Research On Thermodynamic Characteristics Of Planetary Magnetosheaths

Magnetosheaths are important sources of plasmas in the planetary magnetospheres,they partially control the thermal state of magnetospheric plasma and play a critical role in the dynamical evolution of the planetary magnetosphere.Therefore,research on the planetary magnetosheaths can help people understand how the coupling of the solar wind and the planetary magnetosphere determines the dynamics of the planetary magnetosphere.The solar wind originates in and is driven by the corona and propagates outward through interplanetary space.The upstream solar wind is compressed by the bow shock,forming the downstream magnetosheath between the bow shock and the magnetopause,so the plasma temperatures of planetary subsolar magnetosheaths are closely related to the temperature of the corona.A theoretical investigation is presented on the expansion of the solar corona,the propagation of the solar wind,and the compression of the planetary magnetosheath by bow shocks with Bernoulli’s theorem.The method used is general and fits the dynamics of multiple components,thermal anisotropy,and non-Maxwellian plasmas in the steady state.The approximate relation between the temperature of the corona and the temperature of the subsolar magnetosheaths of planets is derived by Bernoulli’s equation.The result shows that the mean temperature of the subsolar magnetosheaths of planets are roughly the same and close to the mean temperature of corona.On the basis of theoretical analysis,this thesis uses the data of Messenger,MMS,Cassini and Voyager 2 to make observations and statistics on the temperature of the subsolar magnetosheaths of Mercury,Earth,Saturn and Jupiter respectively.The observations for these four planets confirm the theoretical analysis result of this thesis.The plasma temperature in the subsolar magnetosheath of the planet without inherent magnetic field(Mars)is analyzed using the data of MAVEN.The results show that the mean temperature in the sub solar magnetosheath of Mars is significantly lower than that of other planets with intrinsic magnetosphere(Mercury,Earth,Jupiter and Saturn).In addition,this research have also analyzed the relationship between the ratio of ion to electron temperatureTi/Te and the ion bulk velocity V.Results show that the average value of TiTe when Vi<300 km/s is considerably higher than when Vi>300 km/s.By analyzing the relationship between Ti and Vi and the relationship between Te and Vi this research have found that the reason for the large difference in the average value of Ti/Te is mainly due to the fact that the ions are not sufficiently thermally heated when Vi>300 km/s.Finally,this thesis has made a research on the relationship between the total plasma β and Vi.The result show that the average value of total plasmaβ reaches its maximum value when Vi is in the range of 250-300 km/s,which indicates that the conversion efficiency of solar wind kinetic energy into internal energy is the highest at this time.Combined with the data of MMS and OMNI,the per particle characteristic energy of the upstream and downstream of the bow shock is calculated in a large number of cases.The calculation results show that the relative deviations of the per particle characteristic energy in the upstream solar wind and downstream magnetosheath is very small.These cases include different solar wind conditions(high-speed solar wind and low-speed solar wind),and different regions of the Earth’s magnetosheath(subsolar point(0°≤Φ≤30°)and far from subsolar point(60°≤Φ≤90°)).Considering some approximations made in the calculation process and the measurement errors of the instruments,the calculation results show that the general Bernoulli equation is completely applicable to the energy conservation relationship between the upstream solar wind and downstream Earth’s magnetosheath of the Earth’s bow shock.Compared with the R-H jump condition,the general Bernoulli equation provides a simpler energy conservation formula for the solar wind on both sides of the Earth’s bow shock,which is more suitable for studying the energy transport of the solar wind to the magnetosphere.