[show abstract][hide abstract] ABSTRACT: Aims. Sunspot numbers form a benchmark series in many studies, but may still
contain inhomogeneities and inconsistencies. In particular, an essential
discrepancy exists between the two main sunspot number series, Wolf and group
sunspot numbers (WSN and GSN, respectively), before 1848. The source of this
discrepancy has remained unresolved so far. However, the recently digitized
series of solar observations in 1825-1867 by Samuel Heinrich Schwabe, who was
the primary observer of the WSN before 1848, makes such an assessment possible.
Methods. We construct sunspot series, similar to WSN and GSN but using only
Schwabe's data. These series, called here WSN-S and GSN-S, respectively, were
compared with the original WSN and GSN series for the period 1835-1867 for
possible inhomogeneities. Results. This study supports the earlier conclusions
that the GSN series is more consistent and homogeneous in the earlier part than
the WSN series. We show that: the GSN series is homogeneous and consistent with
the Schwabe data throughout the entire studied period; the WSN series decreases
by roughly 20% around 1848, which is caused by the change of the primary
observer from Schwabe to Wolf and an inappropriate individual correction factor
used for Schwabe in the WSN; this implies a major inhomogeneity in the WSN,
which needs to be corrected by reducing its values by 20% before 1848; the
corrected WSN series is in good agreement with the GSN series.
[show abstract][hide abstract] ABSTRACT: The Medium Energy Proton and Electron Detector (MEPED) instruments onboard the NOAA/POES satellites have provided a valuable long-term database of low-altitude energetic particle observations spanning from 1978 to present. Here we study the instrumental problems of the NOAA/MEPED electron detectors and present methods to correct them. It is well known that the MEPED electron detectors are contaminated by protons of certain energy range. Using the recently corrected MEPED proton fluxes, we are now able to reliably remove this contamination. Using a simple simulation model to estimate the response of the MEPED electron detectors to incoming electrons and protons, we show that efficiencies of (Space Environment Monitors) SEM-1 and SEM-2 versions of the detectors have large differences due to different detector designs. This leads to a systematic difference between the SEM-1 and SEM-2 measurements and causes a significant long-term inhomogeneity in measured MEPED electron fluxes. Using the estimated efficiencies, we remove the proton contamination and correct the electron measurements for nonideal detector efficiency. We discuss the entire 34 year time series of MEPED measurements and show that, on an average, the correction affects different energy channels and SEM-1 and SEM-2 instruments differently. Accordingly, the uncorrected electron fluxes and electron spectra are severely distorted by nonideal detector efficiency and proton contamination, and their long-term evolution is misrepresented without the correction. The present correction of the MEPED electron fluxes over the whole interval of NOAA/POES measurements covering several solar cycles is important for long-term studies of, e.g., magnetospheric dynamics, solar activity, ionospheric research, and atmospheric effects of energetic electrons.
Journal of Geophysical Research 10/2013; 118:6500–6510. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: Aims. The heliospheric magnetic field (HMF) has long been hemispherically asymmetric so that the field in the northern hemisphere is weaker and the area larger than in the south. This asymmetry, also called the bashful ballerina, has existed during roughly three-year intervals of the late declining to minimum phase of solar cycles 16–22. We study the HMF and its hemispheric asymmetry during the exceptional solar cycle 23.
Methods. We use NASA National Space Science Data Center OMNI database, which contains all solar wind and HMF observations at the Earth’s orbit, and coronal field predictions by Wilcox Solar Observatory. We present a new method to study the global hemispheric asymmetry by using the power n of the radial decrease of the radial field from the coronal source surface to 1 AU.
Results. We find that the HMF is exceptional at low latitudes in solar cycle 23: while the typical latitudinal variation was attained in the north in 2008, it did not take place in the south until Spring 2009. Thus, the Rosenberg-Coleman rule is abnormally delayed or broken for the first time in 50 years. The n-values verify the clear northern dominance in cycles 21–22. However, the low-latitude observations depict a considerably smaller asymmetry in cycle 23, although Ulysses observations at high latitudes show an equally large asymmetry in 2007 and in 1994–1995. We argue that the weak low-latitude visibility of the asymmetry in cycle 23 is due to the exceptionally weak polar fields, leading to large tilt angle and a wide current sheet.
Conclusions. We note that the exceptional properties of cycle 23 (weak dynamo, large tilt, small asymmetry) agree with the long-term evolution of hemispheric asymmetry viewed at the Earth. The active Sun is seen as more asymmetric at the Earth than the quiet Sun because the polar coronal holes with unipolar fields extend closer to the equator, allowing their asymmetry to be viewed even at low latitudes. We suggest that, after the period of weak activity and small asymmetry at 1 AU that started with cycle 23, the hemispheric asymmetry will again, with the increasingly active cycles, become better visible at 1 AU but the asymmetry will be oppositely oriented, including a northward shifted current sheet, and larger areas but weaker intensities in the south. Thus, the ballerina should no longer be systematically bashful for some 100–150 years.
Astronomy and Astrophysics 08/2013; 525(L12). · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Sun's long-term magnetic variability is the primary driver of space
climate. This variability is manifested not only in the long-observed
and dramatic change of magnetic fields on the solar surface, but also in
the changing solar radiative output across all wavelengths. The Sun's
magnetic variability also modulates the particulate and magnetic fluxes
in the heliosphere, which determine the interplanetary conditions and
impose significant electromagnetic forces and effects upon planetary
atmospheres. All these effects due to the changing solar magnetic fields
are also relevant for planetary climates, including the climate of the
Earth. The ultimate cause of solar variability, at time scales much
shorter than stellar evolutionary time scales, i.e., at decadal to
centennial and, maybe, even millennial or longer scales, has its origin
in the solar dynamo mechanism. Therefore, in order to better understand
the origin of space climate, one must analyze different proxies of solar
magnetic variability and develop models of the solar dynamo mechanism
that correctly produce the observed properties of the magnetic fields.
This Preface summarizes the most important findings of the papers of
this Special Issue, most of which were presented in the Space Climate-4
Symposium organized in 2011 in Goa, India.
Journal of Space Weather and Space Climate. 06/2013;
[show abstract][hide abstract] ABSTRACT: Samuel Heinrich Schwabe made 8486 drawings of the solar disk with sunspots in
the period from November 5, 1825 to December 29, 1867. We have measured sunspot
sizes and heliographic positions on digitized images of these drawings. A total
of about 135,000 measurements of individual sunspots are available in a data
base. Positions are accurate to about 5% of the solar radius or to about three
degrees in heliographic coordinates in the solar disk center. Sizes were given
in 12 classes as estimated visually with circular cursor shapes on the screen.
Most of the drawings show a coordinate grid aligned with the celestial
coordinate system. A subset of 1168 drawings have no indication of their
orientation. We have used a Bayesian estimator to infer the orientations of the
drawings as well as the average heliographic spot positions from a chain of
drawings of several days, using the rotation profile of the present Sun. The
data base also includes all information available from Schwabe on spotless
Monthly Notices of the Royal Astronomical Society 05/2013; 433(4). · 5.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: Context. Solar active longitudes and their rotation have been studied
for a long time using various forms of solar activity. However, the
results on the long-term evolution of rotation rates and the hemispheric
asymmetry obtained by earlier authors differ significantly from each
other. Aims: We aim to find a consistent result on the long-term
migration of active longitudes of sunspots in 1877-2008 separately for
the two hemispheres. Methods: We used a dynamic, differentially
rotating reference system to determine the best-fit values of the
differential rotation parameters of active longitudes for each year in
1877-2008. With these parameters we determined the momentary rotation
rates at the reference latitude of 17° and calculated the
non-axisymmetries of active longitudes. We repeated this with five
different fit intervals and two weighting methods and compared the
results. Results: The evolution of solar surface rotation in each
hemisphere suggests a quasi-periodicity of about 80-90 years. The
long-term variations of solar rotation in the northern and southern
hemisphere have a close anti-correlation, leading to a significant
80-90-year quasi-periodicity in the north-south asymmetry of solar
rotation. The north-south asymmetry of solar rotation is found to have
an inverse relationship with the area of large sunspots. The latitudinal
contrast of differential rotation is also found to be anti-correlated
with the sunspot area. Different fit and weight methods yield similar
results. Conclusions: Our results give strong evidence for the
anti-correlation of the rotation of the two solar hemispheres. The
long-term oscillation of solar rotation suggests that a systematic
interchange of angular momentum takes place between the two hemispheres
at a period of about 80-90 years.
Astronomy and Astrophysics 04/2013; · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: Here we compare the traditional analog measure of geomagnetic activity, Ak, with the more recent digital indices of IHV and Ah based on hourly mean data, and their derivatives at the auroral station Sodankylä. By this selection of indices we study the effects of (i) analog vs. digital technique, and (ii) full local-time vs. local night-time coverage on quantifying local geomagnetic activity. We find that all other indices are stronger than Ak during the low-activity cycles 15–16 suggesting an excess of very low scalings in Ak at this time. The full-day indices consistently depict stronger correlation with the interplanetary magnetic field strength, while the night-time indices have higher correlation with solar wind speed. The Ak index correlates better with the digital indices of full-day coverage than with any night-time index. However, Ak depicts somewhat higher activity levels than the digital full-day indices in the declining phase of the solar cycle, indicating that, due to their different sampling rates, the latter indices are less sensitive to high-frequency variations driven by the Alfvén waves in high-speed streams. On the other hand, the night-time indices have an even stronger response to solar wind speed than Ak. The results strongly indicate that at auroral latitudes, geomagnetic indices with different local time coverage reflect different current systems, which, by an appropriate choice of indices, allows studying the century-scale dynamics of these currents separately.
Advances in Space Research 09/2012; 50(6):690–699. · 1.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report here on new problems in the NOAA/MEPED instruments and the
related energetic proton dataset. These problems are solved, and the
implied modifications to the earlier recalibration of the dataset are
evaluated and adopted. We show that, besides degrading due to radiation
damage, the NOAA-12 and NOAA-08 satellites suffer from increased
electronic noise in the back detector of the proton instrument. We
correct the effects of the noise, and present improved estimates for the
effective energy thresholds of the MEPED proton detectors which are now
determined more robustly than previously. We show that the cumulative Ap
index can be used to produce a refined estimate for the temporal
evolution of the effective MEPED energy thresholds. The derived energy
thresholds of all MEPED instruments increase systematically with the
cumulative particle fluxes, and this increase is similar in all
satellites. Using the improved energy thresholds we obtain a uniform
series of corrected MEPED energetic proton fluxes above 120 keV
from 1979 onwards. We find that, due to the effect of the radiation
damage and noise, the uncorrected fluxes at these energies were
underestimated in the worst case by more than an order of magnitude, and
that the earlier correction method also occasionally led to
underestimation of the fluxes by nearly an order of magnitude. Such
underestimation becomes severe already 1-2 years after the
launch of the satellite.
Journal of Geophysical Research 09/2012; 117(A9):9204-. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: It is well known that, in addition to the ring current, also other
current systems like the magnetopause currents and the tail current have
a significant contribution to the Dcx index. While the effect of the
magnetopause currents are typically removed by correcting for the solar
wind pressure, the effects of the tail current are less well understood
and have received less attention. Still, some recent studies have shown
that the tail current can have a significant and even a dominant
contribution to the Dst index at least during the main phase of moderate
storms. We have developed a semi-empirical model that expresses the
Dcx index as a sum of ring current, tail current and magnetopause
contributions. In the model, the tail current is monitored by observing
the location of the night-side isotropic boundary of energetic protons
using the MEPED energetic particle instrument onboard NOAA/POES
satellites. We briefly present here the model paying particular
attention to the tail current and the solar wind parameters driving it.
We apply the model to a set of magnetic storms driven by coronal mass
ejections (CME) and high speed solar wind streams (HSS), and discuss the
differences in the tail and ring current response between these two
[show abstract][hide abstract] ABSTRACT: Several studies suggest that energetic particle precipitation into the
upper atmosphere can cause notable changes in ion and neutral chemistry
in the upper and middle atmosphere. During polar winter these changes
can last long enough to descend down to the stratospheric levels. Some
recent studies also suggest a link between global geomagnetic activity
and changes in stratospheric and tropospheric temperatures. Geomagnetic
activity was used in these studies as a proxy of energetic particle
precipitation in lack of a homogeneous long term energetic particle
dataset. We have recently recalibrated the NOAA/POES energetic particle
fluxes, which now form a homogeneous series over the last 30 years.
Using this data set, we have studied the statistical relationship
between the northern hemisphere winter surface air temperatures and
energetic electron precipitation in 1980-2010. We study the effects of
local energetic electron precipitation in addition to precipitation
averaged globally over the whole polar region. Using monthly surface air
temperature maps we find strong positive (negative) correlation between
the surface temperatures and precipitation in northern Eurasia
(north-western Atlantic). The range of temperature variations
attributable to variations in electron precipitation can be nearly 10 K.
We also discuss possible mechanisms behind the observed surface
temperature variations caused by electron precipitation.
[show abstract][hide abstract] ABSTRACT: When studying the effects of geomagnetic variability on the Earth's
atmosphere reliable sources of quality long term data are essential.
Most past studies concerning the effects of energetic particles
precipitating from the magnetosphere into the atmosphere have been
forced to use geomagnetic activity indices such as Kp (and Ap) as
proxies for the intensity of this particle precipitation. Despite their
good long term coverage these indices are only a crude proxy for the
particle fluxes. Accordingly, there is a great need for a quality long
term dataset of precipitating energetic particles. The polar orbiting
NOAA/POES satellites have measured energetic particles with their MEPED
instrument nearly continuously for more than 30 years. Up to now the
reliable usage of the data has been severely restricted by several
instrumental problems including degradation due to radiation damage,
detector noise and contamination of electron measurements by protons. We
have extensively studied these issues in the data and have now for the
first time produced a new long term NOAA/MEPED database that corrects
the data for all the above mentioned problems. Here we briefly review
the problems that have plagued the NOAA/MEPED measurements and discuss
the methods used to address them. We show how the corrections change the
long term time series of the energetic particle fluxes dramatically. We
will also discuss the relation between the energetic particle fluxes and
geomagnetic indices to emphasize the difference between the two.
[show abstract][hide abstract] ABSTRACT: We investigate the average properties of geomagnetic storms using the
global and local Dxt indices at four Dst stations in 1932-2009.
Imposing the condition of complete data availability during storms, our
study includes 1268/362/134/59 storms with Dxt minimum less than
-50/-100/-150/-200 nT, respectively. The global
Dxt minima were, on an average,
-94/-156/-216/-275 nT, while deepest storm-time
local Dxt minima were -137/-214/-285/-350 nT.
Accordingly, the local Dxt minima are typically 25-30% stronger
than the global Dxt minima. The distribution of largest storm-time
disturbances is strongly peaked at 18 local time (LT), challenging local
midnight as the dominant ion source. Relative timing of local minima
verifies that stations at earlier LT hour observe their minimum a couple
of hours after the deepest minimum, in agreement with westward drift of
ions. Storm-time maximum asymmetries were found to increase with storm
intensity level from about 70 nT to 150 nT for -50 to -200
nT storms. However, strong storms are relatively more symmetric than
weak storms when compared to the typical level of local disturbance.
During individual storms the asymmetry can be more than 200 nT. The rate
of evolution of storm-time asymmetry is found to be roughly twice as
fast for large storms. We emphasize that the unique database of local
Dxt indices proves to be very useful in studying the average spatial
distribution and temporal evolution of storms.
Journal of Geophysical Research 03/2012; 117:3201. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: We reanalyze the observations of the heliospheric magnetic field (HMF) made by the Pioneer 10 and 11 and Voyager 1 and 2 heliospheric probes since 1972, and calculate the HMF sector occurrence ratios and tangential component strengths in the different regions of the heliosphere. Observations at the distant probes and at 1 AU show a very consistent picture of the HMF sector structure in the entire heliosphere, and even beyond the termination shock. HMF observations by the probes also support the southward shift of the heliospheric current sheet (the bashful ballerina phenomenon), which is observed earlier at 1–2 AU by the Ulysses probe and Earth-orbiting satellites, and verify the HCS shift over a wide range of radial distances until the distant heliosphere. Pioneer 11 and Voyager 1 show that the development of northern polar coronal holes was very systematic and active during all the four solar minima since mid-1970s, while Voyager 2 observations show a less systematic and delayed development of southern coronal holes in 1980s, 1990s and 2000s. This delay in the evolution of southern coronal holes with respect to the rapid and systematic evolution of northern coronal holes leads to a larger extent of northern coronal holes and the southward shift of the HCS for a few years in the late declining phase of each solar cycle. Although evidence for the connection between the different evolution of polar coronal holes and the bashful ballerina phenomenon is obtained here only for three solar cycles, this may be a common pattern for solar coronal hole evolution since the southward shift of the HCS has occurred at least since solar cycle 16.
Journal of Geophysical Research 01/2012; · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: There is increasing evidence that various manifestations of solar activity are non-axisymmetric and mainly occur in two preferred longitude ranges, so called active longitudes. We have earlier analyzed the longitudinal occurrence of solar X-ray flares observed by GOES satellites using a specially developed dynamic, differentially rotating coordinate system. In this frame, the longitude distribution shows two persistent preferred longitudes separated by about 180 degrees whose strength alternates in time according to the so called flip-flop phenomenon. Here we make the first global statistical analysis to find the best fitting values for parameters describing the differential rotation of active longitudes of X-ray flares. We find that the new analysis greatly improves the earlier values for the rotation parameters, making them consistent between the three different classes of X-ray flares. The improved parameters also yield a systematically higher level of non-axisymmetry for the longitudinal distribution, thus increasing the statistical significance of the existence of active longitudes. Accordingly, a significant amount of X-ray flares of different classes are produced by the same two active longitudes. We also find a significant difference between the rotation rates in the two solar hemispheres, with active longitudes rotating faster than the Carrington rate in the northern hemisphere and slower than the Carrington rate in the southern hemisphere.
Journal of Atmospheric and Solar-Terrestrial Physics 02/2011; 73:258-263. · 1.42 Impact Factor
[show abstract][hide abstract] ABSTRACT: We test the Kalman filter technique to define solar regular variationAk-type activity index based on Kalman method outperforms previous estimatesKalman algorithm may be used to define a wide variety of activity indices
Journal of Geophysical Research 01/2011; 116. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: Recently, the surface differential rotation parameters were found to
vary differently with time for the northern and southern hemispheres of
the Sun. Both sunspots and flares strongly suggest that the northern
hemisphere rotated considerably faster than the southern during the last
three solar cycles, showing a strong north-south asymmetry in solar
surface rotation. In order to study the long-term variation of solar
surface differential rotation, the location of sunspots during 1877-2009
separately in the two hemispheres. The variation of the rotation of the
northern hemisphere is found to have an anti-correlation with that of
the southern hemisphere and the variation suggests a period of 10-12
[show abstract][hide abstract] ABSTRACT: Context. Active longitudes have been found in various manifestations of solar activity. The longitudinal distribution of, e.g., sunspots and solar X-ray flares shows two persistent preferred longitudes separated by roughly 180 degrees. We previously studied solar X-ray flares using an improved version of a dynamic, differentially rotating coordinate system and found enhanced rotational asymmetry and rotation parameter values that are consistent for the three classes of X-ray flares. Aims. We aim to find the optimal values of rotation parameters of active longitudes of sunspots for several different time intervals and separately for the two solar hemispheres. Methods. We perform a global study of the longitudinal location of sunspots (all sunspots and first appearance sunspots) using a refined version of a dynamic, differentially rotating coordinate system. Results. We find that the rotation parameters for sunspots are in good agreement with those obtained for X-ray flares using the same method. The improved method typically finds somewhat faster equatorial rotation with better accuracy. The improved treatment also leads to a larger non-axisymmetry. Rotation parameters for all sunspots and first appearances closely agree with each other, but non-axisymmetry is systematically larger for all sunspots than for first appearances, suggesting that strong fields follow more closely the pattern of active longitudes. The refined method emphasizes hemispheric differences in rotation. Over the whole interval, the mean rotation in the southern hemisphere is slower than in the north. We also find significant temporal variability in the two rotation param-eters over the 136-year interval. Interestingly, the long-term variations (trends and residual oscillations) in solar rotation are roughly the opposite in the northern and southern hemispheres. Conclusions. Rotation parameters vary differently with time in the northern and southern hemispheres. Both sunspots and flares strongly suggest that the northern hemisphere rotated considerably faster but the southern hemisphere slightly slower than the Carrington rotation rate during the last three solar cycles.
Astronomy and Astrophysics 01/2011; 529(23). · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: The MEPED instruments onboard the low-altitude polar orbiting NOAA/POES satellites have measured energetic particles since 1978, offering a nearly continuous series of energetic particle fluxes in the magnetosphere during three solar cycles. However, there are several problems in using these data for long-term studies, the most significant one being that the solid state detectors of the MEPED proton instruments suffer significant radiation damage. This causes the effective energy thresholds of the instrument to increase, leading to underestimated particle fluxes already a couple of years after satellite launch. Before the MEPED data can reliably be used in any long-term study the data has to be recalibrated taking into account the decay of the detectors. In this paper we present quantified estimates of the degree of radiation damage for all NOAA/POES satellites, a method for correcting the MEPED proton measurements, and give an estimate of energetic proton fluxes from 1978 to present.
Journal of Atmospheric and Solar-Terrestrial Physics 01/2011; 73(2). · 1.42 Impact Factor