One Solar Cycle of Heliosphere Observations with the Interstellar Boundary Explorer: Energetic Neutral Hydrogen Atoms Observed with IBEX-Lo from 10 eV to 2 keV

Abstract

The Interstellar Boundary Explorer (IBEX) is a NASA satellite in Earth orbit, dedicated to observing both interstellar neutral atoms entering the heliosphere and energetic neutral atoms (ENAs) from the interstellar boundaries from roughly 10 eV to 6 keV. This work presents the averaged maps, energy spectra, and temporal variability of heliospheric ENA intensities measured with the IBEX-Lo instrument at 1 au at energies between 10 eV and 2 keV, covering one entire solar cycle from 2009 through 2019. These results expand the range in time and energy for studying the globally distributed ENA flux and the IBEX Ribbon. The observed ENA intensities exceed model predictions, in particular below 500 eV. Moreover, the ENA intensities between 50–200 eV energy show an unexpected rise and fall around the year 2015 in most sky regions.

The Discrepancy between Observed and Predicted Heliospheric Energetic Neutral Atoms below Solar Wind Energy

“…For this study, we concentrate on IBEX-Lo data. We rely on 11 yr average ENA intensities measured with IBEX-Lo (Tables 4 and 6 in Galli et al 2022b); any IBEX-Hi data are taken from McComas et al (2020). These data were corrected for the proper motion of the spacecraft and for ENA survival probabilities, and thus represent ENA intensities at 100 au in the solar inertial reference frame.…”

“…These data were corrected for the proper motion of the spacecraft and for ENA survival probabilities, and thus represent ENA intensities at 100 au in the solar inertial reference frame. Note, however, that the downwind and port tail lobe regions were sampled from IBEX-Lo anti-ram observations, leading to larger error bars (Galli et al 2022b).…”

“…All energy spectra from Galli et al (2022b) used for this study are listed in Table 1. The tabulated energies differ for different directions because the applied Compton-Getting corrections depend on viewing direction.…”

“…The tabulated energies differ for different directions because the applied Compton-Getting corrections depend on viewing direction. Below 50 eV, the ENA intensities derived from IBEX-Lo are challenging to constrain against the background sources (Fuselier et al 2014;Galli et al 2014), and the derived energy spectra entries below that energy are interpreted only as upper limits (Galli et al 2022b). The ENA intensity reconstructed from the count rates in the lowest energy bin (;15 eV) is omitted from these tables and from all model comparisons because of the large uncertainties introduced by the Compton-Getting and survival probability correction in combination with possible local backgrounds at ENA energies below 50 eV.…”

Interstellar Conditions Deduced from Interstellar Neutral Helium Observed by IBEX and Global Heliosphere Modeling

“…The IBEX-Lo instrument (Fuselier et al 2009) on the Interstellar Boundary Explorer (IBEX; McComas et al 2009) provides high-statistics observations of ISN atoms (Möbius et al 2009a(Möbius et al , 2009b. Despite the narrow energy distribution of ISN helium atoms in the solar frame (Sokół et al 2015), their observations span over the four lowest electrostatic analyzer (ESA) steps (Swaczyna et al 2018;Galli et al 2022). This broad energy distribution occurs because helium atoms sputter a broad energy distribution of negative ions (mostly H − ) off of the instrument's conversion surface.…”