Spherical Harmonic Representation of Energetic Neutral Atom Flux Components Observed by IBEX

TL;DR

This paper introduces a spherical harmonic method to separate and analyze the global and ribbon components of ENA flux maps from IBEX, revealing their spatial scales and temporal evolution over the solar cycle.

Contribution

It presents a novel spherical harmonic approach to decompose IBEX ENA maps into GDF and ribbon components without pixelization, enabling detailed spatial and temporal analysis.

Findings

GDF has larger spatial scales than the ribbon.

Identified small-scale signals in the GDF region.

Temporal evolution of components correlates with the solar cycle.

Abstract

The Interstellar Boundary Explorer (IBEX) images the heliosphere by observing energetic neutral atoms (ENAs). The IBEX-Hi instrument onboard IBEX provides full-sky maps of ENA fluxes produced in the heliosphere and very local interstellar medium (VLISM) through charge exchange of suprathermal ions with interstellar neutral atoms. The first IBEX-Hi results showed that in addition to the anticipated globally distributed flux (GDF), a narrow and bright emission from a circular region in the sky, dubbed the IBEX ribbon, is visible in all energy steps. While the GDF is mainly produced in the inner heliosheath, ample evidence indicates that the ribbon forms outside the heliopause in the regions where the interstellar magnetic field is perpendicular to the lines of sight. The IBEX maps produced by the mission team distribute the observations into $6\deg\times6\deg$ rectangle pixels in ecliptic coordinates. The overlap of the GDF and ribbon components complicates qualitative analyses of each source. Here, we find the spherical harmonic representation of the IBEX maps, separating the GDF and ribbon components. This representation describes the ENA flux components in the sky without relying on any pixelization scheme. Using this separation, we discuss the temporal evolution of each component over the solar cycle. We find that the GDF is characterized by larger spatial scale structures than the ribbon. However, we identify two isolated, small-scale signals in the GDF region that require further study.