypothesis suggested, which is likely
caused by the equatorward expansion of the auroral elec-
trojets and the longitudinal station gaps [Ahn et al., 2000].
However, even the seasonal and diurnal variation of AE
index is quite distinctive, AE variation with positive/negative
IMF B
y
still shows identical feature as the R-M effect with
positive/negative IMF polarity: when IMF B
y
is negative, AE
index is higher around the spring equinox and relatively
lower around the fall equinox; when IMF B
y
is positive, it is
Figure 9. Seasonal and diurnal variation of Dst index before and after SSCs under different IMF B
z
conditions.
Table 2. Summarization of Correlation Coefficients Between Geomagnetic Activity Variation Before and After SSCs With Different
IMF B
z
Conditions and Two Hypotheses
Time Period IMF Bz Direction The qAngle The yAngle
The probability of vB
s
>1mV/mbefore SSCs IMF B
z
southward 0.11 0.02
after SSCs IMF B
z
southward 0.60 0.39
The probability of Q<10 nT/hbefore SSCs IMF B
z
southward 0.30 0.15
after SSCs IMF B
z
southward 0.57 0.62
The Dst index before SSCs IMF B
z
southward 0.42 0.13
IMF B
z
northward 0.27 0.01
after SSCs IMF B
z
southward 0.51 0.52
IMF B
z
northward 0.26 0.48
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
9of15
higher around the fall equinox and lower around the spring
equinox.
[35] The correlation coefficients of the variation before/
after SSCs with positive/negative IMF B
y
and corresponding
R-M effect are given in Table 3. For vB
s
we can find that the
R-M effect with positive/negative IMF polarity can explain a
large part of seasonal and diurnal variation of vB
s
, and the
correlation coefficients are up to 0.90, which has been
improved significantly compared to the R-M effect without
distinguishing IMF polarity. For Q and Dst, the correlation
coefficients are also improved a lot. AE index, which
doesnt show clear semiannual variation and the correlation
coefficient of AE variation and the R-M effect before/after
SSCs is only 0.0003/0.21, indeed show features identical
with the R-M effect with positive/negative IMF polarity
when distinguishing the polarity of IMF B
y
, and the corre-
lation coefficients are increased up to 0.65.
[36] The results confirm that the R-M effect is indeed
present before and after SSCs and can predict southward
IMF accurately as well as a large part of geomagnetic
activity. Also, it shows the IMF polarity is an important
parameter when investigating semiannual variation as well
as the R-M effect. To compare the R-M effect with the
equinoctial hypothesis, we also calculate the correlation
coefficients of the variation with the yangle, which are also
shown in Table 3.
[37] We find that the correlation coefficients of the variation
and the R-M effect with positive/negative IMF polarity
are much higher than that of the equinoctial hypothesis, which
can demonstrate the R-M effect can predict the geomagnetic
activity more efficiently when we distinguish positive/
negative IMF B
y
; although the equinoctial hypothesis also
explains part of the variation, it cannot explain the difference
between the semiannual variation with positive IMF B
y
and
negative IMF B
y
.
4. Discussion and Conclusions
[38] By defining the R-M effect with positive/negative
IMF polarity according to the IMF polarity, that is, away/
toward the Sun, we find that the patterns of semiannual
variation predicted by them are totally different. The R-M
effect with positive IMF polarity predicts larger IMF B
z
southward component in GSM coordinate system at fall
equinox, which indicates more efficient dayside reconnec-
tion and energy input leading to more intense geomagnetic
activity, while the R-M effect with negative IMF polarity
shows the maximum at spring equinox.
[39] To confirm the validity of the R-M effect with positive/
negative IMF polarity, we plot contours of semiannual var-
iation of probability of vB
s
>1mV/m, probability of large
ring current injection Q<10 nT/h, geomagnetic indices
Figure 10. Seasonal and diurnal variation of the probability of VB
s
>1mV/mbefore and after SSCs
under different IMF B
y
conditions.
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
10 of 15
Dst and AE under different conditions of IMF B
y
, and find
the results are identical with the R-M effect with positive/
negative IMF polarity. It indicates that the R-M effect
predicts the semiannual variation of geomagnetic activity
more accurately, while other hypotheses cannot explain such
difference at equinoxes between different IMF polarity.
[40] Also, the contours show the pattern of diurnal variation
with positive/negative IMF B
y
is similar to the R-M effect
with positive/negative IMF polarity, which indicates that
the R-M effect can also predict part of diurnal variation.
Compared to previous studies [OBrien and McPherron,
2002; Cliver et al., 2000], the correlation coefficients
between contours of geomagnetic activity and the R-M effect
are improved a lot. The correlation coefficient of vB
s
with
the R-M effect with positive/negative IMF polarity in our
paper is 0.91/0.90, and the correlation coefficient of Qwith
the R-M effect with positive/negative IMF polarity is 0.69/
0.72, while OBrien and McPherron [2002] derived the rank
order correlation coefficient of 0.55 0.03 between
possibility of vB
s
>1mV/mwith angle q, which is the
controlling parameter of the R-M effect, and the corre-
lation coefficient of 0.44 0.04 between possibility of
Q<10 nT/hand q. Cliver et al. [2000] calculated the
correlation coefficient between Dst index and R-M angle
and obtained 0.45, and the seasonal and diurnal variation
of AE index in their paper doesnt show a pattern of the
R-M effect clearly, while our correlation coefficients with
Dst index are much higher and the semiannual variation of
AE index with positive/negative IMF polarity also fits the
R-M effect with positive/negative IMF polarity well. The
results show that the R-M effect with positive/negative IMF
polarity explains semiannual variation of geomagnetic
activity more precisely.
[41] From Figure 5, we find that the average value of Dst
index is much larger with positive IMF B
y
when IMF B
z
is
southward, which indicates that compared to the R-M effect
with negative IMF polarity, the R-M effect with positive
IMF polarity can further enhance the geomagnetic activity.
[42] Also, our results indicate that unlike the R-M effect
which assumes the northward IMF has no effect on the geo-
magnetic activity, the northward IMF can also have effects on
the magnetosphere. This is consistent with recent research,
although it is much weaker than southward IMF situation.
Thus the assumption made by Russell and McPherron [1973]
may introduce some errors in the R-M effect.
[43] For the seasonal and diurnal variation of geomagnetic
activity before and after SSCs, from Figure 7, we find that
the R-M effect predicts the seasonal and diurnal variation of
southward IMF B
z
more precisely than the equinoctial
hypothesis after SSCs. But when it comes to ring current
injections as well as Dst index, the equinoctial hypothesis
explains similar amount of variance as the R-M effect, as
Figure 11. Seasonal and diurnal variation of the probability of Q<10 nT/hbefore and after SSCs
under different IMF B
y
conditions.
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
11 of 15
shown in Figures 8 and 9. It indicates that the R-M effect
predicts southward IMF precisely and explains part of
semiannual variation of geomagnetic activity, while the
equinoctial hypothesis also explains part of variance of
geomagnetic activity after SSCs. Our result is consistent
with Cliver et al. [2000], which believes the equinoctial
hypothesis contributes to the semiannual variation of geo-
magnetic activity by reducing the coupling efficiency of
dayside reconnection at solstices, as well as OBrien and
McPherron [2002]. On the other hand, we also find both
the R-M effect and the equinoctial hypothesis can explain
seasonal and diurnal variation of geomagnetic activity more
accurately after SSCs than before SSCs. This may be
because before SSCs IMF B
z
component is too small to
show clear seasonal and diurnal features.
[44] However, as shown in Figures 10, 11, 12, and 13,
before/after SSCs with positive or negative IMF B
y
, the
semiannual variation of the probability of vB
s
>1mV/mand
Q<10 nT/has well as the Dst and AE indices all reveal
clear features as much higher at spring equinox when IMF
B
y
< 0, while much higher at fall equinox when IMF B
y
>0.
We find that the R-M effect with positive/negative IMF
polarity can explain such difference perfectly, while the
equinoctial hypothesis cannot.
[45] The results presented in this paper demonstrate before
and after SSCs, the R-M effect is actually present and
accounts for a large part of semiannual variation of geo-
magnetic activity. It also shows that the IMF polarity, that is,
positive/negative, is a very important parameter when
examining semiannual variation of geomagnetic activity or
the R-M effect.
[46]Crooker et al. [1992] found that differential com-
pression at the shock increases the Parker spiral angle and
increases the projected IMF southward component, which
results in the enhancement of the R-M effect. Compared
our results with OBrien and McPherron [2002], we find
that the probability of vB
s
>1nT as well as Q<10 nT/hof
our study are higher than OBrien and McPherrons, and
the correlation coefficients of variation and the angle qas
well as the angle yof our study are approximately equal
to theirs. It indicates the differential compression at the
interplanetary shock can result in the enhancement of the
R-M effect, as Crooker et al. [1992] show. And compared
the results of the Dst and the AE index with Cliver et al.
[2000], we can also derive the same conclusion.
[47] However, since we calculate the semiannual and
diurnal variation before/after SSCs with positive and nega-
tive IMF B
y
respectively, and derive much higher correlation
coefficients with the R-M effect with positive/negative IMF
polarity, we can demonstrate the presence of the R-M effect
under extreme solar wind conditions more evidently, and
show before/after SSCs, the R-M effect with positive/
Figure 12. Seasonal and diurnal variation of the Dst index before and after SSCs under different IMF B
y
conditions.
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
12 of 15
negative IMF polarity can explain difference of variance
of geomagnetic activity with different IMF B
y
conditions
perfectly while the equinoctial hypothesis cannot.
[48] In summary, we have performed contour plots of
parameters and calculated the correlation coefficients of
each IMF condition with the R-M effect with positive/
negative IMF polarity and interplanetary shock related R-M
effect as well as the equinoctial hypothesis by analyzing a
large amount of the data of magnetic field and geomagnetic
indices from 1968 to 2010 and 1270 SSC (storm sudden
commencement) events. The main results in this paper can be
summarized as:
[49] 1. The polarity of IMF is one of controlling factors for
the semiannual and diurnal variation of geomagnetic activity
or the R-M effect: the geomagnetic activity is much more
intense around fall equinox when the direction of IMF is
away the Sun, while much more intense around spring
equinox when the direction of IMF is toward the Sun.
[50] 2. The average value of Dst index is much higher
under the R-M effect with positive IMF polarity when IMF
Figure 13. Comparison between seasonal and diurnal variation of the AE index before and after SSCs
with different IMF B
y
orientations.
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
13 of 15
B
z
is southward, which indicates that the R-M effect with
positive IMF polarity can indeed enhance the geomagnetic
activity.
[51] 3. After SSC events, the correlation coefficient
between variation of vB
s
under southward IMF and qangle,
the controlling parameter of the R-M effect, is 0.60, while
the correlation coefficient between vB
s
and yangle, the
controlling parameter of the equinoctial hypothesis, is only
0.39. It indicates the R-M effect can explain more variation
of IMF southward component than the equinoctial hypoth-
esis. However, as for Q and Dst index, the equinoctial
hypothesis explains similar amount of variance as the
R-M effect.
[52] 4. The semiannual and diurnal variations under
extreme solar wind conditions with positive and negative IMF
B
y
are quite different: when B
y
is positive, the maximum of
geomagnetic activity appears around the fall equinox; when
B
y
is negative, the maximum appears at the spring equinox.
The correlation coefficients between variance of geomagnetic
activity before/after SSCs with positive/negative IMF B
y
and
corresponding R-M effect are improved significantly: for vB
s
,
Q, Dst and AE, the correlation coefficients after SSCs
with positive/negative IMF B
y
are 0.93/0.92, 0.61/0.62,
0.60/0.61 and 0.65/0.64 respectively.
[53] 5. As for the equinoctial hypothesis, the correlation
coefficients before/after SSCs with positive/negative IMF B
y
are much lower than the R-M effect with positive/negative
IMF polarity. These results indicate that equinoctial hypoth-
esis cannot explain the difference between geomagnetic
activity variation under positive IMF polarity and negative
IMF polarity, and its prediction precision is much lower than
the R-M effect with positive/negative IMF polarity.
[54] Thus, the R-M effect with positive/negative IMF
polarity is more reasonable to explain seasonal and diurnal
variation of geomagnetic activity under different IMF polar-
ity and extreme solar wind conditions.
[55]Acknowledgments. This work was supported by Major Project
of Chinese National Programs for Fundamental Research and Development
(2012CB825603) and National Natural Science Foundation of China
(40831061, 41074117 and 41050110440). We would like to thank Omni-
Web for providing the Omni database.
[56]Masaki Fujimoto thanks the reviewers for their assistance in
evaluating this paper.
References
Ahn, B.-H., H. W. Kroehl, Y. Kamide, and E. A. Kihn (2000), Universal
time variations of the auroral electrojet indices, J. Geophys. Res.,105,
267275.
Araki, T. (1994), A physical model of the geomagnetic sudden commence-
ment, in Solar Wind Sources of Magnetospheric Ultra-low-Frequency
Waves,Geophys. Monogr. Ser., vol. 81, edited by M. J. Engebretson,
K. Takahashi and M. Scholer, pp. 183200, AGU, Washington, D. C.
Bartels, J. (1932), Terrestrial-magnetic activity and its relation to solar phe-
nomena, Terr. Magn. Atmos. Electr.,37,1.
Burton, R. K., R. L. McPherron and C. T. Russell (1975), An empirical rela-
tionship between interplanetary conditions and Dst, J. Geophys. Res.,80,
42044214.
Cliver, E. W., Y. Kamide, and A. G. Ling (2000), Mountains versus
valleys: Semiannual variation of geomagnetic activity, J. Geophys. Res.,
105, 24132424.
Cliver, E. W., Kamide, Y., Ling, A. G., and Yokoyama, N. (2001), Semian-
nual variation of the geomagnetic Dst index: Evidence for a dominant
nonstorm component, J. Geophys. Res.,106, 21,29721,304.
Cliver, E. W., Kamide, Y., and Ling, A. G. (2002), The semiannual varia-
tion of geomagnetic activity: phases and profiles for 130 years of aa data,
J. Atmos. Sol. Terr. Phys.,64,4753.
Colburn, D. S., and C. P. Sonett (1966), Discontinuities in the solar wind,
Space Sci. Rev.,5, 439481.
Cortie, A. L. (1912), Sunspots and terrestrial magnetic phenomena, 1898
1911, Mon. Not. R. Astron. Soc.,73,5260.
Crooker, N. U., and Siscoe, G. L. (1986), On the limit of energy transfer
through dayside merging, J. Geophys. Res.,91, 13,39313,397.
Crooker, N. U., Cliver, E. W., and Tsurutani, B. T. (1992), The semiannual
variation of great geomagnetic storms and the postshockRussell-McPherron
effect preceding coronal mass ejecta, Geophys. Res. Lett.,19,429432.
Echer, E., and W. D. Gonzalez (2004), Geoeffectiveness of interplanetary
shocks, magnetic clouds, sector boundary crossings and their combined
occurrence, Geophys. Res. Lett.,31, L09808, doi:10.1029/2003GL019199.
Echer, E., W. D. Gonzalez, A. Dal Lago, L. E. A. Vieira, F. L. Guarnieri,
A. L. C. Gonzalez, and N. J. Schuch (2005), Interplanetary shocks and
sudden impulses during solar maximum (2000) and solar minimum
(19951996), Adv. Space Res.,36, 23132317.
Echer, E., W. D. Gonzalez, and B. T. Tsurutani (2011), Statistical
studies of geomagnetic storms with peak Dst 50 nT from 1957
to 2008, J. Atmos. Sol. Terr. Phys.,73, 14541459.
Gonzalez, W. D., J. A. Joselyn, Y. Kamide, H. W. Kroehl, G. Rostoker,
B. T. Tsurutani, and V. M. Vasyliunas (1994), What is a geomagnetic
storm?, J. Geophys. Res.,99, 57715792.
Joselyn, J. J., and B. T. Tsurutani (1990), Geomagnetic sudden impulses
and storm sudden commencements: A note on terminology, Eos Trans.
AGU,71(47), 1808.
Table 3. Summarization of Correlation Coefficients Between Geomagnetic Activity Variation Before and After SSCs With Different
IMF B
y
Orientations and Two Hypotheses
Time Period +/IMF B
y
The R-M Effect With +/IMF Polarity The Equinoctial Hypothesis
The probability of vB
s
>1mV/mbefore SSCs B
y
> 0 0.84 010
B
y
< 0 087 0.04
after SSCs B
y
> 0 0.93 0.12
B
y
< 0 0.92 0.05
The probability of Q<10 nT/hbefore SSCs B
y
> 0 0.67 0.13
B
y
<0 0.30 0.12
after SSCs B
y
> 0 0.61 0.59
B
y
< 0 0.62 0.35
The Dst index before SSCs B
y
>0 0.58 0.01
B
y
<0 0.60 0.06
after SSCs B
y
>0 0.60 0.38
B
y
<0 0.61 0.45
The AE index before SSCs B
y
> 0 0.61 0.002
B
y
< 0 0.61 0.07
after SSCs B
y
> 0 0.65 0.20
B
y
< 0 0.64 0.17
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
14 of 15
Jurac, S., J. C. Kasper, J. D. Richardson, and A. J. Lazarus (2002), Geomag-
netic disturbances and their relationship to interplanetary shock para-
meters, Geophys. Res. Lett.,29(10), 1463, doi:10.1029/2001GL014034.
Li, X., D. N. Baker, S. G. Kanekal, M. Looper, and M. Temerin (2001),
Long term measurements of radiation belts by SAMPEX and their varia-
tions, Geophys. Res. Lett.,28, 38273830.
McIntosh, D. H. (1959), On the annual variation of magnetic disturbance,
Philos. Trans. R. Soc. London A,251, 525552.
Mursula, K., E. Tanskanen, and J. J. Love (2011), Spring-fall asymmetry of
substorm strength, geomagnetic activity and solar wind: Implications of
semiannual variation and solar hemispheric asymmetry, Geophys. Res.
Lett.,38, L06104, doi:10.1029/2011GL046751.
OBrien, T. P., and R. L. McPherron (2000), An empirical phase space anal-
ysis of ring current dynamics: Solar wind control of injection and decay,
J. Geophys. Res.,105, 77077719.
OBrien, T. P., and R. L. McPherron (2002), Seasonal and diurnal variation
of Dst dynamics, J. Geophys. Res.,107(A11), 1341, doi:10.1029/
2002JA009435.
Orlando, M., G. Moreno, M. Parisi, and M. Storini (1993), Semiannual var-
iation of the geomagnetic activity and solar wind parameters, Geophys.
Res. Lett.,20, 22712274.
Rostoker, G. (1972), Geomagnetic indices, Rev. Geophys.,10, 935950.
Russell, C. T., and R. L. McPherron (1973), Semiannual variation of geo-
magnetic activity, J. Geophys. Res.,78,92108.
Russell, C. T., Y. L. Wang, and J. Raeder (2003), Possible dipole tilt
dependence of dayside magnetopause reconnection, Geophys. Res. Lett.,
30(18), 1937, doi:10.1029/2003GL017725.
Siscoe, G., and N. Crooker (1996), Diurnal oscillation of Dst: A manifestation
of the Russell-McPherron Effect, J. Geophys. Res.,101, 24,98524,989.
Siscoe, G., V. Formisano, and A. J. Lazarus (1968), Relation between
geomagnetic sudden impulses and solar wind pressure changes: An
experimental investigation, J. Geophys. Res.,73, 48694874.
Smith, E. J., J. A. Slavin, R. D. Zwickl, and S. J. Bame (1986), Shocks and
storm sudden commencements, in Solar Wind-Magnetosphere Coupling,
edited by Y. Kamide and J. A. Slavin, pp. 345365, Terra Sci., Tokyo.
Svalgaard, L. (1977), Geomagnetic activity: Dependence on solar wind
parameters, in Coronal Holes and High Speed Wind Streams, edited by
J. B. Zirker, p. 371, Colo. Assoc. Univ. Press, Boulder, Colo.
Svalgaard, L. (2011), Geomagnetic semiannual variation is not overesti-
mated and is not an artifact of systematic solar hemispheric asymmetry,
Geophys. Res. Lett.,38, L16107, doi:10.1029/2011GL048616.
Svalgaard, L., E. W. Cliver, and A. G. Ling (2002), The semiannual varia-
tion of great geomagnetic storms, Geophys. Res. Lett.,29(16), 1765,
doi:10.1029/2001GL014145.
Tsurutani, B. T., and X. Y. Zhou (2003), Interplanetary shock triggering of
substorms: wind and polar, Adv. Space Res.,31, 10631067.
Tsurutani, B. T., G. S. Lakhina, O. P. Verkhoglyadova, W. D. Gonzalez,
E. Echer, and F. L. Guarnieri (2011), A review of interplanetary disconti-
nuities and their geomagnetic effects, J. Atmos. Sol. Terr. Phys.,73,519.
Wang, C., C. X. Li, Z. H. Huang, and J. D. Richardson (2006), Effect of
interplanetary shock strengths and orientations on storm sudden com-
mencement rise times, Geophys. Res. Lett.,33, L14104, doi:10.1029/
2006GL025966.
Yue, C., et al. (2010), Geomagnetic activity triggered by interplanetary
shocks, J. Geophys. Res.,115, A00I05, doi:10.1029/2010JA015356.
Zhang, J., W. Poomvises, and I. G. Richardson (2008), Sizes and relative
geoeffectiveness of interplanetary coronal mass ejections and the preced-
ing shock sheaths during intense storms in 19962005, Geophys. Res.
Lett.,35, L02109, doi:10.1029/2007GL032045.
Zhou, X., and B. T. Tsurutani (2001), Interplanetary shock triggering of
nightside geomagnetic activity: Substorms, pseudobreakups, and quies-
cent events, J. Geophys. Res.,106, 18,95718,967.
Zong, Q.-G., X.-Z. Zhou, Y. F. Wang, X. Li, P. Song, D. N. Baker, T. A.
Fritz, P. W. Daly, M. Dunlop, and A. Pedersen (2009), Energetic elec-
trons response to ULF waves induced by interplanetary shocks in the
outer radiation belt, J. Geophys. Res.,114, A10204, doi:10.1029/
2009JA014393.
ZHAO AND ZONG: SEMIANNUAL VARIATION A11222A11222
15 of 15
... However, as far as we are aware, there are not many studies that analyzed B s based on measurements. O' Brien and McPherron (2002) gave a contour plot of the probability of vB s > 1 mV/m as do Zhao and Zong (2012) but for positive/negative IMF polarity. Similar contours for B s were given by Lockwood et al. (2016). ...
... Another example is the work by Zhao and Zong (2012). The contour plots shown in their Figure 3, clearly demonstrate that vB s exists in unfavorable seasons; thus, in that times, B s is Content courtesy of Springer Nature, terms of use apply. ...
Article
Full-text available
We investigate the rectified interplanetary magnetic field (IMF) components in two coordinate systems, GSEQ and GSM, focusing on the southward pointing component (Bs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$B_{\text{s}}$\end{document}) in GSM within the period 1999 – 2017. The analysis is performed for different solar activity levels. The obtained results are valuable for the theoretical interpretation of IMF components variations and variations seen in geomagnetic indices. Bs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$B_{\text{s}}$\end{document} ordered according to the polarity exhibits a “pair of spectacles” pattern. This reveals that Bs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$B_{\text{s}}$\end{document} can also exist for toward/away field in fall/spring. The field is reduced, but it is not zero. Thus, in “unfavorable” seasons, geomagnetic activity can be due to reduced Bs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$B_{\text{s}}$\end{document} and not because the field is northward pointing. We show that patterns of the experimental Bs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$B_{\text{s}}$\end{document} fields are not in agreement with the Russell-McPherron model of Bs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$B_{\text{s}}$\end{document} as widely assumed. The present study contributes in understanding the nature and behavior of Bs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$B_{\text{s}}$\end{document}, which is the important element that controls the reconnection process and has a great influence on terrestrial space weather over the solar cycle.
... This type of substorms and ionospheric current activity is known as high intensity, long duration continuous auroral activity (HILDCAA) events (Tsurutani & Gonzalez, 1987). An additional factor that may affect the occurrence and duration of storms is the Russell-McPherron effect (Lockwood et al., 2020;Russell & McPherron, 1973;Zhao & Zong, 2012). Russell and McPherron (1973) showed the varying probability of southward IMF orientation throughout the year as seen by the Earth's magnetosphere that maximizes at the equinoxes. ...
... Table 1 column 4 shows the spring/fall toward/ away IMF sector polarity, indicating whether the storm had a contribution of the Russell-McPherron effect following the "spring-toward fall-away" (STFA) rule (Miyoshi & Kataoka, 2008). Here spring and autumn are defined as the intervals spanning 55 E days from the spring and autumn equinoxes (Zhao & Zong, 2012). The S-T (spring-toward) and F-A (fall-away) labels indicate contribution from the Russell-McPherron effect, while S-A (spring-away) and F-T (fall-toward) give no contribution. ...
Article
Full-text available
This study considers 28 geomagnetic storms with Dst ≤−50 nT driven by high‐speed streams (HSSs) and associated stream interaction regions (SIRs) during 2010–2017. Their impact on ionospheric horizontal and field‐aligned currents (FACs) have been investigated using superposed epoch analysis of SuperMAG and AMPERE data, respectively. The zero epoch (t0) was set to the onset of the storm main phase. Storms begin in the SIR with enhanced solar wind density and compressed southward oriented magnetic field. The integrated FAC and equivalent currents maximize 40 and 58 min after t0, respectively, followed by a small peak in the middle of the main phase (t0 + 4 hr), and a slightly larger peak just before the Dst minimum (t0 + 5.3 hr). The currents are strongly driven by the solar wind, and the correlation between the Akasofu ε and integrated FAC is 0.90. The number of substorm onsets maximizes near t0. The storms were also separated into two groups based on the solar wind dynamic pressure pdyn in the vicinity of the SIR. High pdyn storms reach solar wind velocity maxima earlier and have shorter lead times from the HSS arrival to storm onset compared with low pdyn events. The high pdyn events also have sudden storm commencements, stronger solar wind driving and ionospheric response at t0, and are primarily responsible for the first peak in the currents after t0. After t0+2 days, the currents and number of substorm onsets become higher for low compared with high pdyn events, which may be related to higher solar wind speed.
... This would increase the rate of reconnection, and hence geomagnetic activity. The time of day in which the effective average southward component of the IMF is most negative changes based on what month of the year we are currently in (see Figure 5 in Russell and McPherron (1973)) (Russell and McPherron 1973;Zhao and Zong 2012;Lockwood et al., 2020b). These seasonal variations may be effecting the occurrence of R2Bs. ...
Article
Full-text available
We present a statistical analysis of the occurrence of bifurcations of the Region 2 (R2) Field-Aligned Current (FAC) region, observed by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). Previously, these have been shown to occur as the polar cap contracts after substorm onset, the beginning of the growth phase. During this phase both the Region 1 (R1) and R2 currents move equatorwards as the polar cap expands. Following onset, the R1 FAC region contracts polewards but the R2 FAC continues to expand equatorwards before eventually fading. At the same time, a new R2 FAC develops equatorwards of the R1 FAC. We have proposed that the bifurcated FACs formed during substorms are associated with plasma injections from the magnetotail into the inner magnetosphere, and that they might be the FAC signature associated with Sub-Auroral Polarization Streams (SAPS). We investigate the seasonal dependence of the occurrence of bifurcations from 2010 to 2016, determining whether they occur predominantly at dawn or dusk. Region 2 Bifurcations (R2Bs) are observed most frequently in the summer hemisphere and at dusk, and we discuss the possible influence of ionospheric conductance. We also discuss a newly discovered UT dependence of the R2B occurrences between 2011 and 2014. This dependence is characterized by broad peaks in occurrence near 09 and 21 UT in both hemispheres. Reasons for such a preference in occurrence are explored.
... To examine this further, an analysis of the probability of occurrence versus (month, MLT) is presented in Figure 3 for the 26 sites at latitudes λ = 60°-70°N. This figure shows that in the pre-midnight hours the frequency of occurrence is greatest near the equinoxes, when the geomagnetic field is more favorably oriented for reconnection with the IMF (Russell & McPherron, 1973;Zhao & Zong, 2012). However, for RMS fluctuations (Figures 3e-3h), as τ increases from 1 to 60 min, the greatest frequency of occurrence occurs on the dawn side (03-09 MLT). ...
Article
Full-text available
Using a global database of 125 magnetometers covering several decades, we present occurrence statistics for fluctuations of the horizontal geomagnetic field (dBh/dt) exceeding the 99.97th percentile (P99.97) for both ramp changes (Rn) and the root‐mean‐square (Sn) of fluctuations over periods, τ, from 1 to 60 min and describe their variation with geomagnetic latitude and magnetic local time (MLT). Rates of exceedance are explained by reference to the magneto‐ionospheric processes dominant in different latitude and MLT sectors, including ULF waves, interplanetary shocks, auroral substorm currents, and traveling convection vortices. By fitting generalized Pareto tail distributions above P99.97, we predict return levels (RLs) for Rn and Sn over return periods of between 5 and 500 years. P99.97 and RLs increase monotonically with frequency (1/τ) (with a few exceptions at auroral latitudes) and this is well modeled by quadratic functions whose coefficients vary smoothly with latitude. For UK magnetometers providing 1‐s cadence measurements, the analysis is extended to cover periods from 1 to 60 s and empirical Magnetotelluric Transfer functions are used to predict percentiles and return levels of the geoelectric field over a wide frequency range (2 × 10⁻⁴ to 4 × 10⁻² Hz) assuming a sinusoidal field fluctuation. These results help identify the principal causes of field fluctuations leading to extreme geomagnetically induced currents (GIC) in ground infrastructure over a range of timescales and they inform the choice of frequency dependence to use with dBh/dt as a GIC proxy.
... During solar minimum, weak recurrent storms are observed frequently due to the interaction of the corotating interaction regions (CIRs) of solar origin with the magnetosphere (e.g., Richardson et al., 2001;Tulasi Ram et al., 2010). The annual variation is characterized by a semiannual pattern with equinoctial maxima and solstice minima (e.g., Svalgaard, 2011;Zhao & Zong, 2012). The semiannual variation has been explained using the axial hypothesis (e.g., Cortie, 1912), equinoctial hypothesis (e.g., Azpilicueta & Brunini, 2012;Bartels, 1932) and Russell-McPherron (RM) effect (Russell & McPherron, 1973), the latter two mechanisms involving solar wind-magnetosphere coupling. ...
Article
Full-text available
A quasi‐semidiurnal type pattern was observed earlier in the diurnal UT variation of the geomagnetic storms studied using mainly Kyoto Dst (disturbance storm‐time) index. However, the pattern has been argued as apparent due to uneven longitude distribution of the four Dst observatories. Unlike earlier studies, this paper investigates the diurnal UT variation of the storms automatically identified in six available indices including Kyoto Dst, USGS (United States Geological Survey) Dst, SymH (symmetric‐H), RC (ring current), Dcx (corrected extended Dst), and AER (Atmospheric and Environmental Research) in 50, 50, 36, 21, 5, and 7 years, respectively. The indices are derived using 4, 4, 12, 14, and 15 ground observatories (with maximum longitude separations of ∼120°, 120°, 70°, 110°, and 50°) and four DMSP (Defense Meteorology Satellite Program) satellites, respectively. The UT distribution of the storm intensity (minimum value of an index during the storm main phase) in all indices shows a striking quasi‐semidiurnal type variation with maxima around 06–08 UT and 21–23 UT and minima around 03–05 UT and 13–15 UT. Similar quasi‐semidiurnal variation is also observed in the computed values of the main energy input in the ring current. The variation correlates well with the variations of the dipole tilt angles μ and θ involved in the equinoctial hypothesis and Russell‐McPherron (RM) effect, respectively. These observations indicate that the quasi‐semidiurnal variation is real.
... The observed spring-autumn asymmetry in currents may be related to the Russell-McPherron (RM) effect (Russell & McPherron, 1973). It is a geometrical projection effect between the geocentric solar equatorial (GSEQ) and GSM coordinate systems that increases the likelihood of southward IMF B E z (in the GSM system) if IMF B E y is positive in the fall or negative in the spring (e.g., Lockwood et al., 2020;Zhao & Zong, 2012). Even though the bootstrapping method should remove the RM effect, we see that for IMF B  E z , the spring currents are stronger than autumn currents for B  E y in the NH and vice versa for B  E y (Figure 7), which still follows the expected RM effect. ...
Article
Full-text available
We present a statistical investigation of the effects of interplanetary magnetic field (IMF) on hemispheric asymmetry in auroral currents. Nearly 6 years of magnetic field measurements from Swarm A and C satellites are analyzed. Bootstrap resampling is used to remove the difference in the number of samples and IMF conditions between the local seasons and the hemispheres. Currents are stronger in Northern Hemisphere (NH) than Southern Hemisphere (SH) for IMF By+ in NH (By− in SH) in most local seasons under both signs of IMF Bz. For By− in NH (By+ in SH), the hemispheric difference in currents is small except in local winter when currents in NH are stronger than in SH. During By+ and Bz+ in NH (By− and Bz+ in SH), the largest hemispheric asymmetry occurs in local winter and autumn, when the NH/SH ratio of field aligned current (FAC) is 1.18 ± 0.09 in winter and 1.17 ± 0.09 in autumn. During By+ and Bz− in NH (By− and Bz− in SH), the largest asymmetry is observed in local autumn with NH/SH ratio of 1.16 ± 0.07 for FAC. We also find an explicit By effect on auroral currents in a given hemisphere: on average By+ in NH and By− in SH causes larger currents than vice versa. The explicit By effect on divergence‐free current during IMF Bz+ is in very good agreement with the By effect on the cross polar cap potential from the Super Dual Auroral Radar Network dynamic model except at SH equinox and NH summer.
... Intense substorms occur more in springs and autumns and are even more toward the end of these seasons, in other words, toward April and October. This feature may be related to the Russel-McPherron effect, as the peak occurrence of the R-M effect given by theory is in early April and early October, respectively (e.g., Zhao & Zong, 2012). Therefore, the role of the R-M effect on triggering intense substorms is significantly higher than that of weak substorms. ...
Article
Full-text available
Geomagnetic substorm is one of the essential solar wind‐magnetosphere energy coupling processes. In this study, we present a statistical analysis of substorms with different intensities that occurred from 1982 to 2012. We investigated the substorm occurrence rate, the time evolution of solar wind and geomagnetic parameters, as well as the SME, SMU and, SML indices during these substorms by using OMNI and SuperMag networks data sets. We find that substorms more frequently occur during solar declining phases and in spring and autumn due to the Russel‐McPherron effect. The superposed SME index shows a double‐peak near midnight and dusk during intense substorms, standing for comparable eastward and westward electrojets. We suggest that there is an additional current wedge during intense substorms located near the dusk. During the substorm expansion phase, eastward and westward electrojets are expanding against their current direction. The results presented in this study provide a comprehensive view of substorm characteristics with different intensities and may facilitate the understanding of the development of auroral electrojets in the ionosphere, even the current system in the magnetosphere.
... The Russell-McPherron effect (Russell & McPherron, 1973) dictates higher geomagnetic activity around the equinoxes due to the higher probability of negative IMF B Z which increases the amount of magnetic reconnection at those times, which again leads to a higher daytime plasma density in the ionosphere. Furthermore, Zhao and Zong (2012) compared IMF polarity and 1-h AE indices over a period of 42 years and found that the Russell-McPherron mechanism does yield higher geomagnetic activity at high latitudes. Additionally, it has been shown that the Dst, which is a measurement of ring current activity, is closely related to the dipole tilt angle, that is, the angle between the Earth-sun line and the earth's dipole axis. ...
Article
Full-text available
The Norwegian Mapping Authority operates a network real time kinematic (RTK) system called CPOS, a positioning service providing centimeter level accuracy aimed at commercial users, for example, in civil engineering, excavation, and surveying. CPOS is based on multiple Global Navigation Satellite Systems (multi‐GNSS) in addition to base stations to provide correction data. CPOS position accuracy is subject to disturbances arising from space weather phenomena, which can disturb and disrupt GNSS signals. Studies have shown that CPOS performance is sensitive to the presence of plasma irregularities, usually quantified by the rate of change of total electron content index (ROTI). This study investigates the performance of CPOS over a 3‐year period, and its relationship with ionospheric irregularities. In a statistical analysis, we observe that CPOS position errors have seasonal, diurnal, and latitudinal variations. The most frequent position errors occur around magnetic noon and are of moderate severity, while the largest position errors occur around night‐time, agreeing well with climatology studies on GNSS scintillations. Additionally, we investigate ionospheric irregularities as characterized by the rate of TEC index (ROTI). We find that there is a significant correlation between CPOS accuracy and ROTI, but that there are also other contributing factors.
Article
Full-text available
In this paper we studied the variability of the peak of the critical frequency of the ionospheric E layer (foE) during the minimum and maximum phase of solar cycle 22 (SC22) at Ouagadougou station whose geographical coordinates are: 12.4°N and 358.5°E. We made a statistical study with the aim of highlighting the month which would have the value of foE which best converges towards the average of its corresponding season. We prove that the median months of each season have their critical frequencies (foE) that best converge to the average foE values of each season. Thus, for the winter, spring, summer and autumn seasons, the months best suited for a seasonal study of foE are January, April, July and October respectively at solar minimum and maximum. This study also revealed that foE varies according to the time of day, the season and the phase of the solar cycle at Ouagadougou station.
Preprint
Full-text available
It is well established that phenomena associated with Sun-Earth dynamics oscillate across decades to millenniums, suggesting that indigenous cultures developed millennia ago could provide insights into understanding current phenomena affecting human-global health in the current rapidly changing geophysical environment. Here, using insights from indigenous culture developed during the Neolithic-Mid Holocene period in Eurasia, I investigate the relationship between the sun’s magnetic field activity and the severity and the temporal dynamics of the Coronavirus Disease 2019 (COVID-19) pandemic. I demonstrate that the temporal dynamics of the sun’s magnetic field activity, as measured using sunspot number is synchronous with the temporal dynamics of COVID-19 cases and deaths across the globe. Therefore, I propose that sunspot number may provide a physical parameter for long-range forecasting of the severity and temporal dynamics of the COVID-19 pandemic.
Article
Full-text available
We present a statistical study of the sizes of interplanetary coronal mass ejections (ICMEs) and the preceding shock sheath regions in near-Earth space. The 46 events studied are a subset of the events responsible for intense (Dst <= -100 nT) geomagnetic storms in 1996-2005 in which only a single ICME was responsible for generating the storm. We find that the durations and radial sizes of these ICMEs range from 8.0 to 62.0 hr and 0.08 to 0.63 AU, respectively, with average values of 30.6 hr and 0.37 AU. The sheath durations and radial sizes range from 2.6 to 24.5 hr and 0.03 to 0.31 AU, with average values of 10.6 hr and 0.13 AU. On average, the ICME radial size is 2.8 times that of the sheath. In terms of their relative geoeffectiveness, ICMEs contribute on average about 71% of the total energy input (sheath + ICME) into the magnetosphere.
Article
A homogeneous series of monthly means of terrestrial-magnetic activity for the years 1872 to 1930 is derived and extended backward, in annual means, to 1835. The annual variation of magnetic activity and of the relative sunspot-numbers is discussed by means of new tests for periods. Only the semi-annual wave in magnetic activity is recognized as physically significant. Its maxima prefer the times when the Sun is in the celestial equator, and not, as has been suggested, the times when the Sun's axis is most inclined towards the ecliptic. This view is supported by tests using the harmonic dial and the probable-error circle, and several independent considerations. The close relations between sunspot-numbers and terrestrial-magnetic activity in the annual and monthly means are discussed. Some general statistical aspects are given for the treatment of the correlation between such series with after-effects, for which both solar activity and terrestrial-magnetic activity are typical. The homogeneity of the whole available series for relative sunspot-numbers and for areas of sunspots and faculae is tested; some inhomogeneities are found, apart from a general lag of terrestrial-magnetic activity that has occurred in some sunspot-cycles. A break in the homogeneity of the international magnetic character-figures in recent years is discovered. The individual 27-day recurrences in terrestrial-magnetic activity during 1906–31, and their relations to solar activity are discussed with the help of a graphical day-by-day record. They indicate the existence of persistent active areas on the Sun's surface, called M-regions, which, in many cases, cannot be coordinated to such solar phenomena as are observable by direct astrophysical methods. This holds in particular for the new solar indices which are available for the years 1928–30, and which are found so closely correlated to sunspot-numbers, that they fail to improve the correlation between solar activity and terrestrial-magnetic activity. Observations of terrestrial-magnetic activity yield therefore not only information about geophysical influences of such solar phenomena that may be traced in astrophysical observations, but supplement these direct observations themselves.
Article
The geomagnetic Dst index exhibits a strong semiannual variation with amplitude of 5.3 nT (on an average baseline of -16.4 nT) for 1957-1997. If we consider the five quietest Dst days for each month during this interval, we find that while the average Dst baseline shifts from -16.4 to +4.0 nT, the amplitude of the 6-month wave remains relatively strong at 2.5 nT. Thus ~50% (2.5 nT/5.3 nT) of the seasonal variation of the Dst index results from modulation of its quiet time baseline. The seasonal modulation of Dst appears to consist of both a storm component, resulting from rapid variations of the ring current and other currents, and a slowly varying, nonstorm component. We estimate that the storm component accounts for only ~30-50% of the amplitude of the 6-month wave in Dst versus ~50-70% for the nonstorm component. The classic equinoctial effect appears to dominate the storm component, accounting for 20-40% of the amplitude of the 6-month wave in Dst versus ~10% for the combined axial/Russell-McPherron mechanisms. Candidate mechanisms for the nonstorm component of the 6-month wave in Dst include the Malin-Isikara effect (the seasonal displacement of ring/tail currents by solar wind compression) and a semiannual variation of magnetopause currents.
Article
This volume provides a comprehensive overview of externally generated ULF waves and transients in the Earth's space environment, including practically all the latest advances as of the date of writing. Tutorials are included to give a complete introduction to time-dependent phenomena in the magnetosphere in the ULF frequency range. The material may therefore be used by the interested reader as a starting point for getting involved in this research field, as an overview of the research in progress, and/or as a convenient reference for advanced background knowledge.
Article
An algorithm is presented for predicting the ground-based Dst index solely from a knowledge of the velocity and density of the solar wind and the north-south solar magnetospheric component of the interplanetary magnetic field. The three key elements of this model are an adjustment for solar wind dynamic pressure, an injection rate linearly proportional to the dawn-to-dusk component of the interplanetary electric field which is zero for electric fields below 0.5 mV per m, and an exponential decay rate of the ring current with an e folding time of 7.7 hours. The algorithm is used to predict the Dst signature of seven geomagnetic storm intervals in 1967 and 1968. In addition to being quite successful, considering the simplicity of the model, the algorithm pinpoints the causes of various types of storm behavior. This one algorithm accounts for magnetospheric behavior at quiet and disturbed times and seems capable of predicting the behavior of Dst during even the largest of storms.
Article
The purpose of this review is to outline the definition and method of derivation of the indices of geomagnetic activity that are most commonly used at present. The first part of the review presents the methods of derivation of Kp, ap, AE, and Dst; the second part involves a discussion of the strengths and weaknesses of each index. It is shown that the distribution of magnetic observatories whose data are used in the production of the indices strongly influences the quantitative reliability of the indices in varying degrees. Furthermore, it is found that, in general, indices are only capable of defining the lower limit of geomagnetic activity at any given time. It is concluded that indices should only be used in statistical studies of solar-terrestrial interactions; their use in the study of individual events is generally not advisable.
Article
Statistical features of the annual incidence of magnetic disturbance, over a very wide range of disturbance intensity and latitude, are exhaustively investigated by means of the K index and related 'planetary' indices. Two distinct and physically significant components are identified: (a) an annual component, with summer maximum and winter minimum; (b) a semi-annual component with equinoxial maxima. Both components are found in all parts of the earth. The amplitude of the annual component increases markedly with latitude, while that of the semi-annual component changes little with latitude. The physical causes of the two types of variation are finally considered. The conclusions reached are (a) that the annual component is probably caused by an atmospheric dynamo effect; (b) that the semi-annual component arises because of a systematic annual variation of the angle between the earth's magnetic axis and the sun-earth line, along which travel the solar particles which cause magnetic disturbance.
Article
The semiannual variation of the geomagnetic activity is investigated in connection with a large set of solar wind and interplanetary magnetic field data (4494 daily averages from 1965 to 1987). Our analysis confirms that the geomagnetic activity (described by the aa index), is mainly modulated by the southward component of the magnetic field (BS), as suggested by Russell and McPherron. On the other hand, it is also found that the solar wind velocity (V) has a relevant role in this phenomenon. In fact, the amplitude of the aa modulation is best correlated with the function BSV2. We also explore the linkage between the annual trend of aa and the sunspot activity (1868-1989), showing that the modulation of the geomagnetic activity follows a more regular pattern during the descending phase of the solar cycle than during the rising and maximum parts.
Article
We use Wind solar wind data and Polar UV imaging data to study the nightside magnetospheric/magnetotail responses to interplanetary shocks/pressure pulses. Of 53 interplanetary shock/pressure pulse events that occurred in 1997 and 1998 at Wind, there are 18 cases where Polar near-midnight UV images are available. All of these 18 events are used in this study. The nightside auroral responses can be classified into three types: substorm expansion phase (SS) (or substorm further intensification) events, pseudobreakup (PB) events, and quiescent (QE) events. It is found that the solar wind preconditions determine the causes of the different auroral responses. A ~1.5-hour interplanetary magnetic field (IMF) Bs ``precondition'' (upstream of the interplanetary shock) gives good empirical results. The upstream IMF is strongly southward prior to substorm expansion phase triggerings (44% of all events), the IMF Bz is ~0 nT for PB triggerings (39% of all events), and the IMF is purely northward for quiescent events (17%). The evidence for IMF Bs preconditioning is interpreted in terms of a plasma sheet loading mechanism. The interplanetary shock compression effects on the near-Earth tail are discussed in light of existing substorm/PB triggering models.