Eclipse-Induced Changes to Topside Ion Composition and Field-Aligned Ion Flows in the August 2017 Solar Eclipse: e-POP Observations

We present in situ ion composition and velocity measurements during the August 2017 solar eclipse from the Enhanced Polar Outflow Probe (e-POP), which crossed the path of totality at ~640-km altitude within 10 min of totality passing. These measurements reveal two distinct H⁺ ion populations, an ~40% decrease in topside plasma density, a similar drop in upward but not downward H⁺ ion flux, and a downward O⁺ ion velocity of ~100 m/s. These features are directly linked to changes in the H⁺/O⁺ composition and in interhemispheric or field-aligned light ion flow and to a reduction in the negative spacecraft potential. These observed features were absent on the preceding, noneclipse days and corroborate the reduction in F region plasma density and topside total electron content observed by the Global Positioning System receivers on board. They are attributed to the temporary reduction of photoionization in the eclipsed F region.