Satellite in situ measurements made by the Dynamics Explorer 2 (DE 2) satellite were utilized to describe the nature of plasma structuring at high latitudes caused by the gradient drift instability process. Specifically, by using noon‐midnight and dawn‐dusk orbits of the DE 2 satellite it was found possible to study the simultaneous density and electric field spectra of convecting large‐scale (approximately hundreds of kilometers) plasma density enhancements in the polar cap known as “patches”) in directions parallel and perpendicular to their antisunward convection. Distinct differences were noted in the behavior of the ac and dc electric field structure and short‐scale (<125 m) density irregularities in these two mutually orthogonal directions perpendicular to the geomagnetic field. However, since these two orthogonal directions were not sampled simultaneously, the observed differences cannot be unequivocally related to the direction of convection. Structured plasma density enhancements in the auroral oval (known as “blobs”) were found to have considerable power spectral density at these short scales in the presence of significant Pedersen and Hall conductances in the 10‐ to 20‐mho range. While density irregularity amplitudes (ΔN/N)rms were found to be as large as 15–20% using 8‐s samples of the DE 2 data, the corresponding dc electric field fluctuation ΔE was found to be less than a few millivolts per meter for both patches and blobs. This (ΔN/N)RMS vis‐a‐vis ΔE behavior for the gradient drift process provided a fairly dramatic contrast with velocity shear driven processes where the ΔE magnitudes were found to be at least an order of magnitude larger for the same levels of density irregularities. The electric field spectra for the moderate shear category discussed by Basu et al. (1988a) were also found to have a significantly different spectral index as compared to such spectra associated with the gradient drift process. The results of this paper together with those of Basu et al. (1988a) provide fairly conclusive 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.

The extended solar minimum conditions in 2008 and 2009 presented an opportunity to investigate the ionosphere at lower solar activity levels than previously observed. The Coupled Ion Neutral Dynamics Investigation (CINDI) Ion Velocity Meter (IVM) instrument onboard the Communication/Navigation Outage Forecasting System is used to construct the median meridional (vertical) ion drifts, ion densities, and O+ concentrations during periods of low geomagnetic activity for four characteristic seasons each year spanning late 2008 to 2010. The presence of a large semidiurnal component in the ion drift variation at the equator produced significant differences from typical ionospheric conditions. Instead of upward drifts during the day and downward drifts at night, downward drifts in the afternoon and upward drifts near midnight are observed. This semidiurnal component is present in all seasons though it is strongest during the solstice seasons. It is shown that upward drifts at night correspond to regions with a high occurrence of postmidnight irregularities during the December 2008 and June 2009 solstices. A comparison with vertical ion drifts observed by the Jicamarca Radio Observatory supports the methodology used to extract meridional drifts from the IVM.

Typically the solar radio emission at 10.7 cm is used to scale the critical euv radiation that is absorbed by the Earth's neutral atmosphere. In the latter half of 2008 this radio emission from the Sun was at the lowest levels seen in the last 50 years and the persistence of these low levels has never been recorded before. Here we show that these uniquely low levels of solar radiation produce similarly unique behavior in the Earth's ionosphere and the upper atmosphere. Most remarkably, the altitude extent of the ionosphere is significantly smaller than our present reference models would predict for these levels of solar activity. The transition height resides near 450 km at night and rises to only 850 km during the daytime. At night, this unusually contracted ionospheric shell around the equator has a temperature of only 600 K and prior to sunrise the ion number densities at the transition height fall below 104 cm−3.

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).

Abstract.Dynamics Explorer 2 (DE 2) spacecraft data are used to detect and characterize polar cap "ionization patches", loosely defined as large-scale (> 100 km) regions where the F region plasma density is significantly enhanced ( _>100%) above the background level. These patches are generally believed to develop in or equatorward of the dayside cusp region and then drift in an antisunward direction over the polar cap. We have developed a flexible algorithm for the identification and characterization of these structures, as a function of scale-size and density enhancement, using data from the retarding potential analyzer, the ion drift meter, and the langmuir probe on board the DE 2 satellite. This algorithm was used to study the structure and evolution of ionization patches as they cross the polar cap. The results indicate that in the altitude region from 240 to 950 km ion density en-