JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, A11207, doi:10.1029/2011JA016934, 2011
Behavior of MeV electrons at geosynchronous orbit during last
two solar cycles
X. Li,1,2 M. Temerin,3 D. N. Baker,4 and G. D. Reeves5
Received 16 June 2011; revised 4 August 2011; accepted 9 August 2011; published 10 November 2011.
[1] A comparison of MeV electron measurements at geosynchronous orbit, GEO, with
solar wind shows that the MeV electron prediction model developed for GEO using
data from the declining phase of solar cycle 22 (1995–1996) works well for the declining
phase of solar cycle 23 (2006–2008), indicating that the MeV electron flux has a
predictable and systematic response to the solar wind. The same comparison for solar
maximum (2000–2003) shows that the model works less well partly because it does not
match the high flux cutoff seen in the data and partly because it does not reproduce
the sudden drops in flux that occur when the magnetopause is close to GEO. The model
also reproduces the nonlinear correlation of the solar wind speed with the log of the
MeV electron flux seen at GEO. An examination of 15 yr of solar wind and the MeV
electron data shows that geomagnetic activity driven by a southward orientation of
the interplanetary magnetic field, IMF, is a necessary condition for MeV electron
enhancements at GEO and that high‐speed solar wind are not necessary. The reason
that high‐speed solar wind is almost always associated with the enhancement of MeV
electrons is mainly because high‐speed solar wind almost always has some southward
components of the IMF.
Citation: Li, X., M. Temerin, D. N. Baker, and G. D. Reeves (2011), Behavior of MeV electrons at geosynchronous orbit during
last two solar cycles, J. Geophys. Res., 116, A11207, doi:10.1029/2011JA016934.
1. Introduction
[2] Recently, Reeves et al. [2011] have used newly
available, almost‐continuous MeV electron data at geosynchronous orbit (GEO) to re‐examine the relation between
solar wind speed, VSW, and MeV electron fluxes at GEO.
Motivated by this study and these data, we have re‐examined
our electron prediction model [Li et al., 2001]. Our model
was developed from data during the declining phase of solar
cycle 22 (1995–1996).
[3] The main goals of this report are
[4] 1. To show the results of using the newly available
MeV electron data at geosynchronous orbit (GEO) to test
our previously developed electron prediction model, Li et al.
[2001], against out‐of‐sample data. We show that the model
1
Department of Aerospace Engineering Sciences, Laboratory for
Atmospheric and Space Physics, University of Colorado at Boulder,
Boulder, Colorado, USA.
2
Also at Laboratory for Space Weather, Chinese Academy of Sciences,
Beijing, China.
3
Retired from Space Sciences Laboratory, University of California,
Berkeley, California, USA.
4
Department of Astrophysics and Planetary Sciences, Laboratory for
Atmospheric and Space Physics, University of Colorado at Boulder,
Boulder, Colorado, USA.
5
Space Science and Applications Group, Los Alamos National
Laboratory, Los Alamos, New Mexico, USA.
Copyright 2011 by the American Geophysical Union.
0148‐0227/11/2011JA016934
works well for the declining phase of solar cycle 23
(2006–2009).
[5] 2. To show the results of using the model during solar
maximum. We demonstrate that the model works less well
during solar maximum and discuss why this may be.
[6] 3. To show that the nonlinear relationship between
solar wind speed and the log of the MeV electron fluxes at
GEO recently described by Reeves et al. [2011] is a natural
feature of the model and thus a natural consequence of the
simple physics underlining the model.
[7] 4. To show, by examining the data, that despite the
good correlation of MeV electron fluxes at GEO with solar
wind speed, the essential feature required for enhancements
of MeV electron fluxes at GEO is geomagnetic activity which
in turn requires a southward component of the interplanetary
magnetic field (IMF).
[8] The Li et al. [2001] model uses only the solar wind to
predict MeV electron fluxes at GEO. Knowledge of a correlation between the solar wind and electrons in the outer
radiation belt has a long history. A correlation between VSW
and relativistic electrons in the magnetosphere was identified
soon after the solar wind was first measured. The correlation
was demonstrated by a 27 day periodicity (Sun’s synodic spin
period near the maximum region of sunspots) in the intensity
of trapped electrons in the outer radiation belt for two
energy channels: >280 keV and >1.2 MeV [Williams, 1966].
Paulikas and Blake [1979] more firmly established the correlation between VSW and MeV electrons by showing that
MeV electrons at GEO enhance two days after the passage of
A11207
1 of 11
A11207
LI ET AL.: MEV ELECTRONS AT GEOSYNCHRONOUS ORBIT
A11207
Figure 1. (top) Variations of yearly window‐averaged sunspot numbers (black curve) and weekly window averaged solar wind speed (km/s, red curve). (bottom) Monthly window‐averaged, color‐coded in
logarithm, and sorted in L (L bin: 0.1) electron fluxes of 2–6 MeV (#/cm2‐s‐sr) by SAMPEX since its
launch (3 July 1992) into a low‐altitude (550 × 600 km) and highly inclined (820) orbit. The superimposed
black curve represents every 10 day’s minimum plasmapause location based on an empirical model
[O’Brien and Moldwin, 2003]. The yellow vertical bars on the horizontal axis mark the equinoxes. Calibrated daily averaged electron fluxes from SAMPEX are only available up to day 74 of 2004. We also
have daily count rate from the same instrument from day 1 of 2003 to the end of 2009. For the period where
the flux data and count rate data overlaps, we performed a linear least squares fit independently for each of
the 60 L‐shell bins. This gave two coefficients for each L‐shell bin, which best converted the count rate to
pseudo‐flux (after day 74 of 2004). Then we applied this set of coefficients to the count rate data to
complete the figure.
high‐speed solar wind streams. Concurrently, Baker et al.
[1979] showed that low‐energy electrons (10 s of keV) at
GEO as well as relativistic electrons, respond to large VSW.
Based on this correlation, Baker et al. [1990] developed a
linear filter model to predict MeV electrons at GEO using
VSW. The linear filter was developed from limited intervals
of continuous solar wind and MeV electron flux measurements. The method achieved a prediction efficiency (PE) of
0.52 in their 3 month sample period [Baker et al., 1990].
[9] The location of GEO is viewed as the outer edge of the
outer radiation belt and the gateway into the inner part of
magnetosphere. The peak intensity of outer radiation belt
electrons normally resides inside GEO. Figure 1 shows the
yearly window‐averaged sunspot numbers and the weekly
window‐averaged solar wind speed (km/s) (Figure 1, top)
and the monthly window averaged flux of 2–6 MeV electrons (#/cm2‐s‐sr) from PET instrument on the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX)
[Cook et al., 1993] (Figure 1, bottom). The period is from
SAMPEX’s launch (3 July 1992, into a low‐altitude, highly
inclined orbit) to the end of 2009. The superimposed black
curve represents every 10 day’s minimum plasmapause location (Lpp) based on an empirical model using the Dst index
as input [O’Brien and Moldwin, 2003], the significance of
the correlation between the minimum Lpp and the inner edge
of the outer radiation belt was discussed in [Li et al., 2006]. It
is evident in Figure 1 that the intensity of outer belt electrons
is very well correlated with the solar wind speed. However,
the essential feature required for enhancements of the MeV
electron fluxes at GEO, as we are going to show later, is
geomagnetic activity that in turn requires a southward
component of the IMF.
[10] As part of the International Solar Terrestrial Program,
the solar wind has been almost continuously measured since
December 1994 by the Wind [Acuña et al., 1995] and by the
Advanced Composition Explorer (ACE, launched in 1998;
ACE home page: http://www.srl.caltech.edu/ACE/) spacecraft [Stone et al., 1998]. Using data from 1995–1996, Li et al.
[2001] developed an improved model to predict MeV electrons at GEO based on solar wind velocity and velocity
fluctuations and the south component of the IMF. They
achieved a PE of 0.81 for this two‐year sample period. They
found that though VSW is the most important parameter
governing relativistic electron fluxes at GEO, relativistic
electrons are enhanced even more when the IMF polarity is
predominately southward (BZ < 0).
[11] The physical mechanism for the correlation is under
debate. In one mechanism, solar wind variations perturb the
magnetosphere, generating ultra low frequency (ULF) waves
[Engebretson et al., 1998; Vennerstrøm, 1999], which drive
radial diffusion [Rostoker et al., 1998; Baker et al., 1998a;
Mathie and Mann, 2000, 2001; O’Brien et al., 2003; Barker
2 of 11
A11207
LI ET AL.: MEV ELECTRONS AT GEOSYNCHRONOUS ORBIT
et al., 2005; Ukhorskiy et al., 2006; Li et al., 2009] and thus
energize electrons. Magnetohydrodynamic simulations and
test‐particle tracing have shown that radiation belt electrons
respond to such magnetospheric fluctuations [Hudson et al.,
1999; Elkington et al., 1999; Kress et al., 2008]. Alternatively, the solar wind drives geomagnetic activity, which
produces very low frequency (VLF) waves with frequencies
comparable to the electrons’ local gyrofrequency. These
waves can both energize and pitch angle scatter electrons
[Temerin et al., 1994; Selesnick and Blake, 2000; Meredith
et al., 2001, 2002; Albert, 2002; Horne and Thorne, 2003;
Bortnik and Thorne, 2007; Li et al., 2007; Tu et al., 2009],
energizing some electrons to relativistic energies while precipitating others into the atmosphere. Though it is generally
accepted that both mechanisms work, their relative contribution remains uncertain. It should be noted that a fast
interplanetary shock (regardless the orientation of IMF) can
cause enhancements of MeV electrons at GEO and much
inside GEO [e.g., Blake et al., 1992; Li et al., 1993, 2003].
[12] Reeves et al. [2003] showed that MeV electron
enhancements are associated with geomagnetic storms, but
that MeV electrons increase above their pre‐storm levels for
only half of the storms. For the other half, MeV electrons
either decrease or only recover to their pre‐storm levels.
Weigel et al. [2003] noted that a VSW above 600 km/s is not a
necessary condition for a large enhancement of MeV electrons and that such large solar wind speeds do not precede
many enhancements. Reeves et al. [2011] also concluded that
high solar wind speed was not a necessary condition but
argued that it was a sufficient condition for high MeV electron flux. Li et al. [2005] analyzed the correlation of electrons from 50 keV to 6 MeV to solar wind variations and
found that electron enhancements occur after increased VSW
with a time delay that increases with energy but that also
depends on the average polarity of the IMF, a shorter delay if
the IMF Bz < 0 and a longer delay if the IMF Bz > 0 during
the VSW enhancement. Miyoshi and Kataoka [2008] investigated the variation of MeV electrons after the arrival of
solar wind stream interaction regions and found that the
greatest MeV electron enhancements occur in the highest‐
speed streams with a southward IMF, indicating that a large
VSW is not a sufficient condition for large flux enhancements. More recently, Kataoka and Miyoshi [2010] focused
on the year 2009 and suggested that the extremely weak IMF
of the very slow solar wind of 2009 played an essential role
in diminishing source processes associated with magnetic
storms and substorms which, in turn suppressed the relativistic electron flux at GEO. The MeV electron fluxes were
indeed very low during 2009, as well as during 2006, as is
evident in Figure 1.
[13] Recently, using solar wind data and a considerably
longer data set (1989–2010) from the LANL energetic particle instruments at GEO, Reeves et al. [2011] re‐examined
the relationship between relativistic electron fluxes and VSW.
They found a triangle shaped distribution in which fluxes have
a velocity dependent lower limit but a velocity ‐independent
upper limit rather than a roughly linear relation between VSW
and the log of the MeV electron flux. They also found that
the highest electron fluxes can occur for any value of VSW
with no indication of a VSW threshold and concluded that the
relationship between the log of the radiation belt electron
fluxes and VSW is more complex than a simple linear cor-
A11207
relation. Such similar findings were reported earlier by Baker
et al. [2004], who showed the triangle‐shaped distribution
between total radiation belt electron fluxes (with energy
greater than 2 MeV and over the range of magnetic L shells:
2.5 < L < 6.5) versus solar wind speed and noted the same
complexity and nonlinearity.
[14] These recent studies (Reeves et al. [2011], in
particular) plus the many more years of continuous measurements of the solar wind and of energetic electrons at
GEO, motivated us to re‐examine the correlation between the
solar wind and MeV electrons at GEO, using our electron
prediction model [Li et al., 2001], which was developed from
data during the declining phase of solar cycle 22 (1995–
1996). The declining phase and minimum of solar cycle 23
provide an independent test of the model. This model has
also been revised to make real‐time forecasts one and two
days ahead of >2 MeV electron fluxes at GEO using real‐
time solar wind data from ACE, normalized with real‐time
GOES measurements at GEO [e.g., Li, 2004; Turner and Li,
2008; Turner et al., 2011].
2. Data
[15] The solar wind data used in this study are from the
OMNI database. Solar wind measurements were made at
various locations, mostly near the L1 point, and have been
propagated to the sub‐solar point of the magnetopause (see
http://omniweb.gsfc.nasa.gov/). All solar wind parameters
used in this study have a time resolution of 10 min. The
energetic electron data are from the LANL energetic particle
measurements at GEO, combining daily averages from
LANL‐GEO ESP instruments on all available satellites (up
to seven). The data were inter‐calibrated and processed for a
long‐term study of GEO electron fluxes [Reeves et al.,
2011]. In this report, we use the 0.7–1.8 MeV ESP channel.
3. Model
[16] The GEO electron prediction model in this study is the
same as in Li et al. [2001]. The model uses a diffusion
equation, modeled after radial diffusion, to predict the electron fluxes at GEO, using solar wind parameters as input. The
inner and outer boundary are set at L = 4.5 and L = 11,
respectively. The radial diffusion equation is solved by setting
the phase space density104 times greater at the outer
boundary than at the inner boundary with a constant decay
rate. The diffusion coefficient in the model is a function
mainly of the solar wind velocity and the southward component of the IMF (The functional form of the diffusion
coefficient also includes a velocity fluctuation term but the
effect of that term is small). The result of the diffusion
equation is then adjusted by a solar wind pressure term and by
a Dst term, calculated from a Dst model [Temerin and Li,
2002, 2006], which is also based on solar wind parameters.
Thus the model is completely driven by the solar wind.
[17] The diffusion term in the model smoothes, averages,
and delays the effect of solar wind changes on the predicted
electron flux at GEO. While the diffusion term in the model
is based on radial diffusion theory, any agreement between
the model and the data should not be taken as definitive
proof that radial diffusion is operating since other processes
that smooth, average, and delay the effects of the solar wind
3 of 11
A11207
LI ET AL.: MEV ELECTRONS AT GEOSYNCHRONOUS ORBIT
A11207
Figure 2. A comparison of 3 yr of daily averages of 0.7–1.8 MeV electron flux measured by LANL
instruments at geosynchronous orbit with the predicted results based solely on measurements of the solar
wind. The red line shows electron flux measured at geosynchronous orbit, the black line shows predicted
results. There is an extended data gap in LANL measurements in early 2008. The Horizontal axis shows
the day of the year.
on the GEO electron flux may be modeled with such an
equation. In particular, heating of electrons by VLF hiss, as
mentioned above, can also have such an effect.
[18] All the parameters in the model are also the same as
in Li et al. [2001] except that an overall multiplicative factor
is applied to the model results to normalize the model to the
data. For application of the model to different periods, the
normalization factor is likely different. Even for the period
1995–1996, which was used to determine the parameters by
Li et al. [2001], the normalization is now different because
the electron data have been recalibrated and some data gaps
have been filled.
4. Results
[19] All of the model parameters (except the normalization)
were determined by comparing the predicted results with
LANL‐GEO electron measurements in 1995–1996 by maximizing the prediction efficiency, PE, defined as the fraction
of the variance that is ‘explained’ by the prediction or as
[1‐(mean squared residual)/(variance of data)] where the
residual is the difference between the data and the prediction
[Li et al., 2001]. We also calculated the linear correlation
between the model prediction and the electron data, LC, as
another index. We re‐ran the same model for the year of
1995–1996 using the OMNI solar wind data and the newly
calibrated LANL‐GEO ESP 0.7–1.8 MeV electron data and
achieved a PE of 0.78 and a LC of 0.88. Data gaps are
excluded in the calculation of PE and LC and all PE’s and
LC’s in this report use the logs of the fluxes.
[20] Figure 2 shows the application of the model to out‐
of‐sample data. The figure shows a comparison of the
2006–2008 daily averages of the MeV electron flux measured
at GEO with the model prediction. The overall variations of
the MeV electrons are well reproduced, with a PE of 0.72 and
a LC of 0.85. Solar wind conditions during 2006–2008 were
similar to those during 1995–1996. Both periods were in the
declining phase of the solar cycle approaching solar minimum. It is worth noting that during electron data gaps at
GEO; e.g., early in 2008, the model likely provides a good
estimate of the actual electron flux.
[21] For the 15 yr period (1995–2009), the model achieved
a PE of 0.62 and a LC of 0.79. The model does not work as
well overall mainly because it works less well during solar
maximum. For the solar maximum period, 2000–2003, the
PE equals 0.56 and LC equals 0.76. In particular the model
sometimes over‐predicts the electron flux at levels higher
than the maximum levels seen in the data. Figure 3 shows
the comparison between the measured electron flux and the
predicted flux (the model results are from a 15 yr run and
thus the normalization is based on the whole data set). The
over‐prediction during solar maximum is obvious (Using
the normalization for 1995–1996 would make the over‐
prediction during 2000–2003 13% larger).
[22] Another reason the model may not work as well for
solar maximum is that the dayside magnetopause is often
pushed close to or inside GEO during solar maximum, leading to a rapid loss of MeV electrons through the magnetopause. Figure 4 shows a comparison of MeV electrons at
GEO during solar maximum (2001), top four panels, with
MeV electrons during the solar declining phase (2007),
bottom four panels. During 2001 high‐speed solar wind
occurs irregularly and the IMF Bz component is often larger
than in 2007. These solar wind properties produced much
larger magnetic storms (red line in panel 3) and also pushed
the dayside magnetopause close to or inside GEO more
frequently. The vertical red bars on the top of panel 4 and
panel 8 show the sub‐solar location of the magnetopause in
Earth radii, based on the magnetopause model by Shue et al.
[1998].
[23] The model results are from the same 15 yr run for
both 2001 and 2007. MeV electrons at GEO during 2007 are
well reproduced, except that the modeled electrons decay a
little faster than the measured electrons. Rapid variations,
especially rapid decreases of MeV electrons during 2001 are
not well reproduced by the model, which doesn’t properly
handle the effects of magnetopause crossings.
[24] During solar maximum solar wind speed, density,
pressure, and magnetic field enhancements occur in an
irregular fashion, disrupting the radiation belt. In contrast,
during the declining phase of the solar cycle, solar wind
speed, pressure, and density enhancements occur in a regular
4 of 11
A11207
LI ET AL.: MEV ELECTRONS AT GEOSYNCHRONOUS ORBIT
Figure 3. The same as Figure 2 except for 2000–2003.
Figure 4. Daily averages of measured and predicted 1.7–1.8 MeV electron fluxes at geosynchronous
orbit, selected solar wind parameters, Dst index, and calculated location of magnetopause for the second
half‐year of 2001 (top four panels) and 2007 (bottom four panels).
5 of 11
A11207
A11207
LI ET AL.: MEV ELECTRONS AT GEOSYNCHRONOUS ORBIT
A11207
Figure 5. Scatterplots of daily averages of 0.7–1.8 MeV electron fluxes at geosynchronous orbit, both
measured and predicted for 15 yr (1995–2009) of data and model run, versus solar wind speed with and
without delay.
fashion at the leading edge of recurring high‐speed streams
emanating from coronal holes. Since the model is based on
data from the declining phase, it is not too surprising that it
performed well during the declining phase of another solar
cycle, and didn’t do as well during solar maximum.
[25] It was pointed in Baker et al. [2004], as well as
emphasized by Reeves et al. [2011], that the relationship
between VSW and the log of the MeV electron flux at GEO
is neither linear nor simple. Here we demonstrate that the
complicated relation can be a natural feature of the model
and thus a natural consequence of the simple physics underlining the model. The top two panels of Figure 5 show the
correlation between VSW and MeV electrons at GEO, essentially reproducing the results shown in Reeves et al. [2011]
(though for a lower energy channel). In particular, the two
important features emphasized in Reeves et al. [2011]:
(1) the nonlinear nature of the correlation and (2) the distinct
velocity‐dependent lower limit to the fluxes are reproduced.
With no delay between VSW and the electron flux, the electron
fluxes can be at any level for low values of VSW. With a 1.9‐
day delay (right upper panel ‐ VSW versus electrons 1.9 days
later), the correlation becomes more organized. The model
shows a similar correlation between the VSW and the predicted MeV electrons at GEO, as displayed in the left bottom
panel of Figure 5 (for the 15 yr model run) and that, with a
1.9 day delay, the correlation is also greatly improved, right
bottom panel. The delay in the electron flux (either measured
or modeled) with respect to VSW was calculated by interpo-
lating daily averaged values of VSW to optimize the correlation as a function of the delay.
[26] The main difference between the model and the data
is the greater scatter in the data but the model displays a
similar nonlinear relation between the solar wind speed and
the log of the electron flux. However, the model with a 1.9 day
delay still displays substantial scatter. This is because the
model also does not have a simple relationship between the
VSW and its output: the model depends also on the southward
component of the IMF and through the diffusion equation
effectively averages the solar wind input over time. In addition, the model has corrections for solar wind pressure and the
modeled Dst. Also the model diffusion term has a nonlinear
dependence on VSW.
[27] Another difference shown in Figure 5 is in the
saturation of the measured electron flux. The data have
few points above a flux level of about 106 (/cm2‐s‐sr‐keV).
Reeves et al. [2011] concluded, with some uncertainty, that
the saturation is more likely physical than instrumental. The
model doesn’t have an explicit saturation mechanism and the
model results occasionally show higher flux levels mostly
during solar maximum. Whatever the saturation mechanism
may be [e.g., Mauk and Fox, 2010], the model implies it
need not be very large, since even in the model, the flux
only exceeds the 106 (/cm2‐s‐sr‐keV) level by a small
amount and only rarely. In the model the flux is limited by
the boundary conditions and a decay term with a constant
decay rate.
6 of 11
A11207
LI ET AL.: MEV ELECTRONS AT GEOSYNCHRONOUS ORBIT
Figure 6. Scatterplot of daily averages of 0.7–1.8 MeV
electron fluxes at geosynchronous orbit, measured versus
predicted for 15 yr (1995–2009) of data and model run.
A11207
[28] Figure 6 shows measured electron fluxes versus the
model‐predicted fluxes. It is clear that the highest values of
the predicted fluxes are greater than the measured fluxes as
discussed before. The measured fluxes also have more very
low values, some of which are from the magnetopause
having been pushed near or inside GEO, causing precipitous
electron loss.
[29] There exists substantial confusion as to the essential
solar wind cause of MeV electron enhancements in the outer
radiation belt. On one hand, much of the literature has
emphasized the correlation between the high‐speed solar
wind and outer radiation belt electrons [e.g., Paulikas and
Blake, 1979; Baker et al., 1979; Reeves et al., 2011]. On
the other hand, such electron enhancements have been discussed as the product of magnetospheric storms [e.g., Baker
et al., 1998a; Reeves et al., 2003], which are known to be
crucially dependent on a large and southward oriented IMF.
Of course, much of this confusion arises because most
magnetic storms are associated with high‐speed solar wind
and the high‐speed solar wind compresses the IMF at its
leading edge producing the large southward IMF fields
required for a magnetic storm.
[30] However, there are cases when the compressed IMF
at the leading edge of high‐speed stream is almost wholly
Figure 7. Daily averages of measured and predicted 1.7–1.8 MeV electron fluxes at geosynchronous
orbit, selected solar wind parameters, calculated solar wind speed fluctuations, and Dst index for a
three‐month period, starting on 1 December 1995.
7 of 11
A11207
LI ET AL.: MEV ELECTRONS AT GEOSYNCHRONOUS ORBIT
Table 1. Examples of High‐Speed Solar Wind With VSW Over
500 km/s Without Appreciable Effect on Relativistic Electrons at
GEO (Less Than a Factor of Two Enhancement)
A11207
Table 2. Examples of Significant Enhancement of MeV Electrons
at GEO (Greater Than a Factor of Five) Without a High‐Speed
Solar Wind (VSW < 450 km/s)
Dates
Dst (nT) >
Dates
Dst (nT) <
98/04/17
98/05/16
00/03/29
00/06/04
00/06/23
04/04/28
06/03/16
07/01/11
08/03/20
09/01/31
−30
−25
−20
−15
−10
−15
−10
−20
−20
−10
95/08/25
95/12/15
97/01/21
98/04/11
00/01/23
01/01/02
01/02/22
04/10/05
05/09/28
09/02/04
−50
−40
−37
−48
−99
−35
−32
−38
−35
−50
northward. As pointed out by Blake et al. [1997], for one
example, a high‐speed stream with a predominately northward IMF needs not cause a MeV electron enhancement.
Another example occurred early in January of 1996 and is
shown as the second highlighted period in Figure 7, where
VSW was greater than 500 km/s but no significant enhancement of the relativistic electrons occurred with respect to the
previous electron fluxes. In terms of the relative enhancement of the electron fluxes, we found several additional
examples of high‐speed solar wind with VSW over 500 km/s
with no appreciable effect (less than a factor of two
enhancement) on relativistic electrons. In nearly all these
cases (more than 10), Bz was predominately northward and
there was no magnetic storm (Dst > −30 nT). Table 1 lists the
dates and the minimum Dst values for these events.
[31] On the other hand, in some rare cases magnetic
storms can occur with slow solar wind speeds, such as the
first highlighted period in Figure 7, where VSW was below
450 km/s and there was a significant enhancement of MeV
electrons (BZ is mostly negative). Furthermore, the highlighted period in Figure 8 shows a magnetic storm and an
electron enhancement (that our model failed to predict) without
a high‐speed stream—VSW never exceeded 360 km/s. We
also found 10 examples where the MeV electron flux
enhanced by a least factor of five without a high‐speed
solar wind (VSW was less than 450 km/s). In all these cases
(total 10), Bz was predominately southward and there was
enhanced magnetic activity (Dst < −30 nT). Table 2 lists
the dates and the minimum Dst values for these events.
Figure 8. Daily averages of measured and predicted 1.7–1.8 MeV electron fluxes at geosynchronous
orbit, selected solar wind parameters, and Dst index for a three‐month period, starting on 1 April 1997.
8 of 11
A11207
LI ET AL.: MEV ELECTRONS AT GEOSYNCHRONOUS ORBIT
[32] Some geomagnetic activity and thus some negative
Bz is required to produce a relativistic electron enhancement
at GEO; e.g., late December of 1995 as shown in the first
highlighted period in Figure 7. Not all electron enhancements are preceded by magnetic storms but there always
needs to be some geomagnetic activity, preferably associated
with a high‐speed stream, to produce a substantial enhancement of the electron flux at GEO. After all, the threshold for
a magnetic storm is arbitrary. As an example, based on our
survey of 15 yr of data, for VSW less than 450 km/s, a
minimum Dst of −30 nT is required for an enhancement of a
factor of five for the 0.7–1.8 MeV electron flux over previous flux level at GEO (see Table 2 for some examples).
At the leading edge of high‐speed streams, the IMF is usually
enhanced and fluctuates. If predominately northward, it may
not cause a magnetic storm but if its fluctuations produce at
least some southward IMF, there will be some geomagnetic
activity and the electron flux may be enhanced. In this manner
some high‐speed streams produce electron enhancements at
GEO without producing magnetic storms.
[33] In summary: high‐speed solar winds that produce little
geomagnetic activity because of associated small or northward IMF’s do not affect MeV electrons at GEO. On the other
hand, slow‐speed solar winds with large southward IMF’s
can produce magnetic storms and can substantially enhance
the MeV electron flux at GEO. Thus we conclude that the
essential cause of MeV electron enhancements at GEO is
enhanced geomagnetic activity, such as negative Dst, driven
by southward oriented IMF, not the high‐speed solar wind.
Another way to interpret this would be the following: Substorms require southward IMF (high‐speed solar wind certainly helps) to produce 100s’ keV electrons. Only with
source population do we get the subsequent MeV electrons
enhancement [e.g., Baker et al., 1998b and Li et al., 2009].
However, a combination of a long lasting high‐speed solar
wind and a southward oriented IMF produce highest fluxes
of MeV electrons at GEO.
[34] The model reflects these facts in part since the diffusion coefficient is a function of both the VSW and the
southward component of the IMF. However, the model will
produce some electron enhancement even for purely northward IMF. This is probably because the model parameters
reflect average conditions rather than rare events.
5. Discussion
[35] Our understanding of the relation between the solar
wind and MeV electron fluxes is imperfect. Particularly
confusing is the relation of magnetic storms and high‐speed
streams to MeV electron flux enhancements. Reeves et al.
[2003] have shown that only about half of storms lead to
electron flux enhancements from the pre‐storm level and that
there is little correlation between the pre‐storm and post‐
storm electron flux levels. This should not be taken as an
indication that no enhancement of the electron flux occurs
during the storm. Rather, storms usually deplete the pre‐
existing electrons before enhancing them. If the pre‐existing
electron flux is small, chances are the storm will enhance
the flux and vice versa.
[36] In the declining phase of the solar cycle, most high‐
speed streams lead to electron flux enhancements from the
pre‐high‐speed stream level because the relatively regular
A11207
temporal spacing of the high‐speed streams allows time for
the electron flux enhancement from the previous high‐speed
stream to decay. Many of these electron flux enhancements
during the declining phase are not preceded by storms but
are rather the products of an enhanced and prolonged level
of geomagnetic activity, which creates and sustains a high
level of electron flux. During solar minimum, when weak
IMF’s and slow solar wind speeds persist, a prolonged and
enhanced level of geomagnetic activity cannot be produced,
leading to low MeV electron fluxes; e.g., 1996 and 2009,
particularly for 2009, as shown in Figure 1 and also as discussed by Kataoka and Miyoshi [2010], who suggested that
the extremely weak IMF of the very slow solar wind during
2009 led to the very low MeV electron fluxes at GEO.
[37] The electron flux is relatively well correlated with the
solar wind speed and poorly correlated with the IMF and yet
the enhancement of the electron flux requires geomagnetic
activity that in turn requires a southward component of the
IMF. How can this be? A partial answer is that the high‐speed
solar wind typically has enhanced levels of IMF fluctuations
with both southward and northward components that average
to zero. So correlating with the average IMF is not very
useful. Only using the southward component of the IMF is
more useful but too large a southward component will lead
to storms that can also deplete the electron flux degrading
the correlation with southward component of the IMF. The
southward component is far more effective in driving geomagnetic activity when combined with a VSW (The electric
field, often considered the main driver of reconnection at
the magnetopause, is a cross product of the velocity and
magnetic field). Our model can help clarify the relative
importance of high solar wind speeds and southward IMF’s.
The model contains both solar wind speed and the southward component of the IMF. In the model the southward
component is a significant term and yet removing the southward component from the model and relying only on the solar
wind speed (with a readjustment of the parameters) only
slightly degrades the performance of the model. One can see
why this is by imagining that each high‐speed solar wind has
an equal amount of both southward and northward IMF’s
whose magnitudes are functions of the speed, as, in fact, is
approximately the case. Under such conditions the solar wind
speed would serve as perfect proxy for the combined effects
of the solar wind speed and of the IMF and so adding an
explicit IMF term would not help the model. It is only because
some high‐speed solar wind streams have more northward
IMF and some more southward IMF that including the IMF
enhances the performance of the model. Figure 9 shows a
comparison between predicted results (identical to the prediction results in Figure 2) and a controlled run where Bz is
set northward (positive) for the entire interval, which is
equivalently in our model to leaving the Bz term out, since
the Bz term in the model is given by Bz‐term = 1 + (−VxBz +
abs(VxBz))2. The second term goes zero when Bz is positive
(Vx is always negative). The diffusion coefficient is significantly reduced and so is the final result. The normalization
factor is kept the same, thus we can see the difference of the
modeled results with and without the Bz term. From a casual
glance, the difference may appear constant throughout. But
actually the difference between the black and red lines on
Figure 9 vary with time, because some high‐speed solar wind
9 of 11
A11207
LI ET AL.: MEV ELECTRONS AT GEOSYNCHRONOUS ORBIT
A11207
Figure 9. A comparison of 3 years of daily averages of 0.7–1.8 MeV electron flux predicted results
based solely on measurements of the solar wind (black line, the same as in Figure 2) and a controlled
model run (red line) where the IMF Bz term in the model is removed.
streams have more northward IMF and some more southward IMF as discussed above.
6. Summary
[38] In this report we have shown that the MeV electron
prediction model developed for geosynchronous orbit using
data from the declining phase of solar cycle 22 (1995–1996)
works well for the declining phase of solar cycle 23 (2006–
2008), indicating that the MeV electron flux at geosynchronous orbit has a predictable and systematic response to
the solar wind. For solar maximum (2002–2003) the model
works less well partly because it does not match the high
flux cutoff seen in the data and partly because it does not
reproduce the sudden drops in flux that occur from interactions of the magnetopause with locations close to geosynchronous orbit. We have also shown that the model
reproduces the nonlinear correlation of VSW with the log of
the MeV electron flux seen at geosynchronous orbit [Reeves
et al., 2011]. We have emphasized that the geosynchronous
MeV electron flux depends not only on the VSW but also on
the orientation of the IMF, the Dst index, which in turn is
complex function of the solar wind [Temerin and Li, 2002,
2006], and the solar wind pressure and that these additional
dependencies explain some of the scatter in the correlation
of the MeV electron flux with the VSW. In addition, we
have shown that geomagnetic activity driven by a southward orientation of the IMF is a necessary condition for MeV
electron enhancements at geosynchronous orbit and, in
agreement with Weigel et al. [2003] and Reeves et al. [2011],
that high‐speed solar wind is not necessary. High‐speed solar
wind is almost always associated with the enhancement of
MeV electrons mainly because high‐speed solar wind almost
always has some southward components of the IMF.
[39] Acknowledgments. This work is supported by NASA grants
(NNX‐09AJ57G, and ‐09AF47G) and NSF grants (CISM, ATM‐
0842388, and ‐0902813). We thank the OMNI group at NASA/Goddard
Space Flight Center and LANL energetic particle group for making solar
wind parameters and MeV electron at GEO data available. This work
was also supported by grants from the National Natural Science Foundation
of China (40921063 and 40728005).
[40] Masaki Fujimoto thanks the reviewers for their assistance in evaluating this manuscript.
References
Acuña, M. H., et al. (1995), The Global Geospace Science Program and its
investigations, Space Sci. Rev., 71, 5–21, doi:10.1007/BF00751323.
Albert, J. M. (2002), Nonlinear interaction of outer zone electrons with
VLF waves, Geophys. Res. Lett., 29(8), 1275, doi:10.1029/2001GL013941.
Baker, D. N., P. R. Higbie, R. D. Belian, and E. W. Hones Jr. (1979), Do
Jovian electrons influence the terrestrial outer radiation zone?, Geophys.
Res. Lett., 6(6), 531–534, doi:10.1029/GL006i006p00531.
Baker, D. N., R. L. McPherron, T. E. Cayton, and R. W. Klebesadel (1990),
Linear prediction filter analysis of relativistic electron properties at 6.6 RE,
J. Geophys. Res., 95, 15,133–15,140, doi:10.1029/JA095iA09p15133.
Baker, D. N., et al. (1998a), A strong CME‐related magnetic cloud interaction
with the Earth’s magnetosphere: ISTP observation of rapid relativistic electron acceleration on May 15, 1997, Geophys. Res. Lett., 25(15), 2975–2978,
doi:10.1029/98GL01134.
Baker, D. N., X. Li, J. B. Blake, and S. Kanekal (1998b), Strong electron
acceleration in the Earth’s magnetosphere, Adv. Space Res., 21, 609–613,
doi:10.1016/S0273-1177(97)00970-8.
Baker, D. N., S. G. Kanekal, and J. B. Blake (2004), Characterizing the
Earth’s outer Van Allen zone using a radiation belt content index, Space
Weather, 2, S02003, doi:10.1029/2003SW000026.
Barker, A. B., X. Li, and R. S. Selesnick (2005), Modeling the radiation
belt electrons with radial diffusion driven by the solar wind, Space
Weather, 3, S10003, doi:10.1029/2004SW000118.
Blake, J. B., W. A. Kolasinski, R. W. Fillius, and E. G. Mullen (1992),
Injection of electrons and protons with energies of tens of MeV into L < 3
on 24 March 1991, Geophys. Res. Lett., 19(8), 821–824, doi:10.1029/
92GL00624.
Blake, J. B., D. N. Baker, N. Turner, K. W. Ogilvie, and R. P. Lepping
(1997), Correlation of changes in the outer‐zone relativistic‐electron
population with upstream solar wind and magnetic field measurements,
Geophys. Res. Lett., 24(8), 927–929, doi:10.1029/97GL00859.
Bortnik, J., and R. M. Thorne (2007), The dual role of ELF/VLF chorus
waves in the acceleration and precipitation of radiation belt electrons,
J. Atmos. Sol. Terr. Phys., 69, 378–386, doi:10.1016/j.jastp.2006.05.030.
Cook, W. R., et al. (1993), PET: A proton/electron telescope for studies of
magnetospheric, solar, and galactic particles, IEEE Trans. Geosci.
Remote Sens., 31, 565–571, doi:10.1109/36.225523.
Elkington, S. R., M. K. Hudson, and A. A. Chan (1999), Acceleration of
relativistic electrons via drift‐resonant interaction with toroidal‐mode
Pc‐5 ULF oscillations, Geophys. Res. Lett., 26(21), 3273–3276,
doi:10.1029/1999GL003659.
Engebretson, M., et al. (1998), The dependence of high‐latitude Pc5 wave
power on solar wind velocity and on the phase of high‐speed solar wind
streams, J. Geophys. Res., 103, 26,271–26,283, doi:10.1029/97JA03143.
Horne, R. B., and R. M. Thorne (2003), Relativistic electron acceleration
and precipitation during resonant interactions with whistler‐mode chorus,
Geophys. Res. Lett., 30(10), 1527, doi:10.1029/2003GL016973.
Hudson, M. K., et al. (1999), Simulation of radiation belt dynamics driven
by solar wind variations, in Sun‐Earth Plasma Connections, edited by
J. L. Burch, R. L. Carovillano, and S. K. Antiochos, pp. 171–182,
AGU, Washington, D. C.
Kataoka, R., and Y. Miyoshi (2010), Why are relativistic electrons persistently quiet at geosynchronous orbit in 2009?, Space Weather, 8, S08002,
doi:10.1029/2010SW000571.
10 of 11
A11207
LI ET AL.: MEV ELECTRONS AT GEOSYNCHRONOUS ORBIT
Kress, B. T., M. K. Hudson, M. D. Looper, J. G. Lyon, and C. C. Goodrich
(2008), Global MHD test particle simulations of solar energetic electron
trapping in the Earth’s radiation belts, J. Atmos. Sol. Terr. Phys., 70,
1727–1737, doi:10.1016/j.jastp.2008.05.018.
Li, L. Y., J. B. Cao, G. C. Zhou, and X. Li (2009), Statistical roles of
storms and substorms in changing the entire outer zone relativistic electron
population, J. Geophys. Res., 114, A12214, doi:10.1029/2009JA014333.
Li, W., Y. Y. Shprits, and R. M. Thorne (2007), Dynamic evolution of
energetic outer zone electrons due to wave‐particle interactions during
storms, J. Geophys. Res., 112, A10220, doi:10.1029/2007JA012368.
Li, X. (2004), Variations of 0.7–6.0 MeV electrons at geosynchronous orbit
as a function of solar wind, Space Weather, 2, S03006, doi:10.1029/
2003SW000017.
Li, X., I. Roth, M. Temerin, J. Wygant, M. K. Hudson, and J. B. Blake
(1993), Simulation of the prompt energization and transport of radiation
particles during the March 24, 1991 SSC, Geophys. Res. Lett., 20(22),
2423–2426, doi:10.1029/93GL02701.
Li, X., M. Temerin, D. N. Baker, G. D. Reeves, and D. Larson (2001),
Quantitative prediction of radiation belt electrons at geostationary
orbit based on solar wind measurements, Geophys. Res. Lett., 28(9),
1887–1890, doi:10.1029/2000GL012681.
Li, X., D. N. Baker, S. Elkington, M. Temerin, G. D. Reeves, R. D. Belian,
J. B. Blake, H. J. Singer, W. Peria, and G. Parks (2003), Energetic particle
injections in the inner magnetosphere as a response to an interplanetary
shock, J. Atmos. Sol. Terr. Phys., 65(2), 233–244, doi:10.1016/S13646826(02)00286-9.
Li, X., D. N. Baker, M. Temerin, G. Reeves, R. Friedel, and C. Shen
(2005), Energetic electrons, 50 keV to 6 MeV, at geosynchronous orbit:
Their responses to solar wind variations, Space Weather, 3, S04001,
doi:10.1029/2004SW000105.
Li, X., D. N. Baker, T. P. O’Brien, L. Xie, and Q. G. Zong (2006), Correlation between the inner edge of outer radiation belt electrons and the
innermost plasmapause location, Geophys. Res. Lett., 33, L14107,
doi:10.1029/2006GL026294.
Mathie, R. A., and I. R. Mann (2000), A correlation between extended
intervals of ULF wave power and storm‐time geosynchronous relativistic
electron flux enhancements, Geophys. Res. Lett., 27(20), 3261–3264,
doi:10.1029/2000GL003822.
Mathie, R. A., and I. R. Mann (2001), On the solar wind control of Pc5
ULF pulsation power at mid‐latitudes: Implications for MeV electron
acceleration in the outer radiation belt, J. Geophys. Res., 106(A12),
29,783–29,796, doi:10.1029/2001JA000002.
Mauk, B. H., and N. J. Fox (2010), Electron radiation belts of the solar
system, J. Geophys. Res., 115, A12220, doi:10.1029/2010JA015660.
Meredith, N. P., R. B. Horne, and R. R. Anderson (2001), Substorm dependence of chorus amplitudes: Implications for the acceleration of electrons
relativistic energies, J. Geophys. Res., 106(A7), 13,165–13,178,
doi:10.1029/2000JA900156.
Meredith, N. P., R. B. Horne, R. H. A. Iles, R. M. Thorne, D. Heynderickx,
and R. R. Anderson (2002), Outer zone relativistic electron acceleration
associated with substorm‐enhanced whistler mode chorus, J. Geophys.
Res., 107(A7), 1144, doi:10.1029/2001JA900146.
Miyoshi, Y., and R. Kataoka (2008), Flux enhancement of the outer radiation
belt electrons after the arrival of stream interaction regions, J. Geophys.
Res., 113, A03S09, doi:10.1029/2007JA012506.
O’Brien, T. P., and M. B. Moldwin (2003), Empirical plasmapause models
from magnetic indices, Geophys. Res. Lett., 30(4), 1152, doi:10.1029/
2002GL016007.
O’Brien, T. P., K. R. Lorentzen, I. R. Mann, N. P. Meredith, J. B. Blake,
J. F. Fennell, M. D. Looper, D. K. Milling, and R. R. Anderson (2003),
Energization of relativistic electrons in the presence of ULF power and
MeV microbursts: Evidence for dual ULF and VLF acceleration,
J. Geophys. Res., 108(A8), 1329, doi:10.1029/2002JA009784.
A11207
Paulikas, G. A., and J. B. Blake (1979), Effects of the solar wind on magnetospheric dynamics: Energetic electrons at the synchronous orbit, in
Quantitative Modeling of Magnetospheric Processes, Geophys. Monogr.
Ser., vol. 21, edited by W. P. Olson, pp. 180–202, AGU, Washington, D. C.
Reeves, G. D., K. L. McAdams, R. H. W. Friedel, and T. P. O’Brien
(2003), Acceleration and loss of relativistic electrons during geomagnetic
storms, Geophys. Res. Lett., 30(10), 1529, doi:10.1029/2002GL016513.
Reeves, G. D., S. K. Morley, R. H. W. Friedel, M. G. Henderson, T. E.
Cayton, G. Cunningham, J. B. Blake, R. A. Christensen, and D. Thomsen
(2011), On the relationship between relativistic electron flux and solar
wind velocity: Paulikas and Blake revisited, J. Geophys. Res., 116,
A02213, doi:10.1029/2010JA015735.
Rostoker, G., S. Skone, and D. N. Baker (1998), On the origin of relativistic
electrons in the magnetosphere associated with some geomagnetic storms,
Geophys. Res. Lett., 25(19), 3701–3704, doi:10.1029/98GL02801.
Selesnick, R. S., and J. B. Blake (2000), On the source location of radiation
belt relativistic electrons, J. Geophys. Res., 105(A2), 2607–2624,
doi:10.1029/1999JA900445.
Shue, J.‐H., et al. (1998), Magnetopause location under extreme solar wind
conditions, J. Geophys. Res., 103(A8), 17,691–17,700, doi:10.1029/
98JA01103.
Stone, E. C., et al. (1998), The Advanced Composition Explorer, Space Sci.
Rev., 86(1), 1–22, doi:10.1023/A:1005082526237.
Temerin, M., and X. Li (2002), A new model for the prediction of Dst on
the basis of the solar wind, J. Geophys. Res., 107(A12), 1472,
doi:10.1029/2001JA007532.
Temerin, M., and X. Li (2006), Dst model for 1995–2002, J. Geophys.
Res., 111, A04221, doi:10.1029/2005JA011257.
Temerin, M., I. Roth, M. K. Hudson, and J. R. Wygant (1994), New paradigm for the transport and energization of radiation belt particles, Eos
Trans. AGU, 75, 538.
Tu, W., X. Li, Y. Chen, G. Reeves, and M. Temerin (2009), Storm‐dependent
radiation belt electron dynamics, J. Geophys. Res., 114, A02217,
doi:10.1029/2008JA013480.
Turner, D. L., and X. Li (2008), Quantitative forecast of relativistic electron
flux at geosynchronous orbit based on low‐energy electron flux, Space
Weather, 6, S05005, doi:10.1029/2007SW000354.
Turner, D. L., X. Li, E. Burin des Roziers, and S. Monk (2011), An
improved forecast system for relativistic electrons at geosynchronous
orbit, Space Weather, 9, S06003, doi:10.1029/2010SW000647.
Ukhorskiy, A. Y., B. J. Anderson, K. Takahashi, and N. A. Tsyganenko
(2006), The impact of ULF oscillations in solar wind dynamic pressure
on the outer radiation belt electrons, Geophys. Res. Lett., 33, L06111,
doi:10.1029/2005GL024380.
Vennerstrøm, S. (1999), Dayside magnetic ULF power at high latitudes: A
possible long‐term proxy for the solar wind velocity?, J. Geophys. Res.,
104, 10,145–10,157, doi:10.1029/1999JA900015.
Weigel, R. S., A. J. Klimas, and D. Vassiliadis (2003), Precursor analysis
and prediction of large‐amplitude relativistic electron fluxes, Space
Weather, 1(3), 1014, doi:10.1029/2003SW000023.
Williams, D. J. (1966), A 27‐day periodicity in outer zone trapped electron
intensities, J. Geophys. Res., 71(7), 1815–1826.
D. N. Baker, Department of Astrophysics and Planetary Sciences,
Laboratory for Atmospheric and Space Physics, University of Colorado
at Boulder, 1234 Innovation Dr., Boulder, CO 80303, USA.
X. Li, Department of Aerospace Engineering Sciences, Laboratory for
Atmospheric and Space Physics, University of Colorado at Boulder,
1234 Innovation Dr., Boulder, CO 80303, USA. (lix@lasp.colorado.edu)
G. D. Reeves, Space Science and Applications Group, Los Alamos
National Laboratory, ISR‐1, MS D466, Los Alamos, NM 87545, USA.
M. Temerin, Space Sciences Laboratory, University of California, 1062
Woodside Rd., Berkeley, CA 94720, USA.
11 of 11