Dynamics and solar wind control of the recovery of strong geomagnetic storms

Abstract: In this work, we studied the characteristics and dynamical changes during the recovery time of moderate and strong geomagnetic storms (Dst $<-50$ nT). Investigating 57 storms triggered by CMEs/CIRs, we focused on the solar wind's influence on their decay phases. Selected storms were classified into distinct groups based on their recovery characteristics. Using superposed epoch analysis and best fit methods, we scrutinized several interplanetary solar wind plasma and field parameters/functions. The analysis included single, dual, and multiple interplanetary plasma and field parameters/functions. We determined the most representative characteristic time for the storm's recovery profile by fitting an exponential curve. A correlation analysis between Dst and solar wind parameters/functions isolated a coupling function ($\rho^{1/2}$Ey) best describing the decay rate of the ring current. This shows that the electric field term (Ey) coupled with a viscous term ($\rho^{1/2}$) plays a pivotal role in determining the recovery rate of geomagnetic storms. Additionally, we modeled the complex patterns of Dst recovery in relation to solar wind parameters/functions using a second-order polynomial. During the recovery phase, a dynamic correlation between Dst and solar wind parameters/functions was revealed. The three-parameter solar wind-magnetosphere electrodynamical coupling function, combining the viscous term ($\rho^{1/2}$) and the electric field-related function (v$^{4/3}$B) ($\rho^{1/2}$v$^{4/3}$B), significantly impacts the recovery phase of geomagnetic disturbances. Our investigation extended to the relationship between main and recovery phase durations, providing valuable insights into the solar wind's control over the decay of geomagnetic disturbances. These findings advance our comprehension of the complex relationship between solar wind dynamics and the evolution of geomagnetic disturbances.