MIST

By Victor Steinwand (Imperial College London)

The ESA/Rosetta mission escorted comet 67P/Churyumov─Gerasimenko for 2 yr, exploring its plasma environment across diverse outgassing conditions. Plasma density observations from the Rosetta Plasma Consortium (RPC) are broadly categorized into two regimes for the ion dynamics, linked to the presence of a diamagnetic cavity at Rosetta's location. With a diamagnetic cavity present, ions detected by Rosetta are accelerated with respect to the neutral coma. Without a diamagnetic cavity present, at lower outgassing, and nearer the nucleus, ions co-move with the neutrals. We examine the transition between regimes following Rosetta's last detection of the cavity in 2016 February. During this transition, global 3D plasma models of the cometary ionosphere underestimate plasma densities. To investigate this underestimation, we assess the sensitivity of cometary ion densities to different parameters using a 3D collisional ion test particle model, driven by electromagnetic fields from hybrid modelling. We show that considering cometary electron cooling is necessary to model cometary ion dynamics within 100 km of the surface. Electron temperatures derived from collisional electron modelling affect ion dynamics via the ambipolar electric field, increasing ion number densities. We further show that the cometary electron cooling exobase organizes Rosetta plasma density observations; different ion dynamics regimes are linked to the position of Rosetta relative to the exobase. These findings demonstrate that Rosetta was below this exobase for much of the post-perihelion period. They justify the absence of ion acceleration in plasma density assessments and the use of uniform electron-impact ionization frequencies between Rosetta and the surface during post-perihelion.

See publication for more details:

https://academic.oup.com/mnras/article/550/2/stag1256/8723309

Two-dimensional histograms of post-perihelion number density data from 2016 February 17 to 2016 September 30, for a total ion production rate 19.85 ≤ log10(νQ[s−2]) ≤ 20.15, showing the organization of the data in the parameter space r/rexo against u_i r/rexo = (ν/100σn_i)(1 − rc/r). The boxes and points are values of (ν/100σni)(1 − rc/r) binned in r/rexo. The local gradient in this parameter space gives the local ion velocity u_i. The straight line represents ions moving at the speed of the neutral background u_n. Below the exobase, the data are clustered around the ui = un line, showing the utility of the acceleration-free model inside the flattened region of the ambipolar potential well. They depart from the line above r ∼ rexo, indicating acceleration.

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.

By Rosie Hodnett (University of Leicester)

Omega bands are wave-like structures in the aurora which drift eastward in the auroral dawn sector. Omega bands carry pairs of upward and downward field aligned current (FAC). This moving current structure causes ground-based magnetic perturbations, which can be observed in magnetometer data, especially in the Y/eastward component (b). The perturbations can be large, resulting in large spikes of dB/dt (c). Spikes in dB/dt can cause geomagnetically induced currents (GICs) in ground-based infrastructure such as in the power grid, and so can be damaging.
In this paper, we investigate three omega band events which have different values of dB/dt. Using ground-based magnetometer data, we show that the events with larger spikes in dB/dt occur when the eastward speed of the omega bands is faster. This occurs when there is strong driving of the magnetosphere, for example during a geomagnetic storm, which leads to greater ionospheric convection speeds and hence greater omega band speeds. Additionally, EISCAT (European Incoherent SCATter radar) data shows large enhancements of electron density at low altitudes (a).

We also find that omega bands are associated with both electron (f) and proton (e) emissions, suggesting that they have a complicated current structure. Additionally, we show that omega bands are visible in the region 1 region 2 FAC boundary in AMPERE (Active Magnetosphere and Planetary Electrodynamics Response Experiment) data (d).

See publication for more details:
Hodnett, R. M., Milan, S. E., Vines, S. K., Gjerloev, J. W., & Paxton, L. J. (2026). A Multi-event comparison of dB/dt resulting from omega band aurora. Journal of Geophysical Research: Space Physics, 131, e2026JA035740. https://doi.org/10.1029/2026JA035740

(a) EISCAT very high frequency electron density measurements of omega band aurora on 2012-09-05. (b-c) Tromsø magnetometer data and dB/dt for 2012-09-05. (d) Keogram of AMPERE region 1/ region 2 FACs at 06 MLT (dawn sector) on 2012-07-15. (e-f) DMSP SSUSI data showing omega band aurora on 2012-07-14.