Most operational spacecraft orbit Earth within 7 Earth radii. Over recent decades, a few research missions have sampled the plasma and energetic particle environment out to 20 Earth radii. From the limited (but growing) number of observations beyond geosynchronous orbit, Denton et al.  have created an empirical model from the CLUSTER mission, a quartet of orbiting spacecraft with identical instruments.
This model supports prediction of electron flux in the energy range ~45 eV to ~325 keV, as a function of local-time and radial distance from the Earth near the equatorial plane. This is a key energy band associated with spacecraft charging. The model is parameterized by a geomagnetic activity index (Kp index) that ranges from 0 to 9. This new model supports international efforts in benchmarking extreme particles fluxes that may damage spacecraft or individual space-based instruments.
Citation: Denton, M. H., Taylor, M. G. G. T., Rodriguez, J. V., & Henderson, M. G. . Extension of an empirical electron flux model from 6 to 20 Earth radii using Cluster/RAPID observations. Space Weather, 17. https://doi.org/10.1029/2018SW002121
—Delores J. Knipp, Editor in Chief, Space Weather