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

Abstract

[1] 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.

Observations in equatorial anomaly region of total electron content enhancements and depletions

“…The ion density data given by Kil and Heelis (1998) also reveals a number of ion density enhancements along with the depletions in the ion density. But they did not emphasize these enhancements, as has been pointed out by Le et al (2003). Recently, Le et al (2003) have reported localized regions of enhancements in the ion density, occurring with or without the plasma density depletions.…”

“…The plasma density enhancements observed by Oya et al (1986), Kil and Heelis (1998) and Le et al (2003) also could not be compared with those observed with the ATS series satellites. The observations of Oya et al (1986), Kil and Heelis (1998) and Le et al (2003) are based on the instruments on board the satellites that had a circular orbit (650 km for Hintori, 370 km and 400 km for AE-E, 600 km for ROC-SAT and 850 km for DMSP satellite).…”

“…This is also the case with the GPS satellites and hence these two cases are comparable. Le et al (2003) have reported enhancements in the ion density in the topside equatorial F region at 600 km and 850 km. As noted earlier, these measurements have been made using the instruments on board the ROCSAT-1 and DMSP satellites, respectively.…”

“…The observations of Oya et al (1986), Kil and Heelis (1998) and Le et al (2003) are based on the instruments on board the satellites that had a circular orbit (650 km for Hintori, 370 km and 400 km for AE-E, 600 km for ROC-SAT and 850 km for DMSP satellite). These satellites are thus revolving at the same altitude and cover a wide range of latitude and longitude.…”

“…The TEC enhancements have also been observed along with the TEC depletions. The TEC enhancements have been interpreted as the manifestation of the plasma density enhancements reported by Le et al (2003). …”