Signatures of the Long Duration Prompt Penetration Electric Field in the 18 MHz HF Radar Observations Over Thumba

One of the best proxies to estimate the daytime ionospheric zonal electric field from the dynamo region is by probing the drift of the plasma irregularities in the ionospheric E-region by coherent radars, although there have not been many at HF frequencies. The present study is based on the observations of the anomalous variability in the zonal drift of 8.3 m scale size plasma irregularities in the ionospheric E-region during the daytime on 12 October 2021, and 28 November 2022, as observed by an 18 MHz HF radar at Thumba (8.5°N, 77°E, and dip lat. = 1.96°N). The wind perturbations from the lower atmosphere and the electric field penetrating from the solar wind, can both cause the perturbations in the E-region electric field. Based on a comparison with the predicted Prompt Penetration Equatorial Electric Field (PPEF) as ascribed by Manoj et al. (2008, https://doi.org/10.1029/2008ja013381), and the quiet-time dynamo electric field over the longitude of Thumba, we conclude that the perturbations in the electric field on these two events are caused by PPEF mapping to the equatorial region, solely driven by IMF B_Z oscillations. The former event is a result of a Coronal Mass Ejection, whereas the latter event was caused by a high-speed solar wind associated with coronal hole. The fluctuations in the strength of the Equatorial Electrojet (EEJ) calculated from the magnetic field observations from a pair of equatorial (Tirunelveli) and off-equatorial (Alibag) stations are also found to be well in agreement with the radar observations and modeled PPEF, thereby verifying the E-region observations.