Space disturbance effect on equatorial sporadic-E during sunrise

The space weather event of 25 September 1998 and its effect in the E-region near magnetic equator has been studied. The geomagnetic H field variations recorded at low latitudes was normal on 24 and 26 September 1998 but there was a geomagnetic storm on 25 September with sudden commencement at 0445 hrs LT (75° EMT). There was a strong counter electrojet after sunrise associated with the magnetic storm. Solar wind speed and ion density and the interplanetary magnetic field (IMF) data show the IMF BZ turning towards south in the morning on 25 September 1998 and solar wind velocity was 850 kms-1. Quarter hourly ionograms at Thumba, located close to the dip equator were examined during morning hours on 24-26 September 1998. On 24 and 26 September 1998, E-region irregularities (Es-q) were first generated about an hour after the appearance of fresh E-region ionization following sunrise and by the time DH at Trivandrum started increasing above the corresponding value at Alibag. On the disturbed day, 25 September 1998, Es-q appeared later due to occurrence of counter electrojet after sunrise. This is caused by electric field changes associated with the magnetic storm rather than the late reversal of the electric field in the morning