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Relativistic electrons in the Earth's outer radiation belt are a significant space weather hazard. The belt, which lies at altitudes from 13,000 to 40,000 km in the Earth's magnetic equatorial plane, is highly dynamic with fluxes of relativistic electrons varying by orders of magnitude on timescales ranging from minutes to months. This variability is controlled by a variety of transport, acceleration and loss processes. One particularly important process is the interaction with a plasma wave, known as chorus. These waves play a significant role in both the acceleration and loss of energetic electrons. In this study we derive new diffusion rates for these interactions and find that the rates can exceed those in our earlier models by up to a factor of 10. The new diffusion rates will be incorporated into the British Antarctic Survey Radiation Belt Model to produce better space weather models and forecasts.

In a case study using Van Allen Probe B, we investigate chorus wave observations near the western edge of a plasmaspheric plume characterized by steep density gradients. Initially, wave vectors are oriented anti-Earthward, but they become very oblique and eastward as the probe approaches the plume boundary. Treating the plume boundary as an azimuthal density gradient, ray tracing can reproduce the observed wave vector directions. Ray tracing shows that the azimuthal density gradient strongly inclines the wave vectors eastward. Consequently, waves are reflected upon reaching the Gendrin angle and cannot enter the plume. We establish an analytical criterion for the azimuthal density enhancement, determining the condition for chorus waves to enter plumes near the equatorial region. Our results partly explain the oblique chorus near plumes observed by Hartley, Chen et al. (2022, https://doi.org/10.1029/2022GL098710), offering insight into wave-particle interactions by chorus waves with the influence of azimuthal density structures.

Observations from Van Allen Probes are analyzed to obtain the statistical wave normal distribution of lightning-generated whistlers (LGWs). An automatic algorithm is developed to identify burst mode waveform data with LGW signals and analyze the wave polarization information for these LGW signals. The spatial distribution of the LGW occurrence and the probabilities of different propagation types demonstrate that most LGWs can be observed in the low L-shell region (L < 3), where the dominant propagation type is oblique outward. Parallel propagation dominates in the high L-shell region, but the LGW occurrence is very small. Additionally, a small group of oblique but inwardly propagating LGWs are observed at low altitudes (<0.2RE). A ray tracing simulation is performed not only to confirm predominant oblique and outward wave vectors in the vast region of the plasmasphere but also to verify the existence of these inward propagating LGW signals at low altitudes near the topside ionosphere.

The proton radiation belt contains high fluxes of adiabatically trapped protons varying in energy from ∼one to hundreds of megaelectron volts (MeV). At large radial distances, magnetospheric field lines become stretched on the nightside of Earth and exhibit a small radius of curvature RC near the equator. This leads protons to undergo field line curvature (FLC) scattering, whereby changes to the first adiabatic invariant accumulate as field strength becomes nonuniform across a gyroorbit. The outer boundary of the proton belt at a given energy corresponds to the range of magnetic L shell over which this transition to nonadiabatic motion takes place, and is sensitive to the occurrence of geomagnetic storms. In this work, we first find expressions for nightside equatorial RC and field strength Be as functions of Dst and L* to fit the TS04 field model. We then apply the Tu et al. (2014, https://doi.org/10.1002/2014ja019864) condition for nonadiabatic onset to solve the outer boundary L*, and refine our expression for RC to achieve agreement with Van Allen Probes observations of 1–50 MeV proton flux over the 2014–2018 era. Finally, we implement this nonadiabatic onset condition into the British Antarctic Survey proton belt model (BAS-PRO) to solve the temporal evolution of proton fluxes at L ≤ 4. Compared with observations, BAS-PRO reproduces storm losses due to FLC scattering, but there is a discrepancy in mid-2017 that suggests a ∼5 MeV proton source not accounted for. Our work sheds light on outer zone proton belt variability at 1–10 MeV and demonstrates a useful tool for real-time forecasting.

The outer radiation belt is a region of space comprising highly energetic electrons. During periods of extreme space weather, the number and energy of these electrons can rapidly vary. During these periods as the electron energies and numbers become enhanced, they can pose a threat to satellite and space infrastructure. While we have an excellent understanding of the physical processes which drive radiation belt electron dynamics, we still have a limited ability to model and forecast radiation belt dynamics; this is a result of the complexity of Earth's radiation belt system. One of the key processes controlling radiation belt dynamics is Ultra Low Frequency (ULF) wave radial diffusion. In this work we detail the development a new model quantifying the strength of ULF wave radial diffusion in the outer radiation belt utilizing space base observations of the electric and magnetic fields in Earth's magnetosphere. Accurately quantifying ULF wave radial diffusion is fundamental to understanding radiation belt dynamics and any improvement or refinements in radial diffusion models can help to provide a better understanding of the complex radiation belt system and importantly improve hindcasts, nowcasts, and forecasts.

The propagation of fast magnetosonic (MS) waves from high to extremely low L shells and their conversion into electromagnetic ion cyclotron (EMIC) waves is investigated with a ray tracing model and a full-wave model. The ray tracing simulations show that MS waves in the vicinity of the local H÷-H÷ crossover frequency at low L shells have a latitudinal range from −20°; to 20°; with wave normal angles from 0°; to 180°;, both of which provide the opportunity for their mode conversion to EMIC waves. Results from ray tracing are fed into the full-wave model as initial conditions. The full-wave simulations show that the incoming MS waves with small (20°;) and intermediate (40°;) wave normal angles may be efficiently converted to right-handedly polarized H÷ band EMIC waves, which may be further converted to right-handedly polarized He÷ band EMIC waves or to guided left-handedly polarized H÷ band EMIC waves through bi-ion resonance reflection and polarization reversal. With intermediate (40°;) or larger wave normal angles, via polarization reversal and left-handed polarization cut-off reflection, the incoming MS waves may be efficiently converted to guided left-handedly polarized H÷ band EMIC waves and the optimal wave normal angles corresponding to the maximum conversion efficiencies decrease with magnetic latitudes. Our study verifies the mode conversion mechanism of MS waves to EMIC waves proposed with a previous ray tracing study and examines the dependence of the efficiency of the wave energy transfer on the magnetic latitudes and wave normal angles, which may help to understand the wave properties and distribution of MS and EMIC waves observed at extremely low L shells.

Owing to the ever-present solar wind, our vast solar system is full of plasmas. The turbulent solar wind, together with sporadic solar eruptions, introduces various space plasma processes and phenomena in the solar atmosphere all the way to Earth’s ionosphere and atmosphere and outward to interact with the interstellar media to form the heliopause and termination shock. Remarkable progress has been made in space plasma physics in the last 65 years, mainly due to sophisticated in situ measurements of plasmas, plasma waves, neutral particles, energetic particles, and dust via space-borne satellite instrumentation. Additionally, high-technology ground-based instrumentation has led to new and greater knowledge of solar and auroral features. As a result, a new branch of space physics, i.e., space weather, has emerged since many of the space physics processes have a direct or indirect influence on humankind.

Extreme ultraviolet images taken by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory make it possible to use deep vision techniques to forecast solar wind speed - a difficult, high-impact, and unsolved problem. At a four day time horizon, this study uses attention-based models and a set of methodological improvements to deliver an 11.1% lower RMSE and a 17.4% higher prediction correlation compared to the previous work testing on the period from 2010 to 2018. Our analysis shows that attention-based models combined with our pipeline consistently outperform convolutional alternatives. Our study shows a large performance improvement by using a 30 minute as opposed to a daily sampling frequency. Our model has learned relationships between coronal holes’ characteristics and the speed of their associated high speed streams, agreeing with empirical results. Our study finds a strong dependence of our best model on the phase of the solar cycle, with the best performance occurring in the declining phase.

We conduct a global survey of magnetosonic waves and compute the associated bounce and drift averaged diffusion coefficients, taking into account co-located measurements of fpe/fce, to assess the role of magnetosonic waves in radiation belt dynamics. The average magnetosonic wave intensities increase with increasing geomagnetic activity and decreasing relative frequency with the majority of the wave power in the range fcp < f < 0.3fLHR during active conditions. In the region 4.0 ≤ L* ≤ 5.0, the bounce and drift averaged energy diffusion rates due to magnetosonic waves never exceed those due to whistler mode chorus, suggesting that whistler mode chorus is the dominant mode for electron energisation to relativistic energies in this region. Further in, in the region 2.0 ≤ L* ≤ 3.5, the bounce and drift averaged pitch angle diffusion rates due to magnetosonic waves can exceed those due to plasmaspheric hiss and very low frequency (VLF) transmitters over energy-dependent ranges of intermediate pitch angles. We compute electron lifetimes by solving the 1D pitch angle diffusion equation including the effects of plasmaspheric hiss, VLF transmitters and magnetosonic waves. We find that magnetosonic waves can have a significant effect on electron loss timescales in the slot region reducing the loss timescales during active times from 5.6 to 1.5 days for 500 keV electrons at L* = 2.5 and from 140.4 days to 35.7 days for 1 MeV electrons at L* = 2.0.

During magnetospheric storms, radiation belt electrons are produced and then removed by collisions with the lower atmosphere on varying timescales. An efficient loss process is microbursts, strong, transient precipitation of electrons over a wide energy range, from tens of keV to sub-relativistic and relativistic energies (100s keV and above). However, the detailed generation mechanism of microbursts, especially over sub-relativistic and relativistic energies, remains unknown. Here we show that these energetic electron microbursts may be caused by ducted whistler-mode lower-band chorus waves. Using observations of equatorial chorus waves nearby low-altitude precipitation as well as data-driven simulations, we demonstrate that the observed microbursts are the result of resonant interaction of electrons with ducted chorus waves rather than nonducted ones. Revealing the physical mechanism behind the microbursts advances our understanding of radiation belt dynamics and its impact on the lower atmosphere and space weather.

We analyse a set of events in which both electron flux dropouts caused by magnetopause shadowing and geosynchronous magnetopause crossings (GMCs) are observed. These observations are compared to event-specific last closed drift shell (LCDS) models derived from the TS05 and TS07 external field models and magnetopause standoff distance. The LCDS models show good association with losses due to magnetopause shadowing, but fail to reproduce observations of GMCs on the timescale of minutes. We show that different satellites in geostationary orbit observe different trends in electron flux during storm events on timescales of less than a day due to their separation in longitude. These differences demonstrate that both satellite L* and MLT must be taken into account when modelling rapid variations in the outer radiation belt, and at least three satellites in geostationary orbit, ideally more, may be required for accurate forecasting and reconstruction of these events on timescales shorter than days.

Efforts to model and predict energetic electron fluxes in the radiation belts are highly sensitive to local wave-particle interactions. In this study, we use multi-point measurements of precipitating and trapped electron fluxes to investigate the dynamic variation of chorus wave-particle interactions during the 17 March 2013 storm. Quasilinear theory characterizes the chorus wave-particle interaction as a diffusive process, with the diffusion coefficients depending on the particle energy and pitch angle, as well as the background plasma parameters such as the wave intensity and plasma density. These plasma parameters in the radiation belts are spatially localized and time-varying, so we construct event-specific diffusion coefficients using MEPED (onboard POES/MetOp) measurements of electron fluxes at low Earth orbit. This new method provides realistic diffusion coefficients for chorus waves that account for changes in the wave intensity, the plasma density, and the magnetic field strength in the outer radiation belt. We show that the inferred chorus intensity is significantly lower than previous estimates that use MEPED observations since the same amount of increased precipitation by 30–300 keV electrons can be explained by a change in the plasma density. This technique therefore allows for us to create time varying, global maps of the plasma-gyrofrequency ratio (fpe/fce), and therefore plasma density, in the outer radiation belts using the MEPED measurements. The global density estimates compare reasonably well to in situ density measurements from RBSP-B.

Kinetic wave-particle interactions in Earth's outer radiation belt energize and scatter high-energy electrons, playing an important role in the dynamic variation of the extent and intensity of the outer belt. It is possible to model the effects of wave-particle interactions across long length and time scales using quasilinear theory, leading to a Fokker-Planck equation to describe the effects of the waves on the high energy electrons. This powerful theory renders the efficacy of the wave-particle interaction in a diffusion coefficient that varies with energy or momentum and pitch angle. In this article we determine how the Fokker-Planck equation responds to the temporal variation of the quasilinear diffusion coefficient in the case of pitch-angle diffusion due to plasmaspheric hiss. Guided by in-situ observations of how hiss wave activity and local number density change in time, we use stochastic parameterisation to describe the temporal evolution of hiss diffusion coefficients in ensemble numerical experiments. These experiments are informed by observations from three different example locations in near-Earth space, and a comparison of the results indicates that local differences in the distribution of diffusion coefficients can result in material differences to the ensemble solutions. We demonstrate that ensemble solutions of the Fokker-Planck equation depend both upon the timescale of variability (varied between minutes and hours), and the shape of the distribution of diffusion coefficients. The uncertainty in the ensemble results increases for longer timescales of variability, and when the average diffusion coefficient at that location is high. We discuss time and length scales of wave-particle interactions relative to the drift velocity of high-energy electrons and confirm that arithmetic drift-averaging is can be appropriate in some cases. In other cases, further parameterisation is required to reduce uncertainty in the solution. We demonstrate that in some lo