Superposed epoch analyses of thermospheric response to CIRs: Solar cycle and seasonal dependencies

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /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. McPherron 6 Received 1 November 2011; revised 3 May 2012; accepted 3 May 2012; published 20 June [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 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,, doi: /2011ja 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) American Geophysical Union. All Rights Reserved. 1. Introduction [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 1of18

2 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/N2isclosetoafixed 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 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; Fuller- Rowell 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 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 nm and N2 Lyman-Brige-Hopfield dayglow emission above an altitude where the N2 column density is cm 2 [Christensen et al., 2003; Strickland et al., 2004], from the Website: 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 It samples the same local time for several consecutive days, taking about four months for 2of18

3 Figure 1. Variations of (a) F10.7 index in unit of Wm 2 Hz 1 and (b) Ap index during the year 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 = Wm 2 Hz 1 ) (Figure 1a) and (b) Ap index during the year , 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 , then turns to decrease gradually and reaches the minimum in the year 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 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 (left) and solar minimum (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 3of18

4 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 (left) and solar minimum (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 ( ) and minimum ( ) 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 4of18

5 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 byhighspeed 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/ 5of18

6 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 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 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 6of18

7 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 7of18

8 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., 8of18

9 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 ; 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 , 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 9of18

10 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. 10 of 18

11 Table 1. Stream Interface List During the Years and YY/MM/DD UT YY/MM/DD UT YY/MM/DD UT Events 2001/1/4 10: /12/3 4: /6/19 2: /1/10 20: /12/15 20: /7/1 5: /1/21 22: /12/24 4: /7/5 11: /1/29 5: /1/10 10: /7/12 9: /2/28 9: /1/20 1: /8/9 8: /4/24 9: /1/25 18: /9/4 3: /5/23 4: /2/5 15: /9/16 13: /6/1 22: /2/11 10: /10/7 9: /6/9 1: /3/4 13: /10/14 12: /6/19 13: /3/11 23: /10/24 9: /7/31 2: /3/30 6: /11/1 23: /8/10 0: /4/11 1: /11/11 4: /8/21 5: /4/27 18: /11/21 5: /9/3 9: /5/27 11: /11/29 1: /9/11 10: /6/2 0: /12/6 23: /9/15 1: /6/8 14: /12/14 13: /10/8 14: /6/16 6: /12/26 23: Events 2007/1/1 19: /8/26 16: /4/30 21: /1/29 8: /8/31 20: /5/3 11: /2/12 13: /9/6 21: /5/20 19: /2/27 8: /9/14 21: /5/23 23: /3/6 7: /9/20 23: /5/28 5: /3/11 22: /9/27 17: /6/6 12: /3/25 2: /10/3 8: /6/14 16: /3/27 16: /10/18 16: /6/20 0: /4/1 0: /10/25 14: /6/25 20: /4/9 8: /10/29 23: /7/5 10: /4/23 3: /11/13 5: /7/11 12: /4/27 17: /11/20 11: /7/22 11: /5/7 12: /11/22 18: /7/27 19: /5/18 9: /11/24 16: /8/9 5: /5/23 13: /12/10 23: /8/18 5: /6/2 17: /12/17 7: /9/3 23: /6/9 9: /12/27 6: /9/15 3: /6/13 19: /1/5 6: /9/30 17: /6/21 10: /1/13 11: /10/11 10: /6/29 17: /1/25 1: /10/22 17: /7/3 20: /1/31 16: /10/28 21: /7/11 1: /2/10 7: /11/7 8: /7/14 17: /2/18 13: /11/15 21: /7/20 10: /2/28 16: /11/25 3: /7/26 15: /3/8 16: /12/3 19: /7/29 1: /3/26 12: /12/22 21: /8/6 22: /4/4 21: /12/30 23: /8/10 13: /4/16 10: /8/15 3: /4/23 5:07 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 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 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 11 of 18

12 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. 12 of 18

13 Figure 10. The same as Figure 8 but for the nighttime condition. 13 of 18

14 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. 14 of 18

15 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. 15 of 18

16 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. 16 of 18

17 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 ( , , , and ), the CMA grant GYHY the Specialized Research Fund for State Key Laboratories, and NASA NNX10AE62G. The thermospheric mass density measured by CHAMP is obtained from the Website data.html. The O/N2 measured by TIMED/GUVI is derived from guvi.jhuapl.edu/. References Ahn, B.-H., S.-I. Akasofu, and Y. Kamide (1983), The joule heat production rate and the particle energy injection rate as a function of the geomagnetic indices AE and AL, J. Geophys. Res., 88(A8), , doi: /ja088ia08p Borovsky, J. E., and J. T. Steinberg (2006), The calm before the storm in CIR/magnetosphere interactions: Occurrence statistics, solar wind statistics, and magnetospheric preconditioning, J. Geophys. Res., 111, A07S10, doi: /2005ja Bruinsma, S., J. M. Forbes, R. S. Nerem, and X. Zhang (2006), Thermosphere density response to the November 2003 solar and geomagnetic storm from CHAMP and GRACE accelerometer data, J. Geophys. Res., 111, A06303, doi: /2005ja Burns, A. G., W. Wang, T. L. Killeen, and R. G. Roble (2004), The solar-cycle-dependent response of the thermosphere to geomagnetic storms, J. Atmos. Sol. Terr. Phys., 66, 1 14, doi: /j.jastp Christensen, A. B., et al. (2003), Initial observations with the Global Ultraviolet Imager (GUVI) in the NASA TIMED satellite mission, J. Geophys. Res., 108(A12), 1451, doi: /2003ja Crowley, G., A. Reynolds, J. P. Thayer, J. Lei, L. J. Paxton, A. B. Christensen, Y. Zhang, R. R. Meier, and D. J. Strickland (2008), Periodic modulations in thermospheric composition by solar wind high speed streams, Geophys. Res. Lett., 35, L21106, doi: /2008gl Deng, Y., Y. Huang, J. Lei, A. J. Ridley, R. Lopez, and J. Thayer (2011), Energy input into the upper atmosphere associated with high speed solar wind streams in 2005, J. Geophys. Res., 116, A05303, doi: / 2010JA Denton, M. H., T. Ulich, and E. Turunen (2009), Modification of midlatitude ionospheric parameters in the F2 layer by persistent high-speed solar wind streams, Space Weather, 7, S04006, doi: /2008sw Emery, B. A., I. G. Richardson, D. S. Evans, and F. J. Rich (2009), Solar wind structure sources and periodicities of auroral electron power over three solar cycles, J. Atmos. Sol. Terr. Phys., 71, Forbes, J. M. (2007), Dynamics of the thermosphere, J. Meteorol. Soc. Jpn., 85B, , doi: /jmsj.85b of 18

18 Forbes, J. M., R. Gonzalez, F. A. Marcos, D. Revelle, and H. Parish (1996), Magnetic storm response of lower thermospheric density, J. Geophys. Res., 101(A2), , doi: /95ja Foster, J. C., J.-P. St.-Maurice, and V. J. Abreu (1983), Joule heating at high latitudes, J. Geophys. Res., 88(A6), , doi: / JA088iA06p Fuller-Rowell, T. J., M. V. Codrescu, R. J. Moffett, and S. Quegan (1994), Response of the thermosphere and ionosphere to geomagnetic storms, J. Geophys. Res., 99(A3), , doi: /93ja Fuller-Rowell,T.J.,M.V.Codrescu,H.Rishbeth,R.J.Moffett,andS.Quegan (1996), On the seasonal response of the thermosphere and ionosphere to geomagnetic storms, J. Geophys. Res., 101(A2), , doi: / 95JA Kan, J. R., and L. C. Lee (1979), Energy coupling function and solar wind magnetosphere dynamo, Geophys. Res. Lett., 6, , doi: / GL006i007p Lei, J., J. P. Thayer, J. M. Forbes, E. K. Sutton, and R. S. Nerem (2008a), Rotating solar coronal holes and periodic modulation of the upper atmosphere, Geophys. Res. Lett., 35, L10109, doi: /2008gl Lei, J., J. P. Thayer, J. M. Forbes, Q. Wu, C. She, W. Wan, and W. Wang (2008b), Ionosphere response to solar wind high-speed streams, Geophys. Res. Lett., 35, L19105, doi: /2008gl Lei, J., J. P. Thayer, A. G. Burns, G. Lu, and Y. Deng (2010), Wind and temperature effects on thermosphere mass density response to the November 2004 geomagnetic storm, J. Geophys. Res., 115, A05303, doi: /2009ja Lei, J., J. Thayer, W. Wang, and R. McPherron (2011), Impact of CIR storms on thermosphere density variability during the solar minimum of 2008, Sol. Phys., 274, , doi: /s y. Liu, H., and H. Lühr (2005), Strong disturbance of the upper thermospheric density due to magnetic storms: CHAMP observations, J. Geophys. Res., 110, A09S29, doi: /2004ja Liu, J., L. Liu, B. Zhao, W. Wan, and R. A. Heelis (2010), Response of the topside ionosphere to recurrent geomagnetic activity, J. Geophys. Res., 115, A12327, doi: /2010ja Liu, R., H. Lühr, E. Doornbos, and S.-Y. Ma (2010), Thermospheric mass density variations during geomagnetic storms and a prediction model based on the merging electric field, Ann. Geophys., 28, , doi: /angeo Liu, R., S.-Y. Ma, and H. Lühr (2011), Predicting storm-time thermospheric mass density variations at CHAMP and GRACE altitudes, Ann. Geophys., 29, , doi: /angeo Lu, G., A. D. Richmond, B. A. Emery, and R. G. Roble (1995), Magnetosphere-ionosphere-thermosphere coupling: Effect of neutral winds on energy transfer and field-aligned current, J. Geophys. Res., 100(A10), 19,643 19,659, doi: /95ja Lu, G., et al. (1998), Global energy deposition during the January 1997 magnetic cloud event, J. Geophys. Res., 103(A6), 11,685 11,694, doi: /98ja McPherron, R. L., D. N. Baker, and N. U. Crooker (2009), Role of the Russell McPherron effect in the acceleration of relativistic electrons, J. Atmos. Sol. Terr. Phys., 71, , doi: /j.jastp Mlynczak, M. G., F. J. Martin Torres, C. J. Mertens, B. T. Marshall, R. E. Thompson, J. U. Kozyra, E. E. Remsberg, L. L. Gordley, J. M. Russell, and T. Woods (2008), Solar terrestrial coupling evidenced by periodic behavior in geomagnetic indexes and the infrared energy budget of the thermosphere, Geophys. Res. Lett., 35, L05808, doi: / 2007GL Müller, S., H. Lühr, and S. Rentz (2009), Solar and magnetospheric forcing of the low latitude thermospheric mass density as observed by CHAMP, Ann. Geophys., 27, , doi: /angeo Olsson, A., P. Janhunen, T. Karlsson, N. Ivchenko, and L. G. Blomberg (2004), Statistics of Joule heating in the auroral zone and polar cap using Astrid-2 satellite Poynting flux, Ann. Geophys., 22, , doi: /angeo Pedatella, N. M., J. Lei, J. P. Thayer, and J. M. Forbes (2010), Ionosphere response to recurrent geomagnetic activity: Local time dependency, J. Geophys. Res., 115, A02301, doi: /2009ja Picone, J. M., A. E. Hedin, D. P. Drob, and A. C. Aikin (2002), NRLMSISE 00 empirical model of the atmosphere: Statistical comparisons and scientific issues, J. Geophys. Res., 107(A12), 1468, doi: / 2002JA Prölss, G. W. (1980), Magnetic storm associated perturbations of the upper atmosphere: Recent results obtained by satellite-borne gas analyzers, Rev. Geophys., 18(1), , doi: /rg018i001p Prölss, G. W. (1995), Magnetic storm associated perturbations of the upper atmosphere, in Magnetic Storms, Geophys. Monogr. Ser., vol. 98, edited by B. T. Tsurutani et al., pp , AGU, Washington, D. C. Pulkkinen, T. I., M. Palmroth, and R. L. McPherron (2007), What drives magnetospheric activity under northward IMF conditions?, Geophys. Res. Lett., 34, L18104, doi: /2007gl Richardson, I. G., G. Wibberenz, and H. V. Cane (1996), The relationship between recurring cosmic ray depressions and corotating solar wind streams at 1 AU: IMP 8 and Helios 1 and 2 anticoincidence guard rate observations, J. Geophys. Res., 101(A6), 13,483 13,496, doi: / 96JA Richmond, A., et al. (1990), Global measures of ionospheric electrodynamic activity inferred from combined incoherent scatter radar and ground magnetometer observations, J. Geophys. Res., 95(A2), , doi: /ja095ia02p Rishbeth, H., and I. C. F. Müller Wodarg (1999), Vertical circulation and thermospheric composition: A modelling study, Ann. Geophys., 17, , doi: /s x. Rishbeth, H., T. J. Fuller-Rowell, and D. Rees (1987), Diffusive equilibrium and vertical motion in the thermosphere during a sever magnetic storm: A computational study, Planet. Space Sci., 35, , doi: / (87) Sojka, J. J., R. L. McPherron, A. P. van Eyken, M. J. Nicolls, C. J. Heinselman, and J. D. Kelly (2009), Observations of ionospheric heating during the passage of solar coronal hole fast streams, Geophys. Res. Lett., 36, L19105, doi: /2009gl Strickland, D. J., R. R. Meier, R. L. Walterscheid, J. D. Craven, A. B. Christensen, L. J. Paxton, D. Morrison, and G. Crowley (2004), Quiettime seasonal behavior of the thermosphere seen in the far ultraviolet dayglow, J. Geophys. Res., 109, A01302, doi: /2003ja Sutton, E. K., J. M. Forbes, and R. S. Nerem (2005), Global thermospheric neutral density and wind response to the severe 2003 geomagnetic storms from CHAMP accelerometer data, J. Geophys. Res., 110, A09S40, doi: /2004ja Thayer, J. P., J. Lei, J. M. Forbes, E. K. Sutton, and R. S. Nerem (2008), Thermospheric density oscillations due to periodic solar wind high speed streams, J. Geophys. Res., 113, A06307, doi: /2008ja Tsurutani, B. T., W. D. Gonzalez, A. L. C. Gonzalez, F. Tang, J. K. Arballo, and M. Okada (1995), Interplanetary origin of geomagnetic activity in the declining phase of the solar cycle, J. Geophys. Res., 100(A11), 21,717 21,733, doi: /95ja Tsurutani, B. T., et al. (2006), Corotating solar wind streams and recurrent geomagnetic activity: A review, J. Geophys. Res., 111, A07S01, doi: /2005ja Tulasi Ram, S., C. H. Liu, and S.-Y. Su (2010), Periodic solar wind forcing due to recurrent coronal holes during and its impact on Earth s geomagnetic and ionospheric properties during the extreme solar minimum, J. Geophys. Res., 115, A12340, doi: /2010ja Zhang, Y., and L. J. Paxton (2011), Long term variation in the thermosphere: TIMED/GUVI observations, J. Geophys. Res., 116, A00H02, doi: /2010ja Zhang, Y., L. J. Paxton, D. Morrison, B. Wolven, H. Kil, C.-I. Meng, S. B. Mende, and T. J. Immel (2004), O/N 2 changes during 1 4 October 2002 storms: IMAGE SI-13 and TIMED/GUVI observations, J. Geophys. Res., 109, A10308, doi: /2004ja Zhang, Y., L. J. Paxton, and D. Morrison (2010), Auroral and thermospheric response to the 9 day periodic variations in the dayside reconnection rate in 2005, Space Weather, 8, S07001, doi: / 2009SW of 18