Thursday 24 July 2014, by W. Gekelman (Department of Physics and Astronomy, UCLA, California 90095, USA)
Friday 11 January 2013 à 11h00 , Lieu : Salle de conférence du bât. 17
Magnetic flux ropes are due to helical currents and form a dense carpet of arches on the surface of the sun. Occasionally one tears loose as a coronal mass ejection and its rope structure is detected by satellites close to the earth. Current sheets in plasma can tear into filaments and these are nothing other than flux ropes. Ropes are not static, they exert mutual J × B forces causing them to twist about each other and merge. Kink instabilities cause them to violently smash into each other and reconnect at the point of contact. We report on experiments done in in the large plasma device the LAPD (L = 17m, dia = 60cm, 0.3 ≤ B0z ≤ 2.5kG, n ≅ 2 × 1012 cm −3 ) at UCLA on three dimensional flux ropes. Two or three magnetic flux ropes are generated from initially adjacent pulsed current channels or a series of ropes from the tearing of a current sheet. in a background magnetized plasma. The currents and magnetic fields form exotic shapes with no ignorable direction and no magnetic nulls. Volumetric space-time data show multiple reconnection sites with time-dependent locations. Mach probes measure three dimensional plasma flow which jet out from reconnection regions and spiral around the rope(s) magnetic field. The concept of a quasi-separatrix layer (QSL), a tool to understand and visualize 3D magnetic field line reconnection without null points is introduced. We will explain what it is and how it is derived from our data. Here the QSL is a narrow ribbon-like region(s) that twist between field lines. It (they) will be shown in detail from data sets acquired at up to 50,000 spatial locations. Within the QSL(s) field lines that start close to one another rapidly diverge as they pass through one or more reconnection sites. 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. Heating and other co-existing waves (the ropes are Alfvén waves in disguise) will be presented. Flux ropes and QSLs show promise in understanding what 3D reconnection really is!
This work was done at the Basic Plasma Science Facility at UCLA and funded by NSF and DOE
Figure caption: The figure is a rendering of the experimental data. The blue and green tubes are magnetic field lines associated with the flux ropes. (Note several “green” field lines have rotated in space and are smashing into the “blue” ones). The orange field lines are plasma flow. The flow speed is as large as Mach 0.3. The blue surface is a Quasi-Seperatrix layer of value 1000. The flow goes through the QSL at one point and then spirals around the flux ropes. The two axis markers are two meters apart in the z direction (the machine axis). Data was acquired at 50,000 locations over a volume (δx=24 cm, δy = 24 cm, δz = 10m). Many other images and movies will be shown in the talk.
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