The radiation belts and magnetospheres of Jupiter and Saturn show significant intensities of relativistic electrons with energies up to tens of megaelectronvolts (MeV). To date, the question on how the electrons reach such high energies is not fully answered. This is largely due to the lack of high‐quality electron spectra in the MeV energy range that models could be fit to. We reprocess data throughout the Galileo orbiter mission in order to derive Jupiter's electron spectra up to tens of MeV. In the case of Saturn, the spectra from the Cassini orbiter are readily available and we provide a systematic analysis aiming to study their acceleration mechanisms. Our analysis focuses on the magnetospheres of these planets, at distances of L > 20 and L > 4 for Jupiter and Saturn, respectively, where electron intensities are not yet at radiation belt levels. We find no support that MeV electrons are dominantly accelerated by wave‐particle interactions in the magnetospheres of both planets at these distances. Instead, electron acceleration is consistent with adiabatic transport. While this is a common assumption, confirmation of this fact is important since many studies on sources, losses, and transport of energetic particles rely on it. Adiabatic heating can be driven through various radial transport mechanisms, for example, injections driven by the interchange instability or radial diffusion. We cannot distinguish these processes at Saturn with our technique. For Jupiter, we suggest that the dominating acceleration process is radial diffusion because injections are never observed at MeV energies.

The intense inner radiation belt at Jupiter (>50 MeV at 1.5 R J ) is generally accepted to be created by radial diffusion of electrons from further away from the planet. However, this requires a source with energies that exceed 1 MeV outside the orbit of the moon Io at 5.9 R J , which has never been explained satisfactorily. Here we test the hypothesis that this source population could be formed from a very soft energy spectrum, by particle injection processes and resonant electron acceleration via whistler mode chorus waves. We use the British Antarctic Survey Radiation Belt Model to calculate the change in the electron flux between 6.5 and 15 R J ; these are the first simulations at Jupiter combining wave particle interactions and radial diffusion. The resulting electron flux at 100 keV and 1 MeV lies very close to the Galileo Interim Radiation Electron model spectrum after 1 and 10 days, respectively. The primary driver for the increase in the flux is cyclotron resonant acceleration by chorus waves. A peak in phase space density forms such that inside L ≈ 9 radial diffusion transports electrons toward Jupiter, but outside L ≈ 9 radial diffusion acts away from the planet. The results are insensitive to the softness of the initial energy spectrum but do depend on the value of the flux at the minimum energy boundary. We conclude by suggesting that the source population for the inner radiation belt at Jupiter could indeed be formed by wave-particle interactions.

Abstract. Data from the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjärvi, Finland, have been compiled to form statistics of auroral absorption based on seven years of observations. In a previous study a linear relationship between the logarithm of the absorption and the K p index provided a link between the observations of precipitation with the level of geomagnetic activity. A better fit to the absorption data is found in the form of a quadratic in K p for eight magnetic local time sectors. Past statistical investigations of absorption have hinted at the possibility of using the solar wind velocity as a proxy for the auroral absorption, although the lack of available satellite data made such an investigation difficult. Here we employ data from the solar wind monitors, WIND and ACE, and derive a linear relationship between the solar wind velocity and the cosmic noise absorption at IRIS for the same eight magnetic local time sectors. As far as the authors are aware this is the first time that in situ measurements of the solar wind velocity have been used to create a direct link with absorption on a statistical basis. The results are promising although, it is clear that some other factor is necessary in providing reliable absorption predictions. Due to the substorm related nature of auroral absorption, this is likely formed by the recent time history of the geomagnetic activity, or by some other indicator of the energy stored within the magnetotail. For example, a dependence on the southward IMF (interplanetary magnetic field) is demonstrated with absorption increasing with successive decreases in B z ; a northward IMF appears to have little effect and neither does the eastward component, B y .

Electromagnetic ion cyclotron (EMIC) waves play an important role in relativistic electron losses in the radiation belts through diffusion via resonant wave-particle interactions. We present a new approach for calculating bounce and drift-averaged EMIC electron diffusion coefficients. We calculate bounce-averaged diffusion coefficients, using quasi-linear theory, for each individual Combined Release and Radiation Effects Satellite (CRRES) EMIC wave observation using fitted wave properties, the plasma density and the background magnetic field. These calculations are then combined into bounce-averaged diffusion coefficients. The resulting coefficients therefore capture the combined effects of individual spectra and plasma properties as opposed to previous approaches that use average spectral and plasma properties, resulting in diffusion over a wider range of energies and pitch angles. These calculations, and their role in radiation belt simulations, are then compared against existing diffusion models. The new diffusion coefficients are found to significantly improve the agreement between the calculated decay of relativistic electrons and Van Allen Probes data. Plain Language Summary In recent years there have been an increasing number of satellites operating in or traversing the Earth's radiation belts. These belts are composed of charged particles that are largely confined by the Earth's magnetic field, although waves can accelerate and scatter these particles. In the outer belt, electrons can be accelerated up to relativistic energies and pose a threat to satellites. Diffusion-based models are used to simulate the electron population, incorporating the statistical effects of waves on the electrons. Electromagnetic ion cyclotron waves are of particular importance for the relativistic population, effectively scattering them into the atmosphere and removing them from the belts. Previous models of this interaction are based on average plasma and wave observations; however, these do not well represent the range of interactions possible. Here we take a new approach, considering each observation individually to determine their statistical effect. When included into diffusion models, this new approach significantly improves the modeling of the relativistic electron population.

Whistler mode chorus emission is important in the acceleration of electrons and filling of the radiation belts at Jupiter. In this work chorus magnetic intensity levels (frequency‐integrated spectral density, PB) at Jupiter are comprehensively binned and parameterized. The frequency range of chorus under study extends from the lower hybrid frequency, flh, to fceq/2 and fceq/2 < f < 0.8 fceq, where fceq is the cyclotron frequency mapped to the magnetic equator. The goal is to obtain a quantized distribution of magnetic intensity for use in stochastic modeling efforts. Parametric fits of magnetic plasma wave intensity are obtained, including PB versus frequency, latitude, and L shell. The results indicate that Jupiter chorus occurrence probability and intensity are higher than those at Saturn, reaching values observed at Earth. Jovian chorus is observed over most local times, confined primarily to the range 8 < L < 15, outside the high densities of the Io torus. The largest intensity levels are seen on the dayside; however, the sampling of chorus on the nightside is much less than on the dayside. Peak intensities occur near the equator with a weak dependence on magnetic latitude, λ. We conclude that Jovian chorus average intensity levels are approximately an order of magnitude lower than those at Earth. In more isolated regions the intensities are comparable to those observed at Earth. The spatial range of the chorus emissions extends beyond that assumed in previous Jovian global diffusive models of wave‐particle electron acceleration.

At Saturn electrons are trapped in the planet’s magnetic field and accelerated to relativistic energies to form the radiation belts, but how this dramatic increase in electron energy occurs is still unknown. Until now the mechanism of radial diffusion has been assumed but we show here that in-situ acceleration through wave particle interactions, which initial studies dismissed as ineffectual at Saturn, is in fact a vital part of the energetic particle dynamics there. We present evidence from numerical simulations based on Cassini spacecraft data that a particular plasma wave, known as Z-mode, accelerates electrons to MeV energies inside 4 RS (1 RS = 60,330 km) through a Doppler shifted cyclotron resonant interaction. Our results show that the Z-mode waves observed are not oblique as previously assumed and are much better accelerators than O-mode waves, resulting in an electron energy spectrum that closely approaches observed values without any transport effects included.

Because of the role of Z‐mode emission in the diffusive scattering and resonant acceleration of electrons, we conduct a survey of intensity in the Saturn inner magnetosphere. Z mode is primarily observed as “5 kHz” narrowband emission in the lower density regions where the ratio of cyclotron to plasma frequency, fc/fp > 1 to which we limit this study. This occurs at Saturn along the inner edge of the Enceladus torus near the equator and at higher latitudes. We present profiles and parametric fits of intensity as a function of frequency, radius, latitude, and local time. The magnetic field intensity levels are lower than chorus, but the electric field intensities are comparable. We conclude that Z‐mode wave‐particle interactions may make a significant contribution to electron acceleration in the inner magnetosphere of Saturn, supplementing acceleration produced by chorus emission.

Abstract.Extending previous studies, a full-circle investigation of the ring current has been made using Cluster 4-spacecraft observations near perigee, at times when the Cluster array had relatively small separations and nearly regular tetrahedral configurations, and when the Dst index was greater than −30 nT (non-storm conditions). These observations result in direct estimations of the near equatorial current density at all magnetic local times (MLT) for the first time and with sufficient accuracy, for the following observations. The results confirm that the ring current flows westward and show that the in situ average measured current density (sampled in the radial range accessed by Cluster ∼4-4.5 R E ) is asymmetric in MLT, ranging from 9 to 27 nA m −2 . The direction of current is shown to be very well ordered for the whole range of MLT. Both of these results are in line with previous studies on partial ring extent. The magnitude of the current density, however, reveals a distinct asymmetry: growing from 10 to 27 nA m −2 as azimuth reduces from about 12:00 MLT to 03:00 and falling from 20 to 10 nA m −2 less steadily as azimuth reduces from 24:00 to 12:00 MLT. This result has not been reported before and we suggest it could reflect a number of effects. Firstly, we argue it is consistent with the operation of region-2 field aligned-currents (FACs), which are expected to flow upward into the ring current around 09:00 MLT and downward out of the ring current around 14:00 MLT. Secondly, we note that it is also consistent with a possible asymmetry in the radial distribution profile of current density (resulting in higher peak at ∼4-4.5 R E ). We note that part of the enhanced current could reflect an increase in the mean AE activity (during the periods in which Cluster samples those MLT).