Abstract. We analyzed measurements of ion number density made by the retarding potential analyzer aboard the Atmosphere Explorer-E (AE-E) satellite, which was in an approximately circular orbit at an altitude near 300 km in 1977 and later at an altitude near 400 km. Large-scale (>60 km) density measurements in the high-altitude regions show large depletions of bubble-like structures which are confined to narrow local time, longitude, and magnetic latitude ranges, while those in the low-altitude regions show relatively small depletions which are broadly distributed in space. For this reason we considered the altitude regions below 300 km and above 350 km and investigated the global distribution of irregularities using the rms deviation z•V/N over a path length of 18 km as an indicator of overall irregularity intensity. Seasonal variations of irregularity occurrence probability are significant in the Pacific regions, while the occurrence probability is always high in the Atlantic-African regions and is always low in the Indian regions. We find that the high occurrence probability in the Pacific regions is associated with isolated bubble structures, while that near 0 ø longitude is produced by large depletions with bubble structures which are superimposed on a large-scale wave-like background. Considerations of longitude variations due to seeding mechanisms and due to F region winds and drifts are necessary to adequately explain the observations at low and high altitudes. Seeding effects are most obvious near 0 ø longitude, while the most easily observed effect of the F region is the suppression of irregularity growth by interhemispheric neutral winds.

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⊥ is generally small but spatial gradients in V⊥ 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. Furthermore, 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 novel two‐step mechanism for ion energization is applied to typical observations of low‐altitude thermal ion upwelling events.

Ion composition data from the Defense Meteorological Satellite Program (DMSP) F10 have been averaged by geographic longitude and dip latitude for the months of June, September, and December 1993. The data were taken under near solar minimum conditions. Near 800 km at two fixed local times near 0920 hours and 2120 hours, and at all longitudes, significant variation in local time and season are found. Longitude variations are consistent with modulation of the F peak height by meridional and zonal neutral winds. The components of these winds parallel to the magnetic field lines act to raise and lower the height of the F peak and, additionally, at night, to modulate the plasma decay rate. Zonal winds were found to have significant effects in the longitude regions 150°E to 270°E and 300°E to 360°E, where the magnetic declination is significant. Under solstice conditions, the summer to winter meridional winds play a dominant role in regulating the F peak height, with the zonal winds enhancing or opposing the effects of the meridional winds at longitudes with significant magnetic declination. Zonal winds dominate the regulation of the F peak height near equinox, when the meridional winds are fairly symmetric about the dip equator. The longitude variations are most clearly seen in the O+ and H+ concentrations when O+ is the dominant ion and is in equilibrium with H+. These conditions were found during the daytime during all seasons. H+ is frequently the dominant ion near 800 km, and at night, the longitudinal variations clearly seen in the O+ concentrations were not as easily seen in the H+ concentrations due to the larger scale height of H+.

The results of a simulation of the global atmospheric dynamo are presented and compared to observations of the ion drift at Jicamarca and the drifts calculated from previous dynamo models. This simulation produces a global plasma distribution which is self‐consistent with the global potential distribution. We examine the dynamo‐generated potential distribution and the associated E × B drifts induced upon the plasma for two F region winds and a (1,‐2) tide in the E region. By using simple diurnal winds and tides and making the plasma and potential distributions self‐consistent, the agreement of the calculated E × B with the observed drifts at Jicamarca is improved. The use of an F region wind derived from the DE 2 satellite (Herrero and Mayr, 1986) can further improve the agreement between the calculated E × B drifts and the observed low‐latitude plasma drifts. These results lead to the conclusion that the F region dynamo is a very important source of electric fields at low latitudes at all local times. Based on these results, we propose that the postsunset enhancement of the vertical ion drift at Jicamarca is primarily due to a local time gradient in the F region zonal wind in conjunction with the conductivity gradient at the dusk terminator.

We have used data from the ion drift meter and the wind and temperature spectrometer on the DE 2 spacecraft to make statistical comparisons of the zonal ion and neutral drifts at dip latitudes (DLAT) in the ±35° range over all local times. Fourier analysis indicates that the superrotation and the diurnal components of both flows are strongly peaked at the dip equator, with the superrotation term becoming negative for |DLAT| ≥20°. One interesting feature is the presence of a period (2200‐0500 solar local time) in the 300‐400 km altitude region near the dip equator where the ion drift is more strongly eastward than the neutral flow. This would seem to indicate the presence of an electric field source of greater strength than the F region dynamo elsewhere along the geomagnetic field line. Model calculations indicate that a possible mechanism for this source lies in the vertical shear in the zonal neutral wind in the 100‐200 km altitude region.