Consequences of Hot Electrons for the Structure of the Outer Heliosphere

Abstract The temperature of electrons is largely unknown in the outer heliosphere because there is a gap in Voyager measurements from ∼3 eV to 22 keV. Most models assume that the electrons have the same temperature as the bulk cold thermal solar wind (10 4 K beyond about 10 au from the Sun), but some studies argue that the electrons could be orders of magnitude hotter. We perform two global, multi-fluid magnetohydrodynamic simulations of the heliosphere: one with cold electrons (∼10 4 K) and one with hot electrons (∼10 6 K). The hot-electron case leads to an electron-pressure-dominated heliosheath, which expands the thickness of the heliosheath by 50% in the nose direction relative to the cold-electron case. In the tail, electron impact ionization is important with hot electrons and leads to a runaway ionization cascade that increases the plasma density by an order of magnitude. The cold case produces a short, turbulent, split, croissant-like tail, while the hot case creates a long, laminar, comet-like tail. The expansion of the heliosheath seen with hot electrons is inconsistent with the heliosheath thickness from Voyager measurements, but it is clear that the electron temperature has a strong effect on the heliosphere and must be constrained. We also show that the magnetosonic speed is variable in the heliosheath and not constant, which could affect estimates of the heliosheath thickness with pressure sounding methods.