Giant Kelvin-Helmholtz waves at the Boundaries of the Mother's Day 2024 Coronal Mass Ejections Driving Geoeffectiveness and Motivating the sub-L1 Space Weather Measurements

Starting in the evening of 10 May 2024 the Earth's magnetosphere was hit by the coronal mass ejections (CMEs) creating the largest geomagnetic storm since the Halloween Storm of 2003. The CME encounter was characterized by variations of plasma number density and magnetic field. Here, I present the ARTEMIS observations at the lunar orbit during this event and the MMS observations closer to the bow shock. The IMF Bz ranged from −60 to +40 nT both with hour to minutes periodicity with plasma jets propagating in +-z-direction within multi-scale wave structures. Similar signature has been recently reported at the magnetopause by MMS spacecraft (Li et al., 2023, https://doi.org/10.1029/2023GL105539; Nykyri, 2024, https://doi.org/10.1029/2024GL108605) of the Kelvin-Helmholtz (KH) wave observations during a strongly southward IMF. Here, I show that the CME boundaries were KH unstable leading to multi-scale density and magnetic field fluctuations including reconnection jets, with clear density compressions when spacecraft moved from southward ejecta field into oppositely orientated, draped sheath field region -a characterisitic signature of plasma compression driven by the KH waves. The wavelengths varied from 60 to 270 Re, suggesting that the magnetosphere was periodically exposed to successive intervals of strongly northward and southward IMF leading to enhanced mass and magnetic flux loading, enabling the strongest ring current growth in 20-years. The source region of the wave growth, driven by the sheared plasma flows at the CME boundaries by the KH-instability, was estimated to be about ~7 million km usptream of the Earth-Sun Lagrange 1 point, motivating the need for the new sub-L1 spacecraft constellations, allowing ~3-5 hr space weather predictions of the time-scale of the IMF Bz and By  variation estimates.