Re-Interpretation of Supra-Arcade Downflows in Solar Flares

Abstract

Following the eruption of a filament from a flaring active region, sunwardflowing voids are often seen above developing post-eruption arcades. First discovered using the soft X-ray telescope aboard Yohkoh, these supra-arcade downflows (SADs) are now an expected observation of extreme ultra-violet (EUV) and soft X-ray coronal imagers and spectrographs (e.g, TRACE, SOHO/SUMER, Hinode/XRT, SDO/AIA). Observations made prior to the operation of AIA suggested that these plasma voids (which are seen in contrast to bright, hightemperature plasma associated with current sheets) are the cross-sections of evacuated flux tubes retracting from reconnection sites high in the corona. The high temperature imaging afforded by AIA's 131, 94, and 193Å channels coupled with the fast temporal cadence allows for unprecedented scrutiny of the voids. For a flare occurring on 2011 October 22, we provide evidence suggesting that SADs, instead of being the cross-sections of relatively large, evacuated flux tubes, are actually wakes (i.e., trailing regions of low density) created by the retraction of much thinner tubes. This re-interpretation is a significant shift in the fundamental understanding of SADs, as the features once thought to be identifiable as the shrinking loops themselves now appear to be "side effects" of the passage of the loops through the supra-arcade plasma. In light of the fact that previous measurements have attributed to the shrinking loops characteristics that may instead belong to their wakes, we discuss the implications of this new interpretation on previous parameter estimations, and on reconnection theory.

Review on Current Sheets in CME Development: Theories and Observations

“…In the late phase of the event, McKenzie and Hudson (1999) found that the downward motion was represented by a dark void in soft X-ray (SXR) moving at speed ranging from 100 to 200 km s −1 , and that the temperature in the void reached up to 9 × 10 6 K, which was slightly hotter than the surrounding fan-shaped supraarcade plasma, and the density was about 10 8 cm −3 . Later, McKenzie (2000) reported 11 long-life CME-associated events observed by Yohkoh/SXT that displayed similar sunward mass motions at speed ranging from 50 to 500 km s −1 , which were also observed recently by the SDO/AIA (Savage et al 2012a). Because of the weak signal of the X-ray data in the void, the temperature deduced for the dark void by McKenzie and Hudson (1999) was somewhat suspect.…”

“…This is the first time that a survey of ray-like structures has been performed and the results have been compared with CME models, and is also the first time that the CS was recognized by studying the white-light data. With the success of SDO, the CS associated structures right above flare loops have as well been observed in the AIA 131 Å and 94 Å channels in exquisite detail both as rays when viewed edge on (Reeves and Golub 2011), and as fans when viewed face on (Warren et al 2011;Savage et al 2012a;McKenzie 2013). studied the 2002 April 21 event and another two events occurring on July 23 and August 24, 2002, respectively, that all produced X-class (> 1.5) flares with associated fast CMEs at speeds of 1700-2500 km s −1 .…”

“…2, we believe that McKenzie and Hudson (1999) had observed a CS above the flare loops roughly face-on although observing a CS face-on in the Yohkoh era was not a trivial job. On the other hand, nowadays, with the observations from the SDO/AIA, currently the highest resolution solar observatory capable of coronal measurements, identifying a CS face-on has become less difficult (e.g., see Warren et al 2011;Savage et al 2012aSavage et al , 2012bMcKenzie 2013;Hanneman and Reeves 2014;Doschek et al 2014).…”

“…Now terms "SADs", "dark voids" and "dark lanes" are usually used in parallel. Spectroscopic observations indicated for the first time SADs to be voids (Innes et al 2003a), and by analyzing the data from SDO/AIA with high signal level and increased temporal cadence, Savage et al (2012a) argued that SADs were actually density depletions left in the wake of thin flux tubes retracting from a reconnection site in the supra-arcade region, and should no longer be considered cross-sections of newly reconnected, large evacuated flux tubes. Hanneman and Reeves (2014) further found that SADs usually had temperatures very similar to or lower than that of the surrounding flare arcades, which ranged from 3.1 × 10 6 to 1.3 × 10 7 K, and that their emission measure was typically about an order of magnitude lower than the nearby arcade.…”

“…SDO/AIA allows for unprecedented scrutiny of the SADs. Savage et al (2012a) showed that SADs, which once were thought to be identifiable as the shrinking loops themselves, appeared to be "side effects" of the passage of the loops through the supra-arcade plasma, and demonstrated the observational difference of SADs and loop shrinkage.…”