Research Article

Supersubstorms during the May 2024 superstorm

¹ CAS Key Laboratory of Geospace Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, PR China
² Retired, Pasadena, California 91208, USA
³ College of Earth and Planetary Sciences, Chinese Academy of Sciences, Beijing 100049, PR China
⁴ Retired, Navi Mumbai 400703, India

^* Corresponding author: rhajra@ustc.edu.cn; rajkumarhajra@yahoo.co.in

Received: 15 May 2025
Accepted: 10 October 2025

Abstract

We study in detail six isolated supersubstorms (SSSs; SML < − 2500 nT) during the May 2024 superstorm (SYM-H peak = −518 nT), the second largest storm by 1-min SYM-H index (since 1981). We also make comparisons to the largest and third largest magnetic storms, the March 1989 storm (SYM-H = −720 nT) and the November 2003 storm (SYM-H = −490 nT), respectively. Like the 1989 superstorm, the May 2024 superstorm is a complex event associated with multiple sheaths and magnetic clouds (MCs). However, unlike the 1989 superstorm, the May 2024 event had three MCs in the storm recovery phase with four SSSs. This caused the May 2024 event to have the longest and strongest “recovery phase” of the three storms. Because of this, the May 2024 event may be equally large in total energy as the 1989 storm. We revise previously published “tippy bucket” analyses for precursor energy input to assume a 3-h linearly input and subsequent dissipation of solar wind energy into the magnetosphere/magnetotail. The new linear tippy bucket model showed that the SSSs were triggered by the strong solar wind driving of ~ 10¹⁷ J. The Akasofu ε-parameter is used to estimate the solar wind energy input. All six SSS events could be explained by both precursor energy and direct driving. Two of the SSS events were possibly triggered by solar wind density parcels; the other four were not. The SSS events were highly varied in morphology, ranging from an isolated substorm morphology to a storm convection bay scenario. Overall, all six SSS events were unique. We suggest a two-mode nightside convection electric field to explain the nightside Joule heating variability. For the dayside Joule heating, we suggest three possible mechanisms: 1) adiabatic compression of magnetopause boundary layer plasma and dayside ionospheric precipitation, 2) deep penetration of solar wind protons and the generation of boundary layer field-aligned currents, and 3) magnetic reconnection with boundary layer magnetic fields with energy dissipation. It is noted that all three proposed mechanisms would deposit energy well away from the Earth’s ionosphere. They are not measured by the ε-parameter either. The missing energy is due to the viscous interaction mechanism.