Cluster observations of high-frequency waves in the exterior cusp

Y Khotyaintsev,Y Ogawa,P Décréau,M André,B Popielawska,B Lavraud,A Vaivads,S Buchert,H Rème

doi:10.5194/angeo-22-2403-2004

Abstract

Abstract. We study wave emissions, in the frequency range from above the lower hybrid frequency up to the plasma frequency, observed during one of the Cluster crossings of a high-beta exterior cusp region on 4 March 2003. Waves are localized near narrow current sheets with a thickness a few times the ion inertial length; currents are strong, of the order of 0.1-0.5μA/m2 (0.1-0.5mA/m2 when mapped to ionosphere). The high frequency part of the waves, frequencies above the electron-cyclotron frequency, is analyzed in more detail. These high frequency waves can be broad-band, can have spectral peaks at the plasma frequency or spectral peaks at frequencies below the plasma frequency. The strongest wave emissions usually have a spectral peak near the plasma frequency. The wave emission intensity and spectral character change on a very short time scale, of the order of 1s. The wave emissions with strong spectral peaks near the plasma frequency are usually seen on the edges of the narrow current sheets. The most probable generation mechanism of high frequency waves are electron beams via bump-on-tail or electron two-stream instability. Buneman and ion-acoustic instability can be excluded as a possible generation mechanism of waves. We suggest that high frequency waves are generated by electron beams propagating along the separatrices of the reconnection region.

Highlights

Magnetospheric cusps are the primary region of magnetosheath plasma entry; they are very dynamic regions with a variable plasma structure (Haerendel and Paschmann, 1975)
We analyze the spectral character of these waves, the location of the most intense wave emissions with respect to narrow current sheets, and discuss how details of the observations are consistent with the reconnection hypothesis
The missing electron distribution function data does not allow us to observationally identify the free energy source; for us it is important that, in principle, all high frequency wave emissions are local electrostatic instabilities that do not propagate far from the generation region; only slight changes in the electron distribution or electron beam strength can create different type of spectra that are seen in our observations

Summary

Introduction

One can identify several magnetospheric regions when satellites are crossing the high altitude cusp. Langmuir and upper hybrid waves close to the reconnection X-line and separatrices are reported from simulations (Drake et al, 2003), satellite observations at the magnetopause (Matsumoto et al, 2003; Vaivads et al, 2004) and the magnetotail (Farrell et al, 2002) Electrostatic instabilities, such as lower hybrid drift instability (LHDI, Krall and Liewer (1971); Silveira et al (2002)), are commonly observed in association with reconnection current sheets. In this paper we present a multipoint observation of wave emissions in the high altitude cusp region, where wave frequencies are between the lower hybrid and electron plasma frequencies. We analyze the spectral character of these waves, the location of the most intense wave emissions with respect to narrow current sheets, and discuss how details of the observations are consistent with the reconnection hypothesis

Observations

C2 C3 C4

Discussion

Summary and conclusions