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GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L14107, doi:10.1029/2006GL026294, 2006
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Correlation between the inner edge of outer radiation belt electrons
and the innermost plasmapause location
Xinlin Li,1,2 D. N. Baker,1 T. P. O’Brien,3 L. Xie,4 and Q. G. Zong5
Received 13 March 2006; revised 26 May 2006; accepted 5 June 2006; published 27 July 2006.
[1] A remarkable correlation between the inner edge of the
outer radiation belt electrons and the innermost plasmapause
location is demonstrated by using 12 years of measurements
of MeV electrons from the Solar, Anomalous, and
Magnetospheric Particle Explorer (SAMPEX) and the
Combined Radiation and Release Experiment Satellite
(CRRES) during its entire mission (July 1990 – October
1991). An empirical model of the plasmapause location,
Lpp, by O’Brien and Moldwin (2003) as well as the
measured Lpp during the CRRES mission is used for the
comparison. This correlation suggests that the innermost
Lpp is the innermost limit of the outer radiation belt
penetration. We discuss the possible scenarios that might
explain this correlation. Citation: 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.
1. Introduction
[2] While the outer radiation belt is comprised of electrons with energies of 100s’ keV to MeV, the plasmasphere
consists of cold (1 eV) and dense (103 cm 3) plasma.
The inner edge of the outer radiation belt, Lrb, can be
anywhere between L = 2 and L = 5 (see Figure 1), where L
is the radial distance in RE at the equator if Earth’s magnetic
field is approximated as a dipole. The outer boundary of the
plasmasphere, usually referred to as the plasmapause, is
where the plasma density has a steep gradient. This boundary can be at a wide range of L-values, as small as at L = 2
and as high as L = 7 and beyond, strongly dependent on the
level of geomagnetic activity [Chappell, 1972]. Early in
space era, it was reported that the position of the plasmapause was related to the region of enhanced trapped
energetic electrons [e.g., Carpenter et al., 1971].
[3] There has been renewed interest in understanding the
plasmapause and especially its relation to the outer radiation
belt. Moldwin et al. [2002] found that only 16% of the
1
Laboratory for Atmospheric and Space Physics and Department
of Aerospace Engineering Sciences, University of Colorado, Boulder,
Colorado, USA.
2
Also affiliated with Laboratory for Space Weather, Chinese Academy
of Sciences, Beijing, China.
3
Space Science Department-Chantilly, The Aerospace Corporation,
Chantilly, Virginia, USA.
4
Department of Geophysics, Peking University, Beijing, China.
5
Center for Atmospheric Research, University of MassachusettsLowell, Lowell, Massachusetts, USA.
Copyright 2006 by the American Geophysical Union.
0094-8276/06/2006GL026294$05.00
plasmapause crossings encountered by CRRES satellite
(altitude: 350 km 5.2 RE, inclination: 18°) were clean
and sharp ‘classic’ isolated steep density gradients, and that
plasmapause observations with significant structure or density cavities are more common. Nonetheless, based on the
CRRES data, O’Brien and Moldwin [2003] derived an
empirical plasmapause model to describe the averaged
plasmapause location, Lpp, as a function of magnetic
indices. Carpenter and Lemaire [2004] suggested that the
plasmapause region be called plasmasphere boundary layer,
PBL, because the term PBL represents the outer boundary
region of the plasmasphere more objectively and accurately.
[4] It is understood that a broad-band whistler mode
emission known as plasmaspheric hiss inside plasmasphere
is the primary cause of the quiet-time loss of outer radiation
belt electrons, precipitating them into atmosphere due to
pitch angle scattering [Lyons et al., 1972]. During storm
times, the pitch angle scattering of the radiation belt
electrons is more prominent near and just outside the
plasmapause where electromagnetic ion cyclotron, EMIC,
waves are also contributing [Albert, 2003]. O’Brien et al.
[2003] showed that the statistical relationship between the
plasmapause and the Dst index resembles that of the
location of the peak outer zone flux and the Dst index
[Tverskaya et al., 2003]. Recently, Baker et al. [2004]
showed that the Lrb was closely related to the plasmapause
location even during the extreme distortion of the outer
radiation belt during the October – November 2003 storm.
Later, Goldstein et al. [2005] investigated the spatial
relationship between the outer radiation belt and the
plasmapause during the two month period 30 March –
30 May 2001. Focusing on the loss mechanism of
the MeV electrons, they found that the Lrb matched with
a 3-day running average of the innermost plasmapause
location, implying a 3-day loss time scale from pitch
angle scattering by plasmaspheric hiss.
[5] In this letter, we will demonstrate that the penetration
of the outer radiation belt, which usually occurs during the
recovery phase of a magnetic storm, is closely related to the
minimum plasmapause location, which usually occurs during the main phase of a magnetic storm. We will then
discuss the most promising explanations of this correlation
regarding the acceleration and loss mechanisms of the outer
radiation belt electrons.
2. Observations
[6] The bottom plot of Figure 1 shows monthly windowaveraged fluxes of 2 – 6 MeV electrons (#/cm2-s-sr) from
PET instrument on SAMPEX [Cook et al., 1993] since
its launch (July 3, 1992) into a low-altitude and highly
inclined orbit. The superimposed white curve represents
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Figure 1. (top) Variations of yearly window-averaged
sunspot numbers (black curve) and weekly windowaveraged 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 (#/cm2s-sr) by SAMPEX since its launch (July 3, 1992) into a lowaltitude (550 600 km) and highly inclined (82°) orbit. The
superimposed white curve represents every 10-day’s minimum Lpp based on an empirical model [O’Brien and
Moldwin, 2003]. The yellow vertical bars on the horizontal
axis are marks of equinoxes.
every 10-day’s minimum Lpp, based on an empirical model
using the Dst index as input [O’Brien and Moldwin, 2003].
The top plot of Figure 1 shows yearly window-averaged
sunspot numbers and weekly window-averaged solar wind
speed (km/s).
[7] A remarkable correlation is evident in Figure 1
between the Lrb and the innermost position of the Lpp over
a long term period, during which the outer radiation belt
also exhibits a strong solar cycle and seasonal variation. The
outer radiation belt was most intense, on average, during the
descending phase of the sunspot cycle (1993 –1995 and
2003 – 2004) when recurrent high speed solar wind streams
emanating from persistent trans-equatorial coronal holes
were prominent and long-lasting, as shown in the top plot.
The outer belt was weakest during sunspot minimum
(1996 – 1997) when the solar wind speed was also low.
The outer belt became more intense during the ascending
phase of the solar cycle (1997 – 1999). Interestingly, the
outer belt was not most intense approaching or at sunspot
maximum. Seasonally, the outer belt is most intense and
penetrates the deepest around the equinoxes [Baker et al.,
1999; Li et al., 2001], as demonstrated by its correlation
with the yellow bars (marks of equinoxes) on the horizontal
axis. Despite the variations in the time scale and magnitude
of the outer electron belt, the depth of its penetration into
the inner magnetosphere remains closely tied to the innermost Lpp.
[8] Figure 2 shows daily-averaged electron fluxes of 2 –
6 MeV (#/cm2-s-sr) by SAMPEX during 1998. The superimposed white curve represents the daily minimum Lpp.
The superimposed black curve represents the Lrb, which is
defined as the L-value where the electron intensity is 40%
of its peak. It is evident that the initial penetration of outer
radiation belt correlates well with the innermost location of
the Lpp and stays inside the Lpp even when the Lpp
recovers to large L. The deep penetration of the outer
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radiation belt usually corresponds to an overall enhancement of the electrons since the innermost location of Lpp
and the value of the Lrb correlates well during enhancements of the outer radiation belt. If we define an enhancement event as a total electron flux (integrating over all L
values) increase over 30% from the previous day’s value,
then 92 days in 1998 satisfy the criteria. During these 92 days
the correlation coefficient between the innermost model Lpp
and the Lrb is 0.510. Including all days in 1998, the correlation coefficient is 0.354.
[9] We must note that the empirical model of Lpp
[O’Brien and Moldwin, 2003] does not reproduce the
detailed structure of the plasmapause motion and there is
considerable scatter between the model and the data from
which the model was derived [see O’Brien and Moldwin,
2003, Figure 1]. Nonetheless, here we demonstrate that
there exists a close relationship between the outer radiation
belt and the modeled outer boundary of plasmasphere even
though it may be only in an averaged sense.
[10] Figure 3 shows orbitally-averaged electron fluxes
from CRRES during its entire mission, July 1990– October
1991. The superimposed white curve represent every twoday’s minimum Lpp from the same plasmapause model and
the superimposed black curve is a five day window-averaged daily minimum Lpp measured by CRRES [Moldwin et
al., 2002]. The modeled Lpp and the measured Lpp do not
always track each other well (as discussed by O’Brien and
Moldwin [2003]) having a typical root-mean-square (RMS)
error of about 1 L. Also there are significant data gaps for
the measured Lpp [Moldwin et al., 2002]. Nonetheless, the
correlation between the inner edge of the outer belt and the
inner most Lpp, both measured and modeled, is evident.
The new belt centered at L 2.5 starting on March 24,
1991 (DOY starting 1990: 448) was caused by a strong
interplanetary shock impacting on the Earth [Blake et al.,
1992; Li et al., 1993]. Lower energy electrons (not shown
here) usually penetrate deeper than the electrons shown
(1.13– 1.23 MeV).
3. Discussion
[11] Several significant points from the above observations warrant more discussion. As noted above, the Lpp
shown in Figures 1 – 3 and used in the correlation calcula-
Figure 2. Daily electron fluxes of 2 – 6 MeV (#/cm2-s-sr)
by SAMPEX, color-coded in logarithm, and sorted in L (L
bin: 0.1) for 1998. The superimposed white curve represents
daily minimum L of the empirical Lpp. The superimposed
black curve represents the inner edge of the outer radiation
belt (see text for details).
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Figure 3. Orbitally averaged, color-coded in logarithm,
and sorted in L (L bin: 0.1) electron fluxes (#/cm2-s-sr) from
CRRES for its entire mission, July 1990 – October 1991.
The superimposed white curve represents every 2-day’s
minimum Lpp based on an empirical model [O’Brien and
Moldwin, 2003] and the black curve is the 5-day windowaveraged daily minimum Lpp measured from CRRES
[Moldwin et al., 2002].
tion is the averaged L-value calculated from an empirical
model and there is considerable scatter between the model
and the data [O’Brien and Moldwin, 2003]. If the local time
dependence is included, the minimum Lpp would be a little
smaller. It is important to emphasize that the Lpp values
shown in Figures 1 – 3 are heavily smoothed. The inner edge
of the outer radiation belt does not follow the plasmapause
all the time. Only the initial penetration of the outer belt
follows the plasmapause erosion. After the plasmapause
moves outward, the most inward electrons stay at small L
shells and slowly decay inside the plasmasphere. This also
means that the outer radiation belt co-exists with the
plasmasphere after a deep penetration. It is most visible
from Figures 2 and 3 that the electron flux inside plasmasphere continues to diffuse inward and slowly decay.
[12] We consider three possible explanations for the
observed correlation. First, as examined by Baker et al.
[2004] and Goldstein et al. [2005], the plasmasphere may
shape the outer zone by providing enhanced storm-time
losses immediately interior to the plasmapause. The dominant loss process would be storm-enhanced EMIC and hiss
waves, combining to deplete the MeV electrons [Albert,
2003]. Both EMIC and hiss are enhanced during the storm,
but would not remain a strong loss process during and after
the recovery phase.
[13] The electrons in Figures 1– 3 are spin-averaged and
cover all local pitch angles, both trapped or precipitating.
Xie et al. [2006], separating measured electrons (30 keV to
>300 keV) into trapped and precipitating during Sept. 10–
Nov. 10 of 2003 from the NOAA-17 satellite (in a low
altitude polar orbit), found that the inner edge of the
precipitating electrons traces the empirical Lpp well and
stays outside of the Lpp. However, the inner edge of the
trapped electrons often stays inside the empirical Lpp. This
suggests that pitch angle scattering of electrons by VLF
waves responsible for precipitation is more significant just
outside the Lpp. This is also consistent with the theoretical
analysis that the combined EMIC waves, often occurring
outside plasmapause, and plasma hiss lead to enhanced
radiation belt electron precipitation [Albert, 2003].
[14] The second explanation for the plasmapause role in
shaping the outer zone is a possible role of the plasma-
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spheric mass density in modifying the ULF waves that
cause radial transport and energization. What role the PBL
plays in terms of the distribution of global ULF wave
activity is still an unresolved question [Carpenter and
Lemaire, 2004]. However, the plasmapause certainly plays
some role in modifying ULF waves [Takahashi and
Anderson, 1992; Lee, 1996], although not necessarily
those in the Pc5 band. The latter are most often associated
with acceleration of MeV electrons [e.g., Rostoker et al.,
1998]. There is certainly plenty of empirical evidence for the
necessary outward radial gradient in the pre-storm radiation
belt phase-space density [e.g., Hilmer et al., 2000]. Also, even
though ULF power decreases with L, storm-time ULF power
is considerably enhanced over a wide range of L values
[Mathie and Mann, 2001]. A positive radial gradient of the
phase-space density and enhanced ULF power would lead to
enhanced inward radial diffusion.
[15] Finally, the VLF acceleration mechanism may move
inward as the plasmapause is eroded, because of the
coincidence of a local minimum in the ratio of the electron
plasma to cyclotron frequencies with strong lower-band
VLF chorus [e.g., O’Brien et al., 2003]. This mechanism
would lead to a phase-space density peak outside the
plasmapause. Selesnick and Blake [2000] and Green and
Kivelson [2004] have shown evidence for storm-time phasespace density peaks growing out of the more usual outward
gradient. In this scenario, VLF acceleration would be
effective and occur just beyond the plasmapause, as was
argued by Horne et al. [2005] and Shprits et al. [2006] for
the October –November 2003 storm case. The phase-space
density peak would be smoothed out over a range of nearby
L values by radial diffusion. Some of the particles would be
carried to lower L, into the plasmasphere, gaining a small
amount of extra energy.
4. Summary
[16] A remarkable correlation between the inner edge of
the outer radiation belt electrons and the innermost plasmapause location is demonstrated using 12 years of measurements of MeV electrons from SAMPEX as well as from
CRRES for its entire mission (July 1990 – October 1991),
and an empirical model of the Lpp [O’Brien and Moldwin,
2003]. This correlation suggests that the depth of penetration of the outer radiation belt electrons into the inner
magnetosphere is associated with the innermost Lpp and
the inner part of the outer radiation belt co-exists with the
plasmasphere after the recovery of the plasmapause to a
larger L. The electrons inside the plasmasphere may continue to diffuse inward and slowly decay. Lower energy
electrons penetrate deeper than higher energy electrons.
Pitch-angle scattering of electrons by VLF and EMIC waves
is probably responsible for the precipitation occurs mostly
near the Lpp. Recent observational evidence is consistent
with this explanation [Xie et al., 2006].
[17] However, we have also outlined two mechanisms by
which the plasmapause may play a role in modifying the
acceleration processes as well: the plasmasphere may modify the characteristics of the ULF waves that diffuse
particles radially inward and to higher energies, and acceleration by VLF chorus may produce a flux peak just outside
the plasmasphere because chorus is strongest just beyond
the plasmapause. We must note that these mechanisms are
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not mutually exclusive. More likely they act in concert to
produce the above correlation.
[18] Determining the relative contribution of the mechanisms discussed above to the observed correlation will
require further study. In particular, global plasmaspheric
maps from IMAGE EUV will aid in making accurate
determinations of which L values are affected by which
mechanism. Also, a detailed examination of the energy
dependence of the radiation belt topology will likely provide important clues. The acceleration and loss mechanisms
are energy dependent: they do not affect 100 keV, 1 MeV
and >5 MeV electrons in the same way. Our study has not
systematically considered the energy dependence.
[19] Acknowledgments. We thank A. Vampola and M. Moldwin for
providing CRRES/MEA and CRRES Lpp data. We thank M. Thomsen,
M. Moldwin, and J. Bortnik for helpful discussions. This work was
supported by NASA grant (NAG5-13518) and NSF grants (CISM and
ATM-0233302), and also in part by National Natural Science Foundation of
China grant 40125012 and the International Collaboration Research Team
Program of the Chinese Academy of Sciences.
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D. N. Baker and X. Li, LASP/CU, 1234 Innovation Drive, Boulder, CO
80303-7814, USA. (lix@lasp.colorado.edu)
T. P. O’Brien, Space Science Dept.-Chantilly, The Aerospace Corporation
15049 Conference Center Drive, CH3/210, Chantilly, VA 20151-3824,
USA.
L. Xie, Department of Geophysics, Peking University, Beijing 100871,
China.
Q. G. Zong, Center for Atmospheric Research, 600 Suffolk Street,
University of Massachusetts-Lowell, Lowell, MA 01854-3629, USA.
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