The Reconnection Substorm

Abstract

The reconnection model of magnetospheric substorms was designed 20 years ago to rationalize the time-dependent changes in the magnetospheric structure associated with auroral substorms. By 1970, it was becoming apparent that there was a characteristic sequence of events prior to auroral onset: The dayside magnetopause moved earthward, the inner edge of the cross-tail current sheet approached the earth, the field strength in the tail lobes increased, and the magnetic flux in the polar caps and tail lobes increased, all while the evening side aurora were migrating equatorward prior to onset. The increase in lobe magnetic field clearly suggested that the growth phase commenced with an increase in the dayside reconnection rate. By the early 1980s, studies quantitatively correlating the ionospheric electric field with southward interplanetary field had confirmed that the changes in structure accompanied enhanced convection as had been suspected all along. Both were related somehow to substorms. There was no other choice in the reconnection model but to spotlight its two reconnection events, at the dayside magnetopause and in the plasma sheet on the nightside, as the “main events” in the magnetospheric substorm. Dayside reconnection clearly initiated the growth phase, and tail reconnection had to follow with some delay. It seemed natural to associate tail reconnection with the onset of the auroral substorm. Sections 7.2 through 7.6 are devoted to growth-phase phenomenology. Section 7.2 deals with the changes in magnetopause position that follow a single isolated southward shift of the interplanetary field, and Section 7.3 deals with the changes in the geomagnetic tail that occur as a result. These changes take place as the rate of convection builds up in the ionosphere (Section 7.4) and the dayside magnetosphere (Section 7.5). A growth phase that begins with enhanced dayside reconnection has to lead to enhanced tail reconnection—the second key event in the reconnection model of substorms. In an MHD model, the time and place where the new reconnection event takes place is determined by the propagation of waves along the characteristics connecting the dayside reconnection region to the tail lobes (Section 7.6).