Katherine Goodrich - Shocks Under the Microscope: Examining the Microphysics of Collisionless Shocks with High Resolution, Multi-point Space Plasma Measurements
Collisionless shocks are an important and universal phenomenon in astrophysical plasmas. Shocks form when a supersonic plasma flow interacts with an impermeable obstacle. Examples of such interactions include galactic jets or supernova remnants interacting with the interstellar medium, or plasma wind from stars encountering stellar system bodies, such as planets, comets and moons. The shock performs the necessary function of converting kinetic energy to thermal energy, heating the originally supersonic plasma flow until its speed is reduced and it can flow around the obstacle. The energy conversion processes that take place inside collisionless shocks, however, are not not well established and have thus been a subject of interest for several decades.
The closest and perhaps the most relevant collisionless shock to us humans, is the Earth’s bow shock. This shock forms 10-12 Earth radii upstream of our planet when supersonic plasma ejected from the sun, called the solar wind, meets the Earth’s intrinsic magnetosphere. The bow shock has been observed by multiple spacecraft such as ISEE, Cluster, WIND, and THEMIS over three decades. These missions have provided us with a wealth of information on plasma conditions both upstream and downstream of the shock, however, limited instrument capabilities have prevented us from resolving the physical processes active at the shock itself. The energy conversion processes active within shocks are expected to occur at primarily electron spatial and time scales (milliseconds to seconds), which up until recently were beyond the reach of our particle observations. While we can rely on highly time resolved magnetic and electric field measurements from these missions, they provide an incomplete view of the inner workings of the shock.
Observations from the Magnetospheric Multiscale (MMS) mission (launched in 2014) provide particle observations on the order of 10s of milliseconds, providing an unprecedented opportunity to directly observe microscale process that inside collisionless shocks. In this talk, we’ll take a first look at the direct connection between electric and magnetic field signals with electron and ion dynamics in the Earth’s bow shock. We find that energy conversion can occur from multiple processes, some unexpected, within varied areas of the shock. The new dataset provided by MMS enables a new age of collisionless shock analysis as well as motivation for future space missions dedicated to the shock, signaling an exciting time for space physicists!