3.7 Evidence of reconnection and current sheets

In Section 3.2, we discussed the Flux Cancellation flare-CME model involving field line reconnection. A consequence of this process is the formation of a current sheet that connects the outgoing CME/flux rope with the reforming coronal loop arcade near the surface (Figure 21View Image). Evidence for this seems to have been observed by a number of instruments. Yohkoh/SXT and Hinode/XRT observations have provided substantial X-ray evidence of current sheet formation, such as cusp-shaped loops (Shibata, 1999) and supra-arcade downflows (SADs – McKenzie and Hudson, 1999, 2001; Sheeley Jr et al., 2004; Savage et al., 2010Jump To The Next Citation Point), of post-CME reconnection occurring over long-duration flares. The supra-arcade downflows are downward motions that have been observed in Yohkoh, TRACE, Hinode, and SOHO SUMER above post-CME flare arcades. SADs have trajectories which slow as they reach the top of the arcade, consistent with post-reconnection magnetic flux tubes retracting from a reconnection site high in the corona until they reach a lower-energy magnetic configuration. Savage et al. (2010Jump To The Next Citation Point) showed for a single XRT event following a limb CME that SADs can also appear as shrinking loops rather than downflowing voids. In that event, for the first time both the current sheet and the outgoing CME were imaged in soft X-rays and followed into the LASCO C2 field of view.

Sui and Holman (2003) first discussed a hard X-ray event observed by RHESSI that showed a compact X-ray source above the top of a roughly vertical current sheet in which they claimed magnetic reconnection was occurring. Subsequently, Sui et al. (2005), Saint-Hilaire et al. (2009), and others have analyzed similar events in which the coronal sources appear to move both downwards and outwards with time. The hard X-ray emission indicates a very hot source, ∼ 107 – 8 K, with the outgoing packet associated with the magnetic X-point where oppositely-directed field lines reconnect. The downward source is likely hot plasma from the most-recently reconnected arcade loops that are shrinking, as well observed in the soft X-ray observations.

The current within the current sheet is confined to a surface. In MHD theory, an electric current passing through part of the volume of a fluid tends to be expelled by magnetic forces from the fluid, compressing the current into very thin layers within the volume. We now have growing evidence for the existence of such current sheets in the corona trailing CMEs when the observing conditions are appropriate. Following earlier studies of concave-outward structures and reforming helmet streamers after CMEs, Webb (1995; 2004) analyzed SMM CMEs with concave-outward bright regions, finding that about half were followed by coaxial, bright rays suggestive of newly formed current sheets lasting for several hours and extending more than five solar radii into the outer corona.

With the advent of LASCO data, cases of CMEs with rays and Y-shapes were reported by Simnett et al. (1997), and St Cyr et al. (2000) found such features in one third to one half of all LASCO CMEs. Bright narrow features with enhanced temperatures (3 – 6 × 106 K), densities (∼ 5 × 107 cm–3 at 1.5R ⊙), and abundances of elements with low first ionization potentials (FIPs) were observed with the UVCS following slow (∼ 180 km s–1; Ciaravella et al., 2003b) and very fast (1800 km s–1; Ko et al., 2003; Lin et al., 2005Jump To The Next Citation Point) CMEs. Figure 26View Image shows enhanced images of the 18 November 2003 event; the CME had a concave-outward, flux-rope like appearance followed by a rapidly brightening ray (Lin et al., 2005Jump To The Next Citation Point). Blobs moved along the ray at ∼ 1000 km s–1 suggesting bursty reconnection in the current sheet, as MHD modeled for example by Riley et al. (2007). Figure 27View Image shows an example of SOHO observations of a classic three-part CME with narrow enhanced Fe XVIII emission centered under the CME where the current sheet should lie (as in the standard-flare-model cartoon sketched at the bottom of the figure). Yokoyama et al. (2001), Simnett (2004), Sheeley Jr and Wang (2007), Vršnak et al. (2009), and Savage et al. (2010) also identified bi-directional flows in SOHO and Hinode images moving away from a common point in the low to mid-corona that were interpreted in terms of reconnecting current sheets.

Ciaravella and Raymond (2008) were the first to combine UVCS and white light data to derive both the density and thickness in a current sheet, rather than assuming one to estimate the other. Bemporad and Mancuso (2010) derived turbulent speeds and their evolution in time, which is the main constraint for turbulent current sheet models. In these and other results (e.g., Lin et al., 2009), the thickness of the current sheet was calculated to be much larger than classical or anomalous resistivity would predict, possibly indicating an effective resistivity much larger than anomalous resistivity, such as that due to hyperdiffusion. The Petschek reconnection mechanism (Petschek, 1964) and turbulent reconnection is consistent with these results.

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Figure 26: A very fast CME with flux-rope structure followed by a narrow ray on 18 – 20 November 2003. The ray also shows evidence of bursty reconnection in the current sheet (bottom panels). Image reproduced with permission from Lin et al. (2005).
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Figure 27: A LASCO “Light-bulb CME” on 23 March 1998 (see Ciaravella et al., 2003a). The UVCS slit at 1.5 R⊙ reveals hot Fe XVIII emission trailing the CME, an expected spectroscopic signature of a current sheet. Image courtesy A. Ciaravella.

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