A Question of Substorm Size. How Large Must a Magnetic Disturbance Be Before It Can Be Called a Substorm?

Despite a long history of substorm studies, there have been few attempts in the past to place a threshold on the size of a magnetic perturbation associated with auroral brightenings which qualifies it as a substorm. Certainly no attempt has been made to specify specific thresholds for the directly driven and expansive phase components of substorm activity. In this comment, I shall address these two questions in an effort bring this problem to the attention of the substorm research community.