Swarm-E Radio Occultation Measurements of the Topside Ionosphere During the August 2017 Solar Eclipse

Swarm-E (e-POP) was well situated to observe the ionospheric response to the August 2017 solar eclipse, crossing the path of totality at 640 km altitude 10 minutes following lunar umbra. We present observations from Swarm-E's GPS radio occultation receiver during the eclipse. Total electron content (TEC) measurements revealed a topside plasma depletion of 0.8-0.9 TECU (44-56%) above spacecraft altitude, within minutes of totality. Vertical plasma density retrievals revealed a 15-35% reduction in F-region densities and 10-35 km decrease in F-region altitude within 15 minutes of totality, with largest density depletions at F1 altitudes. Radio occultation retrievals 40-60 minutes following totality, within the trailing edge of the lunar shadow, showed F-region recovery and possible enhancement in NmF2.

In addition, detrended occultation TEC of Swarm-E revealed TEC perturbations of 0.2-0.3 TECU in the topside F-region within the lunar penumbra, indicating the presence of medium-scale plasma irregularities. Irregularities with wavelengths of 200 km and 100-200 km were observed near the trailing and leading edge of the umbra, respectively, indicating two distinct regions of irregularity characteristics. These topside irregularities potentially result from atmospheric gravity waves generated by the cooling effect of the lunar penumbra.