Experimental study of the dynamics of a thin current sheet

Abstract

Many plasmas in natural settings or in laboratory experiments carry currents. In magnetized plasmas the currents can be narrow field-aligned filaments as small as the electron inertial length w c pe () in the transverse dimension or fill the entire plasma column. Currents can take the form of sheets, again with the transverse dimension the narrow one. Are laminar sheets of electric current in a magnetized plasma stable? This became an important issue in the 1960s when current-carrying plasmas became key in the quest for thermonuclear fusion. The subject is still under study today. The conditions necessary for the onset for tearing are known, the key issue is that of the final state. Is there a final state? One possibility is a collection of stable tubes of current. On the other hand, is the interaction between the current filaments which are the byproduct endless, or does it go on to become chaotic? The subject of three-dimensional current systems is intriguing, rich in a variety of phenomena on multiple scale sizes and frequencies, and relevant to fusion studies, solar physics, space plasmas and astrophysical phenomena. In this study a long (δz=11 m) and narrow (δx=1 cm, δy=20 cm) current sheet is generated in a background magnetoplasma capable of supporting Alfvén waves. The current is observed to rapidly tear into a series of magnetic islands when viewed in a cross-sectional plane, but they are in essence three-dimensional flux ropes. At the onset of the current, magnetic field line reconnection is observed between the flux ropes. The sheet on the whole is kink-unstable, and after kinking exhibits large-scale, low-frequency (f = f ci) rotation about the background field with an amplitude that grows with distance from the source of the current. Three-dimensional data of the magnetic and electric fields is acquired throughout the duration of the experiment and the parallel resistivity is derived from it. The parallel resistivity, for the most part, is not largest in the reconnection regions, but peaks in the neighborhood of large current gradients. At early times a quasi-separatrix layer (QSL) is observed where the current sheet tears, but later on a QSL of larger value, not obviously associated with reconnection, is measured at the edge of the current sheet. This QSL enhancement is connected with the rapidly spatially diverging magnetic fields in the moving sheet (ropes).

Non-local Ohm's law during collisions of magnetic flux ropes

“…The perpendicular magnetic field can be as large as 30 G or about 1/10 of the background field, B 0z ¼ 330 G. The flux ropes rotate about each other in addition to being linearly kink unstable. As the rope rotation starts at a slightly different time for each experimental shot, a conditional averaging technique 20 was used to define the experimental start time, which differs from the time the rope currents are switched on.…”

“…Reverse currents have been seen in the past in experiments on reconnection. 20 There are axial pressure gradients that can potentially drive reverse currents, but these are measured and were included in the Ohm's law evaluation. Assuming that there is no dynamo, one may question whether Ohm's law can be used at all in these circumstances.…”

On the Fourier Contribution of Strong Current Sheets to the High‐Frequency Magnetic Power SpectralDensity of the Solar Wind

“…Finding #3 in Table 2 points out that the profiles of solar wind current sheets determine the high-frequency magnetic spectra. As noted in Table 2, physical concepts that are important to the spatial profiles of current sheets are current sheet structuring (Gekelman et al, 2016;Ng et al, 2019;Schindler & Birn, 2002;Schindler & Hesse, 2008), plasma expansion and compression (Schindler & Hesse, 2008, 2010, Bohm and gyro-Bohm diffusion (Borovsky & Gary, 2009;Pecseli & Mikkelsen, 1985;Vahala & Montgomery, 1971), finite-gyroradii effects (Schindler & Hesse, 2010), ion versus electron current carriers (Aunai et al, 2013;Sasunov et al, 2016;Schindler & Birn, 2002), Alfvén wave nonlinear processes (Gomberoff, 2007;Tsurutani et al, 2002), plasma waves in current sheets (Huang et al, 2009;Malaspina et al, 2013;Verscharen & Marsch, 2011;Zelenyi et al, 2011), pressure balance Figure 13. The spectral slopes of the individual current sheets are plotted as a function of the Pearson linear correlation coefficient R corr between the logarithm of the power spectral density and the logarithm of the frequency in the frequency range where the spectral-slope fit is made.…”

Nonlocal Ohms Law, Plasma Resistivity, and Reconnection During Collisions of Magnetic Flux Ropes

“…The flux ropes rotate about each other in addition to being kink unstable. Because the rope rotation starts at a slightly different time for each experimental shot, a conditional averaging technique (Gekelman et al 2016b) was used to define the experimental start time, which differs from the time the rope currents are switched on by approximately ±100 μs. Briefly, the technique consists of correlating the temporal sequence of one component (B x ) of the reference probe (fixed at δx=5 cm, δy=0, δz=966 cm) to determine the lag time between motion of the rope past the fixed probe and the rope passing the movable probe.…”