Unraveling the May 2024 Space Weather Events

The first two weeks of May 2024 were characterized by the most significant space weather phenomena since the Halloween events more than 20 years ago. NOAA Active Region (AR) 13644 produced numerous M- and X-class solar flares and Earth-directed Coronal Mass Ejections (CMEs), triggering the most intense geomagnetic storm since October 2003. This AR, in terms of size, was comparable to the AR responsible for the Carrington event and was long-lived, reappearing during the next Carrington rotation with an even larger size.

In this work, we combine observations from multiple viewpoints (SOHO, STEREO, Solar Orbiter) and in situ data points (Wind, ACE, STEREO), simulations, and modeling of the propagation of CMEs in the inner heliosphere, as well as retrospective forecasts of the CMEs’ arrivals at Earth. We begin our analysis by examining the characteristics of the AR, utilizing Space-weather HMI Active Region Patches (SHARPs). Specifically, we calculate the effective connected magnetic field strength parameter, known as Beff, an index of magnetic complexity, which we found to achieve a record high value of nearly 9000 Gauss. We then analyze the kinematic properties of the halo CMEs and, in combination with the flare magnitude, we estimate the probability of a Solar Energetic Particle (SEP) event through the application of a recently published machine learning model (see Paouris et al., 2023). The extraordinary strength of the persistent geomagnetic storm led us to simulate propagation of CMEs using MHD modeling by means of the GAMERA model. Through these simulations, we attempt to explain the storm's intensity based on the characteristics of the interaction and compression between the CMEs. Finally, we address the obvious question: “Why were the CMEs not as extraordinary as the parent AR?”, considering that one might expect CMEs with extreme characteristics in terms of speed and mass.