Plasma Measurements Near the Earth’s Bow Shock: Vela 4
The supersonic flow of solar wind plasma past the earth’s magnetic field is now known to produce a well defined, essentially permanent standing shock wave (Ness et al., 1964; Gosling et al., 1967; Burlaga and Ogilvie, 1968). Because collisional mean free paths in the solar wind are much larger than the shock’s dimensions and therefore cannot play a direct role in the shock’s formation, collective (wave particle) interactions must provide the necessary dissipation. Measurements of the thickness of the magnetic transition (Heppner et al., 1967) indicate that the shock is too thin (much less than an ion cyclotron radius) to result from magnetic wave turbulence such as Alfvén wave turbulence suggested by Kennel and Sagdeev (1967). Tidman(1967) indicated that a likely alternative would be electrostatic ion wave turbulence driven by interpenetrating ion beams, and measurements of high frequency electric and magnetic field fluctuations published by Fredricks et al. (1968) and Scarf et al. (1970) show that electrostatic turbulence is likely to be the dominant dissipation mechanism. However, a necessary requirement for the instability of electrostatic waves with respect to small currents or interpenetrating ion beams is T e≫T i, and recent observations (Montgomery et al., 1969; Montgomery et al., 1970) show that the bow shock remains well defined for T e/T i as small as 0.6. This paper briefly presents measurements of electron and positive ion velocity distributions that provide evidence for electron preheating near the leading edge of the shock. The resulting increase in T e/T i may be large enough to cause significant destabilization of ion waves in the region where ion heating is observed to occur.
KeywordsSolar Wind Solar Wind Plasma Electron Heating Electrostatic Wave Wave Turbulence
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