A Case of Anomalous Electric Field Perturbations in the Equatorial Ionosphere During Postsunset Hours: Insights

During a weak geomagnetic storm (Ap = 15) on 24 December 2014, the penetration electric field perturbations over the Indian dip equatorial sector are found to be anomalous on a number of occasions during postsunset hours. The event is anomalous as the magnitude and polarity of penetration electric fields do not obey the existing paradigm. The penetration electric field perturbations are investigated using the vertical drifts derived from the CADI (Canadian Advanced Digital Ionosonde) measurements at Tirunelveli (8.7 degrees N, 77.7 degrees E, dip angle: 1.7 degrees). During this event, we observed postsunset vertical drift of similar to 42 m s (-1) not only at 18:10 LT but also similar to 36 m s( -1) at similar to 21:00 LT which is anomalous. Interestingly, the dawn-dusk component of interplanetary electric field (IEFy) is relatively less (< 2 mV/m) at similar to 21:00 LT compared to the interval 19:30-20:30 LT (IEFy similar to 3 mV/m). Despite that, the vertical drift observed over Tirunelveli is very close to zero or nominally upward during 19:30-20:30 LT. In addition, the downward drift just after 21:30 LT on this night is found to be exceptionally large (similar to-60 m s( -1)). By combining vertical total electron content over the Indian sector with the OI 630.0 nm airglow intensity from Mt. Abu, chain of magnetometer and Los Alamos National Laboratory geosynchronous satellite particle measurements, it is suggested that the anomalous penetration electric field perturbations on this night arise from the effects of interplanetary magnetic field By and substorm.