MIST

By Ardra Ramachandran (University of Warwick)

Solar wind charge exchange (SWCX) is a process in which X-rays are produced when heavy ions in the solar wind interact with neutrals in the solar system. Earth has a neutral hydrogen exosphere; when solar wind heavy ions interact with this hydrogen, they produce soft X-rays in the range of 0.1–2 keV. Earth’s magnetosphere acts as a shield that prevents the solar wind from penetrating further, making these X-rays a useful way to trace magnetospheric boundaries, including the magnetopause and cusps.

This study discusses the new Gorgon-XIM model, which combines magnetohydrodynamics (MHD) with a test-particle model to simulate SWCX X-rays from Earth’s magnetospheric boundaries. We compare XIM simulations with pure MHD simulations, which are less computationally expensive and treat magnetospheric plasma as a single fluid. We also discuss the line-of-sight (LOS) images and spectra produced by XIM, which enable us to compare the model with observations from space-based X-ray observatories of the magnetosphere, such as the newly launched SMILE mission. We conclude that XIM captures particle kinetic effects and the dependence of SWCX on heavy-ion species, which are not represented in the MHD model. Variations in the X-ray spectra can also be used to diagnose plasma dynamics and distinguish between different types of solar wind.

See publication pre-print for more details:

https://doi.org/10.48550/arXiv.2609.35098

Line-of-sight (LOS)-integrated SWCX emission maps. Panels (a)–(c) show the results for Mg¹¹⁺, C⁵⁺, and the MHD-only emission for C⁵⁺, respectively, for a satellite at (−10 Rₑ, 0 Rₑ, 10 Rₑ). The red box in these panels represents the SMILE field of view (FOV) for reference. Panels (d)–(f) show the corresponding results for a satellite at (−10 Rₑ, 10 Rₑ, 0 Rₑ). Both cases look toward (−10 Rₑ, 0 Rₑ, 0 Rₑ).

By Sam Farr (Lancaster University)

In 2017, Cassini executed its final twenty-two low-altitude passes over the auroral regions of Saturn. This enabled the determination of Saturn's field-aligned currents (FACs) from magnetometer measurements at lower altitudes than previously achieved. We investigate southern dusk-side FAC behaviour with four objectives: how in situ MAG measurements compare to the highly variable auroral oval previously imaged, what the typical behaviour of the FACs is, the degree of influence Saturn's unique PPO FAC systems have in this region, and how these regions' FACs respond to the solar wind.

We found that this region consists of a mix of thin, filamented, field-aligned currents flowing upward or downward. A preference toward upward FACs near the poleward edge of our data set was observed, with those upward FACs showing greater magnitudes than downward ones at similar latitudes. The PPO systems drive periodic auroral emission and fluctuations in the magnetic field. We modelled their expected contributions and found that they aren't major drivers of filamented FAC behaviour at dusk, but do influence broader structures. During periods when the solar wind compresses Saturn's magnetic field, the FAC structure initially exhibits stronger, broader filaments. After two Saturn rotations beyond compression onset, these enhanced filaments are no longer observed. We see evidence of a broad increase in upward-directed FACs across the auroral region during these compressions, and of PPO-driven shifting of the auroral oval.

See publication for more details:

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2026JA035533

Distribution of current sheet density magnitude versus footprint colatitude for two regions roughly corresponding to pre-dusk (16-19LT) and post-dusk (19-22LT). A smoothing algorithm was applied to determine FAC boundaries, and current sheet density was determined at set colatitude points. This process was repeated for all orbits. Panel (a) combines upwards and downwards field-aligned currents in the pre-dusk sector in orange and teal, respectively. Panel (b) is the same for post-dusk. The horizontal black lines in each bin show the median values for the positive and negative distributions independently; the darker-coloured shaded region shows the interquartile range; and the light-coloured shaded region shows the full range. Small crosses show individual datapoints.