How the CME on 21 April 2023 Triggered the First Severe Geomagnetic Storm of Solar Cycle 25 (2503.00705v2)

Abstract: The first severe (G4) geomagnetic storm of Solar Cycle 25 occurred on 23-24 April 2023, following the arrival of a Coronal Mass Ejection (CME) on 23 April. The characteristics of this CME, measured from coronagraphs (speed and mass), did not indicate that it would trigger such an intense geomagnetic storm. In this work, our aim is to understand why this CME led to such a geoeffective outcome. Our analysis spans from the source active region to the corona and inner heliosphere through 1 au using multiwavelength, multi-viewpoint remote sensing observations and in situ data. We find that rotation and possibly deflection of the CME resulted in an axial magnetic field nearly parallel to the ecliptic plane during the Earth encounter, which might explain the storm's severity. Additionally, we find that imaging away from the Sun-Earth line is crucial in hindcasting the CME Time-of-Arrival at Earth. The position (0.39 au) and detailed images from the SoloHI telescope onboard the Solar Orbiter mission, in combination with SOHO and STEREO images, helped decisively with the three-dimensional (3D) reconstruction of the CME.

Analysis of the 2023 CME-Induced Severe Geomagnetic Storm in Solar Cycle 25

The paper "How the CME on 21 April 2023 Triggered the First Severe Geomagnetic Storm of Solar Cycle 25" provides a comprehensive examination of the coronal mass ejection (CME) event on April 21, 2023, which precipitated a severe geomagnetic storm. The research employs an extensive array of remote sensing observations and in situ data to dissect the occurrence and unravel why this particular CME, despite its typical coronal characteristics, resulted in significant geoeffective consequences, reaching a Dst index minimum of -213 nT with a Kp index of 8+.

Key Findings

1. CME Characteristics and Initial Observations: Despite the CME possessing unremarkable features such as moderate speed and mass, typically not indicative of severe geomagnetic disturbances, its subsequent impact belied initial expectations. The 3D reconstruction using multi-point imaging with the GCS model revealed CME propagation along a low to mid-range latitude with significant tilt alteration during its transit, elucidating the rotation’s pivotal role in enhancing Earth-directed geoeffectiveness.
2. Source Region and Solar Activity Observations: NOAA active region 13283, despite giving rise to the triggering CME, exhibited modest magnetic complexity with a maximum $B_{eff}$ of 350 G. The simplicity of the region, alongside moderate CME speeds of 1000 km/s, contrasted with the severe geomagnetic impact observed.
3. In Situ Analysis and CME Deflection Factors: The divergence between precursor CME observations and L1 in situ data derived from Wind and STEREO-A, suggested additional factors at play. The investigation identified significant rotation and possible deflection toward the solar equator influenced by adjacent coronal holes as potential amplifiers of its geoeffectiveness.
4. Importance of Off-Sun-Earth-Line Observations: The application of SoloHI’s high-resolution images and uniquely off-axis viewing perspective significantly enhanced 3D CME modeling, ultimately leading to a more precise reconstruction and successful ToA prediction with merely a 1.8-hour disparity from the CME's actual Earth impact. This evidences the critical role of comprehensive angular observation coverage in reliable space weather prediction frameworks.

Theoretical and Practical Implications

The potential underestimation of geomagnetic storm severity based on traditional CME parameters signifies a need for an enriched understanding of coronal and heliospheric dynamics, especially when evaluating CME propagation alterations through deflections and rotations. The findings underscore the need for holistic observation strategies, encompassing multiple vantage points far afield from the Sun-Earth line, to preemptively identify and mitigate space weather impacts.

Future advancements in AI capabilities could augment prediction precision by assimilating expanding data troves via advanced algorithms and learning models. This could enhance our capacity to anticipate complex dynamical behavior in CMEs with high fidelity.

Conclusion

In conclusion, this paper of the April 21, 2023 CME exemplifies the nuanced interplay of solar phenomena influencing space weather outcomes. It calls for advancements in observational practices and models, pressing the necessity for integrating wide-field, multi-perspective data assimilation. High-resolution remote sensing, coupled with robust theoretical frameworks, remains vital to advancing space weather forecasting precision and contributing to our greater understanding of solar-terrestrial interactions.