LRO/LAMP Study of the Interplanetary Medium via the HeI 58.4 nm Resonance Line

Recent measurements by IBEX and modeling have changed our understanding of the flow of the interstellar medium through the Solar System. In particular, a time dependence of the direction of the interstellar medium flow has been proposed, and a new population of helium atoms, called the "warm breeze", has been discovered. We used the sensitive LAMP extreme-UV imaging spectrograph onboard the Lunar Reconnaissance Orbiter to constrain the structure of the interplanetary medium close to the downwind focusing cone, and to confirm the new wind direction. We measured the brightness of the emission line from interstellar helium atoms resonantly scattering solar photons at 58.4 nm and compared it to a "modified cold model" for interplanetary helium (HeI) sky brightness (as a function of ecliptic latitude and longitude) based from the latest interstellar flow parameters obtained by the IBEX spacecraft. We find that the LAMP observations agree with our model, which can therefore be applied to other UV spectrographs as well. Moreover, we have modified this model to include the shift in ecliptic longitude and the brightness from the "warm breeze". LAMP observations are consistent, within error, with both the IBEX parameters and the shift in ecliptic longitude, since the latter has negligible effect on the brightness of the interplanetary HeI emission line. For the same reason, the "warm breeze" was not detected by LAMP. We also noted a discrepancy between solar irradiances measured by TIMED/SEE and those measured by SDO/EVE. We recommend using values from SDO/EVE. Finally, we derive a value of LAMP sensitivity at the EUV wavelength (58.4 nm) of 0.53±0.04 Hz/Rayleigh.