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

Abstract

Simultaneous satellite in situ measurements of density (ΔN/N) and electric field fluctuation (ΔE) 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 ΔN/N and ΔE 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 ΔE spectra steepened to an index of −3±0.5 while the spectral index of ΔN/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 ΔE 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 ΔN/N and ΔE 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).