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The two Voyager spacecraft have now been immersed in the very local interstellar medium for several years. Both spacecraft carry a plasma wave instrument capable of detecting plasma waves that yield electron density through the determination of the electron plasma frequency. Recent observations by Voyager 1 show increases in density at shocks and pressure fronts that are commensurate with increases in the magnetic field at these structures. Voyager 1 has not observed electron plasma oscillations, thought to be a signature of a nearby shock, since 2019, although Voyager 2 continues to observe these as recently as 2022 November. Voyager 1 also detects a faint thermal emission at the electron plasma frequency that shows the evolution of the plasma density as Voyager moves deeper into the medium. Here, we show the most recent observations from both Voyagers showing the increasing densities in the region upstream of the heliopause. We also investigate the fate of solar transients as they move ever deeper into the interstellar medium.

 

Abstract Large-scale disturbances generated by the Sun’s dynamics first propagate through the heliosphere, influence the heliosphere’s outer boundaries, and then traverse and modify the very local interstellar medium (VLISM). The existence of shocks in the VLISM was initially suggested by Voyager observations of the 2-3 kHz radio emissions in the heliosphere. A couple of decades later, both Voyagers crossed the definitive edge of our heliosphere and became the first ever spacecraft to sample interstellar space. Since Voyager 1’s entrance into the VLISM, it sampled electron plasma oscillation events that indirectly measure the medium’s density, increasing as it moves further away from the heliopause. Some of the observed electron oscillation events in the VLISM were associated with the local heliospheric shock waves. The observed VLISM shocks were very different than heliospheric shocks. They were very weak and broad, and the usual dissipation via wave-particle interactions could not explain their structure. Estimates of the dissipation associated with the collisionality show that collisions can determine the VLISM shock structure. According to theory and models, the existence of a bow shock or wave in front of our heliosphere is still an open question as there are no direct observations yet. This paper reviews the outstanding observations recently made by the Voyager 1 and 2 spacecraft, and our current understanding of the properties of shocks/waves in the VLISM. We present some of the most exciting open questions related to the VLISM and shock waves that should be addressed in the future.