Nonlinear heating of the reconnection layer by strong lower hybrid instabilities

Abstract

Magnetic reconnection was first proposed in the 1950s in solar physics in order to explain solar flares, especially the conversion of the magnetic energy to the plasma kinetic and thermal energy. Recently, computer simulations and laboratory experiments provide the opportunity to study reconnection in great detail. However, the original reason to study reconnection, which is the energy conversion problem, seems to be much less discussed. So far, no consistent theory has been developed to explore plausible heating mechanism in the reconnection layer. In this thesis, a self-consistent theory is built to fill this gap. This theory proposes a mechanism for ion heating and probable electron heating in the current layer of the Magnetic Reconnection Experiment (MRX).

This thesis is constructed as follows: (1) a local linear theory, in which multiple plasma waves are found and one of them is unstable, and we also discuss the mechanism of this unstable wave; (2) a local nonlinear theory, which indicates the nonlinear wave-wave interaction process; and (3) a global nonlinear theory, which gives the ion and probable electron heating due to the waves found in the linear theory. The numerical examples show that the theory can generate sufficient ion heating in the current layer, which keeps the layer wide enough for fast reconnection. To the author’s knowledge, so far this mechanism is the only self-consistent theory that explains in detail the plasma heating process.

The unique idea of the theory emerges in the nonlinear calculation. It involves two different kinds of plasma waves. One wave is linearly unstable and the other is damped. However, the damped wave can be driven unstable by its nonlinear wave interaction with the unstable wave. In other words, the unstable wave (which itself does not provide heating) works as the source to pump energy to the otherwise damped wave (which does provide heating). Since the damped wave is THE wave that heats the plasma, the heating process is genuinely a two-step procedure. This idea challenges the traditional thinking that only single linearly unstable waves can be excited and meaningful in the system.