MIST
Magnetosphere, Ionosphere and Solar-Terrestrial
Analysis of Chorus Wave Power on Burst‐Mode Timescales During the Van Allen Probes Era
Analysis of Chorus Wave Power on Burst‐Mode Timescales During the Van Allen Probes Era
By Rachel Black (University of Exeter/British Antarctic Survey)
Interactions between whistler‐mode chorus waves and electrons are a key driver of dynamics in Earth’s radiation belts. These global dynamics are often described using Fokker‐Planck diffusion models. Whilst, in many cases, such models effectively describe the large scale changes within the region, they often rely upon spatially and temporally averaged representations of the wave properties. However, observations have shown that whistler‐mode chorus can display large sub‐second powers that challenge model assumptions and potentially give rise to non‐diffusive processes.
In this work, we investigate the power of whistler‐mode chorus on sub‐second timescales using the high‐resolution data capture mode on the Van Allen Probes’ Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS). We show that peak chorus power on sub‐second timescales is regularly larger than the corresponding spacecraft “survey” power by over a factor of 100. The work also explores the magnetospheric conditions under which the largest sub‐second power variability of chorus waves is observed, and we find that trends vary across different chorus frequency bands. Notably, the largest powers are observed in the lower‐band frequency range during active conditions and between 21:00–12:00 MLT, where >46% of burst samples contain an instantaneous wave intensity that exceeds 2.25 × 104 pT2. Further, binning the lower‐band power by the ratio of plasma‐to‐gyrofrequency separates the waves into two distinct low and high variability populations. The results quantify sub‐second wave power variability that may influence energetic electron dynamics not currently captured in time‐averaged wave models.
See publication for more details:
Black, R., Allanson, O., Meredith, N. P., Hillier, A., & Hartley, D. P. (2026). Analysis of chorus wave power on burst-mode timescales during the Van Allen Probes era. Journal of Geophysical Research: Space Physics, 131, e2026JA035082. https://doi.org/10.1029/2026JA035082
Chorus-containing records in the burst-mode measurements from the Van Allen Probes' EMFISIS instruments when at equatorial latitudes ($|\lambda_m|<$6.$^\circ$). Chorus emissions are divided into low frequency, lower-band and upper-band frequency ranges. For each frequency range, the subpanels show (a)-(c) average chorus power for corresponding survey-mode events; (d)-(f) maximum chorus power from burst-mode events; (g)-(i) ratio between the maximum burst power and the survey power; and (j)-(l) the normalized inter-quartile range ($\frac{Q3-Q1}{Q2}$) for each burst record.
Soft X-Ray Emission from Saturn's Magnetosheath II: Solar Wind Driving
Soft X-Ray Emission from Saturn's Magnetosheath II: Solar Wind Driving
By Dan Naylor (Lancaster University)
Saturn’s magnetosphere is dominated by Enceladus-sourced, water-group neutrals that form a torus and extend into the magnetosheath. Soft X-ray emission can be generated in the magnetosheath due to charge exchange between highly charged solar wind ions and the neutrals. Imaging of the soft X-rays is an emerging technology that aims to provide a more global and dynamic view of the magnetosheath and, for example, give insights into the driving of the magnetosphere by the solar wind. The ESA/CAS SMILE mission has now launched and aims to image the terrestrial magnetosheath. We, along with Rogan et al. (2026, https://doi.org/10.1029/2025JA034462), explore the viability of soft X-ray imaging at Saturn. We consider charge exchange between Enceladus-sourced H, O and OH and solar wind ions O7+ and O8+ to estimate the emission rates from the system and the flux detected by a soft X-ray imager (SXI) at the system. We also vary solar wind dynamic pressure to test the effect of changing solar wind conditions on X-ray production. X-ray volumetric emission rate is on the order of 10-11 to 10-10 photon cm-3 s-1 for slow and fast solar winds. For a SMILE-like SXI imaging the system from around 50 RS, >100 photons could be detected within a quarter of a planetary rotation. A hypothetical future instrument with increased FOV and effective area significantly increases photon count rate, highlighting that X-ray imaging may be a useful technique to better understand Saturn’s magnetosphere and neutral environment on a potential future mission.
See publication for more details:
Naylor, D., Ray, L. C., Rogan, P. C., Dunn, W. R., & Smith, H. T. (2026). Soft X-ray emission from Saturn's magnetosheath II: Solar wind driving. Journal of Geophysical Research: Space Physics, 131, e2025JA034461. https://doi.org/10.1029/2025JA034461
Emission rate slices (a, b, c) in the y-z, x-y and x-z planes and modelled intensity maps (d, e, f) for a nose-on, top-down and side-on view of the system, for a SMILE-like soft X-ray imager at ~50 RS from Saturn.
Which Kelvin-Helmholtz waves grow along the spatially-varying magnetopause flanks and why?
Which Kelvin-Helmholtz waves grow along the spatially-varying magnetopause flanks and why?
By Harley Kelly (Imperial College London)
The Kelvin-Helmholtz instability mediates the viscous-like solar-terrestrial interaction, allowing solar wind plasma and energy to penetrate our magnetic shield through generating magnetopause surface waves that quickly become non-linear. Determining when and where this should occur and which wave modes grow has remained challenging. This is because the underlying theory has concentrated on local wave growth, where the locally most-unstable linear wave dominates. However, these waves travel along the boundary into new regions where the instability is still able to amplify these perturbations despite the different background properties. Two possible paradigms exist, waves are either:
(a) locally generated, being those predicted by the simple theory
(b) originate further upstream, having travelled and grown along the way
We address this conundrum by applying a machine learning technique, Dynamic Mode Decomposition, that efficiently extracts distinct wave modes from a simulation of the entire magnetosphere. This shows Kelvin-Helmholtz waves do grow quickly out of some points on the boundary, signaling local generation. However, their energy persists as they travel down the tail, slowly growing in both amplitude and spatial extent in the process due to the accelerating flow around the magnetosphere and its effect on the instability. Therefore, both effects play a role in which waves are dominant at any point.
These results may explain why longer wavelengths are observed in the tail than local theory predicts and motivates further exploration of tangential inhomogeneities in basic Kelvin-Helmholtz theory. We also highlight that Dynamic Mode Decomposition may prove a powerful technique for studying other forms of waves, instabilities and turbulence across the heliosphere.
See publication for more details:
Kelly, H. M., Archer, M. O., Eastwood, J. P., Heyns, M., Eggington, J. W. B. and Chittenden, J. P (2026). Superposition of Doppler-Shifting Magnetopause Kelvin-Helmholtz Modes Through Dynamic Mode Decomposition of a Global MHD Simulation. Geophysical Research Letters, 53, e2025GL120284, https://doi.org/10.1029/2025GL120284
Comparison of dynamic mode decomposition modes along equatorial magnetopause tangent showing (a) integrated energy densities and (b) polynomial-fit wavelengths. (c) Cartoon depicting key results.