Exploring the development of Kelvin-Helmholtz instability at the Earth's magnetospheres during intense space weather events

Extreme Space Weather (SWE) events have a crucial role in shaping the dynamics of Earth's magnetospheric boundary layer. Under such conditions, several plasma processes can be triggered, including the Kelvin-Helmholtz instability (KHI). This instability arises from the velocity shear at the boundary of two regions: the nearly stagnant magnetosphere (MSP) and the anti-sunward streaming magnetosheath (MSH).

KHI can grow into finite-amplitude Kelvin–Helmholtz waves (KHWs), which may subsequently roll-up into large-scale vortices (KHVs). These vortices can twist magnetic field lines and trigger vortex-induced tearing mode instability (TMI). In the context of planetary magnetospheric dynamics, such instabilities are fundamental because they (i) drive substantial mass, energy, and momentum transport from the MSH into the MSP; (ii) generate ultra-low-frequency magnetospheric waves; and (iii) drive field-aligned currents (FACs), that are the major means of coupling the magnetosphere to the ionosphere, contributing to the auroral activity.

In this work, we analyze two prominent SWE events that occurred in January and November 2025, during which the Sun produced some of the strongest flares of Solar Cycle 25, associated with Earth-directed coronal mass ejections (CMEs).

In the January event case, we combine in-situ magnetospheric observations from MMS with ionospheric measurements from Swarm in order to assess the generation of FACs (Figure 1) due to the twisting and shearing of magnetic flux tubes by KHVs. To confirm the development of the vortices and the correpsonding currents firstly we use a magneto-hydrodynamic (MHD) model (Figure 2), MIM (Biasiotti et al. 2024, Ivanovski et al. 2011), and then we examine ionospheric irregularities using data from a network of ground-based observatories, including GNSS receivers providing total electron content (TEC) and scintillation measurements, as well as ground magnetometer observations of the horizontal equivalent ionospheric currents (EIC) and the vertical spherical elementary current (SEC).

Whereas, for the November 2025 SWE, we investigate the development of KHI following the arrival of a interplanetary CME combining in-situ observations from THEMIS (Figure 3) with magneto-hydrodynamic (MHD) simulations, in order to assess wheter the periodic variations of the IMF Bz expose the magnetospheric system to successive intervals of magnetic flux and mass loading, which can enhance the ring current  (and hence the strength of the geomgnetic storm) through plasma energization driven by tail reconnection, as proposed by Nykyri (2024). More specifically, intervals of southward IMF enable magnetic flux entry via dayside reconnection, while subsequent northward IMF can facilitate plasma transport from the flanks into the plasma sheet due to KHI.

Figure 1. Geographic maps of J acquired by the Swarm constellation during orbits 19 and 20 (from T₀ at 16:55 UT) in the Northern hemisphere. The red and yellow points mark the ionospheric region corresponding to that identified by the TA15 model, and the longitude of the subsolar point, respectively.

Figure 2. Temporal evolution of plasma density (upper panel) and j_z (lower panel), from 1 to 15 s. Kelvin-Helmholtz vortices are formed rapidly at 5 s. As the KHVs develop, twin-vortex structures, i.e. oppositely rotating current cells, are generated.

Figure 3.  Overview of ACE real-time solar wind data and THEMIS observations. SYM-H index from OMNI (a), ACE data of magnetic field magnitude and components (b), ion density (c), ion bulk velocity (d), and ion temperature (e). Position of THA and THD in respect to the magnetopause (f). THA/THD observations of ion energy spectrogram (g/m), ion velocity (h/n), ion density and temperature (i/o), magnetic field components and magnitude (j/p), the mangetic field normal direction estimated with MVAE method (k/q) and the total pressure of the plasma (l/r) during the interval 14:05-15:05 UT on 13 November 2025. The coloured bars at the top show a classification of the THEMIS data based on (Grimmich et al., 2023), indicating whether it is more likely to be magnetosheath data (orange) or magnetosphere data (blue).

 

Acknowledgment:

This research has been carried out within the framework of the Space It Up project funded by the Italian Space Agency, ASI, and the Ministry of University and Research, MUR, under contract n. 2024-5-E.0 - CUP n. I53D24000060005.

 

References: 

Biasiotti L, Ivanovski S, Calderone L, et al., 2024, Evidence of Kelvin-Helmholtz and tearing mode instabilities at the magnetopause during space weather events, Front. Astron. Space Sci., 11:1395775, doi: 10.3389/fspas.2024.1395775

Grimmich, N., Plaschke, F., Archer, M. O., et al., 2023, Study of extreme magnetopause distortions under varying solar wind conditions, Journal of Geophysical Research: Space Physics, 128, e2023JA031603, https://doi.org/10.1029/2023JA031603

Ivanovski, S., Kartalev, M., Dobreva, P., et al., 2011, Coupled Kelvin-Helmoltz and tearing mode instabilities in the magnetopause layer, JTAM, 41(3), 31–42.

Nykyri, K.,2024, Giant Kelvin-Helmholtz (KH) waves at the boundary layer of the coronal mass ejections (CMEs) responsible for the largest geomagnetic storm in 20 years, Geophysical Research Letters, 51, e2024GL110477, https://doi.org/10.1029/2024GL110477