Origin of the Weak Plasma Emission Line Detected by Voyager 1 in the Interstellar Medium: Evidence for Suprathermal Electrons

Abstract

Abstract Recently, a very weak, nearly continuous plasma wave emission line has been discovered in the nearby interstellar medium at the electron plasma frequency. The new observations were made by the plasma wave instrument on the Voyager 1 spacecraft, which crossed into the interstellar medium in 2012 August. Several questions remained unanswered after the initial discovery. Why was the emission line not observed until several years after Voyager 1 entered the interstellar medium, what is the wavelength of the plasma oscillations responsible for the emission line, and what is the origin of the oscillations? Here, we provide answers to these questions. On the most important question, namely the origin of the oscillations, the evidence strongly suggest that the emission is driven by suprathermal electrons that excite plasma oscillations comparable to the quasi-thermal noise (QTN) that is commonly observed by space plasma wave instruments with long, thin electric dipole antennas. These results imply the existence of a relatively dense population of suprathermal electrons that could contribute significantly to the overall pressure in the interstellar medium. Although the similarities to the previous QTN observations are impressive, there is no certainty that the emissions are driven by thermal excitation, and other sources should be explored, such as the possibility that they are driven by pressure fluctuations associated with the short-wavelength cascade of interstellar turbulence.