The particularly clean response of the Indian sector to a strong but short lived geomagnetic storm

Abstract

Using data from multiple sources/techniques, we have studied the low latitude ionospheric response to an intense geomagnetic storm (Dst ∼ −132 nT) in the Indian sector. The storm, which occurred on 30–31 May 2005, had the onset near noon in the Indian sector. It led to strong and sustained activity at high latitudes for twelve hours. Magnetometer data located near the magnetic equator produced clear evidence for a strong Prompt Penetration Electric Field (PPEF). Based on the TEC data going as far north as Delhi, there were clear indications that the PPEF was followed by a Storm Enhanced Density (SED) event. On the next day, i.e., 31 May, there was a strong counter-electrojet in the morning, accompanied by a factor 3 reduction in the plasma density in the Equatorial Ionization Anomaly (EIA) region and beyond. This was consistent with the notion of a vanishing eastward electric field at the equator and the direct impact of an upper thermospheric equatorward wind associated with high latitude Joule heating from the auroral regions. One unexpected feature from the afternoon of 31 May was an undulation in the height of the peak F-region density at the magnetic equator in Trivandrum. A closer look at the ionosonde data revealed an F3 layer until mid-afternoon, strongly indicative of equatorward winds in the upper thermosphere. However, by 16:00 IST, the F3 layer quickly vanished; and with this, the height of the peak-F region density fell by 200 km in two hours. The only plausible explanation was that the winds must have stopped blowing toward the equator by 16:00 IST.