MIST

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.

By Samuel Wharton (University of Leicester)

Magnetosheath jets are regions of enhanced solar wind density that form inside the magnetosheath and can locally compress the magnetopause, generating space weather effects. The higher density of the jet should result in it being brighter in soft X-rays due to the greater occurrence of the solar wind change exchange mechanism. It has been theorised by many authors they might be visible to a soft X-ray imager.
We simulated magnetosheath jets and calculated the X-ray flux expected from them at a soft X-ray imager. We found that it was difficult to resolve the jets due to the LOS integration effect and the noise within the images. However, viewing jets is more likely from viewing angles where the path length through the magnetosheath is short and there is a strong contrast between the jet and its surroundings in the image. Strong solar wind driving is also required so the emission is greater than the astrophysical background. This could be achieved with a larger telescope than SMILE-SXI that would be practical to build.

See publication for more details:
https://academic.oup.com/rasti/article/doi/10.1093/rasti/rzag059/8760917

Simulations of a magnetosheath jet seen from SMILE-SXI from three different viewing positions. The left column shows the intensity of X-rays entering the telescope. The middle column shows the expected count rate on the detector without noise. The right column shows a realistic image with Poisson noise applied.

By Mike Lockwood (University of Reading)

We have studied the evolution of the debris cloud generated by the high-altitude Anti-Satellite (ASAT) test on the Fengyun-1C satellite over the subsequent 18 years. More of the objects at sizes above 10cm have survived than was predicted four years after the ASAT test, despite the average space weather activity being higher than was assumed in making those predictions. We show how debris accelerated by the test into high-apogee orbits has acted as a reservoir, giving a long-lived supply of objects ready to start the reentry spiral. The average rate of descent varies with the solar cycle and with the solar rotation period because of the variation in EUV heating of the thermosphere. Geomagnetic storms
have a short-lived but significant effect on the altitude decay of the debris, the upper decile of geomagnetic activity accounting for 12% of the lost altitude at 400 km, rising to 19% at 700km. A second, more subtle, geomagnetic effect is seen to be present, identified as being driven by the Russell-McPherron effect on solar wind-magnetosphere couplingand thermospheric heating. The contribution of this mechanism to the semi-annual variation in average debris descent speed is greatest at high altitudes but other thermospheric effects, such as wave and tide dissipation and circulation changes, are more important at lower altitudes. At times of low solar activity the EUV heating is the dominant effect but the total geomagnetic contribution is more significant during disturbed times. We discuss how these results are of importance to controlled de-orbiting of space junk.

References
M. Lockwood, C. J. Scott, J. O'Donoghue, M. J. Owens, and L. A. Barnard (2026)
The contribution of the Russell-McPherron effect to the semiannual variation in thermospheric density, J. geophys. Res. Space Phys., 131, e2025JA034732, doi: 10.1029/2025JA034732
M. Lockwood (2025) The Celestial Rubbish Dump, Astronomy and Geophysics, 66 (3), 3.36–3.42, doi: 10.1093/astrogeo/ataf023

See publication for more details:
M. Lockwood, M.J. Owens, C. Saha, L.A. Barnard, C.J. Scott, and J. O’Donoghue (2026) Space Weather, Submitted

Comparison of days with predominant positive (left) and negative (right) IMF Y-component in the GSEQ frame. Parts A and D show the FY-1C debris descent speed, v, as a function of altitude, h, and fraction of a calendar year, F, where its annual variation due to Sun-Earth distance a (as a function of h) has been subtracted to reveal the semi-annual variation. Parts C and F show the height profiles of (v-a) at the dates of peak Russell-McPherron effect in March (green) and September (mauve). Parts B and E show the variation with F of the average (v-a) over the full height range. The Russell-McPherron effect enhances the March/September peak when the Y-component in GSEQ is negative/positive and so can be clearly seen to be driving a semi-annual variation in debris descent speed at higher altitudes.