Interstellar neutral helium in the heliosphere from IBEX observations. V. Observations in IBEX-Lo ESA steps 1, 2, & 3

TL;DR

This study analyzes IBEX-Lo data from 2009-2015 across three ESA steps to improve understanding of interstellar neutral helium in the heliosphere, confirming previous temperature and velocity estimates and planning further analysis of hydrogen contributions.

Contribution

Extended analysis of IBEX-Lo observations across multiple ESA steps, modeling instrument sensitivity as a function of impact speed, and providing insights into ISN He and H signals.

Findings

Sensitivity increases with ESA step and decreases with impact speed

Current ISN He temperature and velocity estimates remain valid

Hydrogen contribution may influence signal interpretation

Abstract

Direct-sampling observations of interstellar neutral (ISN) He by Interstellar Boundary Explorer (IBEX) provide valuable insight into the physical state of and processes operating in the interstellar medium ahead of the heliosphere. The ISN He atom signals are observed at the four lowest ESA steps of the IBEX-Lo sensor. The observed signal is a mixture of the primary and secondary components of ISN He and H. Previously, only data from one of the ESA steps have been used. Here, we extended the analysis to data collected in the three lowest ESA steps with the strongest ISN He signal, for the observation seasons 2009-2015. The instrument sensitivity is modeled as a linear function of the atom impact speed onto the sensor's conversion surface separately for each ESA step of the instrument. We found that the sensitivity increases from lower to higher ESA steps, but within each of the ESA steps it is a decreasing function of the atom impact speed. This result may be influenced by the hydrogen contribution, which was not included in the adopted model, but seems to exist in the signal. We conclude that the currently accepted temperature of ISN He and velocity of the Sun through the interstellar medium do not need a revision, and we sketch a plan of further data analysis aiming at investigating ISN H and a better understanding of the population of ISN He originating in the outer heliosheath.