The Interstellar Mapping and Acceleration Probe: A glimpse at the first heliospheric ENA energy spectra measured below solar wind energy

The Interstellar Mapping and Acceleration Probe (IMAP) is a heliophysics NASA mission to study the acceleration of charged particles and the interaction of the solar wind with the local interstellar medium. IMAP combines images of the plasma boundary regions of the heliosphere by means of Energetic Neutral Atoms (ENAs) with in-situ measurements of the interstellar neutrals flowing into the heliosphere and of the local plasma and dust environment in the solar wind. IMAP was launched in September 2025 and begins its nominal science phase in February 2026. At the same time, the Interstellar Boundary Explorer (IBEX, launched in 2008, IMAP's predecessor in terms of ENA imaging) is still active and continues its observations of heliospheric ENAs. IBEX covers an energy range of roughly 10 eV to 6 keV whereas IMAP covers a wider energy range from roughly 10 eV to 300 keV with three different types of ENA instruments.

In this presentation, we concentrate on the energy spectrum of heliospheric ENAs measured at solar wind energies all the way down to the lowest energy bins of IMAP and IBEX. This energy spectrum is crucial to understand the plasma pressure balance of the heliosphere with respect to the surrounding Very Local Interstellar Medium and to understand the various plasma populations and acceleration mechanisms in the heliosphere. IBEX observations indicate ENA intensities 1-2 orders of magnitude higher than most heliospheric ENA model predictions at energies between 50 eV and 500 eV. The spectrum seems to drop below a power law and start rolling over near 100 eV, but a definite answer is thwarted by the large error bars below 100 eV. Before the advent of IMAP, three explanations have been investigated to explain the discrepancy between ENA measurements with IBEX and model predictions: Additional ENA sources from beyond the heliopause, heliosheath turbulence unaccounted for in models, and/or local foreground near IBEX or in the inner solar system. IMAP will help resolve these outstanding questions thanks to the improved geometric factors of its ENA instruments and the lower background levels expected for the IMAP location at L1 compared with IBEX in Earth orbit.

Based on the knowledge from the first few months of IMAP science observations, we show the first raw data of the ENA spectral intensities measured with IMAP between roughly 100 eV and 1 keV, and we discuss the necessary analysis steps to compare these data with the energy spectra measured with IBEX and those predicted by models. These steps include the correction for the spacecraft motion with respect to the Sun and the correction for ionization losses. Both corrections become more important at lower ENA energies: the typical ENA speed is no longer much faster than the speed of the spacecraft  relative to the Sun, and an increasing fraction of ENAs does not reach IMAP or IBEX because they are ionized by charge-exchange, photo-ionization, or electron impact ionization on their years-long travel from the edge of the heliosphere toward the inner solar system.