C. Z. Cheng

Electron and Ion Heating Characteristics during Magnetic Reconnection in the MAST Spherical Tokamak

Electron and ion heating characteristics during merging reconnection start-up on the MAST spherical tokamak have been revealed in detail using a 130 channel yttrium aluminum garnet (YAG) and a 300 channel Ruby-Thomson scattering system and a new 32 chord ion Doppler tomography diagnostic. Detailed 2D profile measurements of electron and ion temperature together with electron density have been achieved for the first time and it is found that electron temperature forms a highly localized hot spot at the X point and ion temperature globally increases downstream. For the push merging experiment when the guide field is more than 3 times the reconnecting field, a thick layer of a closed flux surface form by the reconnected field sustains the temperature profile for longer than the electron and ion energy relaxation time ~4-10 ms, both characteristic profiles finally forming a triple peak structure at the X point and downstream. An increase in the toroidal guide field results in a more peaked electron temperature profile at the X point, and also produces higher ion temperatures at this point, but the ion temperature profile in the downstream region is unaffected.

Relationship between wave-like auroral arcs and Pi2 disturbances in plasma sheet prior to substorm onset

Wave-like substorm arc features in the aurora and Pi2 magnetic disturbances observed in the near-Earth plasma sheet are frequently, and sometimes simultaneously, observed around the substorm onset time. We perform statistical analyses of the THEMIS ASI auroral observations that show wave-like bright spot structure along the arc prior to substorm onset. The azimuthal mode number values of the wave-like substorm arcs are found to be in the range of ~100–240 and decrease with increasing geomagnetic latitude of the substorm auroral arc location. We suggest that the azimuthal mode number is likely related to the ion gyroradius and azimuthal wave number. We also perform correlation study of the pre-onset wave-like substorm arc features and Pi2 magnetic disturbances for substorm dipolarization events observed by THEMIS satellites during 2008–2009. The wave-like arc brightness structures on the substorm auroral arcs tend to move azimuthally westward, but with a few exceptions of eastward movement, during tens of seconds prior to the substorm onset. The movement of the wave-like arc brightness structure is linearly correlated with the phase velocity of the Pi2 δBy disturbances in the near-Earth plasma sheet region. The result suggests that the Pi2 transverse δBy disturbances are related to the intensifying wave-like substorm onset arcs. One plausible explanation of the observations is the kinetic ballooning instability, which has high azimuthal mode number due to the ion gyroradius effect and finite parallel electric field that accelerates electrons into the ionosphere to produce the wave-like arc structure.

Physical processes of driven magnetic reconnection in collisionless plasmas: Zero guide field case

The key physical processes of the electron and ion dynamics, the structure of the electric and magnetic fields, and how particles gain energy in the driven magnetic reconnection in collisionless plasmas for the zero guide field case are presented. The key kinetic physics is the decoupling of electron and ion dynamics around the magnetic reconnection region, where the magnetic field is reversed and the electron and ion orbits are meandering, and around the separatrix region, where electrons move mainly along the field line and ions move mainly across the field line. The decoupling of the electron and ion dynamics causes charge separation to produce a pair of in-plane bipolar converging electrostatic electric field ( E→es) pointing toward the neutral sheet in the magnetic field reversal region and the monopolar E→es around the separatrix region. A pair of electron jets emanating from the reconnection current layer generate the quadrupole out-of-plane magnetic field, which causes the parallel electric field ...