Stationary “nose-like” ion spectral structures in the inner magnetosphere: Observations by Van Allen probes and simulations

The “Nose-like” ion spectral structures are characterized by the deep inward penetration of ions from the near-Earth plasma sheet and can extend to low L shells in the inner magnetosphere. The stationary “Nose-like” ion structures are believed to be driven by the stationary electric and magnetic field during quiet geomagnetic times. However, the global distribution picture, especially for each MLT, for both single and double stationary nose structures is still not reached. In this study, we statistically investigate the stationary “Nose-like” ion spectral structure based on 2 years of Van Allen probes observations. We find a clear MLT dependence of the number of stationary noses, with a higher occurrence of single-nose structures from 13 to 03 MLT, while a higher occurrence of double-nose structures from 03 to 13MLT. This is confirmed by calculating the backward drift time using backward tracing method based on the dipole magnetic field and Weimer 96 electric field model for each MLT. Simulation results indicate that the ion spectral gap, which is formed due to long drift time or existence of the ion forbidden region, controls the nose structure at different MLT during geomagnetic quiet times. A possible physical explanation of this MLT dependence can be a combination of the ion spectral gaps created by the region of long drift time in the dayside within the nose energy range and the persistent dawn-dusk asymmetries in the convection electric field.