THE EXISTENCE AND NATURE OF THE INTERSTELLAR BOW SHOCK

Abstract

We report a new diagnostic between two different states of the local\ninterstellar medium (LISM) near our solar system using a sensitivity study\nconstrained by several distinct and complementary observations of the LISM,\nsolar wind, and inner heliosphere. Assuming the Interstellar Boundary Explorer\n(IBEX) He flow parameters for the LISM, we obtain a strength of $\\sim\n2.7\\pm0.2$ $\\mu$G and a direction pointing away from galactic coordinates $(28,\n52)\\pm 3^\\circ$ for the interstellar magnetic field as resulting from fitting\nVoyager 1 & 2 in situ plasma measurements and IBEX energetic neutral atoms\nribbon}. When using Ulysses parameters for the LISM He flow, we recently\nreported the same direction but a strength of $2.2\\pm0.1$ $\\mu$G. First, we\nnotice that with Ulysses He flow, our solution is in the expected hydrogen\ndeflection plane (HDP). In contrast, for the IBEX He flow, the solution is\n$\\sim 20^{\\circ}$ away from the corresponding HDP plane. Second, the long-term\nmonitoring of the interplanetary H I flow speed shows a value of $\\sim 26$ km/s\nmeasured at upwind from the Doppler-shift in the strong Lyman-$\\alpha $ sky\nbackground emission line. All elements of diagnostics seem therefore to support\nUlysses He flow parameters for the interstellar state. In that frame, we argue\nthat reliable discrimination between superfast, subfast, or superslow states of\nthe interstellar flow should be based on most existing in situ and remote\nobservations used together with global modelling of the heliosphere. For\ncommonly accepted LISM ionization rates, we show that a fast interstellar\nbow-shock should be standing-off upstream of the heliopause.\n