Global Imaging of the Outer Heliosphere over a Full Solar Cycle by IBEX – A Brief Overview

Abstract:

NASA’s Interstellar Boundary Explorer (IBEX) mission has operated in space for over a full solar activity cycle, spanning the 11+ years from 2009 into 2020 and leading to well over 300 refereed scientific papers. In this time, IBEX Energetic Neutral Atom (ENA) observations exposed the global three-dimensional structure of the outer heliosphere and its interaction with the very local interstellar medium for the first time. Here we provide a brief overview of some of these observations and a few of the many “firsts” and discoveries produced by the IBEX mission. We also provide an update on the most recent IBEX observations, which track the continuing expansion of the outer heliosphere’s response to the large solar wind pressure increase in late 2014. We find that the intensification of ENAs from the heliosheath continued to expand progressively over time to directions farther from the initial, closest direction to the heliospheric boundaries, ~20° south of the upwind direction. This expansion extended beyond the southern pole in 2018 and northern pole in 2019, demonstrating that the termination shock and heliopause are closer in the south. By the end of 2019, the heliotail had not yet responded, indicating that the boundaries are significantly farther away in the downwind, compared to the upwind direction. Finally, the slow solar wind (~1 keV) ENAs just started to intensify from the closest regions of the IBEX Ribbon in late 2019. This is about two and a half years after the initial response from heliosheath ENAs and about four and a half years after the increase in solar wind output observed at one au, both clearly implicating a “secondary ENA” source in the draped interstellar magnetic field region within a few hundred au beyond the heliopause.