Understanding the response of the ionosphere‐magnetosphere system to sudden solar wind density increases

The different responses of the magnetosphere to sudden solar wind density enhancements are investigated. The majority of the responses are observed to have two phases: the first is due to an inductive E field propagating through the system, while the second is due to a vortex formation in the magnetosphere. However, during some events, the system displays a single‐phase response. In order to understand the controlling parameters in the system response, statistical analyses of several solar wind external characteristics as well as the internal index D st are conducted. Results of these analyses show that none of those characteristics seem to be associated with the way the geospace system responds to the abrupt solar wind density increase. Owing to the lack of statistics in the analysis, simulations investigating the effect of the solar wind Alfvén Mach number on the magnetospheric response have been conducted. The simulation results indicate that the two‐phase response is always present but that a low solar wind Mach number causes an indistinct two‐phase response, which is difficult to identify as having two phases. Compression signals following the sudden compression at the magnetopause are carried by a fast mode wave in the magnetosphere, which propagates toward the Earth and is reflected back toward the magnetopause by the plasmapause or ionosphere. The wave is subsequently reflected back again, leading to a secondary Earthward propagation. These multiple bounces of the fast mode wave result in two groups of the two‐phase responses in the ionosphere, although the second group is extremely difficult to identify in ionospheric data.