Radial distributions of equatorial phase space density for outer radiation belt electrons

We present the first direct observations of equatorial electron phase space density (PSD) as a function of the three adiabatic invariants throughout the outer radiation belt using data from the Solid State Telescopes on THEMIS‐D. We estimate errors in PSD that result from data fitting and uncertainty in the calculation of the second and third invariants based on performance‐weighted results from seven different magnetic field models. The PSD gradients beyond geosynchronous orbit (GEO) are energy dependent, revealing different source regions for the relativistic and non‐relativistic populations. Specifically, the PSD distribution of outer belt relativistic electrons is peaked near L* ≈ 5.5. These features are typical for the outer belt, based on a survey of a two‐month period from 01 Feb.–31 Mar. 2010. The results are consistent with previous studies, which were based on off‐equatorial observations, but remove the high uncertainties introduced from mapping by using truly equatorial measurements (i.e., within only a few degrees of the magnetic equator) and quantifying the error in PSD. The newly calibrated THEMIS‐SST dataset forms a powerful tool for exploration of the near‐Earth magnetosphere, especially when combined with the upcoming RBSP mission.