A geomagnetic storm is a phenomenon of weakening of Earth's magnetic field from a few hours to several days, which directly or indirectly affects navigation, transportation, communication, power grids, satellite electronics, etc. In general, the duration of the recovery phase is larger. Thus, its cumulative effect is greater than the main phase. Therefore, it is important to understand the physical processes involved during the recovery phase. In this paper, we have studied the recovery phase of 31 extreme geomagnetic storms that occurred from 1990 to 2020. Each storm demonstrates two distinct features of the recovery phase that is, initial fast and later slow recovery phase. During the fast recovery phase, the rate of recovery either linearly or non‐linearly depends on SYM−H, which has been characterized by an exponential or hyperbolic decay function in various reported studies. In our study, we have noted that the hyperbolic function explains the complete recovery phase of only 11 extreme events. Furthermore, both decay functions fail to explain the late recovery phase of storms. Interestingly, we have found that the rate of recovery during the slow phase shows steady variation (independent to SYM−H), that is, d(SYMbadbreak−H)dt0.3333em=0.3333emconstant. Our analysis suggests that the magnitude of the recovery rate during the late phase is proportional to the magnitude of the storm.

This paper reports, for the first time on a statistical basis, on the key role played by the Alfvénic fluctuations in modulating the recovery phase of the geomagnetic storms, slowing down the restoration of the magnetosphere toward its pre-storm equilibrium state. Using interplanetary and geomagnetic measurements collected over more than one solar cycle, a high correlation between the durations of Alfvénic streams and concurrent recovery phases is found, pointing to a clear coupling between Alfvénic turbulence and magnetospheric ring current dynamics. By exploiting current solar wind models, these observations also provide space weather opportunities of predicting the total duration of any geomagnetic storm induced by any solar driver provided that it is followed by an Alfvénic stream, a crucial piece of information for ground technologies and infrastructures that are affected by time-integrated effects throughout the duration of the storm.

Generally, interplanetary coronal mass ejection (ICME) triggers intense and strong geomagnetic storms. It has been established that the ICME sheath-moulded planar magnetic structure enhances the amplitude of the storms. Alfvén waves embedded in ICME magnetic clouds or high solar streams including corotating interacting regions (CIRs) in turn extend the recovery phase of the storm. Here, we investigate a geomagnetic storm with a very complex temporal profile with multiple decreasing and recovery phases. We examine the role of planar magnetic structure (PMS) and Alfvén waves in the various phases of the storm. We find that fast decrease and fast recovery phases are evident during transit of PMS regions, whereas a slight decrease or recovery is found during the transit of regions embedded with Alfvénic fluctuations.

No abstract

No abstract