In this paper we study equatorial electrodynamics and plasma irregularities linked with the 17 March 2015 severe magnetic storm in the Indian sector by using common volume observations made by the Gadanki Ionospheric Radar Interferometer, airglow imager, Digisonde, and GPS receiver established at Gadanki (13.5°N, 79.2°E). Observations show that with the initiation of the storm at ~06:00 UT on 17 March, which happened to be midday in the Indian sector, the low‐latitude ionosphere responded in tune with the storm‐induced electric field and by the sunset time the base of the F layer ascended to an altitude of 470 km with a peak upward velocity of 50 m s−1 eventually manifesting equatorial plasma bubble and irregularities causing strong GPS scintillation. The most important finding found in this study is the confinement of plasma bubble and irregularities in a narrow longitude zone of 69°E–98°E. Results also show reversal of zonal drift of the irregularities from ~120 m s−1 eastward drift to ~120 m s−1 westward drift in a time span of ~30 min. Both observations are shown to be linked with very special electrodynamical conditions induced by the magnetic storm‐related electric field in the dusk sector. Intriguing details of the longitudinally localized electrodynamics and plasma irregularities are discussed in terms of prompt penetration and disturbed dynamo electric field effects.

An observational evidence of medium‐scale traveling ionospheric disturbances (MSTIDs) reaching to magnetic latitude as low as ~3.5° over the Indian sector is provided for the first time based on OI 630‐nm airglow imaging observation from a low‐latitude station, Gadanki (13.5°N, 79.2°E; 6.6° magnetic latitude), on 12 January 2016. The horizontal wavelength, horizontal phase velocity, and period of the MSTID are found to be 160 ± 6 km, 138 ± 14 m/s, and 19.5 ± 3 min, respectively. These phase fronts are observed to move toward southwest with a propagation angle of ~235° ± 1° with respect to north. In addition to the MSTID, a strong quasiperiodic southward moving wave (QPSMW) from the evening to midnight interval and a small‐scale southward moving wave structure with wavelength and periodicity different from the QPSMW are also detected on the same night. Horizontal wavelength, horizontal phase velocity, and period of the QPSMW are estimated to be 367 ± 14 km, 131 ± 18 m/s, and 46.7 ± 13 min, respectively, and those of the small‐scale southward moving wave are found to be 157 ± 4 km, 121 ± 17.8 m/s, and 21.7 ± 3.4 min, respectively. Global Positioning System‐total electron content maps suggest that the weak and asymmetric equatorial ionization anomaly helped deep ingression of the MSTID on this night. The descent of the F layer seems to have caused the dissipation of the MSTID and QPSMW closer to the dip equator on this night. Therefore, the present investigation shows that the midlatitude MSTIDs can influence the F region plasma processes even over very low latitudes under favorable background conditions.

Using a whole atmosphere–ionosphere coupled model GAIA (ground-to-topside model of atmosphere and ionosphere), we have investigated which parameters mainly control day-to-day variation of vertical plasma drift at the evening terminator over magnetic equator, so-called pre-reversal enhancement (PRE). Day-to-day variations of the peak PRE are compared with those of electron density, eastward current density and eastward neutral wind in the E- and F-region over Chumphon (10.7° N, 99.4° E; 0.86° N magnetic latitude), Thailand during equinoctial months in 2011–2013. Eastward neutral wind in the F-region shows positive correlation with peak PRE, indicating that the F-region winds control the peak PRE through the mechanisms of the F-region dynamo (including E- and F-region coupling processes). Daytime eastward electric current at an altitude of 110 km, corresponding to equatorial electro jet (EEJ), is also positively correlated with the peak PRE. Correlation between the EEJ and PRE is the largest at 1700 LT, approximately 1.5 h prior to the peak PRE.

The nature of the variability of the Total Electron Content (TEC) over Europe is investigated during 2009 and 2019 Northern Hemisphere (NH) SSW events in this study by using a combination of Global Navigation Satellite System (GNSS) based TEC observations and Thermosphere‐Ionosphere Electrodynamics General Circulation Model (TIE‐GCM) simulations. To simulate the SSW effects in TIE‐GCM, the dynamical fields from the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X) simulations of 2009 and 2019 SSWs are specified at the TIE‐GCM lower boundary. The observed and simulated TEC are in overall good agreement and therefore the simulations are used to understand the sources of mid‐latitude TEC variability during both SSWs. Through comparison of TIE‐GCM simulations with and without geomagnetic forcing, we find that the TEC variability during the 2019 SSW event, was predominantly geomagnetically forced, while for the 2009 SSW, the major variability in TEC was accounted for by the changes in vertically propagating migrating semidiurnal solar (SW2) and lunar (M2) tides. By comparing the TIE‐GCM simulations with and without the SW2 and M2 tides, we find that these semidiurnal tides contribute to ∼20%–25% increase in the quiet background TEC.

An observational evidence of a unique plasma depletion event was captured by an O( 1 D) 630.0 nm airglow imager on 13 June 2018 over a transition region of geomagnetic low-mid latitude, Hanle, Leh Ladakh, India (32.77°N, 78.97°E; Mlat.~24. 1°N). The observed plasma depletion structures are tilted at an angle of 13°± 2°west of the geomagnetic north and drifted toward west. Collocated Global Navigation Satellite System-Total Electron Content measurements confirm that the structures are indeed associated with TEC depletions. Simultaneous ionosonde measurements from Delhi, India (28.70°N, 77.10°E; Mlat.~20.2°N) shows spread-F signatures confirming that these structures are associated with the ionospheric irregularities. Interestingly, radar observations over the geomagnetic low-latitude station Gadanki, India (13.5°N; 79.2°E; Mlat.~6.5°N) reveal the absence of equatorial plasma bubbles on this night. Therefore, these observations strongly suggest that the observed structures in the airglow images over Hanle are associated with mid-latitude spread-F (MSF). These MSF structures are possibly affected by the shear in the zonal plasma drift that forces the field aligned plasma irregularity structures to tilt toward west. These observations, for the first time, bring out the presence of MSF structures over geomagnetic low-mid latitude transition region. It is suggested that the plasma distribution over low latitudes plays an important role in the occurrence of MSF structures over this transition region. Understanding the source and characteristics of the plasma irregularity structures over this transition region can help in understanding the spatio-temporal evolution of global L-band scintillation in a better way.Plain Language Summary Understanding the spatio-temporal distribution of the ionospheric plasma irregularities is important in the operational forecasting of L-band scintillation and therefore has important ramifications in the satellite-based communication and navigation systems. Traditionally, plasma irregularities in the low and mid-latitudes had received focused attentions in the past with very less attention has been paid over the low to mid-latitude transition region. The present investigation provides an attempt toward that direction and proposes a mechanism on the relationship between the plasma distribution over low latitudes and the occurrence of the mid-latitude plasma irregularities over the geomagnetic low-mid latitude transition region. Comprehensive investigations are further needed in the future to understand and characterize the ionospheric plasma irregularity structures over this region.