Thermospheric Response To Solar Wind-Magnetospheric Momentum And Energy Deposition At High Latitudes

The thermosphere is a layer that connects the lower atmosphere and the outer space.The properties of the thermosphere,especially its density,can greatly influence the accuracy of the orbital determination and lifetime of satellites.The thermosphere and the ionosphere are highly coupled through multiple physical and chemical processes.Therefore,the change of the thermosphere may bring about ionospheric disturbances,and thus affect the radio communication and the satellite navigation.Therefore,the investigation of the thermosphere has important practical significances.The variation of the thermosphere at high latitudes is strongly modulated by the solar wind energy and momentum deposited into the polar region in the upper atmosphere.The purpose of the thesis is to investigate the response of the thermosphere to the energy and momentum input from the solar wind and the involved mechanisms.The thesis focuses on the effect of the solar wind sector polarity reversal and the magnetospheric convection on the thermospheric neutral density.The main content and achievement of the thesis is listed as below:1.Investigation of thermospheric mass densities in response to the solar wind sector structure.40-year long globally average thermospheric mass densities derived from the two-line element sets of about 5000 objects are used in this work.The statistical results show that in March,the globally average thermospheric total mass density at 400 km increases by 23%as compared to the 11-day average value when the sector polarity changes from away from to toward the sun;and decreases by only 12%for the condition of the opposite sector polarity reversal.In September,the increase or decrease of the thermospheric density in response to the sector polarity reversal is opposite to that in March.In the solstice days,the density change is much smaller during the period of the sector polarity reversal.Besides the seasonal dependence,a larger relative density disturbance is produced at the higher altitude and lower solar activity level.However,at the solar minimum,the relative changes of the thermospheric density are similar at 400 and 550 km.This may be due to the declined transition altitude from an O-dominant atmosphere to a He-dominant one.Our results also show that the corotating interaction region(CIR)occurs close to the sector polarity reversal.This leads to a contribution from the CIR to the density enhancement during sector polarity reversal in a statistical sense.And the density decrease during the sector polarity reversal is weakened by about half due to the cooccurrence of the CIR.2.Exploration of the variations of the thermospheric mass density at the geographic poles and the modulation associated with the solar wind sector structure.A statistical analysis of the pole neutral densities during the years from 2002 to 2010 observed by the GRACE satellite showed that the densities at both the South and North Poles had a UT(magnetic local time,MLT)variation.In September-November.the neutral density at the South Pole in an away solar wind sector maximizes at about 17:00 UT(13:30 MLT),which is about 22%above the daily average.In June-August,the neutral density at the North Pole in a toward sector maximizes at about 06:00 UT(12:30 MLT),which is about 13%above the daily average.The magnetic local time of the South and North Poles are about 12:00 at 15:30 UT and 5:30 UT,respectively.That is,they are closest to the cusp region at the corresponding time.Previous studies show that the cusp neutral density in the high-altitude thermosphere is larger than the surrounding area.Therefore,the MLT variation of the pole thermospheric density may be caused by the periodic overlap of the cusp and the poles.The MLT variations of the thermospheric density at the South and North poles are more obvious in the away and toward sectors.This may be caused by the different effect of the IMF By on the thermospheric densities in the Southern and Northern Hemispheres.The statistical results also show that the MLT variation of the pole neutral density is not obvious in the winter hemisphere.This may be due to the fact that the cusp ion precipitating energy flux in the summer hemisphere is larger than that in the winter hemisphere,and the average particle energy is lower in the summer hemisphere.These may cause stronger cusp density enhancement in the summer hemisphere.3.Simulation study of the altitudinal dependence of the thermospheric low density cell at high latitudes.The high-latitude density cell is referred to as the density enhancement or depletion as compared to the surrounding area.Previous studies have shown that the density cell can only occur below 350 km.The global-ionosphere-thermosphere model(GITM)is used in this work to investigate the mechanism for the absence of the density cell at high altitude.The simulation results show that in both the low-and high-altitude thermosphere,the ion convection produces cyclonic and anticyclonic structures on the dawnside and duskside of the Magnetic Pole,respectively.Inside the cyclone,a downward flow occurs as a result of mass conservation.This downward flow results in a density decrease of the same relative magnitude at the center of the cyclone,which is independent of altitude.In the low-altitude thermosphere,this density decrease is shown as an isolated low density cell.In the high-altitude thermosphere,the density gradient in the day-night direction is so strong that the ion drag-driven density decrease is relatively insignificant.Therefore,the isolated low density cell does not stand out in the high-altitude thermosphere.4.Investigation of the UT/longitudinal variation of the thermospheric low density cell at high latitudes and the underlying mechanism.The GITM simulation results show that,when the thermosphere is in a diurnally reproducible state,the magnitudes of the ion drag-driven neutral cyclone and the corresponding low density cell vary with UT/longitude.As the magnetic field in the model is replaced with a dipole one,the UT/longitudinal variation disappears.The displacement of the magnetic and the geographic poles could play a major role in producing the UT/longitudinal variation.It is found that the plasma has a larger density on the dayside than nightside,which may be due to the difference of the solar radiation.In the process of the Earth's rotation,the magnetic pole is periodically located on the dayside and nightside.The background ion mass density is larger when the magnetic pole is on the dayside,and smaller when it is on the nightside.As the ion drag is proportional to the neutral-ion collision frequency and electron density,the ion drag changes periodically due to the cyclic variation of electron density.Consequently,this results in a UT/longitudinal variation of magnitude of the neutral cyclone and the associated low density cell.