The use of ground magnetograms to time the onset of magnetospheric substorms

A fundamental question of magnetospheric research today is what physical process causes the onset of a magnetospheric substorm. An answer to this question depends on the timing of substorm onset. The original definition of substorm onset was the sudden brightening of an equatorward auroral arc near midnight. Later, other phenomena were correlated with this onset, and it was found that they also undergo sudden changes. In this paper, the technique of substorm timing using ground magnetograms is described. Particularly, some of the pitfalls are emphasized because these have led some workers to the error in onset determinations. The paper includes the sections concerning substorm timing with auroral zone magnetograms, timing substorm onset with midlatitude magnetograms, multiple onset substorms, and a phenomenological model for multiple onset substorms. In the previous descriptions of the growth phase model, it has been suggested that magnetospheric reconfigurations lead to substorm onset. The suggested mechanism is plasma sheet thinning brought about by demand for closed field lines in the dayside reconnection region. To determine whether such thinning is of causal importance will require careful studies of satellite data in future. Ground magnetograms will play an essential role in the determination of these onset times. (Wakatsuki, Y.)