The microphysics of collisionless shocks.

Abstract

Shock waves in interplanetary (IP) space are of considerable interest due to their potential to damage ground based electronic systems and their ability to energize charged particles. The energization of charged particles at IP shocks has the obvious extrapolation to supernova shock waves, which are thought to be a candidate for generating the most energetic particles in the universe. The observations and theory behind collisionless shock wave evolution suggest that IP shocks should, for the most part, be stable structures which require energy dissipation. In a regular fluid, like our atmosphere, energy dissipation is accomplished through binary particle collisions transferring the loss of bulk flow kinetic energy to heat. Plasmas are mostly collisionless fluids, thus requiring other means by which to dissipate energy. The studies herein were performed using wave and particle data primarily from the Wind spacecraft to investigate the microphysics of IP shock energy dissipation mechanisms. Due to their lower Mach numbers, more simplified geometry, and quasiperpendicular nature, IP shock waves are an excellent laboratory to study wave-particle related dissipation mechanisms. Utilization of multiple data sets from multiple high time resolution instruments on board the Wind spacecraft, we have performed studies on the transition region microphysics of IP shocks. The work began with a statistical study of high frequency (&1 kHz) waveform capture data during 67 IP shocks with Mach numbers ranging from ∼1–6 found ion-acoustic wave amplitudes correlated with the fast mode Mach number and shock strength. The ion-acoustic waves (IAWs) were estimated to produce anomalous resistivities roughly seven orders of magnitude above classical estimates. Another study was an examination of low frequency waves (0.25 Hz < f < 10 Hz) at five quasi-perpendicular IP shocks found the wave modes to be consistent with oblique precusor whistler waves at four of the events. The strongest event in that study had low frequency waves consistent with shocklets. The shocklets are seen simultaneously with diffuse ion distributions. Both the shocklets and precursor whistlers are seen simultaneously with anisotropic electron distributions unstable to whistler anisotropy and heat flux instabilities. The IP shock with upstream shocklets showed much stronger electron heating across the shock ramp than the four events without upstream shocklets. Further investigation of the atypical IP shock found the strong heating to be associated with large amplitude (> 100 mV/m) solitary waves and electron Bernstein waves. The observed heating and waveforms are likely due to instabilities driven by the free energy provided by reflected ions at this supercritical IP shock, not the DC macroscopic fields. The particle heating observed for the event with shocklets was observed to be different from other events with similar shock parameters, suggesting a different dissipation mechanism. The work presented in this thesis has helped increase the understanding of the microphysics of IP shocks in addition to raising new questions regarding the energy dissipation mechanisms dominating in the ramp regions. The initial work focused on a statistical study of high frequency waveforms in IP shock ramps. The study results suggested a re-evaluation of the relative importance of anomalous resistivity due to wave-particle interactions. This assertion was further strengthened by the atypical particle heating observed in the 04/06/2000 event which we claimed clearly showed a dependence on the observed waveforms. Thus, the nearly ubiquitous observations of large amplitude IAWs in the ramp regions of IP shocks raises doubts about ignoring these high frequency fluctuations. In addition to these findings, we also observed a low frequency wave mode which is only supposed to exist upstream of quasi-parallel shocks with small radii of curvatures. All of these findings have increased our knowledge of collisionless shock energy dissipation, but they have raised many questions regarding our current theories. We have raised doubts regarding the use of the solar wind electron distributions as one particle population. We have showed evidence to support the energy dependence of wave-particle interactions between low frequency whistler waves and ≤1 keV electrons. Thus, we conclude that in the analysis of IP shocks the microphysics can no longer be disregarded.