Magnetic Decreases (MDs) and mirror modes: two different plasma β changing mechanisms

B T Tsurutani,O P Verkhoglyadova,E Echer,F L Guarnieri,G S Lakhina

doi:10.5194/npg-17-467-2010

Abstract

Abstract. We discuss two different physical processes that create localized high β plasma regions. One is nonlinear wave-steepening, generating magnetic decreases (MDs) by a ponderomotive force. The other is the mirror instability generating alternating high and low β plasma regions. It is demonstrated that MDs and mirror modes are observationally quite different structures. MDs spatially occur in interplanetary space and mirror modes primarily in planetary magnetosheaths. MDs are characterized by: 1) variable (exponentially decreasing number with increasing) angular changes, 2) variable (exponentially decreasing) thicknesses, and 3) no characteristic inter-event spacings. In sharp contrast, mirror modes are characterized by: 1) little or no angular changes across the structures, 2) a characteristic scale size, and 3) are quasiperiodic in nature. Arguments are presented for the recently observed magnetic dips in the heliosheath being mirror mode structures. The sources of free energy for instability are discussed. Both structures are important for energetic particle transport in astrophysical and heliospheric plasmas.

Highlights

Arguments are presented for the recently observed magnetic dips in the heliosheath being mirror mode structures
Burlaga did not intend for readers to assume that magnetic dips in interplanetary space and in the heliosheath were necessarily generated by the same physical process
The potential physical mechanisms leading to their generation have been discussed with main references included so that interested researchers may contribute to further understanding of these important physical processes

Summary

Introduction

Arguments are presented for the recently observed magnetic dips in the heliosheath being mirror mode structures. The sources of free energy for instability are discussed. Both structures are important for energetic particle transport in astrophysical and heliospheric plasmas. Magnetic dips have been detected by the Voyager spacecraft in the heliosheath (Burlaga et al, 2006, 2007). Burlaga et al (2006) have called these structures “magnetic holes” as a descriptive term Burlaga did not intend for readers to assume that magnetic dips in interplanetary space and in the heliosheath were necessarily generated by the same physical process. We will show that the heliosheath structures are different and are generated by a different mechanism, the mirror instability. The expression for instability is Eq (1): β⊥/β|| > 1 + 1/β⊥

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