Magnetospheric substorms

A magnetospheric substorm is a transient process initiated on the night side of the earth, in which a significant amount of energy derived from the solar wind‐magnetosphere interaction is deposited in the auroral ionosphere and magnetosphere. Substorms are controlled by the interplanetary magnetic field, ceasing when it points northward for long intervals and reappearing when it turns southward. In the simplest case a substorm appears to have three phases: a growth phase in which energy is extracted from the solar wind and stored in the magnetotail; an expansion phase in which energy is explosively released; a recovery phase in which the magnetosphere relaxes to a quiet state. More commonly, however, weak auroral disturbances called pesudobreakups can occur during the growth phase, or more than one expansion phase may follow a single growth phase. Often multiple onsets are so close together that only one expansion phase is evident. At times the magnetosphere undergoes a “convection bay” in which quiet‐time processes are much enhanced but transient processes are not observed in the midnight sector. The processes responsible for onset of the expansion phase are not known although various mechanisms have been suggested such as sudden commencement pressure pulses, interplanetary magnetic field direction fluctuations, near earth plasma sheet thinning, onset of tail current instabilities and the tearing mode instability. The most commonly accepted explanation of substorms is based on a reconnection model in which interplanetary magnetic field lines merge with geomagnetic field lines and draw them back over the polar cap into a long tail. A sudden onset of reconnection at an x‐type neutral line in the near‐earth tail reconnects the earth's field lines, releasing a portion of the tail's magnetic flux and injecting particles into the night side magnetosphere. Many details of substorm morphology are subjects of considerable controversy and alternative explanations not involving reconnection also exist.