Formation of the Low‐Energy “Finger” Ion Spectral Structure Near the Inner Edge of the Plasma Sheet

• Published in: Geophysical research letters 2020-11, Vol.47 (22), p.n/a
• Main Authors: Wang, Y. B., Kistler, L. M., Mouikis, C. G., Zhang, J. C., Lu, J. Y., Welling, D., Rastaetter, L., Bingham, S., Jin, Y. W., Wang, L., Miyoshi, Y.
• Format: Article
• Language: English
• Subjects: Approximation; Charge exchange; Computational fluid dynamics; Convection; Dipoles; Drift; Earth magnetosphere; Electric field; Electric fields; Energy; Fluid flow; Heavy ions; Helium; Hydrogen; Magnetic field; Magnetic fields; Magnetic storms; Magnetohydrodynamics; Modelling; Oxygen; Particle physics; Space weather; Spectra; Storms
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL089875

We present a case study of the H+, He+, and O+ low‐energy “finger” structure observed by the Van Allen Probe A Helium, Oxygen, Proton, and Electron (HOPE) spectrometer on 26 October 2016. This structure, whose characteristic energy is from approximately tens of eV to a few keV, looks like a “finger” that is rich in O+ and He+, faint in H+ on an energy‐time spectrogram. By using the Space Weather Modeling Framework (SWMF) and Weimer05 electric fields, combined with a dipole or more self‐consistent magnetohydrodynamic (MHD) magnetic field, backward tracing of O+ reveals that the structure is formed by ions with a long drift time from the plasma sheet during the magnetic storm main phase to the inner region with trajectories dominated by eastward drift motion, and the formation depends on the convection electric field model. The heavy ion dominance of the feature is explained by charge exchange losses along the long slow drift paths. Plain Language Summary Since the early 1970s, a common way to display data from satellite missions in the Earth's magnetosphere is the energy spectrogram, where the flux of observed particles is displayed as a function of particles' energy and the observation time. In this format, many different spectral features have been observed in different regions of the magnetosphere and simulations have been used to understand the formation of these features. Often, these simulations use empirical electric models and dipole magnetic field approximation and observed spectral structures are not able to be replicated. This study investigates the formation mechanism of a type of feature named low‐energy “finger” observed by Van Allen Probe A, using the Space Weather Modeling Framework (SWMF), the Weimer05 electric field as a comparison, combined with a dipole or MHD magnetic field. We find that the inner boundary and the enhanced convection electric field, the source population and the charge exchange losses along the drift paths play a very important role for the formation of this “finger” structure. Key Points A low‐energy “finger” structure deep in the inner magnetosphere is formed by drift of ions from the plasma sheet during storm main phase Simulations using the SWMF electric field reproduce the structure much better than using the Weimer05 model The heavy ion dominance of the feature is explained by the charge exchange losses along the long slow drift paths