Examining the effects of a pre-noon annular Solar Eclipse on equatorial electrodynamics: Evidence for a subsequent post-noon enhancement in plasma density

Abstract

On 26 December 2019, an annular solar eclipse occurred in the southern part of the Indian subcontinent, which falls within the Equatorial ElectroJet (EEJ) region. This presented an opportunity to study the effects of a high-obscuration annular solar eclipse (93.3%) and of the special tides that accompany an eclipse. Eclipses are known to affect the electrodynamics of the equatorial region and the distribution of plasma in the low-latitude ionosphere. On December 26, 2019, different phenomena were indeed seen over different time scales. Around the time of the eclipse, there was a depletion in the total electron content (TEC) of the ionosphere, as inferred from GNSS receiver systems placed along the eclipse path in and around the EEJ region. Magnetometer measurements indicated a weakening of the EEJ current in this area not just when the eclipse took place but for the rest of the day as well. Observations from a digital-ionosonde located in Trivandrum (8.52°N, 76.94°E; 0.33°N dip-latitude) revealed that the ionospheric F-layer moved downwards just after obscurity maximum. Shortly after the F region was observed to move back upward, a one-hour long strong blanketing sporadic E-layer was triggered even though there was only a weakening of the EEJ but no reversal in the associated magnetic perturbations. Simulations from our quasi-two dimensional model clearly show that though there was no Counter Electrojet, there was indeed a weakening of the zonal field on 26 December 2019 during the eclipse and the post-eclipse period. The overall weakening trend lasted until late afternoon. Other notable features were uncovered on a time scale of several hours after the passage of the eclipse. While the day-long weakening of the EEJ is consistent with special tides related to the lunar trajectory, we also observed unusually large amplitude undulations in both vertical-TEC and F2-region peak plasma density over a five-hour time scale, starting around 2:00 PM LT. These undulations were consistent with the zonal electric field variations reconstructed from the magnetometer data.