How Solar Wind Controls the Recovery Phase Morphology of Intense Magnetic Storms

Geomagnetic storms are critical space weather phenomena resulting from the interaction between the solar wind and the Earth's magnetosphere. However, most studies focus on the main phase of magnetic storms, leaving the morphology of the recovery phase an open question. In this study, we analyze 82 intense magnetic storms with the minimum Dst index <= -100 nT between 1995 and 2018, finding that these storms can be classified into two distinct types: one-stage recovery storms that exhibit a single rapid exponential recovery and two-stage recovery storms that are characterized by a rapid exponential recovery in the early recovery phase and a slow linear recovery in the later recovery phase. We find that the two-stage recovery storms are dominant, accounting for approximately 60% of the events. Interestingly, the proportion of two-stage recovery storms peaks during solar minimum. The two-stage recovery storms tend to be accompanied by more Alfven waves with long-duration and intense southward interplanetary magnetic fields. In addition, we find that the decay rate of the Dst index in the later recovery phase is correlated with the average BZ of the interplanetary magnetic field when the solar wind has a high degree of Alfvenicity. Overall, our results shed new light on the recovery phase morphology of intense magnetic storms and highlight the role of Alfven waves in this process. Geomagnetic storms are severe space weather phenomena that can cause significant impacts on our technological infrastructure. Compared to the main phase, rare emphasis has been paid to the recovery phase. However, considering the long duration, understanding the storm recovery phase is essential to predict when the effects of space weather will subside. In this study, we analyzed 82 intense magnetic storms with the minimum Dst index <= -100 nT during the last two solar cycles, focusing on the statistical characteristics of the recovery phase morphology. These intense storms can be categorized into two distinct types: one-stage recovery storms with a rapid exponential recovery, two-stage recovery storms with a rapid exponential recovery in the early recovery phase, and a slow linear recovery in the later recovery phase. In addition, two-stage recovery storms are dominant and often accompanied by long-duration Alfven waves with intense southward interplanetary magnetic fields. The decay rate of the Dst index in the later recovery phase correlates with the strength of the southward magnetic field when the solar wind presents a high degree of Alfvenicity. Our findings provide new insights into the recovery phase of intense magnetic storms and highlight the significance of Alfven waves in this process. Intense storms contain two types of recovery morphology, with a dominant two-stage recovery type of 60% Southward IMF plays a key role in the recovery phase morphology, and the Alfven wave is an important interplanetary origin Average IMF BZ of the continuous Alfven wave positively affects the recovery rate in the later recovery phase