17:15 〜 18:45
[PEM13-P13] Low-Energy Ion Flux Enhancements with the Negative Spacecraft Charging in the Plasmasphere’s Eclipse Region
キーワード：Low-energy ion flux enhancement, Spacecraft charging, Eclipse region, Van Allen Probe satellite
When a spacecraft body is negatively charged, surrounding ions can be attracted into the spacecraft, causing them to accelerate to higher energies. This phenomenon was observed as an increase in low-energy ion flux (E < 10 eV), measured by the HOPE instruments onboard the Van Allen Probes A and B in the eclipse region within the plasmapause. During the same period, a clear discrepancy was confirmed between the cold plasma density estimated by the Electric Field and Waves (EFW) instruments and those derived from the upper hybrid gyrofrequency. This difference is attributed to the spacecraft's potential. Despite the negative averaged potential, indicating a positively charged spacecraft body, the enhanced ion flux shows characteristics of a thermalized distribution with the Maxwellian distribution from the fitting analysis. These results imply that the ambient cold ions were electrostatically attracted toward the satellite and accelerated. A two-year statistical analysis, from January 2016 to December 2017, confirms that the thermalized low-energy ions in the eclipse region are observed under negative averaged potential variations, similar to our previous result. Therefore, these results suggest that the spacecraft's body is negatively charged indeed, and previously undetectable cold ions, called "hidden ions" in space, can be observed.