Studies Of The Kinetic Properties Of Interplanetary Shocks

Interplanetary(IP)shocks are important plasma structures in the Sun–Earth space,which are often driven by coronal mass ejection(CME)and fast solar wind.When a CME propagates in the interplanetary space,it is called an interplanetary CME(ICME).This dissertation investigates the kinetic properties of two IP shocks propagating respectively inside an ICME and typical solar wind.Investigations at kinetic scales of the IP shocks propagating inside distrinct plasma environments may be of significance for the development of microphysics of collisionless shocks.The IP shock inside an ICME was observed by Wind on 1998 August 6.The authors analyze the in situ solar wind magnetic field and plasma data with minimum variance analysis(MVA),bandpass filters,spectral analysis and particle velocity distribution fitting method in order to obtain how the shock–ICME interaction behaves at kinetic scales:(1)Gyrating ions are observed around the shock,which is consistent with theoretical predictions and bow shock observations,and give evidence that particle reflection may occur even at low Mach number shocks.Reflected ions observed in this case may provide a venue for energy dissipation around the shock together with the waves observed near the shock.The shock lacks an obvious foot structure;however,the existence of gyrating ions implies that the foot structure may be under-sampled.In addition,although the shock produces enhanced proton temperature anisotropy in the downstream of the shock,the shocked ICME plasma is under the thresholds of the ion cyclotron and mirror-mode instabilities because of the low plasma ? inside the ICME.(2)The disappearance of BDEs downstream of the shock characterizes the interaction between the shock and ICME.This is probably caused by the pitch-angle scattering of the electrons by the waves observed near the shock ramp.The electron distribution around the shock meets the criteria to excite whistler heat flux instabilities,which may contribute to the wave generation and help explain the disappearance of the BDEs downstream of the shock.Another mechanism that may also help explain the disappearance of the BDEs inside the ICME is the normal betatron acceleration that occurs across the shock.(3)The waves around the shock are thought to be whistler waves,as the higher-frequency wave envelopes occur earlier and all the wave events show RH polarization with respect to the ambient magnetic field.The whistler waves are probably associated with the electron distribution unstable to whistler heat flux instabilities observed around the shock.The waves may originate from the shockramp,since the amplitudes of the wave packets decrease from the shock ramp to the upstream and the downstream.This is consistent with the result that the whistler heat flux instabilities are excited at the shock ramp.The whistler waves share a similar characteristic with the shocklet(steepened magnetosonic waves)whistlers,which likely suggests that the shock may be decaying due to the shock-ICME interaction.The shock has propagated inside the ICME for about 5 hours,so the shock may have begun to decay because of the strong magnetic field inside the ICME.Combining the analysis method mentioned above and wavelet analysis,the authors study the kinetic properties of an IP shock propagating in the typical solar wind.Key results are:(1)The shock may be a supercritical shock with a fast magnetosonic mach number Mf=2.35.The gyrating ions are observed at the same time range with magnetic foot and overshoot structure.The shock may rely upon particle reflection mechanism for energy dissipation together with wave–particle interaction mechanisim.Ion cyclotron and mirror–mode instabilities may be excited in the downstream of the shock,since both Tp?/Tp?and ?pincrease across the shock,which may help explain why there are intense wave activities.(2)Whistler precursors with 2.0Hz<f<4.0Hz are observed near the shock,which may be driven by the whistler heat flux instabilities excited near the shock ramp.The waves may originate from the shock ramp,since the amplitudes of the wave packets decrease from the shock ramp to the upstream.This is consistent with the result that the whistler heat flux instabilities may be the intensest at the shock ramp.In addition,pitch angle scattering of the electrons resulting from the whistler heat flux instabilities and normal betatron acceleration that occurs across the shock may contribute to making the pitch angle distribution of 41.8 e V–689.2 e V electrons more isotropic.(3)Ion cyclotron waves may be driven by ion cyclotron instabilities in the downstream of the shock.Waves in the same frequency range as the ion cyclotron waves observed in the downstream are seen in the upstream of the shock.However,their polarizations are completely inverse and there is also difference between their associated wave vector angles ?k B,?k Vand ?kn.Thus,they may be different wave modes.The field–aligned beam structure and intense gyrating ions observed near the shock may help explain why the upstream wave activities are intenser than those of the 1998 August 6 shock event.No direct observational evidence of mirror–mode waves are found near the shock,although the mirror–mode instabilities may be excited across the shock.New findings of this dissertation are as follows:(1)This dissertaion presents the first analysis of kinetic properties of an IP shock inside an ICME,combining the particle(both ion and electron)distributions and wave analysis.(2)The results show how a shock modifies the plasma environment inside an ICME,which provides a new perpective at kinetic scales about ICME–ICME interaction.(3)The authors identify some of the wave modes near the shock in ambient solar wind and determine their associated excitation mechanisms,which may provide observational evidence for the theory and simulation development.