Large-scale Field-aligned Flows in the Heliosheath

Abstract We use a state-of-the-art 3D MHD simulation to show that large-scale thermal pressure gradients in the heliosheath drive suprathermal particles (pickup ions with 1–5 keV) to stream along magnetic field lines faster than the cold bulk solar wind. Large-scale pressure gradients between the nose and flank of the heliopause stem from neutrals flowing in from the nose direction. Hybrid simulations suggest that these gradients will also drive field-aligned flows at higher energies (5–30 keV). The field-aligned flows are stronger within 10 au of the termination shock, where the magnetic field lines in the upwind hemisphere have shorter path lengths in the heliosheath before connecting back to the termination shock. This mechanism explains the large field-aligned flows seen in the data in the first 2.5 yr of heliosheath data when Voyager 2 was above the heliospheric current sheet. This work has implications for how the bulk flows are derived in Voyager 1 from the Compton–Getting effect, where particles at all energies are assumed to move with the same speed.