Interaction of global merged interaction region shock with the heliopause and its relation to the 2‐ and 3‐kHz radio emissions

Abstract

We use Voyager 2 plasma and magnetic field data together with a one‐fluid magnetohydrodynamics model to study the interactions of the 1991 global merged interaction region (GMIR) shock with the termination shock and the heliopause. The 1991 GMIR is an extraordinarily large global solar wind structure in radial, longitudinal, and latitudinal extents. It has a strong shock at the leading edge. After its penetration through the termination shock, the GMIR shock first propagates through the subsonic solar wind, then interacts with the heliopause. The interaction produces a transmitted shock propagating outward in the interstellar medium, and a reflected shock propagating backward in the subsonic solar wind. We identify the reflected shock and the transmitted shock as the possible source of the radio noise detected at Voyagers (Gurnett, et al., 1993). The plasma frequency behind the reflected and the transmitted shock can be responsible for the 2‐ and 3‐kHz radio emissions, respectively. The two bands of radio noise are emitted from sources on both sides of the heliopause starting at about the same time. If the emissions picked up by Voyager are due to fp radiation, then the heliopause is located at R∼130 AU. If the emissions are due to 2fp radiation, then R∼150 AU. Because the relative speed of the interstellar plasma with respect to the sun appears to be sub‐Alfvenic, it is very unlikely there is a fast‐mode bow shock of the heliosphere.