FAST observations of discrete electrostatic waves in association with down‐going ion beams in the auroral zone

Abstract

[1] Ion beams flowing downward, into the ionosphere, along the Earth's magnetic field have frequently been observed by the FAST satellite in the auroral zone. These discrete downward moving ion beams (DFI) have been characterized by Klumpar et al. [1999] who interpreted the horseshoe-shaped distributions as being consistent with acceleration in a parallel potential drop above the satellite, followed by motion into a region of increased magnetic field strength. The down-flowing ion beams are associated with an intense narrowband electrostatic emission at the lower hybrid frequency, polarized perpendicular to the geomagnetic field. Hydrogen cyclotron harmonics both above and below the lower hybrid frequency are also very common. These are the first observations of down-flowing ions and associated waves outside of the cusp, and the physical mechanism producing the ions is very different from the one associated with cusp ion injections. The DFI events that had a monotonic increase in energy were associated with a clear field-aligned current signature. The DFI densities were usually $5-10/cc, whereas the background plasma had densities up to 100/cc. The wave and DFI observations are consistent with linear dispersion relation calculations and simulations with ion ring distributions that show that the instability is due to coupling of the ion Bernstein waves to the lower hybrid wave. In addition, for a few events, electrostatic ion cyclotron waves were observed. Such waves are usually associated with up-going ion beams and have not previously been seen with DFI, which have a very different shape in the distribution.

Ion Bernstein waves in a plasma with a kappa velocity distribution

“…. 3,8,24,25 Depending on the velocity distribution assumed, various authors have discussed ion cyclotron harmonic instability that can be generated by beams, 26 loss-cone, 27 or subtracted Maxwellian distributions, 8 although it should be noted that in some cases, authors refer to the modes as being ion Bernstein waves, even though k k =k ? is small but non-zero.…”

“…[1][2][3][4] In many cases, their detection is confined to a rather small angular wedge, centred on the geomagnetic equator, 1,2 suggestive of propagation perpendicular (or nearly perpendicular) to the ambient magnetic field. The fact that their frequency structure is governed by the ion gyrofrequency and its harmonics and that they propagate perpendicularly to the magnetic field make the ion Bernstein 5 wave a strong candidate for the production of these waves.…”

Wave particle interactions in the high-altitude polar cusp: a Cluster case study

“…Bouhram et al, 2003). Ion beams (Cattell et al, 2002), temperature anisotropy (André et al, 1986) and velocity shears (Lakhina, 1990) are plasma instabilities commonly observed. Bernstein modes and Alfvén modes obviously interact with particles in this region (Kintner et al, 1986;Chaston et al, 2004).…”

Integrable, oblique travelling waves in quasi-charge-neutral two-fluid plasmas

“…Cattel et al, 2002, Mozer et al, 1997, Ergun et al, 1998, and Pickett et al, 2003, Dubinin et al, 2007 has stimulated the development of theoretical models to explain the observations based on travelling waves in multi-fluid plasmas (e.g. Sauer et al, 2001, Verheest et al, 2004, Webb et al, 2005, Dubinin et al, 2007.…”