Weakly Collisional Shocks and Turbulence Observed by Voyagers in the Very Local Interstellar Medium*

This study discusses Voyager 1 and 2 observations of shocks and turbulence in the very local interstellar medium (VLISM). Both spacecraft detected magnetic field jumps in the VLISM up to 150 AU from the Sun, some believed to be shocks. Models suggest these events occur when solar wind disturbances collide with the heliopause, creating perturbations that can develop into shocks.

The passing time of the shocks at Voyager ranges from hours to days, corresponding to spatial scales ~0.05-0.2 AU, i.e. tens of thousands of ion inertial lengths. These weak, quasi-perpendicular shocks have a fast magnetosonic Mach number below 1.5. Coulomb collisions, particularly thermal conduction, were proposed as the dissipation mechanism. However, if the VLISM temperature near the heliopause exceeds 20,000 K-30,000 K, shock thicknesses smaller than the mean free path are observed.

Compressible turbulence and nonlinear quasi-periodic waveforms exist in the VLISM. MHD-scale turbulence is enhanced after shocks, with one 2014 shock showing intense fine-scale turbulence in the foreshock and plasma oscillations. These observations challenge the assumption of a featureless VLISM below the mean free path.

MHD and kinetic simulations have not replicated Voyager's observations due to the wide range of scales involved. Key questions include understanding the problem of disturbance injection in the VLISM, the role of turbulence in preventing wave steepening and shock formation, shock dissipation, and the evolution of shocks in nonuniform, partially ionized plasma.