There have been a number of theories proposed concerning the loss of relativistic electrons from the radiation belts. However, direct observations of loss were not possible on a number of previous missions due to the large field of view of the instruments and often high-altitude orbits of satellites that did not allow researchers to isolate the precipitating electrons from the stably trapped. We use measurements from the ELFIN-L suit of instruments flown on Lomonosov spacecraft at LEO orbit, which allows us to distinguish stably trapped from the drift loss cone electrons. The sun-synchronous orbit of Lomonosov allows us to quantify scattering that occurred into the loss cone on the dawn-side and the dusk-side magnetosphere. The loss at MeV energies is observed predominantly on the dawn-side, consistent with the loss induced by the chorus waves. The companion data publication provides processed measurements.

The radiation belts of the Earth, filled with energetic electrons, comprise complex and dynamic systems that pose a significant threat to a variety of satellite systems. While various models of the relativistic electron flux have been developed for geostationary orbit (GEO), the behaviour of the medium energy (120-600 keV) electrons below GEO remains poorly quantified. In this paper we present a Medium Energy electRon flux In Earth’s outer radiatioN belt (MERLIN) model based on the Light Gradient Boosting (LightGBM) machine learning algorithm. The MERLIN model takes as input the satellite position, a combination of geomagnetic indices and solar wind parameters including the time history of velocity, and does not use persistence. MERLIN is trained and validated on $>$15 years of the GPS electron flux data, and tested on more than $1.5$ years of measurements. 10-fold cross validation (CV) yields that the model predicts the MEO radiation environment well, both in terms of dynamics and amplitudes of flux. Evaluation on the test set yields high correlation between the predicted and observed electron flux (0.8) and low values of absolute error. The MERLIN model can have wide Space Weather applications, providing information for the scientific community in the form of radiation belts reconstructions, as well as industry for satellite mission design, nowcast of the MEO environment and surface charging analysis.

Sustained periods of southward interplanetary magnetic field can result in strong magnetospheric convection, during which, the Alfven layer, separating regions of sunward convection and closed drift paths, migrates earthwards. Plasmasheet electrons then have direct access to the inner magnetosphere, traversing the dawn sector before crossing the magnetopause, and present a potential seed population for the radiation belts. Here we examine, for the first time, whether energetic electrons can be sufficiently energised during their drift, via resonant interactions with whistler-mode chorus waves, so as to pass the Alfven layer prior to leaving the system. We utilise a natural coordinate system for magnetosphere convection, (U,B,K) space, in which we calculate the drift trajectories, electron energies on open drift paths, and drift times. The acceleration time from resonant chorus-wave particle interactions is calculated using the Versatile Electron Radiation Belt model (VERB) first as a 2-D diffusion equation and then in 4-D convection-diffusion mode. Comparing the drift times to the acceleration timescales we find that interactions with chorus waves do result in a portion of the electrons on open drift paths passing the Alfven energy. However, whether this acceleration occurs sufficiently quickly depends on the energy distribution of the electron population.

Fast-localized electron loss, resulting from interactions with electromagnetic ion cyclotron (EMIC) waves, can produce deepening minima in phase space density (PSD) radial profiles. Here, we perform a statistical analysis of local PSD minima to quantify how readily these are associated with radiation belt depletions. The statistics of PSD minima observed over a year are compared to the Versatile Electron Radiation Belts (VERB) simulations, both including and excluding EMIC waves. The observed minima distribution can only be achieved in the simulation including EMIC waves, indicating their importance in the dynamics of the radiation belts. By analyzing electron flux depletions in conjunction with the observed PSD minima, we show that, in the heart of the outer radiation belt (L*<5), up to 45% of multi-MeV electron depletions are associated with PSD minima, demonstrating that fast localized loss by interactions with EMIC waves are a common and crucial process for ultra-relativistic electron populations.

Radial diffusion is one of the dominant physical mechanisms driving acceleration and loss of radiation belt electrons. A number of parameterizations for radial diffusion coefficients have been developed, each differing in the dataset used. Here, we investigate the performance of different parameterizations by Brautigam and Albert (2000), Brautigam et al (2005), Ozeke et al. (2014), Ali et al. (2015, 2016); Ali (2016), and Liu et al. (2016) on long-term radiation belt modeling using the Versatile Electron Radiation Belt (VERB) code, and compare the results to Van Allen Probes observations. First, 1-D radial diffusion simulations are performed, isolating the contribution of solely radial diffusion. We then take into account effects of local acceleration and loss showing additional 3-D simulations, including diffusion across pitch-angle and energy, as well as mixed diffusion. For the L* range studied, the difference between simulations with Brautigam and Albert (2000), Ozeke et al. (2014), and Liu et al. (2016) parameterizations is shown to be small, with Brautigam and Albert (2000) offering the best agreement with observations. Using Ali et al. (2016)’s parameterization tended to result in a lower flux at 1 MeV than both the observations and the VERB simulations using the other coefficients. We find that the 3-D simulations are less sensitive to the radial diffusion coefficient chosen than the 1-D simulations, suggesting that for 3-D radiation belt models, a similar result is likely to be achieved, regardless of whether Brautigam and Albert (2000), Ozeke et al. (2014), and Liu et al. (2016) parameterizations are used.

In this study we investigate two distinct loss mechanisms responsible for the rapid dropouts of radiation belt electrons by assimilating data from Van Allen Probes A and B and Geostationary Operational Environmental Satellites (GOES) 13 and 15 into a 3-D diffusion model. In particular, we examine the respective contribution of electromagnetic ion cyclotron (EMIC) wave scattering and magnetopause shadowing for values of the first adiabatic invariant μ ranging from 300 to 3000 MeV G. We inspect the innovation vector and perform a statistical analysis to quantitatively assess the effect of both processes as a function of various geomagnetic indices, solar wind parameters, and radial distance from the Earth. Our results are in agreement with previous studies that demonstrated the energy dependence of these two mechanisms. Loss from L* = 4 to L* = 4.8 is dominated by EMIC wave scattering (μ ≥ 900 MeV G) and may amount to between 10%/hr to 30%/hr of the maximum value of phase space density (PSD) over all L shells for fixed first and second adiabatic invariants. Magnetopause shadowing is shown to deplete electrons across all energies, mostly between L* = 5 and L* = 6.6, resulting in loss from 50%/hr to 70%/hr of the maximum PSD. We also identify a boundary located between L* = 3.5 and L* = 5.2 clearly separating the regions where each mechanism dominates. Nevertheless, during times of enhanced geomagnetic activity, both processes can operate beyond such location and encompass the entire outer radiation belt.