Extending Observations of Phase-Space Holes and Double Layers to Consider Electron Cyclotron Maser Emission

Abstract:

Electron phase-space holes and double layers are key elements in the dynamics of space plasmas. Double layers in particular play a strong role in particle acceleration, reconnection, and turbulence. The Magnetospheric Multiscale (MMS) mission expects to see double layers directly related to magnetic reconnection events. Data from previous in-situ measurements has shown that double layers energize electron cyclotron maser emission, the mechanism for Earth’s auroral kilometric radiation. Notably, double layers are a primary generation mechanism for electron phase-space holes. These kinetic plasma structures are also important in astrophysical systems far away from Earth and any possibility of in-situ observations. For example, phase-space holes generated by the two-stream instability are cited as an important component of efficient blazar jet dissipation, and double layers are implicated as an acceleration mechanism for cosmic rays. Using Earth’s magnetosphere as a plasma physics laboratory, we can scale up the physical processes observed by missions like MMS to the magnetospheres of larger planets or more extreme astrophysical systems such as neutron stars. Phase-space holes in these environments are expected to be moving at relativistic speeds. Given the correct frame transformation, the hole’s distribution function may have a positive $df/dp_{\perp}$, and is therefore susceptible to the electron cyclotron maser instability. Relativistic phase-space holes may then produce observable coherent radiation indicative of the plasma’s properties. As a first step in investigating this hypothesis, we have developed a special relativistic, electrostatic Vlasov simulation that can form phase-space holes and double layers in any frame of motion parallel to the flow. When electron holes are produced in a relativistic, counter streaming plasma, we predict distributions that are unstable to the electron cyclotron maser instability.