<p>Corotation electric field is important in the inner magnetosphere topology, which was usually calculated by assuming 24h corotation period. However, some studies suggested that plasmasphere corotation lag exists which leads to the decrease of corotation electric field. In this study, we use electric field measurements from Van Allen Probes mission from 2013 to 2017 to statistically calculate the distribution of large-scale electric field in the inner magnetosphere. A new method is subsequently developed to separate corotation electric field from convection electric field. Our research shows electric field is inversely proportional to the square of L, and, with the assumption of dipole magnetic field, the rotation period of plasmasphere is estimated as 27h, consistent to the results by Sandel et al. [2003] and Burch et al. [2004] with EUV imaging of the plasmasphere. Based on the research, a new empirical model of innermagnetospheric corotation electric field was estibalished, which is significant for a more accurate understanding the large-scale electric field in the inner magnetosphere.</p>
Automated determination of electron density from electric field measurements on the Van Allen Probes spacecraft
