On the Formation of Phantom Electron Phase Space Density Peaks in Single Spacecraft Radiation Belt Data

This study examines the rapid losses and acceleration of trapped relativistic and ultrarelativistic electron populations in the Van Allen radiation belt during the September 7–9, 2017 geomagnetic storm. By analyzing the dynamics of the last closed drift shell (LCDS), and the electron flux and phase space density (PSD), we show that the electron dropouts are consistent with magnetopause shadowing and outward radial diffusion to the compressed LCDS. During the recovery phase, an in‐bound pass of Van Allen Probe A shows an apparent local peak in PSD, which does not exist in reality. A careful analysis of the multipoint measurements by the Van Allen Probes reveals instead how the apparent PSD peak arises from aliasing monotonic PSD profiles which are rapidly increasing due to acceleration from very fast inwards radial diffusion. In the absence of such multisatellite conjunctions during fast acceleration events, such peaks might otherwise be associated with local acceleration processes. Plain Language Summary This study presents a thorough analysis of terrestrially trapped electron space radiation during the September 2017 geomagnetic storm. By analyzing the measurements of the trapped electron population, we show that the predominant loss of the relativistic and ultra‐relativistic electrons depleted from the radiation belt at the beginning of the storm arises from outwards loss into the solar wind and not downwards loss into the atmosphere. We also reveal, for the first time, that the signatures of the acceleration processes which refill the belts, after such losses can occur on much faster timescales than previously thought. Moreover, signatures attributed to the actions of high‐frequency plasma waves are actually caused by a different physical phenomenon known as the radial diffusion. The new knowledge of the very fast rate of change of the amount of electron space radiation points to an urgent need to evaluate the processes, which control belt dynamics. As we show here, this can be faster than the orbital period of monitoring satellites. Overall, we show how the limited satellite spatiotemporal coverage may mask and confuse the signatures of the physical processes responsible. Key Points Global positioning system (GPS) electron flux data reveal fast magnetopause shadowing radiation belt losses during the September 2017 geomagnetic storm During the subsequent acceleration, an apparent local peak in electron phase space density is observed by the in‐bound Van