The First Analysis of the Outward H Fluxes measured by IBEX-Lo in 20-50 RE Geocentric Distances

Jeewoo Park ^1,2* Hyunju Kim Connor ²

• ¹ University of Maryland, Baltimore County, Baltimore, Maryland, United States
• ² Goddard Space Flight Center, National Aeronautics and Space Administration, Greenbelt, Maryland, United States

In this study, we analyze the energetic neutral atom (ENA) observations measured in the lowest energy channel (10-21 eV) of the IBEX-Lo instrument on Interstellar Boundary Explorer (IBEX) during two spring seasons, Day of Year (DOY) 101-146, 2009 and DOY 88-178, 2013, confirming the existence of outward hydrogen (H) fluxes at 15 eV. The outward H flux decreases slightly with distance, showing an intensity of approximately 10 6 cm -2 s -1 sr -1 keV -1 . Results also suggest that the outward H fluxes are not influenced by solar radio flux. We compute the expected H ENA fluxes at 15 eV using ion flux measurements from the Helium, Oxygen, Proton, and Electron (HOPE) mass spectrometer aboard the Radiation Belt Storm Probes (RBSP) during the corresponding period of the 2013 spring season, combined with a simple exospheric density model (𝑛 ! = 𝑛 !" (𝑟 " 𝑟 ⁄ ) # , where 𝑟 " = 10 RE). The expected ENA fluxes similarly show a decrease in intensity with increasing geocentric distance, on the order of 10 5 -10 6 cm -2 s -1 sr -1 keV -1 . These consistent features suggest that the outward H fluxes observed by IBEX-Lo are closely related to escaping H ENAs produced within the inner exosphere (< 4 RE).