The Plasma Density in the Interstellar Medium Near and Beyond the Heliopause: Observations from the Voyager 1 and 2 Plasma Wave Instruments

Abstract:

The heliopause is the boundary between the solar and interstellar plasmas. In this presentation I will summarize results concerning the interstellar plasma density from the Voyager 1 and 2 plasma wave (PWS) instruments. The first evidence that the PWS could provide the plasma densities in the interstellar medium came from transient heliospheric 2-3 kHz radio emissions over 30 years ago. One of the characteristic features of these radio emissions was an upward drift in the radio emission frequency with increasing time after the onset of the emission. In 1993 we published a paper proposing that the 2-3 kHz radio emissions were produced when an interplanetary shock from an energetic event at the Sun (coronal mass ejection) reached the heliopause and produced radio emissions at the electron plasma frequency. In 2012 the Voyager 1 spacecraft reached the heliopause at 121.6 AU and the PWS detected electron plasma oscillations ahead of shocks propagating outward from the Sun. The frequencies of these plasma oscillations increased with increasing distance into the interstellar medium, just as predicted in the 1993 paper. These electron plasma oscillations now provide our primary (and possibly only) method of determining local plasma densities in the interstellar medium (via the equation for the electron plasma frequency, f_p = 8980√n_e Hz, where n_e is the electron density in cm^-3). Both the Voyager 1 and 2 PWS instruments now show that the plasma density initially increases rapidly by almost a factor of 2 in the approximately 5 AU after the first detection of plasma oscillations upon crossing the heliopause. After this initial density ramp the density continues to slowly increase to about 0.12 cm^-3 at 160 AU. We do not know if this density, about 0.12 cm^-3_, is the final asymptotic plasma density in the very-local-interstellar-medium, or will eventually reach a peak and decrease, as predicted by some models.