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ISSN 00167932, Geomagnetism and Aeronomy, 2012, Vol. 52, No. 1, pp. 60–67. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © V.L. Zverev, Ya.I. Feldstein, V.G. Vorobjev, 2012, published in Geomagnetizm i Aeronomiya, 2012, Vol. 52, No. 1, pp. 64–72.
60
1. INTRODUCTION
Bright and mobile discrete forms of auroras at high
latitudes are observed almost continually, surrounding
the geomagnetic pole and located along the auroral
oval (Feldstein, 1960, 1963; Khorosheva, 1962). Usu
ally, in the discrete forms of auroras, the most intense
is the 557.7 nm green atomic oxygen (OI) line. Toward
the midlatitudes, the occurence frequency of discrete
forms sharply decreases. In the IGY period (1957–
1959), the main method for observing auroras at mid
latitudes was visual, conducted at many hundreds of
meteorological stations. The results of their systematic
processing at international data centers have been pub
lished in encoded form in (
Annals IGY
, 1964). Intense
auroral glow has been detected at midlatitudes in
magnetic disturbance intervals accompanying the
development of magnetic storms (Chapman, 1957).
Equatorward from the oval, an extensive band of sub
visual atmospheric glow exists (Sanford, 1969; Lui
et al., 1973). This band is visually distinguishable with
difficulty due to the weak glow contrast over the sky,
often of low intensity; low intensity and weak eye sen
sitivity to the main emission in this glow—the red
(OI) doublet of 630.0–636.4 nm (Krasovskii, 1967).
Although green emission does exist, it is extremely
weak. The emission intensity ratio
I
6300
/
I
5577
~ 2–4.
Due to its own peculiarity—absence in glow in large
areas of distinctly pronounced structures—it has been
called diffuse. The diffuse glow intensity decreases
with decreasing latitude and increases in magnetic
disturbance intervals (Akasofu and Chapman, 1962;
Krasovskii, 1967). According to parallax observation
data from the Yakutsk–Tiksi meridional chain of sta
tions (Alekseev et al., 1975), the height of the back
groundglow maximum in 630.0 nm emission encom
passed the height interval from 140 to 317 km. Usually,
the equatorial boundary of diffuse glow is relatively
smooth, but during strong geomagnetic disturbances
largescale wavelike structures can appear at it. The
amplitude and wavelength change with time; on aver
age, the wavelength is 400 km (Lui et al., 1982).
Toward the equator from the diffuse glow at the
midlatitudes, a sharp intensification in monochro
matic 630.0 nm emission is observed, which has been
termed Marcs (Roach and Roach, 1963) or SAR (sta
ble auroral red) arcs (Rees and Roble, 1975). SAR arcs
are one type of auroral glow, since their appearance is
usually related to an intensification in geomagnetic
activity, and its location is related to the geomagnetic
field. The distinctive features of SAR arcs are as fol
lows: a long, up to 10 h, existence (hence the term sta
ble); location along the
L
shells of energetic charged
particle drifts in the geomagnetic field; a length of sev
eral thousand kilometers in longitude; location at a
low (
∼
42
°
) geomagnetic latitude during intense mag
netic storms; nearly monochromatic radiation of
630.0 nm emission (
I
6300
/
I
5577
∼
50) at a height of
~400–450 km; conjugacy in the Northern and South
ern hemispheres. The intensity of SAR arcs in the IGY
period was several kilorayleighs (kR) (Marovich and
Roach, 1963) and during very intense magnetic storms
can achieve tens of kR (Barbier, 1960). So, during the
magnetic storm of September 21, 1957 (
Dst
= –250 nT),
an increase in the intensity of 630.0 nm emission to
125 kR was recorded, and an increase in 557.7 nm
emission to 1.5 kR (Barbier, 1960). Thus, the intensity
ratio
I
6300
/
I
5577
was ~80. Between the SAR arcs and the
oval boundary, there is a break in latitude that reaches
∼
14
°
in which airglow was identified with an intensity
at the level of the airglow of the nighttime sky (Marov
ich and Roach, 1963).
Auroral Glow Equatorward from the Auroral Oval
V. L. Zverev
a
, Ya. I. Feldstein
b
, and V. G. Vorobjev
a
a
Polar Geophysical Institute, Kola Scientific Center, Russian Academy of Sciences, ul. Akademgorodok, Apatity, 184209 Russia
b
Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation, Troitsk, Moscow region, Russia
email: zverev@pgia.ru
Received February 22, 2011; in final form, June 24, 2011
Abstract
—On the basis of observations for the IGY period (visoplots) it is shown, that during magnetic
storms diffuse glow is detected at all latitudes between the lowest latitude of the visually observed auroral glow
at the zenith and the auroral oval. The diffuse glow region spatially coincides with the region of soft electron
precipitation extending equatorward from the boundary of the oval to the latitude of the plasmopause projec
tions along the magnetic force lines to the ionosphere. Using published materials on the diffuse glow dynam
ics and SAR arcs at the Yakutsk meridian, as well as simultaneous measurements of the DMSP F9 satellite, we
discuss the contribution from lowenergy electron precipitation transfered via convection toward Earth from the
magnetosphere’s plasma sheet to excitation of 630.0 nm emission in lowintensity (<1.0 kR) SAR arcs.
DOI:
10.1134/S0016793212010173
GEOMAGNETISM AND AERONOMY
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No. 1
2012
AURORAL GLOW EQUATORWARD FROM THE AURORAL OVAL 61
According to established notions, the formation of
SAR arcs in the magnetosphere near the plasmopause
occurs as a result of the interaction of hot ions of the
ring current with the cold plasma of the plasmosphere.
The waves that thereby emerge heat the cold electrons
of the plasmosphere. Their flux along the force lines to
the ionosphere also causes atmospheric glow (SAR
arc) in red emission at heights of ~400 km. Thus, SAR
arcs have a nature differing from the that of usual auro
ras at higher latitudes. Auroras in the oval and diffuse
glow toward the equator from the oval are a result of
atmospheric precipitation of energetic electrons from
the plasma sheet of the magnetospheric tail and from
the inner magnetosphere as a result of transfer of their
convection directed toward the Earth. SAR arcs arise
due to atmospheric precipitation of cold electrons of
the plasmosphere heated during the interaction of
ringcurrent ions with plasmospheric plasma.
The aim of this work is to demonstrate that during
magnetic storms diffuse glow is recorded at all lati
tudes between the lowest latitude of visually observed
auroral glow and the auroral oval, and that the appear
ance of SAR arcs of relatively small intensity is related
to electron precipitation near the inner (nearEarth)
boundary of magnetospheric convection.
2. THE AURORAL OVAL AND DIFFUSE GLOW
EQUATORWARD FROM THE OVAL
It is known that the minimum latitude of the equa
torial boundary of the auroral oval characterizing the
smallest geocentric distance of the inner edge of the
plasma sheet in the magnetospheric tail falls in the
nearmidnight hours (Starkov, 1993). During mag
netic storms, the boundary shifts toward the equator
and the size of the shift is determined by the intensity
of
Dst
variation. In Fig. 1, points mark Starkov’s
(1993) systematized positions of the equatorial bound
ary of the auroral oval depending on the
Dst
index from
data of different researchers. The dashed line shows
the regression equation, obtained by the leastsquares
method, of the corrected geomagnetic latitude
Φ
'
from the
Dst
intensity:
(1)
Crosses in this figure mark the latitude values of
auroral glow propagation at the zenith, determined by
us from auroral glow visual observation data contained
in visoplots for the IGY period (
Annals IGY
, 1964). For
analysis, all observation intervals with
Dst
< –100 nT
were used. The data primarily encompass longitude
sectors
D
(East Siberia),
G
, and
H
(North American
continent). The regression equations obtained by the
leastsquares method are depicted by the solid line:
(2)
Φ= °−
=
'77398log ,
correlation coefficient 92
. .
0. .
Dst
r
Φ= °−
=
' 74 7 12 5log
correlation coefficient 7
..0 ,
0. .
Dst
r
The regression line for the equatorial boundary of
visual glow was shifted by
∼
7.5
°
southward from the
boundary of the existence region of usual auroras. This
means that during magnetic storms, the boundary of
the region of appearance of auroral glow at the zenith
in visoplots does not reflect the equatorial boundary of
the oval of usual auroras, but characterizes the diffuse
glow located at lower latitudes. With a change in
Dst
from –100 to – 400 nT, the glow boundary according
to visoplots shifts equatorward by
∼
7
°
. Indeed, in the
main phase of magnetic storms, the intensity of the
diffuse glow sharply increases (Akasofu and Chap
man, 1962). As a result, it begins to be fixed by allsky
cameras and in visual observations.
3. VISUAL GLOW AND SAR ARCS
Marovich and Roach (1963) studied the positions
of SAR arcs observed at Rapid City station (corrected
geomagnetic latitude
Φ
' = 53.3
°
) in the IGY period. A
photometer scanned the entire sky in the 557.7 nm
and 630.0 nm emissions. The study presented the arc
observation intervals according to local time (UT =
MLT + 7). From November 1957 through December
1958, arcs were identified in 23 cases. Under the
assumption of an arc height of 400 km, their distribu
tion was obtained: extreme northern arc,
58.7
°
;
median,
53.3
°
; extreme southern arc,
41.5
°
geomag
netic latitude. A significant arc concentration exists at
a latitude of
Φ
' = 53
°
(
L
= 3)
. Such a location of SAR
arcs means that they are mainly observed at a latitude
of approximately
∼
14
°
more equatorial than discrete
forms of auroras in the nighttime sector of the oval.
Isaev (1962) and Pudovkin et al. (1971) focused atten
tion on the existence of a midlatitude maximum in
the occurrence frequency of auroras, relating it to
splitting of the highlatitude auroral zone. According
to Khorosheva (1987), such splitting occurs during
66
64
62
60
48
46
44
42
40
5005010
100
58
56
54
52
50
Geom. latitude, deg.
|
Dst
|, nT
2
1
Fig. 1.
Points show location of the equatorial boundary of
the auroral oval depending on the
Dst
index (Starkov,
1993). Crosses show limit values of the latitude of auroral
glow propagation at the zenith according to visoplots.
62
GEOMAGNETISM AND AERONOMY
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2012
ZVEREV et al.
magnetic storms, reaching
12
°
latitude during intense
magnetic storms. Between the oval and SAR arcs,
there is a gap in glow intensity.
From visual observations in the IGY period (
Annals
IGY
, 1964), at a longitude close to Rapid City in the
G
and
H
sectors, we determined the lowest latitudes of
auroral glow at the zenith for all observation periods of
SAR arcs indicated in (Marovich and Roach, 1963,
Table 2). The regression equations were obtained in
geomagnetic coordinates for the extreme low latitudes
as a function of the
Kp
and
|
Dst
|
indices. The relations
have the form
(3)
(4)
Figure 2 shows the distribution of limit values of
equatorial latitude of glow from visoplots as a function
of
|
Dst|
in the hours of SAR arcs observation. The
median value of the arc position in (Marovich and
Roach, 1963) is
53.3
°
, which differs by
3
°
from the
main position of glow at the zenith of the observation
point according to visoplots. Note that out of 23 events
with SAR arcs, only four events fall on magnetic
storms with 100 nT <
|
Dst
|
< 180 nT. In the majority of
cases, SAR arcs were observed during insignificant
magnetic disturbances with 7 nT <
|
Dst
|
< 100 nT. SAR
arcs were not observed at large
|
Dst
|
values, although in
the IGY period intense magnetic storms were noted.
The review (Roach and Roach, 1963) presented the
same exact table from (Marovich and Roach, 1963)
with SAR arc observation and indication of
Kp
values in
the observation periods. The obtained median value of
Kp
= 4+ coincides well with our visual observation data.
(
)
Φ=°−
=Φ=°=
av av
'635165
'
7; 56 4+
deg . . ;
0. ;
Kp
rKp
(
)
Φ=°−
=Φ=° =
av
av
'59857;
'
82; 56 67 nT
deg . 0.0
0. ; .
st
st
D
rD
Thus, if there is also a break in latitude between the
position of SAR arcs studied in (Marovich and Roach,
1963) in the IGY period and simultaneously visually
observed auroral glow in the same longitude sector,
then its size is
∼
3
°
, but in no way
14
°
.
4. AURORAL GLOW AT MIDLATITUDES
AND SAR ARCS
Consideration of visually observed auroral glow
and SAR arcs in the preceding sections was limited to
the IGY period. This is the result of the availability of
visual observation materials making up the
Annals
IGY
. However, SAR arc observation in the IGY were
performed sporadically and there are no direct data on
localization, intensity, and the spectral composition of
fluxes of particles precipitating into the upper atmo
sphere responsible for glow excitation. Regular obser
vations of diffuse auroral glow and SAR arcs were per
formed at Maimaga station (Institute of Cosmophysi
cal Research and Aeronomy, Yakutsk Scientific
Center, Russian Academy of Sciences (IKFIA YaNTs
RAN),
Φ
'
≈
57
°
,
Λ
'
≈
200
°
, local geomagnetic mid
night at 1540 UT)). Observations were performed with
a twochannel scanning photometer with a field of
vision of
3
°
in the 557.7 and 630.0 nm emissions and
an allsky camera with an electrooptical image
amplifier in the 630.0 nm emission. Glow inhomoge
neities were reliably identified in the 630.0 nm emis
sion with an amplitude larger than 30–50 R. For the
two solar cycles from 1988 through 2006, 225 cases
(nights) of SAR arc occurence were recorded out of
620 nights of observations (Alekseev and Ievenko,
2000; 2008). Therefore, SAR arc occurrence at midlat
itudes is not exotic, but a sufficiently regular phenome
non. Arcs were in a latitude interval of
49
°
<
Φ
' < 61
°
depending on the level of magnetic activity. The most
probable latitude of SAR occurrence is
Φ
'
∼
55.0
°
–
55.4
°
(
L
∼
3)
, and their mean intensity
I
~ 0.15 kR at
mean values of magnetic activity indices of
Dst
~ –40 nT,
AL
~ –230 nT,
K
p
~ 3–4 [Ievenko and Alekseev, 2004].
The correlation coefficients evidence a closer rela
tionship between glow and substorms (
AL
index) than
with storms.
Figure 3a shows the typical case of observation
from evening to morning hours of diffuse glow and
SAR arcs at Maimaga station on December 4, 1989,
considered in detail in (Ievenko and Alekseev, 2004). A
SAR arc whose borders corresponded to a glow inten
sity of the nighttime sky toward the equator and pole
from the maximum intensity in the arc, shifted slowly
equatorward from the nighttime to the morning hours
MLT. Its latitudinal position was determined under the
assumption of a glow height of 450 km. The boundary
of diffuse glow in the green emission northward from
the arc (height of 110 km) is reflected by the isolines of
three values of surface emission brightness (brightness
gradient) and corresponds to the 0.5 kR isoline. Varia
tions of magnetic activity level presented in Fig. 3c tes
tify that during relative quiet magnetic field till
62
60
48
46
180200
100
58
56
54
52
50
Geom. latitude, deg.
|
Dst
|, nT
120 140 16040 60 80
Fig. 2.
Points show location of the equatorial boundary of
glow according to visoplots depending on the
Dst
index in
the period of occurrence of SAR arcs according to
(Marovich and Roach, 1963); solid line is regression line.
GEOMAGNETISM AND AERONOMY
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AURORAL GLOW EQUATORWARD FROM THE AURORAL OVAL 63
~1740 UT SAR arc was weak, its location near zenith
of observation point and intensity ~0.18 kR were rela
tively stable (Fig. 3b). Onset of disturbances was
accompanied with increase of velocity of arcs shift to
the equator and enlargement of intensity with maxi
mum up to ~0.5 kR. Between the boundary of diffuse
glow and the SAR arc, a gap of
∼
1
°
–2
°
in latitude in
the calm interval was recorded. During prolonged sub
storms activity, the SAR arc is separated from the dif
fuse auroral glow and moves equatorward; the width of
the gap increases to
∼
4
°
. The width of the gap decreases
if we take into account that the position of the diffuse
glow boundary is related to the method of its determi
nation. Equatorward from the 0.5 kR isoline, a more
intense diffuse glow exists filling the space at the latitude
of the gap. Thus, the formation of an SAR arc occurs in
the vicinity of the equatorial boundary of the diffuse
glow when there is a minimum latitudinal gap between
the boundaries of the two phenomena.
The position of the SAR arc on December 4, 1989,
was intersected by the DMSP F9 satellite, moving in
an almost circular solarsynchronous polar orbit at a
height of 840 km. The satellite’s instrument suite
included electron and ionflux measurements with
energies from 30 eV to 30 keV. The duration to mea
sure the entire spectrum was 1 s. The most interesting
are data of low energy channels, which characterize
the location of lowlatitude boundary of the auroral
precipitation region.
On the vertical hatched lines in Fig. 3a reflecting
the trajectory of DSMP F9, asterisks mark the equato
rial boundaries of precipitation regions for three passes
of the satellite in the nighttime sector according to the
results of automatic satellite observation processing,
constituting the APL database. Squares mark the bor
ders of precipitation regions for the same passes after
refinement of the database as a result of examining the
original spectrograms. The difference between the data
base and the refined data is from
0.6
°
to
4
°
for different
passes. The DMSP F9 pass fell on ~23 MLT in the
interval of 12–17 UT, when Maigma station was
located in the nearmidnight sector. In all passes, the
refined boundary of the precipitation region coincides
within the limits of
1
°
of latitude with the boundary of
the diffuse glow, coinciding with or being located
1–2
°
north of the highlatitude boundary of the SAR arc. At
~2135 UT, the glow region at the 19.7 meridian MLT
intersected the DMSP F8 satellite. The boundary of
the electron precipitation region was recorded at
Φ
' =
56.6
°
(marked with an asterisk), which is
∼
2
°
north of
the SAR arc boundary, which at these UT hours was
observed in the morning hours. Since SAR arcs nor
mally drift equatorward from the evening to morning
hours, allowance for this circumstance decreased the
divergence between the SARarc boundary and the
boundary of the electron precipitation region.
The glow dynamics in the SAR arc and its relation
to diffuse glow is illustrated by the 630.0 and 557.5
scanning emissions for three time moments on
December 24, 1989, presented in Fig. 4 from
(Ievenko, 1999). The SAR arc distinctly manifests
itself in the red emission. The sharp emission gradient
in the arc and the distinct boundary of diffuse glow
obtained during each scan make it possible to estimate
the slit width between the arc and the diffuse glow. It is
necessary to determine the choice of level for counting
the background glow intensity on which the SAR arc is
superposed. This can be the moment of the 557.5 nm
emission minimum, the intensification of emission
northward from which is the result of diffuse glow, and
southward of which, the result of the Van Rayne effect.
In another variant, this is the background glow mini
mum in the 630.0 nm emission, which changes sub
stantially with time. The glow intensity value at the
SARarc maximum and the location of its boundaries
in this case depend on the adopted level for counting
the background glow intensity. In the first case, the
accepted counting level, the minimum is the same for
all three time intervals and falls on the zenith angle
15
°
to the south. Then the gap between the two types of
glow is absent at 1900 and 1930 UT, but it is 111 km at
1950 UT. In the second variant, the gap is absent at
100
0
–100
201812
14 16 UT
SYMH
(v)
100
ASYH, nT
Kp
334+5
0.2
0
2018
12
14 16 UT
(b)
20
0.4
0.6
kR
December 4, 1989
61
°
630 nm
1
2
0.5 kR
SAR
Λ
c
57
°
N'
Z
S
(a)
53
°
49
°
1
Δ
bg
557.7 nm
DA
Fig. 3.
Modified figure from (Ievenko and Alekseev, 2004):
(a), variations in the location of diffuse glow (DA) in the
557.7 nm emission and SAR arcs (Ievenko and Alekseev,
2004); vertical hatched lines show DMSP F9 and F8
passes. (b) Variations in SARarc intensity (dark circles)
and background glow (open circles) (Ievenko and Alek
seev, 2004). (c) Magnetic activity (
Kp
, ASYH, SIMH).
64
GEOMAGNETISM AND AERONOMY
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2012
ZVEREV et al.
1900 UT, but it is 167 km at 1930 UT and 222 km at
1950 UT. In calculations for determining the distance
L
on the Earth’s surface from the observer to the glow
boundary, the following relation was used:
(5)
where
R
is the Earth’s radius, equal to ~6370 km, and
Н
= 400 km is the height of the emission layer.
The shift in the emission maximum characterizes
the latitudinal motion of the arc. In parallel to the
increase of glow intensity in the SAR arc, there is an
increase in the background level intensity by ~0.2 kR,
from which, according to the technique adopted in
(Ievenko, 1999), the emission intensity in the arc is
counted. This introduces ambiguity into the SARarc
intensity values if they are determined using another
background level.
The DMSP F8 satellite crossed the auroral zone on
December 24, 1989 at ~2015 UT.
The equatorial boundary of diffuse precipitation
was recorded at
Φ
' = 56.5
°
, MLT = 4.8, i.e., near
zenith of the observation point. Therefore, the precip
itation boundary of auroral electrons with energies
larger that 30 eV fell on the vicinity of the diffuse glow
[
]
{
}
18 arcsin sinZ
0()(),
LR Z R RH
=π − +
boundary in the 557.7 nm emission, fixed approxi
mately at the same time by a groundbased apparatus.
We consider the intensity and spectral composition of
the precipitating electron flux as the satellite moved
through the equatorial part of the auroral oval and the
region of diffuse glow. We discuss in detail the pass of
DMSP F9 on December 4, 1989, at 1346–1350 UT.
Another two passes at 1204–1208 UT and 1709–1712 UT
are characterized by similar patterns. Figures 5a–5d show
the electron spectra typical of various auroral struc
tures. At 1345:30 UT (Fig. 5a), the spectrum is quasi
monochromatic, with an electron flux density of
~10
9
(cm
2
sr s) at an energy peak of ~1.4 keV at
Φ
'
∼
65.5
°
. Particle fluxes sharply decrease with distance
from the peak both toward large and low energies.
With energies larger than 9.7 keV, particles are absent
and less than 1.4 keV of their flux decreases by almost
two orders. Such spectra have discrete auroral forms at
the latitudes of the oval. In this pass, this discrete form
outlines the equatorial boundary of the oval and the
boundary between it and the diffuse glow region.
Equatorward, in the diffuse glow region, the spectrum
softens. At 1346:43 (Fig. 5b) at
Φ
'
∼
64.8
°
, the 97 eV
flux increases and the 1.4 keV flux decreases by more
than an order. At 1347:38 UT (Fig. 5c) at
Φ
'
∼
62.0
°
,
softening of the spectrum continues: the electron flux
at energies >1 keV is cut off and the density of the 97
eV flux decreases by an order; however, the electron
flux density of minimal energies with 31 and 45 eV is
preserved at a sufficiently high level of
∼
10
7
/(cm
2
sr s).
At the boundary of the diffuse precipitation region at
1348:29 UT (Fig. 5d) at
Φ
'
∼
59.4
°
, only electrons with
minimal energies with a flux density of
∼
10
6
/(cm
2
sr s)
remain in the spectrum. We conclude that in the dif
fuse region, with a shift toward the equator, softening
of the electron spectrum occurs and a sufficiently high
level of their flux density is retained. In this pass of the
DMSP F9, the diffuse precipitation boundary in the APL
base is shown at a latitude of
Φ
'
∼
63.9
°
at 1347:01 UT;
i.e., it differs by
4.6
°
from the latitude of the boundary
of the precipitation region in the spectrogram.
5. AURORAL GLOW AND PLASMA
IN THE NIGHTTIME MAGNETOSPHERE
The main plasma reservoir of auroral energies in
the magnetosphere is the central plasma sheet located
on both sides of the neutral sheet of the magneto
spheric tail near the equatorial plane of the geomag
netic dipole (Baumjohann et al., 1989, Schödel et al.,
2002). Consider the motion of electrons that drift from
the magnetosphere tail on trajectories along which are
conserved the total energy
Е
, and the first
μ
=
m
v
2
/2
B
and second
J
=
adiabatic invariants. In the
remote magnetosphere, electrons move primarily
toward the Earth under the influence of a homoge
neous electric field directed from morning to evening,
which occurs within the magnetosphere under the
interaction of the geomagnetic field with the solar
pds
∫
N 60
°
30
°
S, deg.Z
557.7 nm
1000 R
1
2
3
1
– 1900
2
– 1930
3
– 1950 UT
MZ
630.0 nm
1
2
3
500 R
I SAR
December 24, 1989
Fig. 4.
Scanning images in the 630.0 and 557.7 nm emis
sions on December 24, 1989 at 1900 (
1
), 1930 (
2
), and
1950 UT (
3
) according to (Ievenko, 1999).
GEOMAGNETISM AND AERONOMY
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AURORAL GLOW EQUATORWARD FROM THE AURORAL OVAL 65
wind. With convection falling in the region of more
and more intense magnetic fields and less extended
force lines with conservation of the adiabatic invari
ants, electrons are accelerated and their energies in the
plasma sheet increase. This process occurs up to a cer
tain geocentric distance, which characterizes the loca
tion of the inner boundary of the plasma sheet in the
nighttime sector (Vasyliunas, 1970; Frank, 1971).
Near the inner boundary are also the energeticelec
tron trapped boundary with
Е
~ 25–40 keV (O’Brien,
1963) and the equatorial boundary of the oval (Feld
stein and Starkov, 1970). Nearer the Earth, in the
inner magnetosphere, lowenergy electrons continue
to drift toward Earth along the isolines of the potential.
Simultaneously, there is an increase in the azimuthal
drift component, conditioned by the magnetic field
gradient and the curvature of its force lines. The direc
tion of azimuthal drift for particles with energies of
several keV are determined by the sign of the charge:
electrons move to the morning side, and ions, to the
evening side. Azimuthal division of different charges
leads to generation of the electric field from the
evening side to the morning side, which is accompa
nied by screening of the initial electric field.
The azimuthal drift velocity for electrons depends
on their energy, and the more energetic of them begin
to be carried to the morning side at large geocentric
distances. As a result, as they approached Earth, the
fraction of particles with lower energies increases. The
fraction of energetic particles becomes smaller. Such a
process continues right up to a geocentric distance at
which the energy of electrons coming from the plasma
sheet becomes close thermal. At this distance, plasma
motion from the plasma sheet toward Earth ceases.
The boundary at which the plasma from the plasma
sheet stops drifting toward Earth has been termed the
plasmopause (Nishida, 1966). It restricts the plasmo
sphere—the domain of cold plasma. The characteris
tic features of the plasmopause—a geocentric dis
tance of
∼
4
R
E
, its change over the course of a day, and its
approach toward Earth in intervals of magnetic distur
bances—have found explanation in the model (Nishida,
1966). In the plasmosphere, the plasma is enlarged
owing to the transfer of cold plasma from the ionosphere
and participates in corotational movement together with
the Earth. At the plasmopause, observations revealed the
existence of a jump in plasma density (Carpenter, 1963;
Gringaus, 1960). Such a jump could naturally be
expected in the case of a different nature and character
of plasma convection near the Earth and at large dis
tances. During observations from the Earth’s surface,
the boundary of the plasmosphere was identified as a
trough in the plasma density at heights of the
F
region of
the ionosphere (ionospheric trough) (Nishida, 1967).
The lowenergy auroral plasma at geocentric dis
tances between the boundary of the plasma sheet and
the plasmopause precipitates along magnetic force
lines to heights of the ionosphere. Precipitation of
electrons with energies of 10–1000 eV and <10 eV are the
most effective in exciting diffuse auroral glow. Such elec
trons lose their energy at heights of 200–500 km, excit
ing metastable
О(
1
D
)
and
O(
1
S
)
emissions of, respec
tively, red and green atomic oxygen. At such large
heights, there is low deactivation of excited metastable
states. The potentials of excitation of
О(
1
D
)
emission
are low (~5 eV), and the cross sections for excitation
processes near the excitation thresholds are large. All of
this favors the emission of the red line of atomic oxygen
during lowenergy electron precipitation.
Glow excitation is a consequence not only of the
direct interaction of oxygen atoms with the flux of
electrons from the magnetosphere. Electrons within
energies <10 eV transfer part of their energy to iono
sphere electrons, which also become sources of red
emission excitation.
The typical features in the parameters of corpuscu
lar fluxes in the inner magnetosphere and above the
midlatitude ionosphere are similar to a significant
7
6
5
2 3 4
Lg
E
, eV
1348:29 UT
(d)
7
6
1347:38 UT
(c)
6
1346:43 UT
(b)
7
8
7
1346:30 UT
(a)
8
9
Lg
F
, 1/cm
2
s sr
Fig. 5.
Electron spectra obtained by the DMSP F9 satellite
on December 4, 1989 at 1346:30 (a), 1346:43 (b), 1347:38
(c), and 1348:29 UT (d).
66
GEOMAGNETISM AND AERONOMY
Vol. 52
No. 1
2012
ZVEREV et al.
degree: softening of the spectrum, a decrease in the
energy density of the precipitating flux, and weakening
of intensity at lower latitudes. As well, the energy spec
tra of electron fluxes at the latitude of diffuse glow and
in the inner magnetosphere are identical in their char
acter (Meng, 1978). Therefore, it seems natural to
relate the occurrence of SAR arcs at the equatorial
edge of the diffuse precipitation region to lowenergy
electron fluxes, bearing in mind the high efficiency of
redemission excitation processes by electrons with
energies <10 eV. Moreover, referring glow to SAR arcs
is to a certain degree arbitrary because there are no
commonly accepted restrictions on the size of the
I
630.0
/
I
557.7
emission ratio, latitudinal shifts, 630.0 nm
emission brightness, and duration of the phenome
non. Classification should be largely based on the
physics of the occurrence of auroral glow sources than
on the morphological or spectral features of the glow.
The limit energy of electrons measured by equip
ment on the DMSP satellites is 31 eV. In connection
with the absence of measurements of electron fluxes
with energies of ~10 eV, we estimate, according to
(Rassoul et al., 1993), the density of the electron flux
creating the 630.0 nm emission with an intensity of
~0.2 kR. During direct excitation, such emission is cre
ated by an electron flux of ~0.1 erg/cm
2
s or
6
⋅
10
9
elec
trons/cm
2
s with an energy of 10 eV. Owing to excita
tion by heated electrons of the ionosphere, the emis
sion intensity is <0.01 kR. Thus, an intensity of ~0.2 kr
is created by a particle flux of
6
⋅
10
9
, which is at a level
of particle flux in diffuse glow of medium intensity.
The intensities of lowenergy electron fluxes pre
cipitating to heights of 295–950 km above SAR arcs
are presented in (Slater et al., 1987) from measure
ments onboard the Dynamics Explorer 2 satellite. The
results of coordinated measurements by groundbased
photometers of 630.0 nm emission, and of lowenergy
electron fluxes by spectrometers onboard the satellite,
have demonstrated the presence of a close relationship
between both phenomena. It has been demonstrated
that electrons with
Е
< 10 eV are the main source of
energy necessary for the observed values of atmo
spheric heating and emission intensity in SAR arcs.
According to measurements onboard the satellite,
electron flux densities with energies of 5.1 and 8.8 eV
amount to ~2
×
10
8
–10
9
particles/(cm
2
s eV), exceed
ing by three orders electron fluxes of 20–30 eV. The
intensity of red emission created by such fluxes is ~2 kR.
6. CONCLUSIONS
1. The diffuse glow intensity increases equatorwards
from discrete forms during magnetic disturbances. The
limit latitude of such a glow at the zenith according to
visual observations (visoplots) characterizes the equato
rial boundary of the auroral glow region.
2. According to visoplot data, glow has been
recorded at all latitudes between the limit latitude of
observation of auroral glow and the oval. The latitude
interval between them is filled by diffuse glow resulting
from precipitation of auroral particles into the upper
atmosphere.
3. The region of diffuse glow spatially coincide with
that of soft electron precipitation extending equator
ward from the boundary of the oval to the latitude of
the plasmopause projection to heights of the iono
sphere. The energy flux density of electrons and their
energy decrease with decreasing latitude, which leads
both to a decrease in diffuse glow intensity and an
increase in its height at lower latitudes.
4. The existence of SAR arcs of relatively small,
<1.0 kR, intensity can be related to precipitation of
lowenergy electron of the plasma sheet. Arcs are
located near the equatorial boundary of the region of
diffuse electron precipitation, where their energies
decrease to <10 eV. Such an arrangement of weak SAR
arcs is a result of both the character of the change in
energy of auroral particles during a largescale plasma
convection in the nighttime magnetosphere and the
character of excitation and deactivation processes of
630.0 nm emission in the upper atmosphere.
ACKNOWLEDGMENTS
The work was financially supported by the Russian
Foundation for Basic Research (project nos. 0905
00818) and the RAS Presidium Program no. 4. DMSP
F8 and F9 satellite data were obtained via the Internet
(http://sdwww.jhuapl.edu/).
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