Locations of the termination shock and the heliopause

Abstract

The locations of the termination shock and the heliopause are restudied by taking into account the effects of pickup protons. The study uses available plasma and magnetic field data from Voyagers over a 14‐year period (1978–1991) and Voyager observations of the 1992–1993 radio emission event, outside 30 AU, pickup protons have a significant influence on dynamical structures of the outer heliosphere. The solar wind is treated as a mixture of electrons, solar wind protons, and interstellar pickup protons. If the magnitude of the interstellar magnetic field Bint is given, one can quantitatively study the motion and location of the termination shock. We find that the location is anticorrelated with the sunspot number. The absolute mean of the shock speed is 19 km/s, and the quadratic mean of the shock speed is 24 km/s. Because Bint is poorly known, additional information is needed in studying the termination shock. Cummings et al. (1994) have used observations of anomalous cosmic rays to estimate the location of the shock. The observations of the 1991 global merged interaction region (GMIR) and GMIR shock and the 1992–1993 radio emission event provide another handle for the study of the termination shock and the heliopause. After its penetration through the termination shock, the GMIR shock continued to propagate in the subsonic region of the solar wind and eventually interacted with the heliopause. This interaction produced a transmitted shock propagating outward in the interstellar medium and a reflected shock propagating inward toward the Sun in the subsonic solar wind. The plasma frequencies behind the reflected and the transmitted shocks can be responsible for the 2‐ and 3‐kHz radio emissions, respectively. We assume that the impingement of the GMIR shock at the heliopause occurred at the time when Voyagers started receiving the radio emissions. Taking into account the effects of pickup protons, we find that the locations of the termination shock and the heliopause in 1991–1992 are at approximately 66 AU and 150 AU, respectively.