The geomagnetic storms are associated with substorms, however, isolated substorms are also known to occur through the magnetospheric processes without an immediate or explicit forcing from the sun (Liou et al., 2013;Nishida & Kamide, 1983;Vorobjev et al., 2016). Additionally, there may be cases when the magnetosphere is in presubstorm state and a change in the interplanetary parameters may eventually favor the generation of a substorm (Guo et al., 2012; Sandholt et al., 2014, and references therein).Substorms of varying magnitudes have been customarily identified by using the AL (auroral lower component) index along with the gross night side activity by using the AE (auroral electrojets) index. The denser SuperMAG network of magnetometers has made it possible to define a new pair of indices called SME (super magnetic electrojets) index and SML (lower component of SME) index (Newell & Gjerloev 2011a, 2011b) which can better identify the spatio-temporal occurrence of the substorms. Using the SML data, Tsurutani et al. ( 2015) have found that substorms can occur as extreme events and a threshold of SML < −2,500 nT has been used to declare it as supersubstorms (SSSs). Subsequently, Hajra et al. (2016) have analyzed more than 70 geomagnetic storms with a superposed epoch analysis and found that SSSs may not have a strong relationship with the associated geomagnetic storm and also that seasonal variations show ionospheric dominance during such events. The solar wind sources that drive the extreme substorm events have been studied by Hajra et al. (2016), Adhikari et al. (2017 and Despirak et al. (2019) and it has been shown that the SSSs are sourced majorly from magnetic clouds (MCs) and the interplanetary sheath fields. These studies have also reported that a majority of the SSSs occur during the initial or main phase of the geomagnetic storms. Nishimura et al. (2020) have highlighted the dominance of nightside drivers and magnetotail during the SSSs. They found enhancements in auroral brightening, TEC

Four supersubstorms of the solar cycle 24 are analyzed to investigate the global variations in the H‐component and geomagnetically induced currents (GICs). The response to the storm sudden commencement (SSC) of the 2012 and 2017 events is observed differently over different latitude bands. Initially, the latitude band of 0°–45° shows a step‐like preliminary positive impulse (PPI), 45°–65° shows a Gaussian‐kind of PPI and 65°–90° shows a preliminary reverse impulse, followed by a main impulse all over. The H‐component variations during the supersubstorm periods show a strong north‐south asymmetry over the high latitudes which is attributed to the seasonal dependence of the growth and decay of ionospheric currents respectively in the summer and winter hemispheres. The co‐latitude band of ∼55°–65° shows a complete reversal of phase (i.e., global positive peak) of the H‐component compared to the maximum depression observed from the close‐by latitude bands and peak depressions in the SYM‐H, SML, and AE indices. The low latitude variations exhibit a dominant local time‐dependent control of the perturbation electric fields over the short and long terms during the geomagnetic events. The D‐component variations reflect complex ionospheric contributions during the SSC and the main phase with more asymmetries and variability. The GIC threat represented by the dB/dt peaks during the supersubstorms shows the highest magnitude (∼900 nT/min) in the latitude band of 60°–75° with a secondary peak over the dip equatorial regions. Further, some scattered and prominent peaks over the mid and high latitudes outside the supersubstorm periods are also observed.

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