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

Abstract 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 ∼1,775 km s −1 , consistent with arriving at x = −6 R E slightly prior to the time of substorm onset. The internal shock arrival to x = −10 R E would have been after the substorm onset. Magnetic reconnection for the substorm triggering can be ruled out. Magnetic compression at the midnight sector ground stations at LYC and OUJ (∼61° MLAT) were present prior to the substorm onset. This could have been caused by a magnetosonic wave traveling from the dayside to the nightside just outside the plasmasphere. 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.