Is a Substorm Expansion Required to Initiate a Steady Magnetospheric Convection Event?

Steady magnetospheric convection (SMC) events are periods longer than a typical substorm recovery phase of enhanced convection with an absence of substorm onsets. The prevailing theory for their occurrence is a balance of reconnection rates at the dayside and nightside x-lines. Previous statistical studies used superposed epoch analysis of auroral indices to show there is typically a decrease in AL prior to the start of SMCs, indicating that a preceding substorm may be necessary to initiate balanced reconnection. However, several cases have been published in which there is no evident substorm auroral onset before a long period of steady magnetospheric convection. Using a list of nearly 3000 SMC intervals from 1997 to 2009, we determine the percentage of events that do not have a clear AL onset prior to the start. Multiple datasets for this subset are closely examined for other onset signatures. We present several case studies of SMC without a preceding substorm using the THEMIS spacecraft and All-Sky Imagers, and determine whether these special cases differ from the majority of SMCs.