A slow bow shock ahead of the heliosphere

Current estimates of plasma parameters in the local interstellar medium indicate that the speed of the interstellar wind, i.e., the relative speed of the local interstellar cloud with respect to the Sun, is most likely less than both the fast magnetosonic speed (subfast) and the Alfvén speed (sub-Alfvénic) but greater than the slow magnetosonic speed (superslow). In this peculiar parameter regime, MHD theory postulates a slow magnetosonic shock ahead of the heliosphere, provided that the angle between the interstellar magnetic field and the interstellar plasma flow velocity is quite small (e.g., 15° to 30°). In this likely scenario, our multifluid MHD model of the heliospheric interface self-consistently produces a spatially confined quasi-parallel slow bow shock. Voyager 1 is heading toward the slow bow shock, while Voyager 2 is not, which means that the two spacecraft are expected to encounter different interstellar plasma populations beyond the heliopause. The slow bow shock also affects the density and spatial extent of the neutral hydrogen wall. Key Points The interstellar wind is sub-Alfvenic and superslow A quasi-parallel slow bow shock exists ahead of the heliosphere The slow bow shock determines the spatial extent of the neutral hydrogen wall