Low frequency oscillations in a plasma with spatially variable field-aligned flow

Abstract

The effects of a transverse gradient in the plasma flow velocity parallel to the ambient magnetic field are analyzed. A transverse velocity gradient in the parallel ion flow, even in small magnitude, can increase the parallel phase speed of the ion-acoustic waves sufficiently to reduce ion Landau damping. This results in a significantly lower threshold current for the current driven ion acoustic instability. Ion flow gradients can also give rise to a new class of ion cyclotron waves via inverse cyclotron damping. A broadband wave spectrum with multiple cyclotron harmonics is possible. A combination of the multiple cyclotron harmonic waves can result in spiky electric field structures with their peaks separated by an ion cyclotron period. A spatial gradient in the parallel electron flow is also considered but it is found to play a minimal role in the low frequency regime. Relevance of these to natural plasma environments is discussed.

Origin of the turbulent spectra in the high-altitude cusp: Cluster spacecraft observations

“…For field-aligned flow (such as we mostly detect here (see Fig. 1d and 1f), only electrostatic calculations have been carried out, but they suggest that waves can be excited at many harmonics of the i (Gavrishchaka et al, 2000;Ganguli et al, 2002). However, electrostatic is just a limiting case.…”

“…Figures 9 and 10 of Nykyri et al (2004) show that harmonics appear after 05:25 UT, about 2 min later than the observed large (∼100 km/s) velocity shear at 05:23 UT. It is well known that field-aligned velocity shears can excite waves at many harmonics of the i (Gavrishchaka et al, 2000;Ganguli et al, 2002). However, if this is the case it is surprising why harmonics are not observed when the local velocity shear is the strongest.…”

Identification of broad-band waves above the auroral acceleration region: Cluster observations

“…This may explain why emissions with k || >k ⊥ occur only in short bursts, and why waves with k || <k ⊥ may exist in longer bursts and are more common. We note that in an inhomogeneous plasma, including, for example, a varying particle drift along the geomagnetic field, IA waves may also exist for other temperature ratios (Ganguli et al, 2002 andGavrishchaka et al, 1999). Different directions of the k-vector correspond to different wave modes, assigned different names, but there is no sharp theoretical limit between the different modes.…”

Experiments on Plasma Turbulence Created by Supersonic Plasma Flows with Shear

“…It is well known that there are natural plasma instabilities, such as electrostatic ion acoustic waves, present at the interface of two plasmas streaming with respect to each other [6,7]. These instabilities can conceivably scatter the waves that may transmit through the near-vehicle plasma sheath.…”