Ionospheric Responses to Magnetic Cavities during Northward IMF Conditions

Solar wind magnetic cavities are regions of reduced interplanetary magnetic field and increased solar wind density. They often follow magnetic clouds in the solar wind. We present event studies of ionospheric phenomena associated with magnetic cavities/pressure pulses during strongly northward interplanetary magnetic field (IMF) conditions using solar wind plasma and magnetic field data from Wind and auroral images from Polar spacecraft. This kind of event provides a good opportunity to study the ionospheric responses to the magnetic cavities/pressure pulses, because contributions from magnetic field merging can be ignored. It is found that there is one-to-one correspondence between the magnetic cavities/pressure pulses and the ionospheric responses, such as the auroral electrojets, the cross polar cap potential, and the global auroral power. The ionospheric responses were also simultaneous and transients and the duration of the responses were about the same time scale as the cavities. Surprisingly, the ionospheric responses induced by magnetic cavities/pressure pulses can be as significant as those caused by magnetic field merging. In a few instances the auroral electrojet reached more than 200 nT, a typical substorm level. We present detailed quantitative results and propose possible physical mechanisms for the events studied.