Spontaneous onset of magnetic reconnection in toroidal plasma caused by breaking of 2D symmetry

Abstract

Magnetic reconnection is studied in the collisionless limit at the Versatile Toroidal Facility (VTF) at MIT. Two distinct magnetic configurations are applied in the experiments; an open magnetic cusp and a closed cusp. In the open cusp configurations, the field lines intersect the the vacuum vessel walls and here axisymmetric oscillatory reconnection is observed. Meanwhile, in the closed cusp configuration, where the field lines are confined inside the experiment, the coupling between global modes and a current sheet leads to powerful bursts of 3D spontaneous reconnection. These spontaneous events start at one toroidal location, and then propagate around the toroidal direction at the Alfvèn speed (calculated with the strength of the dominant guide field). The three dimensional measurements include the detailed time evolution of the plasma density, current density, the magnetic flux function, the electrostatic potential, and the reconnection rate. The vastly different plasma behavior in the two configurations can be described using a simple theoretical framework, linking together the interdependencies of the reconnection rate, the in-plane electrostatic potential, and the parallel electron currents. We find that it is the breaking of toroidal symmetry by the global mode that allows for a localized disruption of the x-line current and hereby facilitates the onset of spontaneous reconnection.

The use of the power density for identifying reconnection regions

“…Similar potential structures have been measured in laboratory experiments by Egedal et al [], who showed that a current system is set up in the reconnection region, where ion polarization currents perpendicular to B are closed by parallel currents carried by electrons close to the X line. Investigating the physics behind these potentials, Egedal et al [] showed that a combination of magnetic and electric trapping of electrons can explain the formation of the potential structures and that these are responsible for the anisotropic electron pressure that has been observed in reconnection regions both in the magnetotail and in laboratory experiments.…”

Three‐Dimensional X‐line Spreading in Asymmetric Magnetic Reconnection

“…In the solar wind, a long extended X‐line of over hundreds of has also been reported (Phan et al, ). Laboratory experiments such as the Magnetic Reconnection Experiment (Dorfman et al, , ) and the Versatile Toroidal Facility (Egedal et al, ; Katz et al, ) have also observed X‐line spreading across the device.…”

Guide field dependence of 3‐D X‐line spreading during collisionless magnetic reconnection

“…Interestingly, experimental and satellite observations of systems with a strong guide field reveal strikingly different behavior of X‐line spreading. For example, experiments performed at the versatile toroidal facility (VTF) [ Katz et al , 2010; Egedal et al , 2011] exhibit reconnection beginning in a localized region and spreading bidirectionally in the out‐of‐plane (toroidal) direction at a speed consistent with the Alfvén speed based on the guide field. Another example is bidirectional spreading (or elongating) of ribbons observed during two‐ribbon solar flares [ Qiu , 2009], including the Bastille Day flare [ Qiu et al , 2010].…”