Evidence for accelerated electrons from weakly collisional shocks in the very local interstellar medium

Beginning in the early 1980s, the two Voyager spacecraft were able to monitor the very local interstellar medium (VLISM) via low frequency radio emissions generated via a mode conversion mechanism involving electron plasma oscillations. The plasma oscillations are driven by a suprathermal beam of electrons in what is effectively the electron foreshock of shocks moving through the VLISM. The shocks themselves are the result of solar transients that propagate, merge, and otherwise evolve into disturbances that eventually interact with the heliopause and produce shocks that continue outwards through the VLISM. The post-heliopause shocks are very thick and weakly collisional. It is unclear how they can reflect electrons to create the beams necessary to excite plasma oscillations. Yet, both Voyagers have observed electron plasma oscillations in the VLISM. In this largely experimental examination of the shocks and resulting plasma oscillations, we set the challenge for understanding the mechanism that produces the electron distribution that is unstable to plasma oscillations.