The Role of Reconnection at Magnetic Separators in Complex Solar Flare Ribbons

Abstract

Solar flare ribbons, manifesting as transient brightenings in the chromosphere, are believed to trace out the footpoints of magnetic field lines that are reconnecting higher in the solar atmosphere. These field lines lie in a separatrix or quasi-separatrix layer that separates domains of different magnetic connectivity and hence forms a natural location for reconnection. Solar flares are typically characterized as being circular ribbon flares, two-ribbon flares, or complex ribbon flares based on the number and shape of the ribbons. There are relatively well-developed models to explain the first two types of flares based on the location of the reconnection powering the flare. The case of complex ribbons is less well understood, but is often posited to be a result of reconnection at multiple locations. We demonstrate here that reconnection at a magnetic separator connecting two coronal null points can naturally explain the complex ribbons observed for two events, an M2.9 class flare from NOAA AR 11112 (SOL2010-10-16T19:12), and an X2.2 class flare from NOAA AR 11158 (SOL2011-02-15T01:56).

Figures (9)

• Figure 1: The magnetic skeleton in a potential field model containing two opposite type coronal magnetic null points (red and blue triangles). The background image is the radial component of the model field, and the spine field lines are shown as cyan curves. In the top panel, field lines in the separatrix surfaces of the nulls are plotted in orange and magenta. A separator field line (green) connects the two nulls at the intersection of their fan surfaces. In the bottom panel, the footpoints at the lower boundary of separatrix field lines are shown. The fan traces are open curves, with the end of each fan trace corresponding to the location of the intersection with the lower boundary of a spine field line from the opposite type null.
• Figure 2: Elements of the magnetic skeleton in a PFSS model for NOAA AR 11112 at 17:48 TAI on 2010 October 16 (left) and flare ribbons for the M2.9 flare (right) on 2010-10-16. Selected magnetic null points are shown as red and blue triangles, with color determined by null type, and their spine field lines (cyan curves). The greyscale image is the radial component of the PFSS model, scaled to $\pm 10^3$ G. In the top left panel, field lines in the separatrix surfaces of nulls A1 (magenta) and B1 (orange) are plotted, along with the separator field line (green) connecting the nulls (largely obscured by the spine field line above it, but clearly visible in Figure \ref{['fig:AR11112_skeleton_detail']}) that delineates the intersection of the two separatrix surfaces. In the bottom left panel, the footpoints at the lower boundary of separatrix field lines (fan traces) are shown, along with a salmon contour outlining the flare ribbons present in at least two frames of AIA data. Gaps in the fan traces are spanned by spine field lines (e.g., the gap in the orange fan trace from null B1 is spanned by the spine field lines of null A1). The locations of the flare ribbons, visible in an AIA 1600 Å image at 19:05 TAI in the upper right panel, are color coded by the time of first detection in the lower right panel. In the core of the active region, the locations of ribbon R1 and the fan trace of null B1 match reasonably well, as do the locations of ribbon R2 and one segment of the fan trace of null A1, suggesting that reconnection at the separator connecting the nulls plays a role in this event. Another segment of the fan trace of null A1 is also present in the vicinity of the remote ribbon R3, but the spatial agreement is worse than for the ribbons in the core of the region.
• Figure 3: The same elements of the magnetic skeleton shown in Figure \ref{['fig:AR11112_skeleton']} for the core of NOAA AR 11112 but viewed along the Sun's polar axis from the south. The orange field lines lie in the fan surface of null B1, which forms a cave, with the entrance to the cave following the spine field lines of null A1. The magenta field lines lie in the fan surface of null A1, which forms a tunnel, with the two entrances to the tunnel following the spine field lines of nulls B1, B2. Reconnection at the separator (green curve) connecting nulls A1 and B1 would result in a semi-circular flare ribbon along the fan trace of null B1, and an approximately linear segment along one fan trace of null A1.
• Figure 4: Elements of the magnetic skeleton in a PFSS model for NOAA AR 11158 at 01:00 TAI on 2011 February 15 (left) and flare ribbons for the X2.2 flare (right) on 2011-02-15 in the same format as Fig. \ref{['fig:AR11112_skeleton']}. In the top left panel, field lines in the separatrix surfaces of three nulls are plotted (two shades of orange and magenta), along with the separator field lines (green) connecting the nulls. In the bottom left panel, parts of the fan traces of nulls B1, B2, and A1 are in close proximity to the ribbons labeled R1, R2, and R3. All the flare ribbons are visible in an AIA 1600 Å image at 01:51 TAI shown in the upper right panel. In the bottom right panel, the earliest appearance of the flare ribbons labeled R0 is in the J-shape often associated with a twisted magnetic flux rope.
• Figure 5: Elements of the magnetic skeleton in the PFSS model for NOAA AR 11158 but viewed along the Sun's polar axis from the south. Field lines in separatrix surfaces of only nulls A1 and B1 are shown. Null B1 lies on the far side of its separatrix surface from this direction and thus is mostly obscured. An additional null point, B3, is shown in this plot that was not included in Figure \ref{['fig:AR11158_skeleton']} lying below null A1, and connected to it by a separator. This results in an additional gap in the fan trace of null A1, not easily visible in Figure \ref{['fig:AR11158_skeleton']}.