Properties of MHD Fluctuations in the Heliosheath and in the Earth ’ s

Voyager 2 (V 2) observed a highly variable magnetic field in the heliosheath with complex profiles which varied from day to day. In this paper we analyze high-resolution observations of the magnetic field strength, B, and plasma density, N , measured by V 2 in the post-termination shock (TS) and unipolar regions of the heliosheath. We examine correlations between these parameters on 2-hour subintervals. Finally, we compare these correlation coefficients with the results of a similar analysis in the Earth’s magnetosheath based on 20-minute intervals from Cluster. Introduction The heliosheath exhibits magnetic field fluctuations of very large amplitudes at scales from several hours to tens of days with complex profiles. The fluctuations were described as “turbulence,” although the nature and origin of these fluctuations are not fully understood. Burlaga and Ness [2009] showed that turbulence consists of both coherent (semi deterministic) and random structures as seen in time profiles of the magnetic field strength B(t) on scales from 48 s to several hours, which vary greatly from day to day. They defined two different regions in the heliosheath, (1) the “post-TS region” observed in the interval 2007 DOY 245-301 (just behind the TS, which was crossed by V 2 at least five times from 2007 DOY 242-245) and (2) the “unipolar region” which was observed by V 2 in the interval from 2008 DOY 2-76 in which the direction of the magnetic field was nearly constant and concluded that the fluctuations observed in these two regions differ in some important respects, but share some common properties. They have suggested that in order to understand the turbulence of the heliosheath, it is important to consider whether the TS is rapidly evolving on a wide range of scales including very small scales. Liu et al. [2007] studied data from Voyager 1 (V 1) after the crossing the TS on 2004 December 16 (day 351). The magnetic fluctuations in the heliosheath were characterized by a series of depressions in the field magnitude that have been called magnetic holes [Burlaga et al., 2006a,b]. They showed that the magnetic holes observed by V 1 in the heliosheath are produced by the mirror-mode instability. The fluctuations are similar to those observed downstream of quasiperpendicular bow shocks and that have been identified as mirror-mode structures in planetary magnetosheaths [e.g., Bavassano Cattaneo et al., 1998]. The Earth’s magnetosheath is an extended inhomogeneous region characterized by a variety of waves and ion distributions. In this region, the solar wind is modified by wave-particle interactions before it encounters the Earth’s magnetic field. The existence of the foreshock imposes an asymmetry on the plasma downstream from the shock. Upstream waves are convected by the solar wind while developing into large amplitude structures that form a patchy quasi-parallel shock region through which the plasma is thermalized [Russell , 1988; Schwartz and Burgess, 1991; Thomsen et al., 1988]. Wave modes can be converted [Krauss-Varban, 1994] by the shock and new wave modes can be generated. 149 WDS'10 Proceedings of Contributed Papers, Part II, 149–155, 2010. ISBN 978-80-7378-140-8 © MATFYZPRESS