Cluster observations of high-frequency waves in the exterior cusp

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 0.1-0.5 µA/m 2 (0.1-0.5 mA/m 2 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 1 s. 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.

Energetic electrons in the exterior cusp: identifying the source

“…Correspondence to: B. M. Walsh (bwalsh@bu.edu) The CDC often hosts strong wave activity ranging from ultra low frequencies Le et al, 2001) up to the electron cyclotron plasma frequencies (Pickett et al, 2001;Khotyaintsev et al, 2004). The waves have been shown to be both right and left-handed polarized (Nykyri et al, 2004).…”

Anisotropic scaling features and complexity in magnetospheric-cusp: a case study

“…As already mentioned, the magnetic field in the magnetosheath and in the cusp has a very complex topology, resulting in a "Turbulent Boundary Layer" (TBL) (Savin et al, 1998). TBL is found to be a permanent feature (Klimov et al, 1997;Savin et al, 2002a), and is characterized by transient, highly variable, flows of heated magnetosheath plasma (Savin et al, 2002b,c), an irregular magnetic field, and strong plasma waves (Khotyaintsev et al, 2004). The irregular magnetic field can tentatively be associated with strong (weak) magnetic field structures where magnetic force (plasma pressure) is predominant.…”

Cluster observations of particle acceleration up to supra-thermal energies in the cusp region related to low-frequency wave activity – possible implications for the substorm initiation process

“…The latter process can be identified by the observation of FTEs where recently reconnected flux tubes filled with magnetosheath-like plasma are being dragged around the magnetosphere after reconnection (e.g. Khotyaintsev et al, 2004). Similarly, Błȩcki et al (2003) has suggested that kinetic Alfvén waves at frequencies below the ion cyclotron frequency could be the result of plasma acceleration at a reconnection site.…”

“…Yet, they are very dynamic with variable plasma structure (Haerendel and Paschmann, 1975). Cusps are also associated with strong plasma wave activity ranging from very low frequencies (Le et al, 2001;Savin et al, 2004) up to electron cyclotron and plasma frequencies (Pickett et al, 2001;Khotyaintsev et al, 2004). In these regions fluxes of accelerated magnetosheath-like plasma particles are observed.…”