Effect of plasma density on diffusion rates due to wave particle interactions with chorus and plasmaspheric hiss: extreme event analysis

Wave particle interactions play an important role in controlling the dynamics of the radiation belts. The purpose of this study is to estimate how variations in the plasma density can affect diffusion rates resulting from interactions between chorus waves and plasmaspheric hiss with energetic particles and the resulting evolution of the energetic electron population. We perform a statistical analysis of the electron density derived from the plasma wave experiment on the CRRES satellite for two magnetic local time sectors corresponding to near midnight and near noon. We present the cumulative probability distribution of the electron plasma density for three levels of magnetic activity as measured by Kp. The largest densities are seen near L* = 2.5 while the smallest occur near L* = 6. The broadest distribution, corresponding to the greatest variability, occurs near L* = 4. We calculate diffusion coefficients for plasmaspheric hiss and whistler mode chorus for extreme values of the electron density and estimate the effects on the radiation belts using the Salammbô model. At L* = 4 and L* = 6, in the low density case, using the density from the 5th percentile of the cumulative distribution function, electron energy diffusion by chorus waves is strongest at 2 MeV and increases the flux by up to 3 orders of magnitude over a period of 24 h. In contrast, in the high density case, using the density from the 95th percentile, there is little acceleration at energies above 800 keV at L* = 6, and virtually no acceleration at L* = 4. In this case the strongest energy diffusion occurs at lower energies around 400 keV where the flux at L* = 6 increases 3 orders of magnitude

A new diffusion matrix for whistler mode chorus waves

Global models of the Van Allen radiation belts usually include resonant wave-particle interactions as a diffusion process, but there is a large uncertainty over the diffusion rates. Here we present a new diffusion matrix for whistler mode chorus waves that can be used in such models. Data from seven satellites are used to construct 3,536 power spectra for upper and lower band chorus for 1.5 ≤ L∗ ≤ 10, MLT, magnetic latitude 0o ≤ |λm| ≤ 60o and five levels of Kp. Five density models are also constructed from the data. Gaussian functions are fitted to the spectra and capture typically 90% of the wave power. The frequency maxima of the power spectra vary with L∗ and are typically lower than that used previously. Lower band chorus diffusion increases with geomagnetic activity and is largest between 21:00 and 09:00 MLT. Energy diffusion extends to a few MeV at large pitch angles > 60o and at high energies exceeds pitch angle diffusion at the loss cone. Most electron diffusion occurs close to the geomagnetic equator (< 12o). Pitch angle diffusion rates for lower band chorus increase with L∗ and are significant at L∗ = 8 even for low levels of geomagnetic activitywhile upper band chorus is restricted to mainly L∗ < 6. The combined drift and bounce averaged diffusion rates for upper and lower band chorus extend from a few keV near the loss cone up to several MeV at large pitch angles indicating loss at low energies and net acceleration at high energies

Improved outer boundary conditions for outer radiation belt data assimilation using THEMIS-SST data and the Salammbo-EnKF code

Over the last decade, efforts have been made in the radiation belt community to develop data assimilation tools in order to improve the accuracy of radiation belts models. In this paper we present a new method to correctly take into account the outer boundary conditions at L∗= 8 in such an enhanced model of the radiation belts. To do that we based our work on the Time History of Events and Macroscale Interactions during Substorms/Solid State Telescope data set. Statistics are developed to define a consistent electron distribution at L∗= 8 (in both equatorial pitch angle and energy), and a variance-covariance matrix is estimated in order to more realistically drive the Monte Carlo sampling required by the Ensemble Kalman Filter (EnKF). Data processing is first described as well as caveats avoided, and then the use of these information in a machinery such as the EnKF is described. It is shown that the way the Monte Carlo simulations are performed is of great importance to realistically reproduced outer boundary distribution needed by the physic-based Salammbô model. Finally, EnKF simulations are performed and compared during September 2011 in order to analyze the improvements gained using this new method of defining outer boundary conditions. In particular, we highlight in this study that such a method provides great improvement in the reconstruction of the dynamics observed at geosynchronous orbit, both during quiet and active magnetic conditions. © 2015. American Geophysical Union. All Rights Reserved

Spacecraft surface charging induced by severe environments at geosynchronous orbit

Severe and extreme surface charging on geosynchronous spacecraft is examined through the analysis of 16 years of data from particles detectors on-board the Los Alamos National Laboratory spacecraft. Analysis shows that high spacecraft frame potentials are correlated with 10 to 50 keV electron fluxes, especially when these fluxes exceed 1 × 108 cm−2 s−1 sr−1. Four criteria have been used to select severe environments: 1) large flux of electrons with energies above 10 keV, 2) large fluxes of electrons with energies below 50 keV and above 200 keV, 3) large flux of electrons with energies below 50 keV and low flux with energies above 200 keV, and 4) long periods of time with a spacecraft potential below - 5 kV. They occur preferentially during either geomagnetic storms or intense isolated substorms, during the declining phase of the solar cycle, during equinox seasons and close to midnight local time. The set of anomalies reported in Choi et al. (2011) is concomitant with a new database constructed from these events. The worst-case environments exceed the spacecraft design guidelines by up to a factor of 10 for energies below 10 keV. They are fitted with triple Maxwellian distributions in order to facilitate their use by spacecraft designers as alternative conditions for the assessment of worst-case surface charging