Numerical Experiments on Fine Structure Within Reconnecting Current Sheets in Solar Flares

Abstract

We perform resistive magnetohydrodynamic simulations to study the internal structure of current sheets that form during solar eruptions. The simulations start with a vertical current sheet in mechanical and thermal equilibrium that separates two regions of the magnetic field with opposite polarity which are line-tied at the lower boundary representing the photosphere. Reconnection commences gradually due to an initially imposed perturbation, but becomes faster when plasmoids form and produce small-scale structures inside the current sheet. These structures include magnetic islands or plasma blobs flowing in both directions along the sheet, and X-points between pairs of adjacent islands. Among these X-points, a principal one exists at which the reconnection rate reaches maximum. A fluid stagnation point (S-point) in the sheet appeared where the reconnection outflow bifurcates. The S-point and the principal X-point (PX-point) are not co-located in space though they are very close to one another. Their relative positions alternate as reconnection progresses and determine the direction of motion of individual magnetic islands. Newly formed islands move upward if the S-point is located above the PX-point, and downward if the S-point is below the PX-point. Merging of magnetic islands was observed occasionally between islands moving in the same direction. Reconnected plasma flow was observed to move faster than blobs nearby.

Review on Current Sheets in CME Development: Theories and Observations

“…They concluded that the difference in velocities of plasma blobs could be ascribed to the reconnection processes occurring in the different environments. Similar phenomenon was also observed in numerical experiments on the reconnection process taking place in two-ribbon flares (see also Forbes and Malherbe 1991;Riley et al 2007;Shen et al 2011).…”

“…Panels in Fig. 12 display LASCO C2 images of the same process that had been enhanced by the wavelet technique, which further indicates that discriminating a post-CME CS from a streamer CS is not difficult, although in both cases the tearing mode instability could take place in the same fashion (e.g., see also Einaudi et al 2001) since the occurrence of the tearing mode depends on the length to thickness ratio of the CS (e.g., see Furth et al 1963;Loureiro et al 2007;Ni et al 2010;Shen et al 2011;Bárta et al 2011aBárta et al , 2011bMei et al 2012) no matter whether the CS is stretched by the expanding solar wind, or by the disrupting coronal magnetic field.…”

“…We need to note here that S. Savage (2014, private communications) keeps pointing out that SADs and plasmoids usually appearing in numerical experiments (e.g., see Bhattacharjee et al 2009;Bárta et al 2011a;Shen et al 2011;Mei et al 2012; and references therein) are not the same thing. SADs are low-density regions, while plasmoids show up in simulations as high density structures, and plasmoids appear when we view CSs edge-on, while SADs appear when we view CSs face-on.…”

“…Compared with the anti-sunward flow, the sunward flow is apparently slow. Shen et al (2011) pointed out that this is generally true because the sunward flow is always slowed down by its interaction with the closed flare loops below. Studying the SDO/AIA data, Takasao et al (2012) brought the speed of the anti-sunward flow to the range from 220 to 460 km s −1 , and that of the sunward flow to the range from 250 to 280 km s −1 .…”

“…Since the resistivity is small and the catastrophe takes place much faster than the dissipation at the beginning of the eruption (see discussions of Lin and Forbes 2000;Lin 2002), the CS is able to develop to a large scale, and various plasma instabilities would occur in the CS. Linear theory of the tearing mode instability indicates that the CS becomes unstable as its length exceeds 2π times its thickness (Furth et al 1963;Priest and Forbes 2000), and nonlinear investigations through numerical experiments suggest that this ratio could apparently exceed 10 and even up to 100, before the tearing mode is invoked (e.g., see Loureiro et al 2007;Ni et al 2010;Shen et al 2011;Bárta et al 2011aBárta et al , 2011bMei et al 2012). Plasma instabilities yield turbulence inside the CS that may significantly enhance the dissipation of the magnetic field, resulting in an effective resistivity that is much higher than the traditional Spitzer value (Strauss 1986(Strauss , 1988Ambrosiano et al 1988;Bhattacharjee and Yuan 1995;Shibata and Tanuma 2001;Skender and Lapenta 2010;Bárta et al 2011aBárta et al , 2011bShen et al 2011;Mei et al 2012; and references therein).…”