JOURNAL OF GEOPHYSICAL RESEARCH: SPACE PHYSICS, VOL. 118, 99–104, doi:10.1029/2012JA017967, 2013
Simultaneous observations of plasmaspheric and ionospheric
variations during magnetic storms in 2011: First result
from Chinese Meridian Project
Chi Wang,1,2 Qingmei Zhang,2,1 P. J. Chi,3 and Chuanqi Li4
Received 21 May 2012; revised 29 October 2012; accepted 2 November 2012; published 15 January 2013.
[1] The plasma transport between the plasmasphere and ionosphere during magnetic
storms is a long-standing problem and is still not fully understood. Simultaneous
observations of the plasmasphere and ionosphere are vital to understand the coupling
between the two regions. In this study, using the measurements from the newly developed
Chinese ground-based space weather monitoring network (Meridian Project), we
investigate the plasmaspheric density at L ’ 2 inferred from ground magnetometers and the
ionospheric electron density inferred by digital ionosondes and GPS signals during
magnetic storms in 2011. Five moderate magnetic storms with minimum Dst index
between 47 and 103 nT during this period have been investigated. The observations
show that the plasmaspheric density drops significantly by more than half of the
prestorm value. The ionospheric F2 layer electron density NmF2 and the total electron
content (TEC) show ~20–50% decreases, and the NmF2 and TEC reductions take place
before the plasmaspheric density reaches its minimum. These findings suggest that the
plasmaspheric depletion is very likely due to the reduced plasma supply from the ionosphere
for the five moderate magnetic storms in 2011. Therefore, the plasmasphere dynamics
seems to be controlled by the ionosphere during magnetic storms.
Citation: Wang, C., Q. Zhang, P. J. Chi, and C. Li (2013), Simultaneous observations of plasmaspheric and ionospheric
variations during magnetic storms in 2011: First result from Chinese Meridian Project, J. Geophys. Res. Space Physics,
118, 99–104, doi:10.1029/2012JA017967.
of the plasmasphere during magnetic storms [e.g., Park,
1973; Chi et al., 2000]. Different from the erosion of the plasmasphere, this plasmaspheric depletion occurs within the
eroded plasmasphere during magnetic storms, and the plasma
density in the depleted region can drop by a factor of 4. The
plasmaspheric depletion was often found to be concurrent
with an ionospheric storm by which the ionosphere underwent significant changes. For example, Villante et al.
[2006] used the FLR remote sensing technique to reveal a
significant reduction (by a factor of 1.6–2) in the plasmasphere density at L = 1.7–1.8 during the recovery phase of
a geomagnetic storm. This decrease was accompanied by a
significant negative ionospheric storm phase in which the
critical frequency of the F2 layer (foF2) is depressed below
its median value and was confirmed by vertical total
electron content (TEC) measurements. They pointed out
that each storm event has its own individual manifestation because of the complex nonlinear interactions of the contributing processes and the large variety of the initial and
boundary conditions. Even during magnetic quiet time, there
exists a connection between ionospheric and plasmaspheric
density variations. Clilverd et al. [1991], using the whistler
mode group delays from very low frequency Doppler experiment, showed an annual variation in the equatorial electron
density in the plasmasphere, which has a maximum in
December and a minimum in June/July (maximum-minimum
ratio of about 1.7). This annual variation in equatorial
1. Introduction
[2] Geospace, defined by the extent of the terrestrial magnetic field into space, includes the closely coupled regions of
the middle-upper atmosphere (the thermosphere, ionosphere,
and magnetosphere) and their interactions with the lower
atmosphere. These regions are characterized by dynamic
processes known as space weather which links solar activity to the changes in the near-Earth space environment.
[3] An important process in geospace that is still not fully
understood is the plasma transport between the plasmasphere
and the ionosphere during magnetic storms. Specifically, previous ground-based observations of ducted whistlers and
field line resonance (FLR) have shown the internal depletion
1
State Key Laboratory of Space Weather, Center for Space Science and
Applied Research, Chinese Academy of Sciences, Beijing, China.
2
College of Math and Physics, Nanjing University of Information
Science and Technology, Nanjing, China.
3
Institute of Geophysics and Planetary Physics, University of California,
Los Angeles, CA, USA.
4
Guangxi Normal University, College of Electronic Engineering,
Guilin, China.
Corresponding author: C. Wang, State Key Laboratory of Space
Weather, Center for Space Science and Applied Research, Chinese
Academy of Sciences, Beijing 100080, China. ([email protected])
©2012. American Geophysical Union. All Rights Reserved.
2169-9380/13/2012JA017967
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WANG ET AL.: PLASMASPHERE AND IONOSPHERE
Table 1. Locations of Stationsa
Stations
MHT
MZL
Magnetic
Latitude
Magnetic
Longitude
Geographic
Latitude
Geographic
Longitude
L Value
48.6
44.9
195.9
191.2
53.5
49.6
122.4
117.5
2.29
1.99
a
The resonance point halfway between MHT and MZL has an L value
equal to 2.14. MHT, Mohe station; MZL, ManZhouLi station.
plasmaspheric electron density (Neq) can be modeled from
the combined foF2 medians at each end of the field line by assuming that diffusive equilibrium is maintained from the F2
layer to the equator over long (>1 month) time scales. However, the cause of internal plasmaspheric depletion during
magnetic storms is still uncertain. It could be due to the
downward flux to the ionosphere [Park, 1973] or the reduced
plasma supply from the ionosphere during negative ionospheric storms [Chi et al., 2000]. The negative ionospheric
storms are likely caused by the composition changes of the
neutral atmosphere, especially by the atomic oxygen to molecular nitrogen concentration ratio ([O/N2]) that alters the
balance of the production and loss processes of the ionized
plasma. Additionally, the vertical air motion can also change
the [O/N2] ratio and further affect the negative ionospheric
storm [Rishbeth, 1998]. To help answer this question, more
simultaneous observations of the plasmasphere and the ionosphere are needed to understand the coupling between the
two regions.
[4] In order to monitor the geospace environment, the
Meridian Space Weather Monitoring Project (or Chinese
Meridian Project) [Wang, 2011] operates a chain of 15
ground-based observatories located roughly along the
120 E longitude meridian and the 30 N latitude. Aside
from the station in Antarctica, the stations are located
roughly 4–5 of latitude or about 500 km apart. Each observatory is equipped with multiple instruments to measure
key parameters such as the baseline and time-varying geomagnetic field, as well as the middle and upper atmosphere and ionosphere from about 20 to 1000 km. This
project started test observations in 2011 and began normal
operation after the middle of 2012. In this study, we will
take advantage of the simultaneous observations from the
ground magnetometer, ionosonde, and ionospheric TEC
monitor in Mohe station (MHT) of this project to investigate the plasmaspheric and ionospheric variations during
magnetic storms in 2011.
[5] In this paper, we examine the variations of the plasmaspheric density using the gradient technique [Chi et al.,
2000]. The phase-difference method was first introduced
by Kurchashov et al. [1987] and then was first used by
Waters et al. [1991]. The phase difference of field line resonances observed by two nearby stations along the same longitude maximizes at the eigenfrequencies of the field line
midway between them. Stations from Chinese Meridian
Project are located in the middle to low latitudes with the
geographic latitudes ranging from 18.3 to 53.5 . It is found
that the first harmonic of field line resonances is in the Pc 3–
4 frequency range at mid- and low-latitude, and the gradient
technique can produce clear phase-difference patterns at
eigenfrequencies [Waters et al., 1991, 1994]. If the dipole
magnetic field and plasma density models (r r m, where
r is the geocentric distance) are assumed, we can infer the
plasma mass density from the observed eigenfrequency.
Similarly, the plasma mass density has been studied by
ground-based ULF magnetic field measurements [e.g. Menk
et al., 1999; Clilverd et al., 2003; Kawano et al., 2002;
Berube et al., 2003].
[6] In the same time, the digisonde and TEC monitor give
information about the ionosphere. The foF2 observed by the
digisonde gives the peak of the ionospheric electron density
in F2 layer (NmF2). The profile of the F2 layer is needed to
convert the foF2 value into the TEC value, which is not easy
to know exactly in most cases. Note that the F2 layer may
not be described by a simple Chapman profile in the daytime
close to solar maximum [Rishbeth and Garriott, 1969].
Therefore, real-time ionospheric TEC and scintillation monitors deployed in the meridian chain are common instruments
to infer TEC by GPS signals.
[7] The remainder of the paper proceeds as follows. We
describe the data used in the study and present the observations in section 2, and the discussion and summary is given
in section 3.
2. Observations
2.1. Data
[8] The data of ground magnetometer stations MHT and
ManZhouLi (MZL) are available from the data center of
the Chinese Meridian Project. Fluxgate magnetometers data
sampled at 1 Hz are used in this study. The noise level of the
systems is about 0.1 nT. The magnetic latitudes of MHT and
MZL are 48.6 and 44.9 , respectively, with L ’ 2 (Table 1).
The fluxgate magnetometers at these stations record the geomagnetic H, D, and Z components. The digisonde at MHT
gives 15 min time resolution of the foF2 values in a standard
format, and the TEC monitor provides 30 s time-resolution
data. We search the Dst index in 2011 available from the
World Data Center for Geomagnetism (http://wdc.kugi.
kyoto-u.ac.jp/index.html) for magnetic storms with Dst <
50 nT [Gonzalez et al., 1994]. However, the Chinese Meridian Project did not provide full data coverage for all these
magnetic storms, since it was in its test phase in 2011. In the
end, five magnetic storms with relatively good data coverage
have been selected, which took place on 6 April, 12 April,
10 September, 17 September, and 26 September 2011, respectively. For each event, we infer the plasmaspheric density from the ground magnetometer observations by using
the gradient technique and compare them with the ionospheric electron density variations.
2.2. A Typical Example
[9] On 26 September 2011, an interplanetary shock arrived at the Earth’s magnetosphere at 1144 UT and caused
the sudden commencement (SC) monitored by ground magnetometer stations. A moderate magnetic storm took place
afterward and reached its maximum (the Dst index reached
its minimum of 103 nT) near the end of the day. We take
this event as an example to give details of the data analysis
process. The phase differences between H component of
the magnetic fields at MHT and MZL were used to distinguish FLR events and identify the eigenfrequencies of the
magnetospheric field line midway between their latitudes.
Four days of phase-difference spectrograms are given in
Figure 1. All spectrograms present the time interval
100
WANG ET AL.: PLASMASPHERE AND IONOSPHERE
Density Frequency
TEC
NmF2
3
(1012el/cm2) (105/cm3) (amu/cm )
(mHz)
between 0000 and 1200 UT, which corresponds to the local time interval 0800–2000, since the phase-difference
patterns are usually seen on the dayside. The bright color
corresponds to larger phase differences. FLR signals can
be seen in a short time of the period on 0000–1000 UT
(0800–1800 LT) every day.
[10] On September 26, the FLR signatures appear obviously, and the fundamental mode frequency at about
30 mHz can be clearly identified over the interval 0000–
1000 UT in Figure 1. Additionally, it is slightly higher in
the morning than in the afternoon from 0000 to 1000 UT.
This kind of diurnal variation of the FLR frequency is typical at low-middle latitudes [Waters et al., 1994]. On 27
September, the eigenfrequency at 32 mHz at 0700 UT
(1500 LT) is a little bit higher than 30 mHz at the same
hour on the previous day. From 0600 to 0800 UT,
phase-difference signatures are observed at the high frequencies 60–90 mHz, which corresponds to higher FLR
harmonics. On 28 September, the phase-difference pattern
is identified during 0500–0800 UT and is not as clear as
that on the previous 2 days, indicating much weaker field
line resonance. The corresponding fundamental mode frequency is about 35–50 mHz, and it is 42 mHz at 0700 UT
(1500 LT). On September 29, the eigenfrequency is higher
than that on the first 2 days and is identified at about
47 mHz at 0700 UT (1500 LT). On the following days
of 30 September to 2 October (not shown here), the fundamental mode frequency is identified clearly, and the
Dst
(nT)
Figure 1. Phase-difference spectrograms of BH for the station pair Mohe and ManZhouLi stations
(MHT-MZL) during the time period of the 26 September 2011 magnetic storm event. Each diagram shows
the spectrogram for the local time interval 0800–2000.
50
0
−50
−100
60
50
40
30
20
5000
4000
3000
2000
1000
15
10
5
0
30
20
10
0
9/26
9/27
9/28
9/29
9/30
10/1
10/2
10/3
Days in 2011
Figure 2. From the top to bottom panels, shown are Dst index, fundamental mode frequencies of field line resonances
at L ’ 2 for the local time 1500, equatorial plasma mass densities, the ionospheric F2 layer electron density values
NmF2, and the total electron content (TEC) values during
the time period of the 26 September 2011 magnetic storm
event. The solid and dashed lines in the last panel indicate
the prestorm and the minimum TEC daily peak values,
respectively.
101
WANG ET AL.: PLASMASPHERE AND IONOSPHERE
Dst
(nT)
50
0
TEC
NmF2
Density Frequency
12
2
5
3
3
(10 el/cm ) (10 /cm ) (amu/cm ) (mHz)
−50
60
50
40
30
20
5000
4000
3000
2000
1000
15
10
5
0
30
20
10
0
9/16
9/17
9/18
9/19
9/20
9/21
9/22
9/23
Days in 2011
Figure 3. From the top to bottom panels, shown are Dst index, fundamental mode frequencies of field line resonances
at L ’ 2 for the local time 1500, equatorial plasma mass densities, the ionospheric F2 layer electron density values
NmF2, and the total electron content (TEC) values during
the time period of the 17 September 2011 magnetic storm
event. The solid and dashed lines in the last panel indicate
the prestorm and the minimum TEC daily peak values,
respectively.
eigenfrequency declines slowly and is close to the prestorm value at 0700 UT (1500 LT).
[11] Several more days of phase-difference spectrograms
have been examined, and the results of eigenfrequency measurements at L ’ 2 for the local time 1500 LT (0700 UT) are
given in the second panel of Figure 2 as the same time
frames as the Dst index (top panel). The error bar indicates
the standard deviation of all the eigenfrequency values identified during the day. The eigenfrequency increases with
time during the SC, and at the beginning of the recovery
phase the eigenfrequency is slightly larger than the prestorm
value, reaches a maximum on September 29, and then
decreases closely to the prestorm value during the recovery
phase of the magnetic storm dates from 29 September. Since
the equatorial plasma mass density was derived from the
eigenfrequency values, the third panel of Figure 2 plots the
equatorial plasma mass density at L ’ 2 for the local time
1500 LT (0700 UT). The plasma mass densities were calculated by using the formula given by Schulz [1996], in which
a dipole magnetic field and the plasma density r r m are
assumed, where r is the geocentric distance. In particular,
for this range of latitudes (1.8 < L < 2.2), an index m in the
range 0–2 may be appropriate [Chi et al., 2005;Vellante
and Förster, 2006], and the inferred equatorial r value is
not very sensitive to the particular choice of m within this
range, so we choose the index m = 1 in our calculation.
Figure 2 shows a reduction of the plasma density when
the main phase of the magnetic storm was over, which
is about 12% less than prestorm value and even further
decreases by 58% to the lowest value later in the recovery
phase, then the density gradually increases and recovers,
and it takes about 2 days to return to the prestorm value.
[12] The decreases of the NmF2 and TEC peak values of
the ionosphere are significant during the main phase of the
storm. The bottom two panels show the NmF2 value from
the digisonde and the vertical TEC value from GPS measurements above MHT during this magnetic storm event.
Both parameters can be used as a proxy for the electron density in the ionosphere. In addition to the typical diurnal variation of NmF2 and TEC, which present the highest values
in the early afternoon and the lowest values just before sunrise, a significant drop of the daily NmF2 and TEC peaks by
approximately 56 % and 36 %, respectively, which occurred
on 27 September 2011 during the magnetic storm. However,
unlike the plasmaspheric density at L ’ 2, the daily NmF2
and TEC maxima rose to slightly higher than 98 % and 93 %
of the prestorm value on the next day. After 28 September,
the NmF2 and TEC values recover to the prestorm values.
One of the most striking features is that the NmF2 and TEC
reductions take place before the plasmaspheric density
reaches its minimum. These results are consistent with the
findings by Chi et al. [2000].
2.3. Statistical Results
[13] We apply the same approach to the other four magnetic storms in 2011. First of all, it is noted that the ionospheric electron density reduction indicated by the NmF2
and TEC reductions happened before plasmaspheric density
reached its minimum as well for three other magnetic storms
(i.e., the 6 April, 12 April, and 10 September events) and almost on the same day for only one case (the 17 September
event). The former three cases followed the similar variation
pattern as the above 26 September storm event. The latter
one is plotted in Figure 3, which has the same format as
Figure 2.
[14] The main results about the plasmasphere and ionosphere variation (i.e., density decrease) are summarized in
Table 2. The columns give the date, the hour of the magnetic
storm Dst reached its minimum value, the largest plasmaspheric density reduction percentage from the prestorm
value, the delay time (in days), the largest drop of the daily
NmF2 and TEC peak values from the prestorm peak values,
and the delay time (in days) as well. For moderate magnetic
storms with Dst > 100 nT investigated here, it seems that
the decreases of the plasmaspheric density at L ’ 2, the daily
Table 2. Summary of the Plasmaspheric and Ionospheric Variations During Magnetic Stormsa
No.
1
2
3
4
5
Date
Time (UT)
(Dst)min
Density Reduction
D1
NmF2 Reduction
TEC Reduction
D2
26 Sep
17 Sep
10 Sep
12 Apr
6 Apr
2400
1600
0500
1000
2000
103 nT
63 nT
64 nT
47 nT
61 nT
58%
58%
45%
36%
49%
3
2
3
2
2
56%
41%
42%
55%
19%
36%
22%
17%
46%
21%
1
2
2
1
1
a
D1 and D2 represent the delay time in days when the plasmaspheric density, NmF2, and TEC got the largest reduction after the
Dst reached the minimum value, respectively.
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WANG ET AL.: PLASMASPHERE AND IONOSPHERE
NmF2 peak value, and TEC peak value do not depend on the
strength of magnetic storms, which is indicated by the minimum value of the Dst index. However, we need to investigate more magnetic storm events with different intensity to
make reliable conclusions in the future.
3. Discussion and Summary
[15] In this paper, we estimated the plasmaspheric density
by applying the gradient technique to the H component magnetometer data from two ground stations of the Chinese
Meridian Project, in which the difference in geomagnetic
latitude is 3.7 . However, this is slightly larger than what
is recommended (12 ) for an accurate inference of the
plasmaspheric density. The large difference in geomagnetic latitude of the two stations may be a source of error
even though the exact amount of uncertainty is hard to estimate. Kawano et al. [2002] found that the resonance
width is nearly constant in the geomagnetic latitude range
of 28 to 32 , and its magnitude 4 400 km. For the
event of 26 September 2011, it was estimated that the resonance width is about 3 for the frequency of 32 mHz,
which is close to the difference in geomagnetic latitude
of the two stations. Nevertheless, the clear FLR signatures
during magnetic storms indicate that the method is still
valid.
[16] As mentioned above, the equatorial plasmaspheric
densities increases during the recovery phase to the pre-storm
value after decreasing with the main phase for moderate magnetic storms in 2011 investigated here. The results show the
plasma depletion and refilling of the plasmasphere during
magnetic storms, as pointed out by Chi et al. [2000]. To
roughly estimate the location of the plasmasphere, we employ the empirical formula of the plasmapause location Lpp
from spacecraft and whistler observations at higher L shells
[Carpenter and Anderson, 1992]. The plasmapause location
Lpp was reduced from 5.1 to 2.8 on the 26 September 2011
event in response to an increase of Kp up to 6+ at 18–21 UT
on that day (Kp from 1+ to 6+). This implies that the plasmapause never came inside of L ’ 2 for this moderate magnetic
storm event. In this case, the plasmasphere depletion is unlikely due to the enhanced convection which drives plasma
sunward. Instead, it is a result of the coupling with the ionosphere. In the recovery phase, the plasmasphere is filled
at a similar rate, which is closely related with the plasma
ion of the ionosphere outflow into the newly formed plasmasphere. For the case of the 26 September event, it was
estimated that the mean value of the depletion rate of
the plasmasphere was about 31 amu cm3 hr1 during
the interval between 26 and 29 September, and the plasmasphere was refilled at a rate of 47 amu cm3 hr1 at the
equator after it was drained during the interval between 29
September and 1 October. However, there was also a diurnal
variation of the plasmaspheric density associated with ion
outflow from the ionosphere on the dayside and loss of particles from the flux tubes in the nightside [Chi et al., 2000]. For
example, it can be estimated that the refilling rate was ~210
amu cm3 hr1 during the interval of 0400–0800 UT on 27
September after the TEC value decreased rapidly on 26–27
September. The NmF2 and TEC reductions take place 1 or
2 days before the plasmaspheric density reaches its minimum
in four of the five magnetic storms, and in the case of
exception the density reductions in the ionosphere and in
the plasmasphere occurred on the same day. The results
strongly imply that the plasmaspheric depletion is due to
the reduced plasma supply from the ionosphere.
[17] The comparison of the results from the five magnetic
storm events in 2011 as shown in Table 2 implies that
decreases of the plasmaspheric density at L ’ 2, the daily
NmF2 peak value, and TEC peak value seem to be not affected by the strength of the moderate magnetic storms.
The percentage decreases of the NmF2 and TEC peak values
are generally lower than that of the plasmaspheric density in
the main phase during magnetic storms. These results suggest that a minor change in the ionospheric content may result in a significant impact on the density in the plasmasphere. However, the results from these five moderate
magnetic storms in 2011 do not lead to a general conclusion
in a statistical sense. More investigations with magnetic
storms of different intensities are needed to establish a reliable quantitative relationship between the magnetic storm
strength, the plasmaspheric density variation, and the ionospheric density variation.
[18] In summary, the plasmaspheric density at L ’ 2 and
ionospheric electron density variations during five moderate
magnetic storms in 2011 are investigated using the recent
observations of Chinese Meridian Project during its first year
of test operation. The observations show the plasmaspheric
density drops significantly by about half of the prestorm
values. At the same time, the ionospheric F2 layer electron
density NmF2 and the TEC also show sizeable decreases.
There is no clear relationship between the magnitude of ionospheric depletion and plasmaspheric depletion. For the five
moderate magnetic storms in 2011, it is suggested that the
plasmaspheric depletion is very likely due to the reduced
plasma supply from the ionosphere. The accurate statement
of the plasmaspheric and ionospheric densities can be used
to further explore the dynamics of the plasmasphere and
ionosphere and their coupling. The multiple parameters
observed by the Chinese Meridian Project give us a unique
opportunity to simultaneously examine the changes among
different coupled regions in geospace.
[19] Acknowledgments. We acknowledge the use of data from
Chinese Meridian Project and the Dst index data provided by the World
Data Center for Geomagnetism. Q. M. Zhang would like to thank H. Shen for
helping with TEC calculations. This work was supported by grants 973 program (2012CB825600), NNSFC (40921063, 41231067) and in part by the
Specialized Research Fund for State Key Laboratories of China.
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