Plasma density enhancements associated with equatorial spread F: ROCSAT‐1 and DMSP observations

Abstract

Large‐scale plasma density depletions are typically associated with equatorial spread F (ESF) plasma irregularities in the nightside F region, especially in the postsunset sector. Data gathered on the ROCSAT‐1 spacecraft reveal numerous cases of localized, discrete plasma density enhancements in the nightside low‐latitude region at ∼600 km altitude. In some cases, nearly simultaneous DMSP observations at ∼800 km reveal similar density enhancements in the same local time sector. These density enhancement structures occur in association with ESF plasma depletions, i.e., the density enhancements are observed in the same local time where ESF plasma depletions are also present simultaneously. Within these discrete structures, the plasma density may be enhanced by ∼2–3 times above the background density. The density enhancement regions have sharp, distinct edges with embedded irregularities that appear to have similar scale sizes and density fluctuation spectra as those typically found in plasma depletions. Examples studied here occur at local times about 3 hours after sunset near the equatorial anomaly region, ∼10° to 20° from the magnetic equator. The ion velocity data within the density enhancement regions show upward plasma drifts perpendicular to the magnetic field, similar to those within adjacent plasma depletion regions. The magnetic field‐aligned plasma flows are generally poleward within the density enhancement regions. The observations suggest that density enhancement structures are caused by the polarization electric field which is generated within the equatorial plasma depletions and then maps to the higher latitudes along the magnetic field lines.