Katariina Nykyri

Along the flank magnetopause, magnetosheath flow interfaced with the relatively stationary magnetosphere produces strong flow shears, and the Kelvin‐Helmholtz instability (KHI) occurs. When the interplanetary magnetic field (IMF) is southward, anti‐parallel day‐side reconnection and KHI can interact, which may enhance the reconnection rate as KHI compresses the current layer and increases the length of the x‐line. If this reconnection transports sufficient magnetic flux into the magnetotail, it may disrupt the magnetotail current sheet. This study examines a Multiscale Atmosphere Geospace Environment (MAGE) simulation of the 10–11 October 2024 geomagnetic storm to quantify the contribution of magnetic flux transported into the magnetotail that is generated where KHI and the anti‐parallel x‐line are co‐located. During a 15‐min interval, the KH unstable dawn flank magnetopause reconnects ∼13% of the flux added to the northern lobe from the subsolar region, or ∼5% of the total magnetospheric open flux.

Along with magnetic reconnection, Kelvin‐Helmholtz (KH) waves are the main mechanisms controlling the solar wind‐magnetosphere interaction, enabling plasma transport into the magnetosphere due to secondary reconnection, diffusion and wave‐particle interactions. In this paper we use global magnetohydrodynamical (MHD) simulations for studying how the Kelvin‐Helmholtz instability (KHI) is modulated by interplanetary magnetic field (IMF) during negative dipole tilt. We find that KH wave activity on the magnetopause maximizes in the winter hemisphere and at dawn sector for positive IMF . These asymmetries of KHI are caused by dawn‐dusk asymmetries of velocity shear on the magnetopause and draped IMF in the magnetosheath. These factors are linked to magnetic reconnection geometry, which also creates a broader boundary layer, slowing the growth of KHI at dusk for positive . These results are important, for example, for understanding field‐aligned currents generated by KH vortices and their ionospheric effects during different seasons and IMF conditions.

During fast mode intervals the Magnetospheric Multi‐Scale (MMS) Fast Plasma Investigation (FPI) operates continuously in ion burst capacity at 150 msec cadence. From the burst data and at the same cadence, FPI computes four ion moments (ion density, ion flux) termed trigger data. Only a few percent of the full burst data are downloaded, but trigger data are downloaded over the entire fast mode interval. The trigger data were not envisioned as a science product, so the sign bits of the 3 ion flux components were dropped, and only absolute values were telemetered to the ground. Using 4.5 s FAST mode data and minimization techniques, we reconstruct the ion flux sign bits and computed ion velocity, validating the methodology through comparisons with burst mode velocity for 103 burst mode intervals. We demonstrate that calibrated FPI “trigger” data are scientifically valid in the magnetosheath and its boundary layer, and particularly useful for wave studies whose frequency is greater than 0.1 and less than 3 Hz. As an example of the use of the trigger data, we present an analysis of ion cyclotron waves (ICW) observed by the MMS constellation on 7 October 2015, in the afternoon magnetosheath.

The dayside lunar photoelectron sheath generates near-surface electrostatic fields that reflect a fraction of solar wind electrons and accelerate low-energy electrons. As the Moon passes from the solar wind through the magnetosheath and into the magnetotail plasma sheet, electron temperature ( T _e ) and density ( n _e ) can vary by more than 2 orders of magnitude, which leads to large variations of the lunar photoelectron sheath. In order to prepare for a human presence at lunar orbit, this paper quantifies the extremes of T _e and n _e observed by Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) extending over an entire solar cycle. A statistical analysis of ARTEMIS P1 data shows that in solar wind, T _e is in the range 1.6–76 eV and n _e is in the range 0.06–103 cm ⁻³ . In the magnetotail (including magnetosheath, lobe, and plasma sheet), T _e is in the range 6–1800 eV and n _e is in the range 0.03–20 cm ⁻³ (0.03 cm ⁻³ is the instrument threshold, so the lowest density is likely smaller than this). These values were sustained for 1 minute such that the entire moon was immersed in plasma with a particular T _e or n _e . The solar wind T _e and n _e do not have a strong correlation with the solar cycle, but in the magnetotail near midnight, the range of T _e and n _e is significantly larger at solar minimum compared to solar maximum.

On 10 May 2024, Earth was hit by a CME that drove the largest geomagnetic storm in 20 years. Multi‐spacecraft observations previously showed that the ∼100 nT north‐south IMF variation was driven by Kelvin‐Helmholtz waves with wavelength ∼250 and reconnection jets in the ‐direction (Nykyri, 2024a, https://doi.org/10.1029/2024gl110477 ). Here we show that Magnetospheric Multiscale (MMS), located upstream of the Earth's bow shock at , detected additional reconnection jets not seen by ARTEMIS at lunar orbit ∼60 away, perpendicular to CME propagation. One jet coincided with high‐energy ions and electrons streaming anti‐parallel to the magnetic field. The ions showed dispersion followed by apparent counter‐dispersion, whereas electrons above 100 keV arrived nearly simultaneously. Strong electromagnetic wave activity between these phases is consistent with magnetosonic‐Whistler waves, suggesting that MMS traversed very near the particle acceleration region.

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 jz (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.2024.1395775Grimmich, 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/10.1029/2023JA031603Ivanovski, 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

Identifying the primary physical mechanisms and associated secondary processes (e.g., different plasma wave modes, instabilities and magnetic reconnection), that are responsible for plasma heating at the planetary magnetospheres and astrophysical plasmas is often difficult with a single spacecraft or even with a constellation of spacecraft within a single‐plasma scale. Here, we present a statistical study using data from four Magnetospheric Multiscale (MMS) satellites to characterize the cross‐scale plasma‐heating distributions associated with Kelvin‐Helmholtz instability (KHI) and drift mirror instability (DMI). We find that characteristic heating frequency (CHF) distributions follow distinct tail‐scaling regimes: KHI events exhibit low‐ power‐law‐like tails, whereas DMI events occupy larger‐ regimes closer to quasi‐Gaussian behavior. Comparison of a KHI event with Particle‐in‐Cell (PIC) simulations yields consistent tail‐scaling, supporting the physical interpretation. We also analyze an event with periodic encounters of diamagnetic cavities (DMCs) at the Earth's dayside magnetopause, and find that the CHF scaling together with KH growth‐rate analysis is consistent with KHI modulation of reconnection. These results establish statistical discrimination between KHI‐ and DMI‐associated heating, and can be used in future works to establish an automated identification of primary mesoscale processes driving cross‐scale non‐adiabatic heating in collisionless plasmas.

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.

An important mechanism for the transfer of energy across the boundary between the Earth's magnetic field and the solar wind involves the formation of waves and vortices at the magnetopause. These waves and vortices arise from the Kelvin–Helmholtz instability, which is caused by the difference in velocity between the magnetospheric plasma and the shocked solar wind plasma. From spacecraft observations and simulations, we know that Kelvin-Helmholtz waves can evolve and grow differently depending on their formation conditions and locations. Specifically, evidence from simulations indicates an impact from the foreshock on the development of the waves. However, this has not yet been fully confirmed by observations. Using the extensive in-situ data from the last solar cycle, we can compare the parameters of 3,335 KHI observations under different conditions. By applying different methods to determine the necessary plasma conditions at the boundary in wave parameter calculations, we are able to accumulate statistical evidence indicating whether the wave parameters of Kelvin–Helmholtz-induced waves are altered in the presence of a foreshock region upstream of the magnetopause. Our results suggest that, under certain solar wind conditions, the presence of the foreshock indeed alters the typical wave parameters of Kelvin-Helmholtz waves. This further reaffirms that the presence of the foreshock must be considered when understanding solar-terrestrial interactions.