Large-scale fluctuations of PSBL magnetic flux tubes induced by the field-aligned motion of highly accelerated ions

E E Grigorenko,M Shevelev,T M Burinskaya,J.-A Sauvaud,L M Zelenyi

doi:10.5194/angeo-28-1273-2010

Abstract

Abstract. We present a comprehensive analysis of magnetic field and plasma data measured in the course of 170 crossings of the lobeward edge of Plasma Sheet Boundary Layer (PSBL) in the Earth's magnetotail by Cluster spacecraft. We found that large-scale fluctuations of the magnetic flux tubes have been registered during intervals of propagation of high velocity field-aligned ions. The observed kink-like oscillations propagate earthward along the main magnetic field with phase velocities of the order of local Alfvén velocity and have typical wavelengths ~5–20 RE, and frequencies of the order of 0.004–0.02 Hz. The oscillations of PSBL magnetic flux tubes are manifested also in a sudden increase of drift velocity of cold lobe ions streaming tailward. Since in the majority of PSBL crossings in our data set, the densities of currents corresponding to electron-ion relative drift have been low, the investigation of Kelvin-Helmholtz (K-H) instability in a bounded flow sandwiched between the plasma sheet and the lobe has been performed to analyze its relevance to generation of the observed ultra-low frequency oscillations with wavelengths much larger than the flow width. The calculations have shown that, when plasma conditions are favorable for the excitation of K-H instability at least at one of the flow boundaries, kink-like ultra-low frequency waves, resembling the experimentally observed ones, could become unstable and efficiently develop in the system.

Highlights

The Plasma Sheet Boundary Layer (PSBL) represents an interface region separating hot and mostly isotropic plasma of the Central Plasma Sheet (CPS) and the lobe region, which is void of hot plasma but may contain cold ion beams slowly moving tailward (e.g. Sharp et al, 1981)
The events of the large Poynting flux carried by Alfven waves in the near-Earth PSBL at geocentric distances 4–6 RE were reported by Wygant et al (2000) and by Keiling et al (2000, 2001) for active geomagnetic intervals
In the present paper we report the observations of the Alfven waves propagating earthward in the lobe-PSBL interface with significantly lower frequencies, than the ones studied in the papers mentioned above

Summary

Introduction

The Plasma Sheet Boundary Layer (PSBL) represents an interface region separating hot and mostly isotropic plasma of the Central Plasma Sheet (CPS) and the lobe region, which is void of hot plasma but may contain cold ion beams slowly moving tailward (e.g. Sharp et al, 1981). It was revealed that the sudden increases of drift velocity of cold lobe ions observed near the lobeward edge of the PSBL were always associated with the propagation of high velocity field-aligned plasma flows and kink-like magnetic fluctuations propagating earthward with the practically local Alfven velocity. Electron pitch-angle distribution functions measured within the interval of interest by Cluster-2 (see Fig. 2) are more or less symmetrical along the magnetic field In this event, the high-energy PSBL population may be considered as a high velocity fieldaligned plasma flow, rather than a high velocity ion beam. This means that in these PSBL crossings ion currents were compensated by field-aligned electron currents and the high velocity plasma flows rather than beams propagate towards the Earth in the PSBL-lobe interface

Kelvin-Helmholtz instability in a bounded flow

Discussion

Findings

Conclusion