Magnetic flux closure in the geomagnetic tail

Images of the proton aurora produced by the SI12 imager onboard the IMAGE satellite can be combined with the data from the SuperDARN radar network to estimate the location and motion of the open/close field line boundary at ionospheric altitude, the amount of open flux, and the flux opening and closure rates, that translate to voltages according to Faraday’s law. This method allows us to characterize the reconnection voltage of various auroral phenomena, like substorms, shock-induced auroras, auroral streamers, sawtooth events etc.
During the substorm cycle, open flux is accumulated during the growth phase, as intense magnetic reconnection merges the geomagnetic and interplanetary fields, then the expansion onset takes place, sometimes preceded by pseudobreakups. The closure rate reaches its maximum intensity shortly after onset, as the expansion phase develops. The flux closure voltage then returns to an undisturbed value during the recovery phase. Poleward Boundary Intensifications (PBI’s) are found to be related to weak enhancements of the flux closure rate during the recovery phase.
We show that strong interplanetary shocks can force flux closure as the magnetotail is compressed by the solar wind dynamic pressure increase. Weaker shocks can also trigger flux closure by disturbing the loaded magnetosphere.
Based on ionospheric observations, flux closure also appears to be a key process in the formation of auroral streamers and Bursty Bulk Flows (BBFs), that we propose to be related to a variation of the properties of the plasma entering the reconnection site, either due to an anisotropy of the plasma sheet, or, more likely, a localized motion of the reconnection site itself.
Sawtooth events, often presented as a quasi-periodic sequence of substorms, are also phenomena that have properties that naturally suggest a relation with the flux closure process. Indeed, we found several cases of sawtooth events that take place when the amount of open flux was larger than 1GWb, due to strong flux opening on the dayside. Consequently, a strong loading of the magnetosphere with open flux appears as a condition that favours the development of a sawtooth event.