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Sources of the High-Latitude Thermospheric Neutral Mass Density Variations
Sources of the High-Latitude Thermospheric Neutral Mass Density Variations
Journal of Astronomy and Space Sciences. 2010. Dec, 27(4): 329-335
Copyright ©2010, The Korean Space Science Society
This is an Open Access article distributed under the terms of theCreative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0/)which permits unrestrictednon-commercial use, distribution, and reproduction in any medium,provided the original work is properly cited.
  • Received : November 11, 2010
  • Accepted : November 11, 2010
  • Published : December 15, 2010
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About the Authors
Young-Sil Kwak
Solar and Space Weather Research Group, Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea
yskwak@kasi.re.kr
Arthur Richmond
High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado, USA
Yue Deng
Department of Physics, University of Texas, Arlington, Texas, USA
Byung-Ho Ahn
Department of Earth Science, Kyungpook National University, Daegu 702-701, Korea
Kyung-Suk Cho
Solar and Space Weather Research Group, Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea
Abstract
We investigate the sources of the variation of the high-latitude thermospheric neutral mass density depending on the interplanetary magnetic field (IMF) conditions. For this purpose, we have carried out the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM) simulations for various IMF conditions under summer condition in the southern hemisphere. The NCAR-TIEGCM is combined with a new empirical model that provides a forcing to the thermosphere in high latitudes. The difference of the high-latitude thermospheric neutral mass density (subtraction of the values for zero IMF condition from the values for non-zero IMF conditions) shows a dependence on the IMF condition: For negative By condition, there are significantly enhanced difference densities in the dusk sector and around midnight. Under the positive- By condition, there is a decrease in the early morning hours including the dawn side poleward of -70°. For negative Bz , the difference of the thermospheric densities shows a strong enhancement in the cusp region and around midnight, but decreases in the dawn sector. In the dusk sector, those values are relatively larger than those in the dawn sector. The density difference under positive- Bz condition shows decreases generally. The density difference is more significant under negative- Bz condition than under positive- Bz condition. The dependence of the density difference on the IMF conditions in high latitudes, especially, in the dawn and dusk sectors can be explained by the effect of thermospheric winds that are associated with the ionospheric convection and vary following the direction of the IMF. In auroral and cusp regions, heating of thermosphere by ionospheric currents and/or auroral particle precipitation can be also the source of the dependence of the density difference on the IMF conditions.
Keywords
1. INTRODUCTION
Thermospheric neutral mass density, which primarily determines the atmospheric drag, is an important parameter for satellite operations in the near-Earth space, and is also very useful for understanding the thermosphereionosphere coupling process as well. The thermospheric neutral mass density is highly variable in space and time by various energy and momentum sources. In particular at high latitudes, these sources are directly or indirectly associated with the direction and/or strength of the interplanetary magnetic field (IMF) (McCormac & Smith 1984, McCormac et al. 1985, 1991, Killeen et al. 1985,1995, Meriwether & Shih 1987, Thayer et al. 1987, Rees &Fuller-Rowell 1989, 1990, Sica et al. 1989, Hernandez et al.1991, Niciejewski et al. 1992, 1994, Won 1994, Richmond et al. 2003, McHarg et al. 2005, Zhang et al. 2005, Kwak & Richmond 2007, Kwak et al. 2007, Lühr et al. 2007, Forster et al. 2008). Therefore, there is an intimate relationship between the IMF variation and thermospheric density variation (Crowley et al. 2006, Rentz & Lühr 2008, Kwak et al. 2009).
Recently, Kwak et al. (2009) have carried out the systematic analysis of observed IMF By and Bz influences on the thermospheric density by using the high-latitude southern summer thermospheric total mass density near 400 km altitude, derived from the high-accuracy accelerometer on board the Challenging Minisatellite Payload (CHAMP) spacecraft during October 17, 2001 through February 24, 2002. Their study showed that the thermospheric density distribution depends on the orientation of the IMF. Especially, their study showed that the difference in density distributions, which were obtained by subtracting values for zero IMF from these for non-zero IMF, varied strongly with respect to the direction of the IMF: density differences poleward of -50° tended to be strongest when Bz was negative, and stronger when By was negative than when it was positive. Density differences for negative By showed significant enhancements in the early morning hours and hours around dawn, but showed reduced values in the dusk sector. For positive By the density differences were opposite in sign to those for negative By . Density differences for negative Bz showed significant increases in the cusp region and premidnight sector, but a small decrease in the dawn sector. The density differences at high latitudes tended to be weakest when Bz is positive. Since the behavior of the high-latitude thermospheric densities varies with IMF conditions, one can expect that the sources driving the thermospheric density variations should vary with IMF conditions.
Further study is necessary to identify why the variation of the thermospheric neutral mass density is dependent on the orientation of the IMF. In this paper, we extend the work of Kwak et al. (2009) by conducting model simulations by using the coupled model of the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM) and a new empirical model that provides a forcing of the thermosphere in high latitudes.
2. MODEL SIMULATIONS
The TIEGCM (Richmond et al. 1992) used in this study is an extension of the NCAR-TIGCM (Roble et al. 1988) that incorporates the electrodynamic processes. The TIEGCM simulates self-consistently the neutral winds, conductivities, electric fields and currents. The model calculates the neutral gas temperature and mass mixing ratios of O2, N2, O, N(2D), N(4S) , and NO. It also solves the electron and ion temperatures and the number densities of O+, O2+, NO+, N2+ , and N+ . The numeric grid size of the model is 5° × 5° horizontally and 1/2 scale height vertically, and the vertical coordinate has 29 constant pressure levels from approximately 97-700 km altitude. The external inputs required by the model are the solar extreme ultraviolet (EUV) and UV fluxes, the auroral particle precipitation, the ionospheric convection, and the upward propagating tides from the middle atmosphere.
The model simulations are conducted by using the seasonal and solar conditions on January 5, 2002. The F10.7 index was 224 × 10 -22 W/m 2 /Hz and the total hemispheric power (HP) was 16 GW on that day. The HP is related with Kp index by HP (GW) = -2.78 + 9.33 × Kp (Maeda et al. 1989). The mean Kp index on January 5, 2002 was 2. The prescriptions of the upward propagating diurnal and semidiurnal tides are taken from the Global Scale Wave Model (GSWM) (Hagan & Forbes 2002). The TIEGCM used in this study is coupled with a new empirical model of the high-latitude forcing on the thermosphere including electric field, magnetic potential and Poynting flux, and soft particle precipitation to the polar region (Deng et al. 2008) is considered.
To investigate the response of the thermospheric density to varying IMF, we made five TIEGCM simulations for IMF ( By , Bz ) values of (-4.4, 0.0), (4.4, 0.0), (0.0, -3.4), (0.0, 3.4), and (0.0, 0.0) nT. The magnitudes of the non-zero reference values of By and Bz (4.4 nT and 3.4 nT). The 4.4 and 3.4 nT are the root-mean square values of By and Bz corresponding to the IMF on the day that Kwak et al. (2009) analyzed the CHAMP neutral mass density data. We have chosen those IMF values to compare the model simulation results with the CHAMP data. A 2-min time step is used for the entire simulation, and linear interpolations of input values are made at a given time step. The TIEGCM history is recorded hourly for January 5, 2002.
3. ANALYSIS PROCEDURE
Quantities considered to investigate the potential source of the variation of the neutral mass density in the high latitudes are E × B drifts, neutral winds, and heating terms including auroral heating and joule heating. We first bin each quantity at 400 km altitude for each universal time in magnetic coordinates for different orientations of the IMF; we analyze the quantities in Quasi-Dipole (QD) coordinates (QD-latitude and magnetic local time [MLT]) (Richmond 1995). Especially, for analysis of E × B drifts and neutral winds, the QD-latitude/MLT region poleward of 47.5°S is divided into 145 subregions of approximately equal size, each having 5° width in latitude, and a variable, latitude-dependent, width in MLT. The number of MLT subregions at a given latitude decreases from 32 at 50°S to 4 at 85°S, plus one at the pole.
We then carry out an averaging for 24 UT hours of each quantity in each bin for each of the different IMF conditions. The resultant values can be then mapped out over magnetic latitude and MLT.
4. RESULTS
- 4.1 Comparison between thermospheric neutral mass densities from model and observations
To examine how well the thermospheric densities from the TIEGCM agree with the observations, we compare the results of the TIEGCM with densities derived from
Lager Image
The thermospheric densities at 400 km altitude over the southern hemisphere for interplanetary magnetic field (IMF) (By Bz) values of (left to right) (-4.4 0.0) (4.4 0.0) (0.0 -3.4) and (0.0 3.4) nT from (a) CHAMP observations and (b) the TIEGCM run respectively.
the high-accuracy accelerometer on board the CHAMP spacecraft as analyzed by Kwak et al. (2009).
Figs. 1a and b show the average thermospheric neutral mass densities at 400 km over the southern summer hemisphere in the poleward latitude -50° for IMF ( By , Bz ) values of (-4.4, 0.0), (4.4, 0.0), (0.0, -3.4) and (0.0, 3.4) nT from CHAMP observations and the TIEGCM run, respectively. These projections are as if one were looking up on the thermosphere from below. For all IMF conditions, southern summer high-latitude thermospheric total mass density at 400 km from the CHAMP shows the classic picture, reaching a maximum in the postnoon sector (~14 MLT), and a minimum in the early morning sector (~04 MLT). Density patterns having the postnoon maximum and early morning minimum at high latitudes have been seen in previous studies (Jacchia & Slowley 1968, Liu et al. 2005). And large thermospheric density peaks is clearly visible in the cusp region. This feature is most pronounced when the IMF Bz is negative. This feature and its strengthening with increasing magnetic activity were also reported in previous studies (Lühr et al. 2004, Liu et al. 2005, Rentz & Lühr 2008). The TIEGCM thermospheric density distributions show agreements with the CHAMP observations for the dependence of density on the IMF direction, although variations in the model density are generally weaker than observations and don’t show the significant large peak in the cusp region as do the observations. This difference may be associated with numerical smoothing features of the model.
The general agreement between thermospheric density observations and model results indicates that we can use the model to analyze in more detail the sources that drive the neutral density variations in the summertime high-latitude thermosphere.
- 4.2 Effect of the IMF on the thermospheric neutral mass density variations
Fig. 2 shows the difference of thermospheric neutral mass densities (subtraction of the values for zero IMF condition from the values for non-zero IMF conditions) at 400 km over southern-hemisphere high latitudes for the four IMF ( By , Bz ) values from the TIEGCM run. As shown in Fig. 2 , the thermospheric density variation patterns in the high latitudes depend on the orientation of the IMF. The difference densities for negative By show an increase in almost the whole polar regions. In particular, there are significantly enhanced densities in the dusk sector and around midnight when By is negative. Under the positive- By condition, although there is an increase of difference density in the afternoon including midnight, there is a decrease in the early morning hours including the dawn side poleward of -70°. The difference of the thermospheric densities for negative Bz shows a strong enhancement in the cusp region and around midnight with maximum value of 2.08 × 10 -12 kg/m 3 , but decreases in the dawn sector. In the dusk sector, although densities are not significantly enhanced, those
Lager Image
Difference thermospheric densities at 400 km altitude over the southern hemisphere for interplanetary magnetic field (IMF) (By Bz) values of (left to right) (-4.4 0.0) (4.4 0.0) (0.0 -3.4) and (0.0 3.4) nT from the TIEGCM run. These are obtained by subtracting values with zero IMF from those with non-zero IMF conditions.
values are relatively larger than those in the dawn sector. The positive- Bz difference densities show decreases generally, although there are weak increases in the early morning and evening sectors. The negative- Bz difference densities are more significant than the positive- Bz difference densities, indicating that negative Bz has a stronger effect on the thermospheric density than does positive Bz . Since the behavior of the high-latitude thermospheric densities varies with IMF conditions, one can expect that the sources driving the thermospheric density variations should vary with IMF conditions.
- 4.3 Driving sources for the high-latitude thermospheric density variations
A possible interpretation for different thermospheric density patterns for different IMF conditions is that the high-latitude thermospheric density is strongly determined by thermospheric winds, which are associated with the ionospheric convection and vary strongly with respect to the direction of the IMF. Geostrophic adjust theory, as it applies to the thermosphere (Larsen & Mikkelsen 1983, Walterscheid & Boucher 1984), indicates that winds and horizontal pressure gradients tend to be linked, such that a cyclonic wind vortex tends to have a low pressure and density at its center, while an anti-cyclonic vortex tends to have a high pressure and density at its center.
Fig. 3 shows the difference of neutral winds and difference ionospheric convection velocities at 400 km over southern hemisphere high latitudes for four IMF ( By , Bz ) values. The difference winds reflect well the pattern of the difference ionospheric E × B drifts with different IMF conditions, indicating that the thermsopheric winds are influenced strongly by ionospheric plasma convection
Lager Image
(a) Difference neutral winds at 400 km altitude over the southern hemisphere for interplanetary magnetic field (IMF) (By Bz) values of (left to right) (-4.4 0.0) (4.4 0.0) (0.0 -3.4) and (0.0 3.4) nT from the TIEGCM run. (b) Corresponding difference ionospheric E×B drift. These are obtained by subtracting values with zero IMF from those with non-zero IMF conditions.
through the ion drag. The difference winds for negative B y show a strong high-latitude anticyclonic vortex on the dusk side. The difference winds for positive By show a high-latitude cyclonic vortex on the dawn side. The difference neutral winds for negative Bz have a two-cell structure with evening anticyclonic and morning cyclonic vortices, and extend to subauroral latitudes. There is an equatorward flow in the early morning hours. The negative- Bz difference winds are stronger than the positive- Bz difference winds, indicating that negative Bz has a stronger effect on the winds than does positive Bz .
If the geostrophic adjust theory is applied to the thermosphere, we would expect strong anticyclonic winds on the dusk side for negative- By condition drive high neutral density on the dusk side of the polar cap. We also expect that strong cyclonic winds on the dawn side drive low neutral density on the dawn side for positive- By condition. This speculation generally agrees with the density distributions in Fig. 2 . Similarly, the dawn-side minimum difference density around -70° for negative Bz , and maximum difference density for positive Bz , are related to the strength of the cyclonic vortex there seen in Fig. 3 . In contrast, the difference densities around 18 MLT are not much larger for negative Bz than for positive Bz , despite the fact that the dusk-sector anti-cyclonic winds having high pressure and density shown in Fig. 3 are considerably stronger for negative Bz than for positive Bz . The possible reason can be that the horizontal advection of momentum, or centrifugal force, is strongly divergent in the dusk sector for Bz negative, reducing the need for an outward pressure-gradient force to help balance the Coriolis force, and decreases density in the dusk sector (Kwak et al. 2007). Eventually, high-latitude thermospheric density, especially in the dawn and dusk sectors, can be strongly determined by thermospheric winds, which are associated with the ionospheric convection and vary strongly with respect to the direction of the IMF.
Additionally, thermospheric density variations can be also influenced by the local heating including joule heating and auroral heating associated with ionospheric currents and/or auroral particle precipitation, which vary with IMF conditions. Indeed, the heating may generate upward neutral motion and cause an increase of total mass density.
Fig. 4 shows difference heating terms at 400 km altitude over the southern hemisphere for IMF ( By , Bz ) values. For negative By , a strong difference heating occurs in the auroral region in the afternoon and polar cusp region with the maximum value of 5.28 × 10 5 erg/s/g. For positive IMF By , the distribution of the difference heating is similar to that for negative By condition but the intensity with maximum value of 3.72 × 10 5 erg/s/g is weaker than that for negative By . For negative IMF Bz condition, there is also a strong difference heating regions in the afternoon and polar cusp region with the maximum value of 4.96 × 10 5 erg/s/g. When positive IMF Bz condition, a weaker difference heating than that other IMF conditions occurs around polar cusp region with the maximum value of 3.14 × 10 5 erg/s/g.
If these difference heating distributions are considered with the thermsopheric density distributions from Fig. 2 , we can see the high-latitude thermospheric density variations in auroral and cusp regions, especially near cusp region for negative Bz condition, are also influenced by the local heating associated with ionospheric currents and/or auroral particle precipitation, which vary with IMF conditions. Lühr et al. (2004) suggested Joule heating due to very intense small-scale field aligned currents (FACs) near the cusp region at a lower level causes upwelling and enhanced neutral mass densities at a higher level. In particular, Neubert & Christiansen (2003) pointed out that small-scale FACs are strong in the cusp region when the IMF is strongly southward, which may explain why strong thermospheric density distributions near the cusp region occur for negative Bz . Recently, it has been shown by Rentz & Lühr (2008) that the increased positive anomaly in cusp density is detected for about an hour after the enhancement of the merging electric fields, which are proportional to amplitude of southward IMF.
Lager Image
Difference heating terms at 400 km altitude over the southern hemisphere for interplanetary magnetic field (IMF) (By Bz) values of (left to right) (-4.4 0.0) (4.4 0.0) (0.0 -3.4) and (0.0 3.4) nT from the TIEGCM run. These are obtained by subtracting values with zero IMF from those with non-zero IMF conditions.
5. SUMMARY AND CONCLUSIONS
In this paper, we present the investigations of the driving sources for the high-latitude thermospheric density variations depending on the IMF conditions. For this study, we analyzed steady state condition for different IMF directions, for summer conditions in the southern hemisphere, on the basis of numerical experiments with the NCAR-TIEGCM coupled with a new quantitative empirical model of the high-latitude forcing on the thermosphere.
The difference high-latitude thermospheric densities, obtained by subtracting values with zero IMF from those with non-zero IMF, vary with IMF conditions: There are significantly enhanced difference densities in the dusk sector and around midnight when By is negative. Under the positive- By condition, there is a decrease in the early morning hours including the dawn side poleward of -70°. For negative Bz , the difference of the thermospheric densities shows a strong enhancement in the cusp region and around midnight, but decreases in the dawn sector. In the dusk sector, those values are relatively larger than those in the dawn sector. The density difference under positive- Bz condition shows decreases generally. The density difference is more significant under negative- Bz condition than under positive- Bz condition.
A possible interpretation for different thermospheric density patterns for different IMF conditions is that the high-latitude thermospheric density, especially in the dawn and dusk sectors, can be strongly determined by thermospheric winds, which are associated with the ionospheric convection and vary strongly with respect to the direction of the IMF: Strong anticyclonic winds on the dusk side for By negative condition drive high neutral density on the dusk side of the polar cap. For positive By , strong cyclonic winds on the dawn side drive low neutral density on the dawn side. The dawn-side minimum difference density for negative Bz , and maximum difference density for positive Bz , are related to the strength of the cyclonic vortex. The difference densities around 18 MLT are not much larger for negative Bz than for positive Bz , despite the fact that the dusk-sector anti-cyclonic winds are considerably stronger for negative Bz than for positive Bz . Additionally, the high-latitude thermospheric density variations, especially in auroral and cusp regions, are also influenced by the local heating associated with ionospheric currents and/or auroral particle precipitation, which vary with IMF conditions.
Acknowledgements
This work was supported by the “Development of Korean Space Weather Center”, the project of KASI, the KASI basic research fund, and the Korea Research Foundation Grant (KRF-2005-070-C00059).
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