The Shock-Substorm of 24 September 1998: A Different Type of Substorm Trigger Onset

An interplanetary shock triggered a substorm with a peak intensity SML = −1,781 nT on 24 September 1998. This is called a shock-substorm here to differentiate it from generic substorms. The shock, with a speed of ∼790 km s⁻¹, caused the release of prestored magnetosphere/magnetotail energy plus additional solar wind input energy, the latter unusual for a shock-nonsupersubstorm event. The internal magnetospheric shock/wave had a speed of ∼630 km s⁻¹, consistent with arriving at x = −6 R_E at the time of substorm onset. The internal shock arrival to x = −10 R_E would have been after the substorm onset time, so magnetic reconnection for the substorm triggering can be ruled out. Akasofu (2023, https://doi.org/10.1093/mnras/stac3187) has stated that there are many different types of substorm onsets. We believe that this shock-substorm is different than generic substorms.