JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, A00L10, doi:10.1029/2011JA017315, 2012
Superposed epoch analyses of thermospheric response to CIRs:
Solar cycle and seasonal dependencies
Jing Liu,1,2,3 Libo Liu,1 Biqiang Zhao,1 Jiuhou Lei,4 Jeffrey P. Thayer,5
and Robert L. McPherron6
Received 1 November 2011; revised 3 May 2012; accepted 3 May 2012; published 20 June 2012.
[1] Thermospheric response to Corotating Interaction Regions (CIRs) has been studied
previously; however, its solar cycle and seasonal effects have not been fully investigated.
Thermospheric mass density at 400 km measured by the CHAMP satellite during
2001–2008 and ∑ O/N2 from the TIMED/GUVI instrument covering a period from 2002
to 2008 are used to investigate the solar cycle and seasonal dependencies of the
thermospheric response to CIRs. Our results reveal: (1) solar minimum CIRs compared
to solar maximum counterparts have larger solar wind speeds before and after the stream
interface. However, solar wind dynamic pressure and merging electric field are slightly
larger at solar maximum than solar minimum. (2) CIR-induced variations of ∑ O/N2 are
characterized by high latitude depression and low latitude enhancement, a distinction
from global enhancement of neutral density at a fixed altitude. These relative
thermospheric changes are dependent on solar cycle, with a more pronounced increase
in neutral density at all latitudes and a stronger decrease in ∑ O/N2 at high latitude at solar
minimum than at solar maximum. (3) A seasonal asymmetry is presented in the relative
deviations of thermospheric mass density and composition. On the dayside, the peak
increases of neutral density at high latitudes on average are 40% in the summer
hemisphere and 26% in the winter hemisphere. Nighttime neutral density changes
are more remarkable than that in the same latitudinal bands of daytime and have the same
seasonal preference of enhancement as the dayside. At the daytime, ∑ O/N2 at high
latitudes suffers more reduction in the summer hemisphere than in the winter hemisphere.
At middle latitudes, ∑ O/N2 reduces in the winter hemisphere; nevertheless, it increases
slightly in the summer hemisphere.
Citation: Liu, J., L. Liu, B. Zhao, J. Lei, J. P. Thayer, and R. L. McPherron (2012), Superposed epoch analyses of thermospheric
response to CIRs: Solar cycle and seasonal dependencies, J. Geophys. Res., 117, A00L10, doi:10.1029/2011JA017315.
1. Introduction
1
Beijing National Observatory of Space Environment, Institute of
Geology and Geophysics, Chinese Academy of Sciences, Beijing, China.
2
Also at State Key Laboratory of Space Weather, Center for Space
Science and Applied Research, Chinese Academy of Sciences, Beijing,
China.
3
Graduate University of Chinese Academy of Sciences, Beijing, China.
4
CAS Key Laboratory of Geospace Environment, School of Earth and
Space Sciences, University of Science and Technology of China, Hefei,
China.
5
Department of Aerospace Engineering Sciences, University of
Colorado, Boulder, Colorado, USA.
6
Institute Geophysics and Planetary Physics, University of California,
Los Angeles, California, USA.
Corresponding author: L. Liu, Beijing National Observatory of Space
Environment, Institute of Geology and Geophysics, Chinese Academy of
Sciences, Beijing 10029, China. (liul@mail.iggcas.ac.cn)
©2012. American Geophysical Union. All Rights Reserved.
[2] High-speed streams originating from solar coronal holes
interact with low-speed background solar winds, and generate
the Corotating Interaction Regions (CIRs). This region centered on Stream Interface (SI) is often bounded by forward and
backward shocks, a separatrix for the high-speed and lowspeed streams, and is featured by increases in the solar wind
speed and proton temperature, compression and flow deflection [e.g., Richardson et al., 1996; McPherron et al., 2009].
During the declining phase and solar minimum of solar
cycle 23, CIRs became the dominant drivers producing
periodic variations in the thermosphere and ionosphere
[e.g., Tsurutani et al., 2006; Crowley et al., 2008; Lei et al.,
2008a, 2008b, 2011; Mlynczak et al., 2008; Sojka et al.,
2009; Thayer et al., 2008; J. Liu et al., 2010; Pedatella
et al., 2010; Tulasi Ram et al., 2010]. Periodic energy
inputs associated with CIRs are deposited into the polar
region in the form of Joule heating and auroral precipitation
[e.g., Emery et al., 2009; Zhang et al., 2010; Deng et al.,
2011]. Lei et al. [2008a] and Thayer et al. [2008] reported
that neutral density in the thermosphere oscillates with periods
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of 7 and 9 days as a response to periodic energy injections into
the polar regions connected with high-speed streams during
the descending phase of solar cycle 23. The same periodic
oscillations in ∑ O/N2 have been described by Crowley et al.
[2008]. ∑ O/N2 observed by TIMED/GUVI and neutral
mass density at 400 km showed different behaviors as a consequence of the recurrent geomagnetic forcing. The periodic
variations in neutral density are global, with little difference
between the high and low latitudes, and almost in phase with
geomagnetic Kp index. In contrast, the ∑ O/N2 effect is more
remarkable at high latitudes and is anti-phased with Kp. It is
not surprising to see this discrepancy because the mass density
is observed at a constant height and ∑ O/N2 is close to a fixed
pressure level measurement [Strickland et al., 2004]. Combined contributions from thermal expansion and the vertical
winds result in variations in neutral mass density at a fixed
height [Rishbeth and Müller‐Wodarg, 1999; Lei et al., 2010].
[3] As is known, both thermospheric density and composition show strong solar cycle and seasonal dependencies in
response to geomagnetic forcing [e.g., Burns et al., 2004;
Liu and Lühr, 2005; Sutton et al., 2005; Müller et al., 2009].
Resorting to NCAR-TIEGCM simulations, Burns et al.
[2004] investigated the thermospheric changes under different solar EUV radiation during geomagnetic storms. They
found that the composition disturbance zone is more readily
to expand to lower latitudes in summer and that horizontal
advection becomes more effective with increased solar
activity. In contrast, the storm-time temperature in the thermosphere experiences greater enhancements in the absolute
sense during solar maximum than solar minimum.
[4] Besides the solar cycle dependency, seasonal effects
are another important factor determining the storm-time
morphology of thermospheric mass density and composition.
Fuller-Rowell et al. [1996] summarized the seasonal dependence of the thermosphere response to geomagnetic storms.
On the one hand, the Joule heating rate is generally larger in
the summer hemisphere than in the winter hemisphere
because of higher electrical conductivity in the summer high
latitudes compared to the winter hemisphere. On the other
hand, the prevailing summer-to-winter wind driven by differential solar heating will facilitate the composition disturbance zone’s equatorward propagation in the summer
hemisphere, whereas it restricts the disturbance area at higher
latitudes in the winter hemisphere. Forbes et al. [1996]
revealed that daytime atmosphere density at 200 km exhibited 50–70% enhancement at high latitudes in the summer
hemisphere, being about double of the maximum increase of
the winter hemisphere. The same results were observed by
Sutton et al. [2005] during 29 October to 1 November 2003.
The three superstorms occurring during October–November
2003 showed that the noon average density enhancement is
weaker in winter than in summer, while the seasonal asymmetry in the midnight sector differs from case to case [Liu
and Lühr, 2005; Bruinsma et al., 2006].
[5] Most previous studies on seasonal, latitudinal and daynight dependencies of thermospheric response focused on
severe ICMEs or CIRs storms [e.g., Prölss, 1980; FullerRowell et al., 1994, 1996; Forbes et al., 1996; Liu and Lühr,
2005; R. Liu et al., 2010; Liu et al., 2011; Sutton et al.,
2005; Bruinsma et al., 2006], however, little attention has
been paid to the seasonal dependence of the thermospheric
changes to CIRs, which are expected to drive weak to
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moderate geomagnetic disturbances. As mentioned above,
latitudinal and local time dependencies of the thermospheric
density and composition changes to CIRs have been
revealed in Lei et al. [2008a] and Crowley et al. [2008],
respectively; nevertheless, their solar cycle and seasonal
dependencies are yet to be studied. The contrasting behavior
of neutral composition and density under different seasons
and solar cycles deserves further studies, which may provide
some clues in understanding the thermospheric dynamics.
[6] The primary objective of this work is to investigate the
solar cycle and seasonal dependencies of the thermospheric
mass density and composition response to geomagnetic
forcing during the passage of CIRs, as a follow-up study of
Lei et al. [2008a, 2011] and Crowley et al. [2008]. This will
provide an improved picture regarding the thermospheric
response to CIRs. The paper is organized as follows. The
data and methods used in this work are described in section
2. Section 3 examines the solar cycle and seasonal effects on
the CIRs-induced thermospheric variations. The last two
parts are the discussion and conclusion sections.
2. Data Set and Analysis Methods
[7] Interplanetary solar wind parameters are obtained from
the OMNI 2 data set with hourly resolution (ftp://nssdcftp.gsfc.
nasa.gov/spacecraft_data/omni/). The geomagnetic activity
indices are provided by the World Data Center in Japan. The
AE index roughly represents the energy input into the auroral
region. The Dst index denotes the disturbance state of the ring
current. We follow the methods used by McPherron et al.
[2009] to define the SI within CIRs according to the patterns
presented in the solar wind parameters. CIRs zero epoch time is
taken as the time of the zero crossing of the azimuthal flow
angle. A geomagnetic calm interval tends to occur just prior
to the arrival of high-speed streams [Tsurutani et al., 1995;
Borovsky and Steinberg, 2006; Lei et al., 2011]. In this
regard, the magnetospheric energy input into thermosphere is
the least, leading to the least perturbed thermosphere by
geomagnetic forcing. The satellite samples nearly a constant
local time over a few days. We examine the relative and
absolute deviations of neutral density in percentage for both
the ascending and descending portions of the orbit during the
CIRs relative to the reference of one day prior to the SI.
[8] The Global Ultraviolet Imager (GUVI) is an instrument
on board NASA TIMED satellite to investigate the far ultraviolet airglow of major components from the upper atmosphere. This satellite has been sent into circular polar orbit at
around 630 km since 2001, with an inclination of 74.1 . Thus
the satellite samples almost the same local time over a few
days. The column ∑ O/N2 in the daytime is calculated from OI
135.6 nm and N2 Lyman-Brige-Hopfield dayglow emission
above an altitude where the N2 column density is 1017 cm 2
[Christensen et al., 2003; Strickland et al., 2004], from the
Website: http://guvi.jhuapl.edu/. Each image scanned by
GUVI covers an area 2500 km by 100 km at an altitude of
150 km. Please refer to Zhang et al. [2004] for a more detailed
descriptions regarding to the retrieval of ∑ O/N2.
[9] Neutral mass density used in the present work is measured by the CHAMP satellite. The CHAMP satellite was
sent into a near-circular orbit at about 456 km with inclination
of 87.3 on 15 July 2000. It samples the same local time for
several consecutive days, taking about four months for
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Figure 1. Variations of (a) F10.7 index in unit of 10 22 Wm 2 Hz 1 and (b) Ap index during the year
1998–2010. Histogram of CIR events as a function of (c) year and (d) day of year.
CHAMP to pass through all local times. Thermospheric mass
density is measured by the STAR accelerometer. The detailed
procedures of retrieving neutral density are described by
Sutton et al. [2005]. The in situ measured neutral density at
satellite altitudes are normalized to a constant altitude of
400 km using NRLMSIS-00 empirical model [Picone et al.,
2002].
3. Results
[10] Figure 1 shows the variations of (a) solar index F10.7
sfu (1sfu = 10 22 Wm 2 Hz 1) (Figure 1a) and (b) Ap index
during the year 1998–2010, as well as histogram of CIR
events as a function of (c) year and (d) day of year. The F10.7
index increases from 60 sfu at the beginning of the year
1998 to the maxima during 2000–2002, then turns to
decrease gradually and reaches the minimum in the year
2008–2009. As illustrated in Figure 1b, the geomagnetic
activity is likely to be more active at higher solar activity.
However, CIRs tend to occur in the declining phase and
minimum of solar cycle, which is in agreement with previous
findings [e.g., Richardson et al., 1996; Tsurutani et al.,
2006]. It is shown in Figure 1c and 1d that the CIR has the
lowest occurrence in the years 2000 and 2001, reaches the
maximum in 2007, and shows no seasonal preference of its
occurrence.
3.1. Solar Cycle Dependence of Solar Wind Parameters
and Thermospheric Response to CIRs
[11] Figure 2 depicts, from the top to the bottom, the
superposed epoch results for (a) solar wind velocity V,
(b) solar wind dynamic pressure, (c) merging electric field
Em, (d) the z component of interplanetary magnetic field in
GSM Bz, (e) Dst index, (f) AE index for CIR events at solar
maximum 2001–2002 (left) and solar minimum 2007–2008
(right). The thick solid line stands for the median value, and
the shaded area represents the upper and lower quartiles.
These parameters respond in a similar way to CIRs at both
the high and low solar activities Typical characteristics
of CIRs are evident and in accordance with past conclusions [e.g., Denton et al., 2009; McPherron et al.,
2009; Lei et al., 2011]. An increase in solar wind
velocity is observed around zero epoch time, which is
accompanied by enhanced solar wind dynamic pressure,
resulting in the elevated auroral magnetic activity. It takes
about 4–5 days for the solar wind parameters and geomagnetic indices to recover to pre-event state. The study of R. Liu
et al. [2010] indicated that the Em [Kan and Lee, 1979] is
highly correlated with storm-time neutral density changes at
400 km. Em starts to increase the day before SI, peaks within
a day after SI, and returns to normal level in about 2–3 days.
A weak southward and northward turning of the interplanetary magnetic field appears in GSM coordinates around
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Figure 2. Superposed epoch analyses of (a, g) solar wind velocity V, (b, h) solar wind dynamic pressure,
(c, i) merging electric field, (d, j) the z component of interplanetary magnetic field in GSM Bz, (e, k) Dst
index, and (f, l) AE index for CIRs at solar maximum 2001–2002 (left) and solar minimum 2007–2008
(right). The solid line is the median, and the shaded area is the upper and lower quartiles.
zero epoch time, which is due to the fluctuating nature of
magnetic fields within the body of Alfven waves. The interplanetary Bz component serves as an important signature
discriminating CIRs events from ICMEs events. During
CIRs-driven storms, interplanetary Bz component is highly
variable and fluctuates rapidly between north and south. In
contrast, the ICMEs-driven storms, such as magnetic clouds,
usually have a larger steady southward Bz component. The
Dst index starts to decrease around zero epoch time, reaches a
minimum 20 nT after about 20 h, and increases in the
recovery phase. CIRs-generated storms with 100 nT Dst
are probably a combination of ICME catching up with a
stream interface, which are eliminated by using lists of
ICME.
[12] The organized behavior of solar wind parameter and
energy input into the upper atmosphere connected with CIRs
contribute to the organized behavior of thermospheric mass
density and neutral composition as shown in Figures 3–8.
Figure 3 shows superposed epoch results of relative variations of daytime neutral density at 400 km due to CIRs at (a)
high latitudes (60 –90 ), (b) middle latitudes (30 –60 ), and
(c) low latitude (0 –30 ) bands in magnetic coordinated
from both hemispheres at solar maximum and minimum. In
this work, we group thermospheric variations according to
magnetic latitude because the auroral energy input deposits
mainly in the magnetic frame. The CIR events used in this
work during the solar maximum (2001–2002) and minimum
(2007–2008) are listed in the table in Table 1. The thick
black line represents the median value. In the daytime, as
demonstrated in Figure 3, a striking difference in magnitude
is that the neutral density experiences larger enhancement
after the SI at solar minimum than solar maximum. Figure 4
is plotted in the same manner as Figure 3 but under nighttime conditions. Relative variations of neutral density at both
day and night sides are about the same at high latitudes, but
nighttime variations are larger in magnitude than those on
the dayside at mid and low latitudes.
[13] To further compare the relative variations of neutral
density at different latitudes and solar activities directly, we
depicted median relative variations in neutral density at
400 km for both the daytime and nighttime for different solar
activities in Figure 5. There is a systematic increase in neutral density after the SI, recovering to pre-event values about
3–4 days later. The thermospheric response to CIRs presents
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Figure 3. Superposed epoch results of relative variations in daytime neutral density at 400 km at (a, d)
high latitude (60 –90 ), (b, e) middle latitude (30 –60 ), and (c, f) low latitude (0 –30 ) bands at solar
maximum (left) and solar minimum (right) due to CIRs. The thick solid line is the median value, and
the shaded area represents the upper and lower quartiles.
marked solar cycle and day-night differences. As mentioned
before, the maximum relative increment of neutral density
reaches 60% for nighttime conditions at solar minimum. At
solar maximum, the peak increment in neutral density is about
20% at nighttime, weaker than that of solar minimum. In
addition, the day-night difference appears in relative amplitude
of neutral density, which is larger at nighttime than the dayside. At solar minimum, the relative variations of neutral
density reach the maximum faster at the middle latitudes than
at the low latitudes. At solar maximum, the maximum relative
deviations occur earlier at high latitudes and there is no clear
difference between middle and low latitudes. Absolute changes of neutral density in response to CIRs are also depicted in
Figure 6. As shown in this figure, the CIRs-induced changes of
thermospheric mass density depend partly on the expression.
The thermospheric mass densities in absolute senses are perturbed to greater extents at solar maximum than at solar minimum, which is opposite to the results derived from the relative
differences.
[14] The response of daytime ∑ O/N2 ratio to CIRs is shown
in Figures 7 and 8. Figure 7 shows the superposed epoch
results of relative variations in daytime ∑ O/N2 at (a) high
latitude (60 –90 ), (b) middle latitude (30 –60 ), and (c) low
latitude (0 –30 ) bands in magnetic frame at solar maximum.
The right panels are in the same format as the left but for the
solar minimum condition. The solid line is the median, and the
shaded area represents the upper and lower quartiles. The
nighttime information of ∑ O/N2 cannot be obtained because
only the far ultraviolet day glow is recorded by TIMED/
GUVI. There are apparent solar cycle and latitudinal dependencies of the ∑ O/N2 response to CIRs. At high latitude, the
maximum depression at solar minimum is 17%, which is 9%
larger than that of solar maximum. At mid-low latitudes, there
is no salient difference in the relative changes of ∑ O/N2
over the solar cycle. The modification of ∑ O/N2 by highspeed streams at middle latitudes decrease by 5% and
increase about 8% at low latitudes after the passage of SI on
average, respectively. Both the relative and absolute variations
of ∑ O/N2 during CIR-driven storms conform to the same
pattern in terms of their solar cycle dependencies.
[15] To sum up, there are similarities and differences
between the responses of neutral density at 400 km and ∑ O/
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Figure 4. The same as Figure 3 but for the nighttime condition.
N2 on the constant pressure surface to CIRs with solar cycle.
On the dayside, the thermospheric variations for both neutral
density and ∑ O/N2 are more pronounced at high latitudes at
solar minimum than that at solar maximum. A notable difference exists in that ∑ O/N2 decreases at high latitudes and
increases at low latitudes, while neutral density increases
globally. The observed features of ∑ O/N2 and neutral
density responses will be interpreted in the discussion.
3.2. Seasonal Dependence of Thermospheric Response
to CIRs
[16] Figures 9–11 illustrate the seasonal dependence of the
neutral density response to CIRs. Superposed epoch results
of relative variations in daytime neutral density at 400 km (a)
at high latitude (60 –90 ), (b) middle latitude (30 –60 ), and
(c) low latitude (0 –30 ) bands in magnetic coordinates in
summer are depicted in Figure 9. CIR events during the
years 2002–2008 are used in this part. The right three panels
are for winter conditions. The thick black line represents the
median value. A total of 91 CIRs are selected over May to
August for summer in the Northern Hemisphere and winter
in the Southern Hemisphere, and 83 events in total over
November to February for winter in the Northern
Hemisphere and summer in the Southern Hemisphere. Here
we combine events from the same latitudinal bands of both
hemispheres in the same season and choose the median
value of the relative deviations.
[17] In the daytime, as shown in Figures 9 and 11, the
average density enhancement at the same latitudinal bands is
larger in the summer hemisphere than in the winter hemisphere. The maximum increase at high latitudes on average
is 40% in the summer hemisphere, while it reaches 23%
in the winter hemisphere at middle latitudes. The minimum
enhancement is 20% in the winter hemisphere of the
daytime. It is shown in Figures 10 and 11 that seasonal
effects are also prominent at nighttime, being a stronger
response in the summer hemisphere. In short, the neutral
density suffers larger enhancement on average in the summer hemisphere for both the dayside and nightside during
the passage of CIRs. The summer and winter discrepancies
of CIR effects are more remarkable at high latitudes than
those at mid - low latitudes. It is instructive to compare the
response of the thermospheric mass density to CIRs with
that of geomagnetic storms. They share a common feature in
the daytime that stronger enhancement is observed in the
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Figure 5. The median relative variations in neutral density due to CIRs at 400 km at both the daytime
and nighttime for different solar activities. The abbreviation HS and LS represents solar maximum and
solar minimum, respectively. High, Mid and Low stand for the high, middle and low latitude bands,
respectively.
summer hemisphere than in the winter hemisphere [e.g.,
Forbes et al., 1996; Liu and Lühr, 2005; Sutton et al., 2005;
Bruinsma et al., 2006]. However, in the night sector, the
relative intensity of neutral density during the superstorms
has no clear seasonal dependence, which is distinct from our
results. During superstorms, the abundant energy input into
the auroral region is strong enough to smooth out the seasonal asymmetry.
[18] Figure 12 shows superposed epoch results of relative
variations in daytime ∑ O/N2 at (a) high latitude (60 –90 ),
(b) middle latitude (30 –60 ), and (c) low latitude (0 –30 )
bands in magnetic coordinates in summer. The right three
panels are for winter conditions. The thick black line is the
median value and the shaded area represents the upper and
lower quartiles. As illustrated in Figure 13, the high latitude
∑ O/N2 experiences deeper depression in the summer
hemisphere, approximating to 18%, while it reduces 11%
in the winter hemisphere. An interesting feature is that the
∑ O/N2 decreases at middle latitudes in the winter hemisphere, however, it increases slightly in the summer hemisphere. This is different from our expectation since the
composition disturbance zone, characterized by a reduction
in ∑ O/N2, is thought to propagate to lower latitudes in
summer than in winter [Fuller-Rowell et al., 1994]. Thus it
should be easier to observe the depression in ∑ O/N2 at midlow latitudes in the summer hemisphere than in the winter
hemisphere, however, the observed decrease in ∑ O/N2 at
the middle latitudes of winter hemisphere but not the
summer hemisphere, which cannot be explained in the
context of Fuller-Rowell et al. [1996] and will be explained
in the discussion.
4. Discussion
[19] Thermospheric mass density at a constant height
shows a remarkable enhancement at all latitudes in response
to CIRs, with little latitudinal difference. Nevertheless, the
relative deviation of ∑ O/N2 is depressed at high latitudes and
increases at low latitudes. The discrepancy between thermospheric mass density and ∑ O/N2 should be attributed to the
different properties of the neutral atmosphere at a constant
height versus on a constant pressure surface, which has been
interpreted by Crowley et al. [2008] and Lei et al. [2010].
During geomagnetic quiet and disturbed periods, ∑ O/N2
from TIMED/GUVI tends to represent neutral composition at
nearly a constant-pressure surface, and also varies with
thermal expansion or contraction, which is mainly due to
variations of the reference height of the reference N2 column
density and O density profile [Zhang and Paxton, 2011]. The
observed storm-time decrease in ∑ O/N2 at high latitudes
indicated that the vertical wind effects dominate the effects of
thermal expansion owing to auroral heating [Rishbeth et al.,
1987; Rishbeth and Müller-Wodarg, 1999; Lei et al., 2010];
otherwise, the ∑ O/N2 will increase to some extent. Increment in ∑ O/N2 at low latitudes is as a result of convergence
of winds, bringing O-rich air to lower altitudes crossing
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Figure 6. The same as Figure 5 but for the absolute changes.
through pressure surfaces. Columnar changes in the ∑ O/N2
as a consequence of thermal expansion are not as sensitive as
thermospheric mass density because both O and N2 are
almost equally affected on the constant pressure surface
[Crowley et al., 2008]. Enhancement in the thermospheric
mass density at 400 km may arise from thermal expansion or
upward vertical wind at high latitudes due to elevated energy
input.
[20] To further compare the response of ∑ O/N2 with that of
neutral density to CIRs, one would expect that it is more natural to discuss both neutral density and ∑ O/N2 changes on a
constant pressure surface. In order to address this issue, we
normalize the observed neutral density at satellite altitudes to
the average pressure level on one day before stream interface
for each event using NRLMSIS-00. In the normalization process, we adjust the exospheric temperature so as to match the
NRLMSIS-00 predicted density with the observed values at
satellite altitudes. In this way, neutral density on a constant
pressure surface is then derived after altitudinal profiles of
neutral density/composition and temperature as well are
obtained through this assimilation technique.
[21] As shown in Figure 14, the neutral mass density on the
constant pressure surface is inclined to decrease globally as
the geomagnetic activity becomes more active at both solar
maximum and minimum. The peak reductions of neutral
density on the constant pressure level range from about 7–9%
at solar minimum to 3–6% at solar maximum. This is not
difficult to understand since the ideal gas equation P = rRT/
M (where P, r, R, T, M are the reference pressure level,
thermospheric mass density, universal gas constant, neutral
temperature, and mean molecular weight, respectively)
defines anti-correlation between the neutral mass density and
neutral temperature on the constant pressure level if the mean
molecular weight does not change significantly during the
passage of CIRs. However, M is decreasing at low latitudes
as indicated by the increase in ∑ O/N2, while M is increasing
at high latitudes because of the decrease in ∑ O/N2. So, at
high latitudes on a constant pressure surface the density does
not decrease as much because the increase in M offsets the
increase in temperature. At low latitudes, the density
decreases even more significantly because the M is also
decreasing. This latitude effect is depicted in Figure 14 with
density showing a greater change at low and midlatitudes
than at high latitudes.
[22] As illustrated in Figure 11, the relative intensity of
neutral density is stronger in summer than in winter at both
the daytime and nighttime. The ∑ O/N2 at high latitudes also
suffers a deeper depression in the summer. These characteristics are in agreement with previous findings [e.g., Prölss,
1980, 1995; Fuller-Rowell et al., 1996; Bruinsma et al.,
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Figure 7. Superposed epoch results of relative variations in daytime ∑ O/N2 in response to CIRs at (a, d)
high latitude (60 –90 ), (b, e) middle latitude (30 –60 ), and (c, f) low latitude (0 –30 ) bands at high
solar activity (left) and at low solar activity conditions (right). The thick solid line is the median value,
and the shaded area represents the upper and lower quartiles. The right side is in the same format as the
left but for the low solar activity year 2007–2008.
2006; Forbes, 2007], explained by Forbes et al. [1996] in the
framework of the thermospheric simulation outcomes of
Fuller-Rowell et al. [1996]. Several factors may lead to the
seasonal asymmetry, including uneven magnetospheric
energy input and prevailing summer-to-winter winds. Summer-winter differences in neutral atmosphere variations have
a close association with the unequal magnetospheric energy
input. Essentially it takes more magnetospheric energy input
to produce the same percent change in density for a dense
atmosphere than a less dense atmosphere. Thus the greater
percent change in the more dense summer than winter indicates greater magnetospheric energy input in summer than
winter. The energy dissipations into auroral regions mainly
take the form of auroral precipitation and Joule heating. Joule
heating plays a more important role in the high-latitude
heating than particle precipitation during geomagnetic disturbed periods [e.g., Ahn et al., 1983; Richmond et al., 1990;
Lu et al., 1995]. Estimations of Joule heating have been done
by running empirical and theoretical models, finding that
the Joule heating rate in the summer is generally larger than
in the winter hemisphere [e.g., Fuller-Rowell et al., 1996;
Lu et al., 1998]. This inclusion was supported by the
observations of the Atmosphere Explorer C (AE-C) satellite
during the years 1974–1978, revealing that Joule heating
input is 50% larger in summer than winter [Foster et al.,
1983] and corresponding a larger thermospheric heating
rate in the summer hemisphere.
[23] Difference in thermospheric background wind is
another important factor contributing to summer-winter difference in the neutral atmosphere response. The zonal mean
meridional wind driven by differential solar heating is generally from the summer to winter. The disturbance-driven
circulation is equatorward for both hemispheres. The prevailing summer-to-winter zonal-mean solar-driven circulation tends to facilitate the equatorward expansion of the
density disturbance in the summer hemisphere, and resist its
expansion in the winter hemisphere. Therefore, combined
effects of uneven auroral energy input and asymmetry in
background neutral winds contribute to the larger response in
the summer hemisphere than in the winter hemisphere of
neutral density and composition at high latitudes.
[24] It is illustrated in Figure 5 that, at mid and low latitudes, relative variations of neutral density on the nightside
are larger than those on the dayside. The Joule heating generally is more deposited on the dayside than the nighttime
during disturbed periods as shown in the statistical results of
ionospheric Joule heating pattern based on the measurements
by the Astrid-2 satellite [Olsson et al., 2004] and the simulation outcomes of the Global Ionosphere Thermosphere
Model (GITM) [Deng et al., 2011]. Hence the observed daynight asymmetry of neutral density variation could not be
attributed to uneven day-night Joule heating rate. Thermospheric mass density changes at a constant altitude should be
due to the cumulative effects of thermospheric scale height
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Figure 8. The median relative variations in daytime ∑ O/N2 at different solar activity due to CIRs. The
abbreviation HS and LS represents high solar activity and low solar activity, respectively. High, Mid and
Low stand for the high, middle and low latitude bands, respectively.
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Table 1. Stream Interface List During the Years 2001–2002 and
2007–2008
YY/MM/DD
UT
YY/MM/DD
UT
2001/1/4
2001/1/10
2001/1/21
2001/1/29
2001/2/28
2001/4/24
2001/5/23
2001/6/1
2001/6/9
2001/6/19
2001/7/31
2001/8/10
2001/8/21
2001/9/3
2001/9/11
2001/9/15
2001/10/8
10:09
20:13
22:09
5:07
9:58
9:11
4:32
22:09
1:38
13:27
2:48
0:17
5:54
9:11
10:32
1:27
14:13
2001–2002 Events
2001/12/3
4:21
2001/12/15
20:13
2001/12/24
4:21
2002/1/10
10:32
2002/1/20
1:38
2002/1/25
18:29
2002/2/5
15:00
2002/2/11
10:32
2002/3/4
13:15
2002/3/11
23:54
2002/3/30
6:40
2002/4/11
1:27
2002/4/27
18:17
2002/5/27
11:42
2002/6/2
0:05
2002/6/8
14:48
2002/6/16
6:05
2007/1/1
2007/1/29
2007/2/12
2007/2/27
2007/3/6
2007/3/11
2007/3/25
2007/3/27
2007/4/1
2007/4/9
2007/4/23
2007/4/27
2007/5/7
2007/5/18
2007/5/23
2007/6/2
2007/6/9
2007/6/13
2007/6/21
2007/6/29
2007/7/3
2007/7/11
2007/7/14
2007/7/20
2007/7/26
2007/7/29
2007/8/6
2007/8/10
2007/8/15
19:27
8:48
13:03
8:01
7:15
22:56
2:01
16:56
0:40
8:13
3:34
17:19
12:52
9:46
13:03
17:30
9:34
19:27
10:09
17:30
20:25
1:38
17:19
10:44
15:11
1:27
22:21
13:27
3:23
2007–2008 Events
2007/8/26
16:44
2007/8/31
20:01
2007/9/6
21:23
2007/9/14
21:46
2007/9/20
23:54
2007/9/27
17:54
2007/10/3
8:13
2007/10/18
16:21
2007/10/25
14:13
2007/10/29
23:07
2007/11/13
5:07
2007/11/20
11:42
2007/11/22
18:17
2007/11/24
16:21
2007/12/10
23:54
2007/12/17
7:15
2007/12/27
6:05
2008/1/5
6:29
2008/1/13
11:19
2008/1/25
1:50
2008/1/31
16:21
2008/2/10
7:38
2008/2/18
13:27
2008/2/28
16:21
2008/3/8
16:56
2008/3/26
12:05
2008/4/4
21:23
2008/4/16
10:21
2008/4/23
5:07
YY/MM/DD
UT
2002/6/19
2002/7/1
2002/7/5
2002/7/12
2002/8/9
2002/9/4
2002/9/16
2002/10/7
2002/10/14
2002/10/24
2002/11/1
2002/11/11
2002/11/21
2002/11/29
2002/12/6
2002/12/14
2002/12/26
2:13
5:30
11:54
9:23
8:25
3:23
13:15
9:58
12:52
9:58
23:07
4:56
5:19
1:50
23:54
13:50
23:54
2008/4/30
2008/5/3
2008/5/20
2008/5/23
2008/5/28
2008/6/6
2008/6/14
2008/6/20
2008/6/25
2008/7/5
2008/7/11
2008/7/22
2008/7/27
2008/8/9
2008/8/18
2008/9/3
2008/9/15
2008/9/30
2008/10/11
2008/10/22
2008/10/28
2008/11/7
2008/11/15
2008/11/25
2008/12/3
2008/12/22
2008/12/30
21:00
11:54
19:50
23:54
5:07
12:05
16:56
0:40
20:13
10:09
12:52
11:42
19:15
5:54
5:30
23:07
3:11
17:19
10:32
17:19
21:11
8:25
21:11
3:34
19:15
21:23
23:54
changes in the altitudinal range between the heat source
region and satellite altitude [Lei et al., 2011]. It is expected
that greater changes will be seen of neutral density in percent
in the night than the dayside when the neutral atmosphere is
less dense or equivalently has a smaller scale height due to
colder temperatures.
[25] An interesting feature presented in Figure 13 is that
the ∑ O/N2 decreases at middle latitudes in the winter
hemisphere, however, it increases slightly in the summer
hemisphere. We speculate that this may be caused by the
difference in the satellite sampling local time when it passes
different seasons. According to the notion of Prölss et al.
[1980], composition changes are larger in the nighttime/
early morning than in the afternoon sector because the
composition disturbance is first seen in the nighttime sector
and then rotates into the daytime. Supposed that the satellite
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mainly samples the morning sector in the winter hemisphere,
whereas it passes the afternoon sector most of the time in
summer. In this regard, it is easier for high latitudes composition disturbance to penetrate into middle latitudes in the
early morning sector. We have checked the local time distribution of CIRs events in summer and winter in Figure 13b
when dealing with midlatitude thermospheric composition
changes. There is a subtle difference in the cumulative distribution of the event local times at middle latitudes between
the summer and winter. The CIRs events recorded by the
TIMED/GUVI takes a larger portion during 0600–1000 LT
in winter than in summer, which coincides with our speculation to some extent. However, it is still uncertain whether
this observed discrepancy in composition changes at middle
latitudes is due to effects of the minor difference in the
sampling local times or other unknown mechanisms.
[26] The solar cycle effects on the thermospheric response
to CIRs are prominent, with a larger reflection in neutral
density at lower solar activity at 400 km. Joule heating will be
relatively more important at solar minimum because energy
input from EUV radiation is less in this part of solar cycle if
the magnetospheric energy input is the same at solar maximum and minimum [Burns et al., 2004]. As shown in
Figure 8, the relative variations of ∑ O/N2 also show clear
solar cycle dependence mainly concentrated at high latitudes,
which should be related to stronger upward winds occurring
within auroral region associated with enhanced auroral
energy input during solar minimum. Stronger upward winds
disturb the neutral composition to greater extent, resulting in
greater decrease in ∑ O/N2 at solar minimum. This result is
consistent with the results of Burns et al. [2004]. Through
term analysis of horizontal, vertical advection and molecule
diffusion, they concluded that upward advection is more
effective at solar minimum than at solar maximum. This, in
turn, leads to a larger decrease in ∑ O/N2 at solar minimum
than that at solar maximum. In addition, as depicted in
Figure 2, the solar wind speed is a little larger at solar minimum than solar maximum and the opposite condition
applies for solar wind dynamic pressure and merging electric
field. A little difference in average solar wind speed between
solar maximum and minimum may also make a contribution
to the difference in neutral density response because the
amount of energy input is highly dependent on the solar wind
speed and interplanetary magnetic field [Pulkkinen et al.,
2007].
5. Summary
[27] In this article, we have studied the thermospheric
composition and mass density variations in response to CIRs,
emphasizing the solar cycle and seasonal effects of CIRs on
the thermosphere. The main conclusions are summarized as
follows.
[28] 1. CIRs are inclined to occur at the declining phase
and minimum of the solar cycle 23, and show no conspicuous seasonal preference of its occurrence. In a statistical
sense, the solar wind parameters and geomagnetic indices
present almost the similar pattern in the absolute term,
though differing in magnitude, during the passage of CIRs at
different phase of this solar cycle.
[29] 2. The neutral density expressed in the relative term at
a fixed altitude of 400 km experiences a larger enhancement
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Figure 9. Superposed epoch results of relative variations in daytime neutral density at 400 km in
response to CIRs at (a, d) high latitude (60 –90 ), (b, e) middle latitude (30 –60 ), and (c, f) low latitude
(0 –30 ) bands in summer. The right three panels are for winter condition. The thick solid line is the
median value, and the shaded area represents the upper and lower quartiles.
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Figure 10. The same as Figure 8 but for the nighttime condition.
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Figure 11. The median relative variations in neutral density at 400 km caused by CIRs at both the daytime and night in different seasons. The abbreviations Sum and Win represent summer and winter, respectively. High, Mid and Low stand for the high, middle and low latitude bands, respectively.
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Figure 12. Superposed epoch results of relative variations in daytime ∑ O/N2 due to CIRs at (a, d) high
latitude (60 –90 ), (b, e) middle latitude (30 –60 ), and (c, f) low latitude (0 –30 ) bands in summer. The
right side shows winter conditions. The thick solid line is the median value, and the shaded area represents
the upper and lower quartiles.
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Figure 13. (a) The median relative variations induced by CIRs in daytime ∑ O/N2 in different seasons.
The abbreviations Sum and Win represent summer and winter, respectively. High, Mid and Low stand for
the high, middle and low latitude bands, respectively. (b) Cumulative distributions of the local time in percentage when the satellite passes the midlatitudes in summer and winter.
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Figure 14. The mean relative variations of neutral density on a constant pressure surface due to CIRs at
both the daytime and night at different solar activities. See details in the text.
in percent after the SI at solar minimum than at solar maximum, which is contrary to the results in the absolute sense.
The peak relative increment of neutral density reaches
60% for nighttime conditions at solar minimum, while a
weaker enhancement of neutral density is seen with the
maximum increase of 25% at solar maximum.
[30] 3. At solar minimum, ∑ O/N2 at high latitudes
decreases by 18%, being 9% larger than that of solar
maximum. However, there is no evidence of solar cycle
dependence of ∑ O/N2 changes at mid-low latitudes.
[31] 4. The average neutral density enhancement at the
same latitudinal bands is larger in the summer hemisphere
than in the winter hemisphere. The largest enhancement of
neutral density at high latitudes on average is 40% in the
summer hemisphere, and it reaches 23% in the winter
middle latitudes. The peak reduction of ∑ O/N2 at high
latitudes is more remarkable in the summer hemisphere than
in the winter hemisphere.
[32] Acknowledgments. This research was supported by the Chinese
Academy of Sciences (KZZD-EW-01-3), National Key Basic Research Program of China (2012CB825604), National Natural Science Foundation of
China (41074112, 41174137, 41174138, and 41174139), the CMA grant
GYHY201106011 the Specialized Research Fund for State Key Laboratories, and NASA NNX10AE62G. The thermospheric mass density measured
by CHAMP is obtained from the Website http://sisko.colorado.edu/sutton/
data.html. The ∑ O/N2 measured by TIMED/GUVI is derived from http://
guvi.jhuapl.edu/.
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