Simultaneous density and electric field fluctuation spectra associated with velocity shears in the auroral oval

Abstract

Simultaneous satellite in situ measurements of density (AN/N) and electric field fluctuation (AE) spectra in the high-latitude ionosphere are presented using two orbits of Dynamics Explorer 2 (DE 2) data traversing, respectively, the F region at 350 km altitude and the topside ionosphere at 900 km altitude. The spectral study was primarily confined to large structured velocity regions in the auroral oval. By means of the very complete set of energetic particle, dc and ac electric field, field-aligned current, thermal plasma density, and temperature measurements available from DE 2, we were able to identify two categories of spectra associated with velocity shears irrespective of the height of the satellite. The first category was observed in very intense velocity shear regions of shear frequencies • 10 Hz in conjunction with large field-aligned current densities. Under these conditions the spatial spectra of AN/N and AE had identical power law indices of --1.8 _+ 0.2 between scale lengths of approximately 10 km and 300 m. At scale lengths shorter than 300 m the AE spectra steepened to an index of --3 _+ 0.5 while the spectral index of AN/N remained close to its original value of approximately --1.8 _+ 0.2, with large power spectral densities observed down to 10 m scale lengths. The second category was observed in more moderate velocity shear regions of shear frequencies • 1 Hz in conjunction with weak field-aligned currents. In this case the slopes of the density spectra were essentially unchanged, while the AE spectra had a much steeper slope of --3 _+ 0.5 between 10 km and a few hundred meters. Other factors identifying the two categories are as follows. The first category of spectra was characterized by the existence of upward flowing ions with conic distributions energized to 30 eV and possibly O + ion cyclotron waves and large electron temperature enhancements. The second category of spectra was associated with wave activity in the 4-to 16-kHz range, most probably O + lower hybrid waves, and occasionally large ion temperature enhancements. The observations of AN/N and AE spectral behavior are compared to recent work on two-dimensional plasma turbulence theory and nonlinear simulations of the collisional Kelvin-Helmholtz (KH) instability. In particular, the spectral behavior associated with the moderate velocity shear category agrees well with some recent computations of the spatial power spectra of the KH instability (Keskinen et al., 1988).

Wave power studies of cusp crossings with the Polar satellite

“…Other instabilities such as the gradient drift instability [Cerisier et al, 1985], shear-driven turbulence [Basu et al, 1988[Basu et al, , 1994 …”

Ion temperature anisotropy effects on threshold conditions of a shear-modified current driven electrostatic ion-acoustic instability in the topside auroral ionosphere

“…Satellite data from Dynamics Explorer (DE) 1 and 2 also showed evidence of parallel shears co-located with perpendicular shears (e.g. Basu et al, 1988;Loranc et al, 1991;Lu et al, 1992;Kivanc and Heelis, 1999, and the review by Koepke and Reynolds, 2007). The occurrence of simultaneous and co-located parallel and perpendicular shears suggests that the conditions behind one type of shears may be involved with the other one (Koepke and Reynolds, 2007).…”

On the collocation between dayside auroral activity and coherent HF radar backscatter

“…These have been proposed to be the dominant plasma structuring mechanisms in the southward IMF polar cap ionosphere. Velocity shear driven plasma instabilities are now also known to occur for suciently severe velocity shears (Basu et al, 1988(Basu et al, , 1990 and are believed to dominate the northward IMF polar cap. Keskinen et al (1988) has applied a Kelvin-Helmholtz instability theory to these latter instabilities, adding the re®nement of ionospheric-magnetospheric electrical coupling.…”