Three‐dimensional data assimilation and reanalysis of radiation belt electrons: Observations of a four‐zone structure using five spacecraft and the VERB code

<jats:title>Abstract</jats:title><jats:p>Obtaining the global state of radiation belt electrons through reanalysis is an important step toward validating our current understanding of radiation belt dynamics and for identification of new physical processes. In the current study, reanalysis of radiation belt electrons is achieved through data assimilation of five spacecraft with the 3‐D Versatile Electron Radiation Belt (VERB) code using a split‐operator Kalman filter technique. The spacecraft data are cleaned for noise, saturation effects, and then intercalibrated on an individual energy channel basis, by considering phase space density conjunctions in the T96 field model. Reanalysis during the CRRES era reveals a never‐before‐reported four‐zone structure in the Earth's radiation belts during the 24 March 1991 shock‐induced injection superstorm: (1) an inner belt, (2) the high‐energy shock‐injection belt, (3) a remnant outer radiation belt, and (4) a second outer radiation belt. The third belt formed near the same time as the second belt and was later enhanced across keV to MeV energies by a second particle injection observed by CRRES and the Northern Solar Terrestrial Array riometer network. During the recovery phase of the storm, the fourth belt was created near <jats:italic>L</jats:italic>*=4<jats:italic>R</jats:italic><jats:sub><jats:italic>E</jats:italic></jats:sub>, lasting for several days. Evidence is provided that the fourth belt was likely created by a dominant local heating process. This study outlines the necessity to consider all diffusive processes acting simultaneously and the advantage of supporting ground‐based data in quantifying the observed radiation belt dynamics. It is demonstrated that 3‐D data assimilation can resolve various nondiffusive processes and provides a comprehensive picture of the electron radiation belts.</jats:p>