The existence and nature of the interstellar bow shock

We report a new diagnostic between two different states of the local interstellar medium (LISM) near our solar system using a sensitivity study constrained by several distinct and complementary observations of the LISM, solar wind, and inner heliosphere. Assuming the Interstellar Boundary Explorer (IBEX) He flow parameters for the LISM, we obtain a strength of $\sim 2.7\pm0.2$ $\mu$G and a direction pointing away from galactic coordinates $(28, 52)\pm 3^\circ$ for the interstellar magnetic field as resulting from fitting Voyager 1 & 2 in situ plasma measurements and IBEX energetic neutral atoms ribbon}. When using Ulysses parameters for the LISM He flow, we recently reported the same direction but a strength of $2.2\pm0.1$ $\mu$G. First, we notice that with Ulysses He flow, our solution is in the expected hydrogen deflection plane (HDP). In contrast, for the IBEX He flow, the solution is $\sim 20^{\circ}$ away from the corresponding HDP plane. Second, the long-term monitoring of the interplanetary H I flow speed shows a value of $\sim 26$ km/s measured at upwind from the Doppler-shift in the strong Lyman-$\alpha $ sky background emission line. All elements of diagnostics seem therefore to support Ulysses He flow parameters for the interstellar state. In that frame, we argue that reliable discrimination between superfast, subfast, or superslow states of the interstellar flow should be based on most existing in situ and remote observations used together with global modelling of the heliosphere. For commonly accepted LISM ionization rates, we show that a fast interstellar bow-shock should be standing-off upstream of the heliopause.
Comment: Astrophysical Journal, in press