Adv. Space Res.Vol.4, No.2—3, pp.2 65—275, 1984 0273—1177/84 $0.00 + .50 Printed in Great Britain. All rights reserved. Copyright ©COSPAR RELAXATION OF PLASMA AT THE SHOCK FRONT 0. Vaisberg, S. Klimov, G. Zastenker, M. Nozdratchev, A. Sokolov, V. Smirnov, S. Savin and L. Avanov Space Research institute, U.S.S.R. Academy of Sciences, 84/32 Profsoyuznaya St., 117810 Moscow GSP—7, U.S.S.R. ABSTRACT Brief overview of previous studies of ion thermalization at the shock tran- sition is given. One non—quite typical Prognoz—8 quasi—perpendicular bow shock crossing on 11 February 1981 is considered for which high—time reso- lution data on plasma and ELF electric field fluctuations are available. Strong turbulence in LH—raage that is associated with two—stream ion motion upstream of shock transition is characterized by an exponential growth and saturation of these fluctuations at a level of ‘~‘1O0mV/zn. The heating of ions at the main shock transition is associated with pulse—like increase of these waves amplitude. Relaxation of gyrating beams downstream of the main shock transition appears to be associated with ion—cyclotrG.n waves and addi- tional heating of ions and passes through two phases: h~ydrodynaznic and kine- tic ones. Linear and time scales of the events are estimated. INTRODUCTION Collisionless shock waves in the laboratory and space plasmas are ones of most intriguing phenomena. Theoretical studies /1,2/ have shown that shocks in the magnetized plasma with low ~ are well described by hydromagnetic approximation until Alfven Mach number M~is below some critical number, M 63. The shock profile is laminar at These circumstances and the front tfi~ckness is determined by anomalous resistivity resulting from ion—sound instability of electron current /3,4/. At M~>M ion—sound instability does notprev~tthe steepening and breaking of t~esfi~ck profile as it does not provide sufficient dissipation rate. Ve- locity profile is overturning and multjcomponent flow of ions develops that leads to anomalous viscosity and to necessary dissipation rate /3,5/. The isomagnetic potential jump inside the shock profile associated with ion— sound fluctuations and preventing the velocity profile overturning for Aif— yen Mach numbers between”3 and’v5 was found in laboratory /6/. Observations of Montgomery et al. /7/ on Vela 4. satellite allowed them to reveal for the first time the second proton component upstream, inside and downstream of the shock front. This phenomenon was subsequently considered in more detail in /8—10/. Computer simulation and analytical treatment show that the part of ion flow reflects from the shock front due to combinad ac- tion of magnetic and electric field jumps at the front of strong collision— less shock /11,12,3,13—16/. For shock propagating perpendicular to ambient magnetic field the reflected ion beam returns to the shock due to Lorenz force in the upstream flow /13,15/. More detailed analysis of the gyrating beam kinematics was performed recently for different geometry of shock and magnetic field that showed that condition of specular reflection can be considered as satisfactory explanation although significant deviations were observed /17—19/. Phenomenological classification of the collisionless shocks was given in /8,20,21/ in terms of upstream MA,J~,P IT. and O~, the angle between the vector of magnetic field and the~norma!t~ t~esff~ck. Depending on O~, the shocks ase devided by quasi—parallel, 8 .~4.5 , and. quasi—porpendicul&r, 8 ~4.5 . M and fts values affect the m~netic field profile and the turbu— le~e near tAe shock front • One most important difference between the quasi—parallel shocks and. the quasi—perpendicular ones is that for reflected ions do return back to the shock front and their energy is large 265