Longitudinal And Universal Time Differences In The Parameters Of Ionosphere And Thermosphere

With the development of aerospace technology and radio wave communication technology,the effects of the near-Earth space environment changes on the human's space activities are becoming more and more important.The ionosphere-thermosphere?IT?coupling system is an important regime of the near-Earth Geospace,and a large number of satellites are running in the IT system.Thus,understanding the spatial and temporal distributions of thermospheric and ionospheric parameters?i.e.,neutral winds and density,electron density?have a remarkable application value for the spacecraft orbital operation,communication,navigation and positioning,and the national defense security,and have a great scientific value for the modeling and forecasting of the space weather.Our work systemically investigate the regional differences?longitudinal/universal time variations?of ionospheric and thermospheric parameters,which is an international frontier and challenging topic.This work has outstanding contributions,in which the key roles of solar heating,substorms,SAPS,geomagnetic field structure,and the upward-propagating atmospheric tides are firstly reported.Based on CHAMP and ground-based observations,and numerical simulations from TIEGCM,GITM,and CMIT model,we have investigated the physical mechanisms driving the longitudinal and universal time variations in thermospheric and ionospheric parameters under different geomagnetic activity conditions.The main results are as follow.1.Using satellite observations and numerical simulations,we firstly investigate the effects of upward-propagating atmospheric tides on the longitudinal structures of ionospheric electron density?Ne?at mid?latitudes.We focus on the propagation of tides into E and F layer,and the altitude differences of their effects.The main conclusions are as follow:a)The upward-propagating tides play a role in regulating the longitudinal variations of Ne??Ne?at altitudes above 200 km,with the maximum variations occurring at³00 km.b)?Ne over North America and in the Southern Hemisphere are decreased by migrating tides,whereas a wave-4 pattern in Ne is produced by non-migrating tides in both hemispheres.c)Neutral composition changes?i.e.,ratio between atom oxygen and molecular nitrogen?are the dominate processes causing the?Ne variations for both the migrating and non-migrating tidal driving conditions.d)For the?Ne due to tidal winds,?Ne caused by the tidal zonal winds?about 30%? is weaker than that by tidal meridional winds?accounting for approximately 70%?for both the migrating and non-migrating tides.2.Using satellite observations and numerical simulations,the zonal differences of thermospheric mass density at mid-latitudes during 2007-2009?solar minimum?are studied.The key role of ion drag is firstly reported,and the role of solar heating is also figured out.The new results are as follow:a)The longitudinal profiles of neutral densities exhibit an obvious wave-1 pattern,which is almost 180oout of phase for the two hemispheres.The locations of the peaks and troughs shift eastward with local time.b)The mechanism for the longitudinal variation is the ionospheric plasma density variations,which are too weak to influence the neutral densities in the European-Asian sector through ion drag.c)The differences between of solar zenith angle of the trough and those of the peak region could reach 20o³0o.The non-uniformity of the solar heating source in longitude is another possible reason for the longitudinal variations of the neutral densities.3.Using 2001-2009 CHAMP cross-track wind data and TIEGCM simulations under different geomagnetic field configurations,we firstly investigate the pattern and cause of the longitudinal variations of thermospheric zonal winds at low and mid latitudes in detail with their solar activity independence,seasonal variations and great hemispheric asymmetry.The main findings are shown below: a)There are large longitudinal variations in the zonal winds from the CHAMP cross-track wind data,which do not have substantial solar activity dependence.b)There are strong local time dependence and great hemispheric asymmetry in the longitudinal variations of the zonal winds.In the northern hemisphere,the daytime zonal winds are more westward at longitudes of-90°⁹0°and more eastward at the remaining longitudes.On the other hand,the longitudinal patterns of the nighttime zonal winds are opposite to those during the daytime.In the southern hemisphere,the local time variations of the longitudinal structures of the zonal winds are almost the opposite of those in the northern hemisphere.c)The longitudinal wind patterns at low latitudes during the June solstice are notably different from those in other seasons,indicating a seasonal dependence.d)The main cause for the large longitudinal variations of zonal winds is the geomagnetic field configuration.4.Using the GTIM simulations,the thermospheric zonal wind responses to the UT variations of subauroral polarization streams?SAPS?are investigated under southward IMF Bz.This study can contribute to the understanding of the ionosphere-thermosphere coupling processes under SAPS condition.The new conclusions are as follow:a)The westward winds in the subauroral regions are greatly accelerated at dusk during the SAPS periods,via the ion drag effects.b)The maximum magnitudes of the westward wind enhancements have notable differences,when SAPS commence at different universal times.The wind changes reach their maximum magnitudes at 18 UT and 04 UT,and their minimum values at 04 UT and 16 UT in the Northern and Southern Hemispheres,respectively.c)There is a good correlation between the thermospheric wind changes and cos^0.5 ?SZA?under southward IMF Bz condition.Stronger westward neutral wind disturbances occur under stronger sunlight conditions.d)At altitude of 400 km,the reduction?enhancement?in the electron?neutral? densities is the strongest when the SAPS region corresponds to local maximum solar illumination.5.Using GITM outputs,the UT variations of thermospheric wind responses,with the seasonal variations,to a substorm at high latitudes are systematically studied.The relative effects of ion drag and Joule heating are shown in singular.This study can contribute to the understanding of IT coupling during substorm periods.The main findings are shown below:a)At high latitudes,the substorm-induced thermospheric wind changes in the dusk sector are mainly in the poleward,westward,and upward directions.On the other hand,in the nighttime,they are in the equatorward,westward,and upward directions.b)In the daytime,the wind perturbations are due to ion drag,which is driven by the variations of IMF Bz.The nighttime wind perturbations are the combined effects of the Bz and auroral hemispheric power inputs.c)The wind responses on the nightside are somewhat later than those on the dayside, resulting from the low background ionospheric electron densities.d)During the daytime,ion drag plays the dominant role in the changes of both meridional and zonal winds,whereas pressure gradient in associated with temperature changes caused by Joule heating dominates the vertical wind changes.At night,the meridional and zonal wind changes are controlled by a combination of Joule heating and ion drag.e)The wind changes exhibit large UT variations during equinoxes and local winter.The more the solar illumination is,the stronger the wind disturbances are.In local summer,the weak or even opposite UT variations of the wind responses to a substorm are found.6.The effects of IMF Bz Aflvenic oscillations on thermospheric meridional winds are studied for the first time using simulations from the CMIT.The simultaneous and time delay?short-periodic and large scale TADs?of wind responses have been taken to show the responses of the ionosphere-thermosphere coupling system and its low-pass filter nature.The contributions of ion drag associated with electron density changes and pressure gradient due to periodic energy injection have been concluded using diagnostic analysis.This work can help to the understanding of ionosphere-thermospheric coupling.The new conclusions are as follow:a)The cross polar cap potential and auroral peak electron energy flux are stronger when the IMF Bz oscillation frequency is lower.b)The relatively weak periodic wind responses in the 10-min IMF oscillation case indicates a low-pass filter nature of the magnetosphere-ionosphere-thermosphere?MIT?system.c)Two different thermospheric wind?Vn?responses are revealed in the 30-and 60- min cases.One is the almost simultaneous responses at all latitudes and the other shows a typical traveling atmospheric disturbances?TADs?signature with a delayed response with respect to the latitude for all UTs.The simultaneous Vn responses at all latitudes appear in the daytime Northern Hemisphere,which are mainly caused by the ion drag force.The short-period TADs occurring in the nighttime of both hemispheres and the daytime of the Southern Hemisphere propagate from high to low latitudes are controlled by the pressure gradient in association with the periodic energy injection.