Relative In-flight Response of IBEX-Lo to Interstellar Neutral Helium Atoms

The IBEX-Lo instrument on the Interstellar Boundary Explorer (IBEX) mission measures interstellar neutral (ISN) helium atoms. detection of atoms is made through negative hydrogen (H$^-$) ions sputtered by from conversion surface. energy spectrum ISN broad and overlaps four lowest electrostatic analyzer (ESA) steps. Consequently, response function for does not correspond to nominal step transmission. Moreover, laboratory calibration incomplete because it difficult produce narrow-energy atom beams that are expected Here, we analyze observations in ESA steps 1-4 derive relative in-flight We compare ratios observed count rates as a mean estimated using Warsaw Test Particle Model (WTPM). WTPM uses global heliosphere model calculate charge exchange gains losses estimate secondary population. find modeled energies atoms, unlike their fluxes, very sensitive local medium parameters. obtained responses supplement enable more detailed quantitative studies signal. A similar procedure applied may be used complement next-generation IMAP-Lo Mapping Acceleration Probe (IMAP) mission.