Collisionless shocks

Shock waves

Shock waves are found in regular fluids like air and water here on Earth. Shock waves arise when the flow velocity of an object relative to the fluid is greater than the speed of sound. Shocks are formed for example when an airplane breaks the sound barrier, which causes a sonic boom. Shocks can also be created by running water from a tap into a sink. A termination shock is then formed where the fast water flow transitions into slow flow.

Shocks in collisionless plasmas

Shocks are also found in a wide variety of plasmas in the universe. These plasmas are generally collisionless, which means that energy is dissipated via wave–particle interaction instead of particle–particle collisions. Some examples of shocks in collisionless plasma are shocks around supernova remnants and the Earth's bow shock, which forms when the solar wind encounters the magnetosphere of the Earth.

An important aspect of collisionless shocks is the orientation of the magnetic field relative to the shock. A shock where the magnetic field forms an angle of less than 45 degrees with the normal of the shock surface is called quasi-parallel. Otherwise, it is called quasi-perpendicular. While quasi-perpendicular shocks are characterized by a sharp transition, quasi-parallel shock transitions are usually extended far in space and are highly turbulent.

The mechanism of electron heating across collisionless shocks remains a subject of ongoing research. It has been observed that as the Alfvénic Mach number increases, the electron heating mechanism transitions from predominantly adiabatic to non-adiabatic. This shift indicates that stochastic shock drift acceleration (SSDA) becomes a more dominant process in high Mach number shocks. Analyses of large datasets of quasi-perpendicular shocks suggest that this non-adiabatic heating mechanism is consistent with SSDA, particularly in shocks with higher Mach numbers.

Particle acceleration

Shocks in space plasmas are capable of accelerating charged particles to extremely high energies. For example, cosmic rays are believed to be formed at shock transitions in supernova remnants. The particles are accelerated through a process called diffusive shock acceleration. However, the particles that undergo this process must have higher than thermal energies (suprathermal). Particles must first be accelerated to suprathermal energies before they can gain even higher energies.

A thorough understanding of collisionless shocks requires detailed knowledge of how different ion species are accelerated across the shock. For instance, studies of Earth's bow shock using data from the Magnetospheric Multiscale (MMS) spacecraft after a coronal mass ejection have provided insights into the behavior of various ion species such as protons, alpha particles, and singly charged helium ions. These studies reveal that while protons can be specularly reflected and create quasi-periodic fine structures in their velocity distribution downstream of the shock, heavier ions tend to transit the shock without reflection. The ability to resolve these ion species with high time-resolution is critical for understanding the complex dynamics at play.

We study in detail how particles are accelerated to suprathermal energies and are subsequently injected into the diffusive shock acceleration process. To do this, we use Earth's bow shock as a plasma physics laboratory. Utilizing data from the four European Cluster satellites, we investigate ion acceleration with high temporal and spatial resolution, focusing on how ions are accelerated by reflection off magnetic structures in the solar wind. The MMS satellites, with their enhanced resolution of field and particle data, will provide even deeper insights into the acceleration processes in shocks, potentially refining our understanding of how particles achieve the suprathermal energies necessary for further acceleration.