Solar Wind–Magnetosphere–Ionosphere Coupling Under Extreme External Driving: Characteristics of Dayside Ground Magnetic Disturbances

The objective of the present study is to investigate solar wind‐magnetosphere‐ionosphere coupling (SWMIC) under intense external driving in terms of dayside ground magnetic disturbances. Dayside magnetic reconnection drives a region‐1 sense current system, the SWMIC current system. This current system intensifies, before other magnetospheric currents, if the interplanetary magnetic field becomes strongly southward after sudden storm commencements. During the October 2003 storm, its effect on mid‐ and low‐latitude magnetic fields manifested as a single‐swirl pattern of equivalent currents, which is expected for the field‐aligned current closure through the auroral electrojet. During the July 2000 storm, in contrast, it manifested as a double‐swirl pattern, as expected for ionospheric closure currents extending to lower latitudes. The difference in the equivalent current pattern presumably reflected different distributions of ionospheric conductance. For each storm, it is found that the sequences of ground magnetic disturbances are well approximated as a convolution of external driving and a response function, . This result suggests that the SWMIC current system behaves like an overdamped electric circuit. The e‐folding time is inferred by fitting the convolution to ground magnetic observations, and is estimated at ∼10 min, which is considered L / R with the inductor of the SWMIC current system L , and the ionospheric resistor R . However, the estimation is more challenging if the variation of solar wind driving is not steplike. Moreover, for low‐latitude magnetic disturbances, the effect of other current systems such as the ring current and equatorial electrojet can significantly modify the response of ground magnetic disturbances.