Abstract:
The annular solar eclipse of 15 January 2010 over southern India was studied with a multi-instrument
network consisting of magnetometer, ionosonde and GPS receivers. The presence of a counter electrojet
(weakened or westward zonal electric field) during the eclipse and adjacent days suggests the strong
gravitational tidal effect associated with the exceptional Sun–Moon–Earth alignment around the eclipse
day. With a strong backup of magnetometer recordings on the day of eclipse, its adjacent days and the
normal electrojet day, it is argued that the regular eastward electric field for the whole day at the equator
was not just weakened, but actually was flipped for several hours by the influence of enhanced lunar
tides. The effect of flipping the electric field was clearly seen in the equatorial ionosonde data and
through the large array of GPS receivers that produced the total electron content (TEC) data. The main
impact of flipping the electric field was poor feeding of equatorial ionization anomaly (EIA) due to the
severely weakened fountain effect on the eclipse day, with the regular anomaly crest shifting towards the
equator. The equatorial ionosonde profile was also showing an enhanced F2 region peak in spite of a
reduced vertical TEC. While the plasma density depletion at the lower F region altitude over the equator
was due to the temporary lack of photo-ionization, the reductions in high altitude plasma density beyond
the equator were caused by the electrodynamics taking place around the eclipse. The important finding
of this analysis is that the electrodynamical consequences on the low latitude ionosphere were mainly
due to the combination of eclipse and lunar tides which were far more significant and influenced the EIA
density rather than eclipse alone. Based on these findings, it is argued that the prevailing lunar tidal
impact also needs to be taken into account while seeking to understand the electrodynamical impact of
the solar eclipse on the low latitude ionosphere.
