Lao, Christian John; (2025) Investigating the Influences on Substorm Signatures and their Identification. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Substorms are a rapid release of energy that is redistributed throughout the magnetosphere-ionosphere system, resulting in many observable signals, such as enhancements in the aurora, energetic particle injections, and ground magnetic field perturbations. Numerous substorm identification techniques based on each of these signals have been provided in the literature, but often with no cross-calibration. Since the signals produced are not necessarily unique to substorms and may not be sufficiently similar to be identified for each and every substorm, individual event lists may misidentify events, hindering our understanding of the phenomenon. To gauge the scale of this problem, we quantify the association between lists of substorms derived from SuperMAG SML/SMU indices, midlatitude magnetometer data, particle injections, and auroral enhancements. Overall, although some degree of pairwise association is found between the lists, even lists generated by applying conceptually similar identification techniques to ground magnetometer data achieve an association with fewer than 50\% event coincidence. The poor event agreement between different ground magnetometer-based methods is hypothesised to be due to the poor differentiation between magnetic perturbations due to substorms and magnetospheric convection enhancements. Unlike other substorm identification methods, the SOPHIE technique (Forsyth et al., 2015) attempts to distinguish between negative SML bays produced by a substorm or magnetospheric convection enhancement by examining the SMU index. Despite this, we find evidence that up to 50% of the events originally identified as substorms are misidentified and that the auroral indices alone are insufficient to distinguish between the phenomena. Finally, four event studies are presented. These show a classical substorm, a "classical" magnetospheric convection enhancement, an atypical substorm - from the location of the auroral onset and ground magnetic signature - and a magnetospheric convection enhancement occurring in a location that we would expect only substorms to occur. Even when integrating further geomagnetic indices, there appears to be no differentiator apart from coupled changes in SMU and SML in identifying the convection enhancements from substorms. Furthermore, there seems to be no difference in the driving of the magnetosphere prior to substorms or convection enhancements.

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