I. Jonathan Rae

The generation and propagation of Ultra Low Frequency (ULF) waves are intrinsically coupled to the cold plasma population in the terrestrial magnetosphere. During geomagnetic storms, extreme reconfigurations of the cold plasma create a complex and dynamic system that drastically modifies this coupling. The extent and manner in which this coupling is affected remains an open question. In this report, we assess the coupling between ULF waves and cold plasmaspheric plumes during geomagnetic storms. We present a series of event studies of Van Allen Probes observations that show selected times that clear coupling between plumes and ULF waves was observed. For each event, we use inferred measurements of the cold plasma density during plume crossings, in combination with magnetic and electric field observations of ULF waves. The event studies show very different, and at times contrasting, wave behavior. This includes events where ULF waves appear to be spatially confined within plume structures. We discuss potential physical mechanisms that could generate the observed coupling. The results demonstrate that understanding cold plasma‐ULF wave coupling is a critical area of future investigations.

Coordinated spacecraft and ground-based observations provide evidence of cross-scale wave coupling and energy-dependent modulation of energetic ion and electron populations in the outer radiation belt. Using observations from Van Allen Probe B during 1030 UT - 1100 UT on April 20, 2018, we show that fast-mode compressional Pc5 ultra-low frequency (ULF) waves were excited in the outer radiation belt (5 < L < 5.5) in the post-midnight sector (2.7 < MLT < 3.1) near the magnetic equator (MLAT < 5°), with a dominant periodicity of ∼200 s. During this interval, electron and ion temperature anisotropies were elevated (>1.2), favoring growth of both whistler-mode chorus and He ⁺ -band electromagnetic ion cyclotron (EMIC) waves. Additionally, the ULF waves drove periodic in-phase modulations of electron and ion fluxes in the 10s-100s keV energy range, which produced periodic variations of both chorus and He ⁺ -band EMIC waves at the same periodicity, connecting fluid and ion- and electron-gyroscale electromagnetic phenomena, spanning six orders of magnitude in frequency (mHz-kHz). The calculated pitch angle diffusion coefficients exhibited similar periodic variations in phase with the chorus wave power, suggesting strong pitch angle diffusion. Periodic ionospheric absorption measured by the riometer at the Rabbit Lake station, which was located closest to the magnetic footprint of Van Allen Probe B, independently confirmed the resulting electron precipitation signatures. Collectively, these results demonstrate a coherent, multi-step causal chain linking compressional ULF wave excitation to atmospheric electron precipitation through cross-scale wave coupling, with significant implications for radiation belt electron dynamics and upper atmospheric chemistry.

We present a case study of large amplitude, compressional Pc5 waves observed by GOES 13 and 15 at geosynchronous orbit near noon. These waves were excited near the arrival time of two consecutive solar wind dynamic pressure spikes at the magnetopause, the second of which was associated with a large and rapid southward turning of the Interplanetary Magnetic Field (IMF). Magnetopause oscillations at a similar frequency were observed by the THEMIS spacecraft in the post‐noon magnetosphere, however without clear signatures of compressional waves. The analysis of the ground response near the GOES foot‐points, located close to the Churchill line of magnetometer stations, revealed an increase in wave power following each spike, with slight changes in polarizations. Following the second pressure spike's arrival at the magnetosphere, we found evidence of Field Line Resonances in the Northward component of the magnetic field, which suggests coupling with the compressional waves observed at GOES. We investigated potential Ultra Low Frequency drivers, and identified the most likely driving mechanism to be the drift‐mirror instability; lack of appropriate measurements however prevented us from confirming this hypothesis.

Geomagnetically Induced Currents (GICs) are a key space weather hazard to ground‐based infrastructure, and can cause mis‐operation or even equipment failure. The solar wind interacts with the Earth's geomagnetic field, causing the magnetic field at ground level to vary with time. This variability causes GICs in grounded, conducting networks via coupling with the subsurface geology. Most studies of the hazard focus on short, high amplitude GICs, but long intervals of lower amplitude GIC can also be problematic, representing a rare but known pathway to equipment damage and power failure. We evaluate an interval in June 2015 where the power network in New Zealand experienced long‐duration, steadily increasing GICs across the South Island. These GICs were up to 20A and lasted approximately 90 min during an otherwise moderate storm interval, while New Zealand was in the dawn sector. We investigate the magnetospheric cause, attributing them to a possible diversion of the ring current into the ionosphere. We test the ability of modeling and proxies to capture this hazard, finding that it can be recreated, but is dependent upon the regional geology, and requires detailed knowledge of the network configuration. We model two hypothetical examples, (a) where the magnetic perturbation is larger and (b) where a different mid‐latitude location is exposed, finding that larger currents are possible and that the vulnerability is not unique to New Zealand. Our case study highlights the limitations of geomagnetic indices, which we show to be vulnerable to influence from such rare mid‐latitude phenomena.

We present a suite of VAMPIRE (Van Allen belt Multi‐day Predictions by Implementing a Random Forest for Electrons) models capable of predicting if the outer radiation belt crosses set percentile thresholds. We use Random Forest classification models to predict if the daily ∼2 MeV electron flux level across the outer radiation belt exceeds thresholds from the 60th to the 95th percentiles. Most models show a balanced accuracy of >0.7 (>70%) at nowcasting and ∼0.6 (60%) at forecasting up to 6 days in advance, a longer forecast than current operational models. Using feature importance (mean decrease in impurity), we determine the key inputs that are important in driving increasing flux levels and over what timescales they have an impact. Crucially, we find that only the average AL index from various days beforehand is required to be able to forecast radiation belt fluxes with good skill, meaning that models such as these could be operationally viable for space weather stakeholders. We call this suite a Collection Of VAMPIRE models for Enhanced Notification.

A geomagnetic storm is a significant and prolonged disturbance of the Earth's magnetic field caused by an enhancement of the ring current. Storms are typically identified using magnetic field measurements from multiple ground‐based magnetometers, and the corresponding dates and times of multiple storms can be collated to form a storm list. There is currently no quantitative definition of a geomagnetic storm, so the contents of a particular storm list are highly dependent on the identification criteria that were used to generate it. The different definitions and identification methods can cause the physical properties of storms in different lists to vary, and these variations may be significant. We take several storm lists with different identification methods, and we compare the probability distributions of a range of solar wind variables and geomagnetic indices between those lists. We also compare the temporal behavior of the storm lists, and discuss how the differences may affect the way geomagnetic storms are defined and studied. We find that changing the definition of the quiet time has a direct impact on the start and end times of the storms, and this can result in a storm being significantly shorter when defined using a more negative quiet time definition. We also find that changes to the threshold value of minimum magnetic disturbance have a greater impact on the properties of the identified storms than changes to the quiet time definition. We provide recommendations on which storm lists to use for different scenarios.

We quantify the asymmetry between the Birkeland currents (also known as field‐aligned currents) in the Northern and Southern Hemispheres. We use data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) in 2010–2024 inclusive for both hemispheres, compute the observed asymmetry, and then subtract the modeled asymmetry. We use Block 1 data (based on the original Iridium constellation) and NEXT data (based on Iridium‐NEXT spacecraft), and compare the effect of changing the data processing pipeline on the latter (yielding NEXT v1 and NEXT v2 subdatasets). We find that the asymmetry is significantly smaller in NEXT v2 data than in Block 1 or NEXT v1 data . The previously reported asymmetry seen in Swarm is , indicating that the new data processing pipeline goes from approximately twice the Swarm asymmetry to approximately half. This may indicate that the new AMPERE processing pipeline overestimates the amount of current flowing in the Southern Hemisphere, whereas the old one underestimated this quantity. We use frequency analysis to find that the asymmetry varies with frequencies corresponding to seasonal effects, CME occurrence, Iridium constellation orbit precession, and geomagnetic storms. We find a correlation with IMF , indicating that more current flows in the Northern Hemisphere during positive and vice versa. We find that Block 1 data in 2010–2017 shows correlations with AL, AU, , PCN, and PCS index which indicate that the asymmetry is less pronounced when Dungey Cycle convection is stronger, but NEXT data in 2019–2024 do not show this.

Quasilinear diffusion coefficients can be used to model the response of charged particles to resonant wave‐particle interactions. The calculation of these coefficients is sufficiently complicated and arduous to render it prohibitive to many potential users, because of the expense in time spent developing the code. The PIRAN software package (”Particles In ResonANce”) is written using Python, and allows the user to calculate local and bounce‐averaged relativistic diffusion coefficients in energy and pitch‐angle space via the two main current proposed methods in the literature. The code is predominantly based upon the formalisms and methods presented in Glauert and Horne (2005, https://doi.org/10.1029/2004JA010851 ) and Cunningham (2023, https://doi.org/10.1029/10.1029/2023JA031703 ). We solve for diffusion coefficients using exact relativistic formulae. We use Gaussian spectra in wave frequency and in tangent of the wave normal angle and solve the full cold‐plasma dispersion relation. At present the code supports fully tested calculations for electron diffusion coefficients based on whistler‐mode waves in a fully ionized proton‐electron cold plasma. However the codebase architecture is built such that future developments to include other wave modes and other plasma compositions should involve incremental additions. The initial release of PIRAN may not have the same number of features as some other numerical codes, but is has the advantages of being a fully open‐source diffusion coefficient code that: (a) supports calculation of both local and bounce‐averaged diffusion coefficients via both of the two proposed methods; (b) is written fully in Python; (c) has detailed user pages, commit history and changelog on GitHub.

In this study, we present results using observations from a conjunction of three satellites to study the outer radiation belt electron dynamics during the April 20, 2018, geomagnetic storm. Between 0900 UT and 1230 UT, Van Allen Probe B, Van Allen Probe A, and Arase were located within similar L-shell ranges (5 – 6) but separated in local time, which provided a unique opportunity to study the variation of electron fluxes along their drift trajectory. The electron fluxes exhibited different responses in three energy ranges: at ∼300 keV, the fluxes showed a decrease following the injection closer to local midnight, and an increase further along the drift trajectory toward dawn. To understand the observed flux variations, we calculated the pitch angle and momentum diffusion coefficients and found the results to be consistent with the observations: the pitch angle diffusion coefficients were higher at smaller pitch angles for ~300 keV electrons. The momentum diffusion coefficients were significantly low at all energies. Our results showed that intense chorus waves can drive rapid precipitation of several hundreds of keV electrons on injection timescales (~tens of minutes) and that using multi-spacecraft observations can provide a higher-fidelity picture of the systemic response of the radiation belts to solar wind drivers.

Wave-particle interactions are a fundamental mechanism to control irreversible plasma energization and energy transport throughout the Heliosphere, and universally throughout astrophysical plasma domains. The most tractable paradigm to model the plasma response to perturbations by plasma waves is the 60 year old quasilinear diffusion theory. This paradigm predominates in our understanding, but within the last two decades there has been a sustained resurgence and emergence of fundamental new questions motivated by the discovery of highly variable, intense/energetic and structured electromagnetic plasma waves and wave-particle interaction plasma physics processes by single and multi-point missions. These interactions act and control plasma energization and energy transport from microscale (gyroradius/kinetic) through to the macroscale (system scale), and in addition crucially link these scales via complex coupled fluid/mesoscale plasma physics processes. We discuss recent advances, and highlight some open, fundamental questions for wave-particle interactions that the Plasma Observatory Mission can solve via multiscale observations.