HR: 16:35h
AN: SM44A-03    [Abstracts]
TI: Polytropic Equations of State in a Magnetized Plasma
AU: * Jaeger, T
AF: University of Iowa, Room 203 Van Allen Hall Jefferson at Dubuque Streets, Iowa City, IA 52242 United States
AU: Scudder, J D
EM: jack-scudder@uiowa.edu
AF: University of Iowa, Room 203 Van Allen Hall Jefferson at Dubuque Streets, Iowa City, IA 52242 United States
AB: Closure in the low frequency MHD regime is an old problem. Polytropic one and two fluid closures have been assumed in collisionless MHD codes and low frequency dispersion analysis; double adiabatic closures have also been explored in spite of the presence of parallel electric fields and heat flow. The collisionally motivated adiabatic value for $\gamma \simeq 5/3$ is often inserted. Two years of Polar data collected in the high density regions of the magnetosheath/ entry layers have been analyzed in 15min segments to determine (i) if polytropic relations characterize the electrons and/or ions and (ii) what values of $\gamma_{e,i}$ are implied. Of the possible 7900 intervals attempted, successful fits were obtained for ions in 239 intervals, while 1552 electrons intervals had successful fits. Frequently there was insufficient dynamic range in the density to determine the polytropic index. However, only 122 intervals had good simultaneous polytropic characterizations of ions and electrons. Of those intervals with common fits there is a general trend for higher electron $\gamma_e$ to accompany increases in $\gamma_i$. Most surprising are the distribution of values for $\gamma_j$ ; the good electrons fits are especially bimodal, with a strong peak near 0 and another in the vicinity of 5/3, but a deep void in between. With poorer statistics the ions also show this behavior. The incidence of $\gamma_i = 5/3$ is rare, representing less than 5% of the good ion fits that were possible. Examples of representative observations in the newly defined regime $\gamma_e < 1; \gamma_i < 1$ that have no classical analogue will be presented. A simple model of circumstances that permit these observations will be presented.
DE: 7800 SPACE PLASMA PHYSICS
DE: 7827 Kinetic and MHD theory
DE: 7859 Transport processes
DE: 7871 Waves and instabilities
DE: 2752 MHD waves and instabilities
SC: SPA-Magnetospheric Physics [SM]
MN: 2004 AGU Fall Meeting