Signatures of Reconnection and a Split Heliospheric Tail in High-energy Energetic Neutral Atoms

Astrophysical Journal. 2026. Vol. 1004. No. 1. P. 1–18.

Kornbleuth M., Opher M., Drake J. F., Swisdak M., Yin Z., Dialynas K., Chen Y., Giacalone J., Sokol J. M., Gkioulidou M., Baliukin I., Izmodenov V., Zank G.

The shape of the heliosphere, regarded as comet-like since the 1960s, has recently been the subject of intense debate in the last decade. There is disagreement as to whether the heliospheric tail extends to ∼10,000 au in a comet-like shape or if it is short (∼400 au) with a split. Energetic neutral atom (ENA) maps from Cassini/Ion and Neutral Camera (INCA) at energies from 5.2 to 13.5 keV revealed a global structure extending from the nose to the heliospheric tail, known as the Belt, whose origin has remained largely unexplored. Here, we use a state-of-the-art multi-ion magnetohydrodynamic model and a novel reconnection simulation to establish that the Belt structure is consistent with a split-tail heliosphere but not with a comet-like heliosphere. In a split-tail heliosphere, there is a region of low β (the ratio of thermal to magnetic pressure) in the downwind direction close to the heliopause. Direct simulations of this region reveal that magnetic reconnection is strong and drives the energetic particles that produce the >5.2 keV ENAs measured by INCA in the low-latitude portion of the Belt. Since the comet-like heliosphere does not produce this low-β region and the resultant reconnection-driven mechanism for the >5.2 keV ENAs, our results indicate that the INCA observations are inconsistent with a comet-like heliosphere. Further, these simulations and analysis establish for the first time that magnetic reconnection in the complex magnetic fields, expected in astrospheres across the Universe, is likely to be a source of energetic particles and radiation.

Keywords: heliosphere, heliopause, heliosheath, termination shock, solar system