Abstract: Most of the observable matter in the universe is plasma, and therefore understanding its behavior is important for a variety of space, astrophysical and laboratory applications. An active topic of research in recent years has been magnetic reconnection, a process by which magnetic fields diffuse in plasma, leading to a change in magnetic field topology, an explosive release of stored magnetic energy, and the formation of jets. This process is important in a variety of applications, including solar flares, planetary magnetospheres, astrophysical jets, and sawtooth crashes in controlled fusion devices such as tokamaks. One unresolved question is how the diffusion and energy conversion can occur in plasmas that are effectively collisionless. In 2015, NASA launched the Magnetospheric Multiscale (MMS) mission to study the physics of collisionless reconnection in detail. MMS consists of four identical spacecraft orbiting the earth in a tetrahedral formation, which can probe magnetic reconnection events down to the smaller scales (~1km or less) where collisionless effects occur. One process that warrants further attention is the role of plasma waves and turbulence in mediating magnetic reconnection. To address this, we present the result of two recent studies. The first is a statistical investigation of reconnection events at the dayside of earth’s magnetosphere using MMS data. For each event we identified the plasma wave modes present and determined their location with respect to the reconnection diffusion region. We find that electromagnetic waves known as “whistlers,” long thought to be important in mediating reconnection, are most often observed further away from the diffusion region. Instead, the region is dominated by low frequency electrostatic oscillations called “Lower Hybrid” waves, as well as turbulent distortions of the magnetic field associated with electron vortices. In the second study, we investigate why whistlers may not reach the diffusion region using observations of the turbulent region behind earth’s bow shock, known as the magnetosheath. Two reconnecting current sheets during an interval of strong turbulence are studied in detail. Both current sheets show that the dissipation in the diffusion region is dominated by an electron “acceleration channel,” containing an electric field parallel to the background magnetic field. These alter the electron distribution function in a way that suppresses whistler waves, preventing them from reaching the diffusion region. To close, we discuss the implications of the results terms of the collisionless dissipation of plasma turbulence and acceleration of charged particles.
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