Ion Temperature Anisotropy Thresholds in the Magnetosheath
Wednesday, 3 September 2014: 9:45 AM
Regency Ballroom (Hyatt Regency)
Naiguo Lin1, Christopher Carew Chaston1, Jay Johnson2, Khan-Hyuk Kim3, Ensang Lee4, James P McFadden1, George K Parks5, Jungjoon Seough6, Pavel M. Travnicek7 and Peter Haesung Yoon8, (1)Univ California Berkeley, Berkeley, CA, United States, (2)Princeton Plasma Physics Lab, Princeton, NJ, United States, (3)Kyung Hee University, Yongin, South Korea, (4)Dept. of Astronomy and Space Science, Kyung Hee University, Yongin, Gyeonggi, South Korea, (5)Univ California, Berkeley, CA, United States, (6)Kyung Hee Univ., Yongin, South Korea, (7)University of California Berkeley, Berkeley, CA, United States, (8)Univ Maryland, College Park, MD, United States
Abstract:
Observations of ion temperature anisotropy boundaries in the magnetosheath are studied and compared with the theoretical stability thresholds. Distributions of wave parameters including |dB||/B0|, |dBperp/B0|, and the magnetic compressibility, dB||2/( dB||2+ dBperp2), on the Tperp/T|| vs beta|| plane are examined.  It is found that for compressional waves, dB||, there exist enhancements at temperature anisotropy larger than expected mirror mode threshold, which may indicate evolving process of the unstable plasma, i.e. fluctuations of mirror mode instability before it reaches saturation.  The transverse variations are bounded by the threshold curve of the electromagnetic ion cyclotron (EMIC) wave mode for Tperp>T|| plasma.  These EMIC fluctuations seem to have compressional component, which is likely to happen when the waves propagate obliquely.  There also exist plasma regions in the magnetosheath where T|| > Tperp and beta||>>1, with intense fluctuations. These fluctuations are mostly transverse. They seem to be restricted by firehose instability thresholds, which is largely unstudied in magnetosheath plasma study.  Simulation of the temperature anisotropy-driven instabilities with time-varying local magnetic field shows that evolving mirror mode fluctuations occur at Tperp>T|| above the mirror mode curve, while in the T|| > Tperp and beta||>>1 region the fluctuations are confined by the firehose instabilities thresholds. These simulation results are consistent with the observations and justify our interpretation.
