Three-Dimensional Magnetic Field Line Reconnection
Oct 26, 2011
3:30PM to 4:30PM
Date(s) - 26/10/2011
3:30 pm - 4:30 pm
Title: Three-Dimensional Magnetic Field Line Reconnection
Speaker: Dr. Walter Gekelman – UCLA
Institute: Basic Plasma Science Facility – UCLA
Location: ABB 102
Magnetic field line reconnection is a processes in which magnetic field energy is converted to particle energy and heating accompanied by changes in the magnetic field topology. It occurs near the surface of the sun and is thought to be responsible for coronal heating, Images and data from the sun and several laboratory experiments indicate that reconnection is a fully three-dimensional process. Reconnection events often involve interesting structures called magnetic flux ropes. Magnetic ropes are helical magnetic fields with pitch varying with radius. In the experiments we describe two or three magnetic flux ropes are generated from initially adjacent pulsed current channels in a background magnetized plasma (L = 17m,dia = 60cm,0.3≤B0z≤2.5kG,n=2Ã?1012cm−3)in the large plasma device (LAPD) at UCLA. The flux ropes exert mutual forces causing them to twist about each other and merge. Currents associated with the ropes are not static but move towards and away from each other in time, and when they merge they sunsequently break up into filaments.
Volumetric space-time data show multiple reconnection sites with time-dependent locations. The concept of a quasi-separatrix layer (QSL) is a tool to understand and visualize how magnetic field lines reconnect in 3D. We will explain what a QSL is, and that it is necessary in the description of reconnection in the presence of a strong background magnetic field. In our experiment the QSL is a narrow region(s) between the flux ropes. It was observed for the first time in our laboratory. Its three dimensional development will be shown in movies made from the data. Reconnection of three ropes leading to multiple QSLâ??s, will also be presented. A related experiment involves sheets of current rather than ropes. The currents attract and revolve around another as well as filament into multiple magnetic islands. When the currents move toward or away from one another reconnection three-dimensional reconnection occurs. The magnetic helicity, which is a measure of the linkage of magnetic fields is evaluated from volumetric data in both cases and its rate of change is used to estimate the plasma resistivity.