MIST

By Martin Cafolla (University of Warwick)

The ionosphere is a portion of the upper atmosphere of the Earth consisting of free electrons and ions as a result of exposure to solar radiation. The variability of intensity of this radiation, as well as the differing levels of geomagnetic activity, results in fluctuations in electron number density across the ionosphere, characterised by the Total Electron Content (TEC). We define High Density Regions (HDRs) of TEC, that is regions of enhanced line-integrated electron number density, as the top 1% of TEC measurements from Global Ionospheric Maps (GIMs). We then construct an algorithm that isolates, detects and tracks these regions for 20 years of TEC data in sun-centered geomagnetic coordinates. This produces a contiguous set of uniquely labelled space-time TEC HDRs. We conduct a statistical study to determine reproducible trends in HDR formation location, trajectories and durations for different levels of geomagnetic activity at continental and sub-continental (small) scales.

We find that HDR formation is primarily driven by the sub-solar point, spanning the afternoon ionosphere, and in general occurs around four magnetic latitude clusters. Small HDRs typically move along lines of constant magnetic latitude in the direction of Earth rotation, while continental scale HDRs have much more complex paths. The statistical nature of our results provides a probabilistic prediction on future HDR behaviour, offering an ensemble constraint on enhancements seen in ionospheric models.

Example TEC map for 2009-07-23 at 16:30:00 UTC. Panel (a) plots the geographic TEC map. Each 1 degree by 1 degree grid point plots the Vertical TEC at that longitude/latitude. Black triangles show the locations of ground stations. Panel (b) plots the geomagnetic TEC map with grey contours at the top 1% value of TEC in the map, defining the High Density Regions (HDRs). Panel (c) isolates these HDRs and plots them in black, with green contours drawn around each black region to demonstrate the algorithm’s contour detection. Panel (d) plots the isolated HDRs in SM coordinates with bounding rectangles in blue and centroids marked in black, obtained by the detection/tracking algorithm.

See publication for details:
Cafolla, M. A., Chapman, S. C., Watkins, N. W., Meng, X., & Verkhoglyadova, O. P. (2025). Dynamics of TEC high density regions seen in JPL GIMs: Variations with latitude, season and geomagnetic activity. Space Weather, 23, e2024SW004307. DOI: 10.1029/2024SW004307

By Henrik Melin (Northumbria University)

The molecular ion H3+ is a significant component of the ionospheres of the giant planets. By observing the near-infrared emission from this ion we can remotely diagnose the physical conditions of this region. This layer of the atmosphere is an importance conduit for energy transfer between the space environment, magnetic field, and the atmosphere below, and it is in this region that magnetospheric auroral currents deposit energetic electrons that form enhanced temperatures and densities of H3+.

H3+ was discovered at Jupiter, Saturn, and Uranus over 30 years ago and a great number of studies have been able to characterise the processes that occur in the ionospheres of these planets. However, despite many attempts using telescopes on the ground, H3+ has never been observed from Neptune, in spite of models predicting it should be detectable, based on data from the 1989 Voyager 2 encounter.

The James Webb Space Telescope (JWST) is the most powerful telescope ever put into space, designed to observe the first galaxies formed in the early Universe. We can leverage this extraordinary sensitivity to explore our own cosmic backyard, by pointing the telescope at the giant planets. The first JWST observations of Neptune were taken in June 2023, and we were able to detect H3+ for the first time (yay!), as well as localised H3+ emissions about the magnetic pole. In other words, we were able to detect the ionosphere and aurora of Neptune for the first time, exactly 100 years after it was discovered at the Earth (Appleton & Barnett, 1925).

See publication for details:
Melin et al., (2025), Nat. Astro., doi: https://doi.org/10.1038/s41550-025-02507-9

By Simona Nitti (University of Leicester)

Continuous monitoring of the solar wind ion composition is vital for understanding solar-terrestrial interactions, particularly through the study of Solar Wind Charge Exchange (SWCX) emission. SWCX produces soft X-rays (<2 keV) when highly charged solar wind ions (e.g., O7+, O8+, C6+) interact with neutral atoms. This emission is ubiquitous across the solar system, occurring wherever the solar wind encounters interstellar neutrals or interacts with planetary environments, including those of Earth, Mars, Venus, Jupiter, and Pluto.

In this study (https://doi.org/10.1029/2024JA033323), we investigated whether SWCX emission from Earth’s exosphere, observed by the XMM-Newton telescope, can be used to infer solar wind composition. By comparing spectral line intensities extracted from XMM data with ion abundance measurements from the ACE spacecraft at L1, we found that OVIII emission closely tracks O8+ abundances. In contrast, other ions involved in the SWCX process—such as O7+, C6+, C5+, and Mg11+—do not exhibit a consistent correlation between their abundance and X-ray emission.

To explore whether XMM data still encodes information about the solar wind state, we employed a Random Forest Classifier to predict solar wind types, following the classification scheme by Koutroumpa (2024). Incorporating XMM features alongside proton parameters significantly improved model performance, with a macro-averaged F1 score of 0.80 ± 0.06, compared to 0.55 when using proton data alone. Notably, XMM emission features ranked among the top five most important inputs. Moreover, XMM emission features ranked among the top five most important predictors. This suggests that, while individual ion abundances cannot currently be inferred directly from emission fluxes, XMM still provides valuable insight into the solar wind type, from which an average compositional profile may be inferred. This work is particularly timely given the degradation of the heavy ion spectrometer onboard the Advanced Composition Explorer (ACE), which has left the scientific community without reliable near-Earth ion composition measurements since 2011.

(a) SWCX periods ACE O8+/p (where p is proton density) 2D histograms, averaged over their occurrence between 2000 and 2009, in a (O7+/O6+)×(C6+/C5+) space, with black lines separating different solar wind types.
(b). Occurrence rate of ACE O8+/p for the Streamer (black), Outlier (red) wind and ICMEs (green).
(c) (d) Same as left plots but for OVIII ion line fluxes from the XMM SWCX data set.

See publication for details:

Nitti, S., Carter, J. A., Sembay, S. F., Milan, S. E., Zhao, L., Lepri, S. T., & Kuntz, K. D. (2024). Can XMM-Newton be used to track compositional changes in the solar wind? Journal of Geophysical Research: Space Physics, 129, e2024JA033323. https://doi.org/10.1029/2024JA033323 

By Cristian Radulescu (Mullard Space Science Laboratory, UCL)

Pick‐up ions (PUIs) are charged particles that have been accelerated by electric and magnetic fields beyond the speed of the bulk plasma and then proceed to thermalize through wave‐particle interactions. In this paper we investigate the distribution of PUIs in Saturn’s magnetosphere. We specifically look at water group PUIs as these are expected to be the most prevalent due to the majority of plasma in the inner to middle magnetosphere of Saturn coming from the moon Enceladus. Data from the Cassini spacecraft’s CAPS instrument is used, with the bulk plasma population and penetrating radiation subtracted, spanning radial distances from 3 to 10 Saturn radii (R_S) and the time period between 2005 and 2012. Ten-minute intervals of this data are plotted on radial v_perp vs v_parallel plots. The width in pitch angle (PA) and velocity are measured and then distributed into 0.2 R_S by 2° bins on global maps of the Saturnian magnetosphere. We find that the PUI distributions broaden with increasing latitude creating a “Pacman”-like signature. This pattern is approximately anti-correlated with the amplitude of ion cyclotron waves (ICWs). The explanation for this phenomenon lies in cyclotron resonance between the PUIs and ICWs. ICWs get driven/damped by the PUIs resulting in energy transfer between them and scattering of the PUIs in PA. The strongest resonance, and hence scattering,  is expected in areas of high ICW amplitude, which is close to the equator. We find that this is not the case here because the PA scattering time is much longer than the PUI bounce period meaning ions reach high latitudes by the time they get scattered by the equatorial ICWs. In contrast, the half-widths of the velocity distributions remain mostly constant, increasing only past the orbit of Rhea, likely because of elevated plasma temperatures.

Global maps of the Saturn magnetosphere with top-down (left) and side (right) views. a)-b) show the PA distributions shown as the full width at half maximum (FWHM) of
the distribution at the injection velocity at the middle of the measured interval. c)-d) show the velocity distributions shown as half width at half maximum (HWHM) of the distribution. e)-f) show the average amplitude of ion cyclotron waves (reproduced from Long et al. (2022)). The grey bins in f) are bins where measurements were made, but no waves were detected. In the top down view, the inner black circle is the orbit of Enceladus, the white circle is the orbit of Dione and the outer black circle is the orbit of Rhea.

See publication for details:
Radulescu, C. R., Coates, A. J., Simon, S.,Verscharen, D., & Jones, G. H. (2025).Pick‐Up ion distributions in the inner andmiddle saturnian magnetosphere. Journalof Geophysical Research: Space Physics,130, e2024JA033390. https://doi.org/10.1029/2024JA033390