Equinoctial asymmetry in the occurrence of equatorial spread-F over Indian longitudes during moderate to low solar activity period 2004-2007

The seasonal pattern of equatorial spread-F (ESF) was observed at the magnetic equatorial station Trivandrum and low latitude station SHAR in the Indian sector using ionosonde data. A clearly higher ESF occurrence percentage is observed during vernal equinox compared to autumnal equinox at both locations. Post-sunset vertical drift velocities were observed to be nearly comparable at Trivandrum indicating the comparable electrodynamical forcing on the post-sunset F-layer during the two equinoxes. At the off equatorial station SHAR, post-sunset vertical drift exhibits clear asymmetric pattern, with higher values in the vernal equinox, indicating the possible role of meridional neutral winds in modulating the layer height. The meridional winds estimated during the period just prior to ESF at the two equinoxes reveal much larger poleward winds to be prevalent during autumnal equinox compared to that in vernal equinox. Large neutral winds are shown to be related directly to asymmetry factor of the equatorial ionisation anomaly (EIA) derived from single station Coherent Radio Beacon Experiment (CRABEX) data. The larger neutral winds in autumnal equinox, results in an increase in off equatorial E-region conductivity, thereby, inhibiting or weakening the F-region dynamo. This study, thus, focuses on the role of neutral dynamics, as one possible candidate producing the asymmetric occurrence pattern of ESF, during the two equinoxes over Indian longitudes.

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