Auroral substorms and the interplanetary magnetic field

The Alaska meridian all-sky photographs from 1966 and 1970 are compared with the corresponding interplanetary magnetic field data. More than 50 auroral substorms are identified, and their onset times (T = 0) determined. Some of the important findings in this correlative study are as follows: (1) During quiet periods, auroral substorms are quite common along the contracted oval even when the north-south component Bz is positive; such substorms occur beyond the field of view (both photographically and magnetically) of an AE station. (2) During moderately disturbed periods, even when allowance is made for the IMF signal transition time, it is difficult to find any consistent IMF signature that can be readily identified as that which can be related to the onset of auroral substorms; specifically, we examined |B|, Bz > 0, Bz < 0, ∂Bz/∂t, and so forth. From (1) and (2) we conclude that Bz < 0 is not always a necessary condition for triggering mechanisms and that Bz < 0 is primarily responsible for shifting the oval equatorward to the latitude of AE stations.