Observations obtained by the Freja satellite at altitudes around 1700 km in the high-latitude magnetosphere are used to study ion energization perpendicular to the geomagnetic field. Investigations ...

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JA00855

Ion energization mechanisms at 1700 km in the auroral region

A kinetic theory in the form of an integral equation is provided to study the electrostatic oscillations in a collisionless plasma immersed in a uniform magnetic field and a nonuniform transverse electric field. In the low temperature limit the dispersion differential equation is recovered for the transverse Kelvin-Helmholtz modes for arbitrary values of K parallel, where K parallel is the component of the wave vector in the direction of the external magnetic field assumed in the z direction. For higher temperatures the ion-cyclotron-like modes described earlier in the literature by Ganguli, Lee and Plamadesso are recovered. In this article, the integral equation is reduced to a second-order differential equation and a study is made of the kinetic Kelvin-Helmholtz and ion-cyclotron-like modes that constitute the two branches of oscillation in a magnetized plasma including a transverse inhomogeneous dc electric field.

https://apps.dtic.mil/sti/pdfs/ADA193404.pdf

Kinetic theory for electrostatic waves due to transverse velocity shears

Recent observations indicate that low-altitude (below 1500 km) ion energization and thermal ion upwelling are colocated in the convective flow reversal region. In this region the convective velocity V(sub perpendicular) is generally small but spatial gradients in V(sub perpendicular) can be large. As a result, Joule heating is small. The observed high level of ion heating (few electron volts or more) cannot be explained by classical Joule heating alone but requires additional heating sources such as plasma waves. At these lower altitudes, sources of free energy are not obvious and hence the nature of ion energization remains ill understood. The high degree of correlation of ion heating with shear in the convective velocity (Tsunoda et al., 1989) is suggestive of an important role of velocity shear in this phenomenon. We provide more recent evidence for this correlation and show that even a small amount of velocity shear in the transverse flow is sufficient to excite a large-scale Kelvin-Helmholtz mode, which can nonlinearly steepen and give rise to highly stressed regions of strongly sheared flows. Futhermore, these stressed regions of strongly sheared flows may seed plasma waves in the range of ion cyclotron to lower hybrid frequencies, which are potential sources for ion heating. This novle two-step mechanism for ion energization is applied to typical observations of low-altitude thermal ion upwelling events.

Coupling of microprocesses and macroprocesses due to velocity shear: An application to the low-altitude ionosphere

The nonlinear evolution of the electrostatic Kelvin-Helmholtz instability, resulting from velocity-sheared plasma flows perpendicular to an ambient magnetic field, was studied including Pedersen conductivity effects (i.e., ion-neutral collisions). The Kelvin-Helmholtz instability develops in a distinctly different manner in the nonlinear regime with Pedersen coupling than without it. Specifically, the Pedersen coupling effects, in conjunction with a neutral wind and density gradient, (1) result in an increased time scale for Kelvin-Helmholtz instability wave growth, (2) inhibit Kelvin-Helmholtz vortex formation, (3) lead to nonlinear structures that can be described as breaking waves, and (4) generate, in the nonlinear regime, small-scale turbulence by means of secondary instabilities growing on the primary waves. The spatial power spectra of the electrostatic potential and density fluctuations, and it is found that there is a tendency for the potential and density to become shallower when Pedersen conductivity effects are included. The results are compared with recent Dynamics Explorer satellite observations of velocity sheared plasma flows in the high-latitude, near-earth space plasma and good agreement is found.

Nonlinear evolution of the Kelvin-Helmholtz instability in the high-latitude ionosphere. Interim report

Using results of the in situ measurements made by the DE 2 satellite, the nature of plasma structuring at high latitudes, caused by the gradient drift instability process, is described. Using noon-midnight and dawn-dusk orbits of the DE 2 satellite, it was possible to examine the simultaneous density and electric field spectra of convecting large-scale plasma density enhancements in the polar cap known as 'patches', in directions parallel and perpendicular to their antisunward convection. The results provide evidence for the existence of at least two generic classes of instabilities operating in the high-latitude ionosphere: one driven by large-scale density gradients in a homogeneous convection field with respect to the neutrals, and the other driven by the structured convection field itself in an ambient ionosphere where density fluctuations are ubiquitous.

Plasma structuring by the gradient drift instability at high latitudes and comparison with velocity shear driven processes

The electrostatic ion instabilities are studied for oblique propagation in the presence of magnetic field-aligned currents and transverse localized electric fields in a weakly collisional plasma. The presence of transverse electric fields result in mode excitation for magnetic field aligned current values that are otherwise stable. The electron collisions enhance the growth while ion collisions have a damping effect. These results are discussed in the context of observations of low frequency ion modes in the auroral ionosphere by radar and rocket experiments.

Electrostatic ion instabilities in the presence of parallel currents and transverse electric fields

Consideration is given to the suggestion by Basu and Coppi (1988, 1989) that the DE-2 observations of broadband turbulence associated with sheared ion flows can be explained by electrostatic waves driven by a sheared ion flow along the magnetic field. It is pointed out that such a theory ignores the stronger shear in the ion flow transverse to the magnetic field, and that, when this shear is taken into account, the modes described by Basu and Coppi are easily destabilized. The theory of Basu and Coppi is shown to break down even when the shear in the parallel flow exceeds the shear in the transverse flow.

d.c. electric field stabilization of plasma fluctuations due to a velocity shear in the parallel ion flow

Observations from the DE 2 satellite indicate the existence of a broadband turbulence associated with sheared ion flows. A recent attempt [Basu and Coppi, 1988, 1989] to explain the observed spectrum by electrostatic waves that are driven by a sheared ion flow along the magnetic field ignores the stronger shear in the ion flow transverse to the magnetic field. We show that when the shear in the transverse ion flow is also considered the modes described by Basu and Coppi [1988, 1989] are easily stabilized and their theory breaks down even when the shear in the parallel flow exceeds the shear in the transverse flow. Hence, the explanation of the DE 2 observations based on these modes is questionable.

G. Ganguli

Papers

Electrostatic ion instabilities in the presence of parallel currents and transverse electric fields

d.c. electric field stabilization of plasma fluctuations due to a velocity shear in the parallel ion flow

d.c. electric field stabilization of plasma fluctuations due to a velocity shear in the parallel ion flow. [in ionosphere