Re-interpretation of Supra-Arcade Downflows in Solar Flares
RRe-interpretation of Supra-Arcade Downflows in Solar Flares Sabrina L. Savage, David E. McKenzie, & Katharine K. Reeves NASA/Goddard Space Flight Center (Oak Ridge Associated Universities), 8800 GreenbeltRd Code 671, Greenbelt, MD 20771, USA Department of Physics, Montana State University, PO Box 173840, Bozeman, MT59717-3840, USA Harvard-Smithsonian Center for Astrophysics, 60 Garden Street MS 58, Cambridge, MA02138, USA
ABSTRACT
Following the eruption of a filament from a flaring active region, sunward-flowing voids are often seen above developing post-eruption arcades. First dis-covered using the soft X-ray telescope aboard
Yohkoh , these supra-arcade down-flows (SADs) are now an expected observation of extreme ultra-violet (EUV) andsoft X-ray coronal imagers and spectrographs (e.g,
TRACE , SOHO /SUMER,
Hinode /XRT,
SDO /AIA). Observations made prior to the operation of AIAsuggested that these plasma voids (which are seen in contrast to bright, high-temperature plasma associated with current sheets) are the cross-sections of evac-uated flux tubes retracting from reconnection sites high in the corona. The hightemperature imaging afforded by AIA’s 131, 94, and 193 ˚A channels coupled withthe fast temporal cadence allows for unprecedented scrutiny of the voids. For aflare occurring on 2011 October 22, we provide evidence suggesting that SADs,instead of being the cross-sections of relatively large, evacuated flux tubes, areactually wakes (i.e., trailing regions of low density) created by the retraction ofmuch thinner tubes. This re-interpretation is a significant shift in the fundamen-tal understanding of SADs, as the features once thought to be identifiable as theshrinking loops themselves now appear to be “side effects” of the passage of theloops through the supra-arcade plasma. In light of the fact that previous mea-surements have attributed to the shrinking loops characteristics that may insteadbelong to their wakes, we discuss the implications of this new interpretation onprevious parameter estimations, and on reconnection theory.
Subject headings:
Magnetic reconnection — Sun: corona — Sun: flares — Sun:coronal mass ejections (CMEs) — Sun: magnetic topology — Sun: UV radiation a r X i v : . [ a s t r o - ph . S R ] F e b
1. Introduction
Magnetic reconnection is a ubiquitously occurring process in the universe and is widelyaccepted to be important for energy release in solar eruptions. However, it is expected tooccur in regions of low emission measure, and therefore, observations have tended to beindirect (e.g., McKenzie 2002). Retracting loops and sunward-flowing plasma voids abovepost-eruption flare arcades are often observed throughout long duration events using instru-ments sensitive to emissions from high temperature plasmas or white-light scattering fromdensity structures in the corona (
TRACE , SOHO /SUMER,
SOHO /LASCO,
Hinode /XRT,
SDO /AIA) (see Savage & McKenzie 2011, Figure 1 therein, for example images of thesefeatures). Both supra-arcade downflowing loops (referred to as SADLs in Savage et al. 2010,for example) and supra-arcade downflows (SADs: plasma voids) have been interpreted asthe outflows created during the re-organization of the magnetic field during the reconnectionprocess.To reconcile the difference in observational appearance between the plasma voids (SADs)and shrinking loops, a geometrical explanation was offered based on the line of sight toSADs or SADLs above curved polarity inversion lines (Savage & McKenzie 2011; Warren,O’Brien, & Sheeley 2011). SADs were suggested to be the cross-sections of the retractingpost-reconnection flux tubes, viewed along a line of sight tangential to their axes, while theshrinking loops were viewed from a line of sight orthogonal to their axes. Many voids andshrinking loops have been reported in observations taken prior to the launch of
SDO /AIA(McKenzie & Hudson 1999; McKenzie 2000; Innes, McKenzie, & Wang 2003; Asai et al. 2004;Sheeley, Warren, & Wang 2004; Khan, Bain, & Fletcher 2007; Reeves, Seaton, & Forbes 2008;McKenzie & Savage 2009; Savage et al. 2010; Savage & McKenzie 2011).In this Letter, we present new evidence from recent AIA observations of the 2011 October22 flare (SOL2011-10-22T10:00:00L045C065) that support an alternative interpretation ofvoid-type SADs as wakes behind thin retracting loops, rather than cross-sections of muchlarger, evacuated loops. For the purpose of this reference, we will refer to the densitydepletions that trail behind the shrinking loops as “wakes” (i.e., disturbances in the densityof the current sheet, apparently resulting from the passage of the shrinking loops). Thedensity within the thin loops is unknown due to their small size; however, the fact that theyare emitting implies that they are not evacuated. The key observation is depicted in Figure 1and in the movies accompanying this Letter.To be clear, SADs were also associated with the idea of a wake in the previous inter-pretation: The shape of the plasma voids has been likened to that of a tadpole – typicallyconsisting of an oblong, tear-drop shaped trough at the head of the downflow followed bya thin, often waving “tail” (Savage & McKenzie 2011, Figure 1 therein). In the traditional 3 –Fig. 1.— Sequence of AIA 131 ˚A images (reverse-scaled) focusing on the evolution of oneSAD. The top panels show the descent of the leading edge of a large void. Panel (a) indi-cates the position of two initial SADs that appear to merge in subsequent frames. Around12:06 UT, two thin retracting loops appear at the leading edge of the void as indicated inthe bottom panels, which have been differenced and scaled to enhance motion. Panel (h)provides the clearest example image of the two leading loops. We refer the reader to theonline movies accompanying this Letter for clearer evidence of this process. The white boxin the overlay panel contains two large SADs shown to be regions of density depletion inFigure 4. (Animations of this figure are available in the online Astrophysical Journal.)interpretation, the head of the flow is the cross-section of a large evacuated flux tube, whilethe trailing tail has been explained as the wake of this large loop. In the new interpretation,the tail is still considered part of the wake, but now the head of the flow is also completely a wake and not a loop cross-section. 4 –An unresolved problem with the previous line-of-sight interpretation is the fact thatthe shrinking loop areas have been consistently measured to be much smaller than the voidareas. Loop areas measured with
Yohkoh /SXT,
Hinode /XRT, and TRACE range from ∼ (median: ∼ ) versus the range for void areas of ∼ (median: ∼
25 Mm ) (Savage & McKenzie 2011). The new interpretation we present here would resolvethe disparity in sizes between shrinking loops and downflowing plasma voids; however, thesenew findings call into question previously-reported parameter estimations such as fluxes andshrinkage energies. 5 – Observations
On 2011 October 22 at about 9:18 UT,
SDO /AIA (Lemen et al. 2011) observed theslow eruption of a filament from AR 11314 (GOES M1.3) between ∼ ◦ in front of the westlimb. The passbands of interest for the observations reported herein are 131, 94, and 193 ˚Abecause the hot plasma in the supra-arcade region emits sufficiently at those wavelengthsand the flows are clearly observable. The plasma temperatures to which the narrow bandAIA 193, 94, and 131 ˚A bandpasses admit significant response under flaring conditions areapproximately 6 & 20 MK, 7-10 MK, and 11-14 MK, respectively (O’Dwyer et al. 2010). Theangular resolution of the telescope is ∼ ∼
435 km).AIA continuously observes the full Sun in all wavelengths at a 12 second cadence.The top panel of Figure 2 shows a GOES light curve with downflow initial detectiontimes overlaid at top. The downflows are classified as either shrinking loops only or loopsleading large voids (the latter will be explained below). The shaded region in Figure 2indicates the presence of a fan of hot plasma in the supra-arcade region as seen in theAIA 131, 94, and 335 ˚A channels (and to a lesser degree in the 193 ˚A bandpass). Theshrinking loops are primarily observed prior to the presence of this plasma while the voidsare only observed flowing through the fully-developed fan. As the plasma is filling the regionafter ∼ STEREO-A /SECCHI 195 and 304 ˚A filters, is depicted in the bottom panel of Figure 2.The times for each wavelength panel with respect to the GOES light curve are indicatedby the arrows. This sequence of images shows that there is no significant change in theorientation of the arcade’s axis throughout the event. In addition, the arcade’s axis appearsto be simple and straight, not allowing for a single line of sight (as viewed from Earth) tobe simultaneously tangent and perpendicular to the polarity inversion line. 6 –Fig. 2.—
Top : GOES light curve with downflow initial detection times overlaid at top(red: shrinking loops; blue: shrinking loops leading voids). The shaded region indicates thepresence of a fan of hot plasma in the supra-arcade region as seen primarily in the AIA 131,94, and 335 ˚A channels. The large peaks in the light curve after 14:00 UT are due tosolar activity from other areas of the solar disk.
Bottom : Sequence of
STEREO-A /SECCHIimages taken with the 195 and 304 ˚A filters. The image times per wavelength with respectto the GOES light curve are indicated by the arrows. The white dashed curve represents thesolar limb as seen from Earth’s perspective. A color version of this figure is available online. 7 –
2. Analysis
Fig. 3.—
Top : Sequence of AIA 131 ˚A showing the development of the arcade with nosignificant change in the orientation of the polarity inversion line.
Bottom : (e) Image takenof the full active region with extractions (indicated by the box) shown in Panels (f) and(g). (f) Shrinking loops (indicated by the arrows) are observed during the early phase ofthe flare ( ∼ − ∼ − , consistent with speeds previously reported (e.g., Savage & McKenzie 2011;Warren, O’Brien, & Sheeley 2011, and references therein). The sizes of the SADs rangebetween ∼ , while the shrinking loops are nearer to ∼ prior to thedevelopment of the fan in the supra-arcade region. These areas are also consistent with thosepreviously reported using TRACE , a similarly equipped instrument.The perspective of this flare, nearly perpendicular to the axis of the arcade as determinedfrom analysis of the
STEREO-A data (Figure 2), combined with AIA’s high temperaturecoverage, resolution, and cadence, allows for further examination of the relationship betweenshrinking loops and SADs. Figure 1 shows an image sequence of a large descending SADled by shrinking loops. The bottom set of images has been differenced from the median ofsurrounding images and scaled so as to accentuate the moving features. There are two loopsleading the void which may be due to the merger of two SADs creating the large descendingvoid. The arrows in the top sequence indicate the position of the SAD while those in thebottom sequence point out the loops as they become apparent at the leading edge of thevoid. The positions of the two initial SADs are indicated in Panel (a). Admittedly, the lowerSAD is much fainter and more difficult to distinguish in the single frame but is more readilyidentifiable in a high-cadence movie of the entire region. (In order to enhance the visibilityof the slow-moving loops, a subset of images with temporal spacing of 48 s between frameswas selected.) Panel (h) provides the clearest example image of the two leading loops. (Werefer the reader to the online movies accompanying this Letter.)The descending void shown in Figure 1 and the associated shrinking loops are observablebetween 11:58 and 12:45 UT beginning at a height of ∼
150 Mm above the solar surface untilthe loops reach a potential configuration just above the cooler arcade ( ∼
60 Mm). Most of theretraction occurs prior to 12:20 UT as the flows decelerate significantly near the arcade. Eventhough SADs shrink considerably in size during their descent, this large void has a diameterthat is much larger than the leading loops ( ∼
20 pixels [9 Mm] versus only ∼ − − xrt_dem_iterative2 routine and the AIA response curves publishedin SolarSoft (Freeland & Handy 1998). We estimate the error in the observations by using anempirical formula that approximates Poisson statistics for low count rates, but approachesGaussian statistics for high count rates (see, e.g., Gehrels 1986).Figure 4 (a) shows an emission measure map constructed from the DEMs by integratingthe DEM in each pixel over the 10-13 MK temperature range. Figure 4 (b) shows a plot ofthe emission measure for several temperature bins (5-8, 8-10, 10-13, and 13-20 MK) along aline that cuts through the SAD indicated with the arrows in Figure 1. The emission measurein the SAD for the 10-13 MK temperature range is lower than the emission measure of thesurrounding plasma by a factor of four. Similar results are seen in a map of the emissionmeasure at 8-10 MK. In addition, there is no discernible increase across the SAD in the lowertemperature bins, and there is no emission seen in the 1600 & 304 ˚A images (which were notused to calculate the DEMs), indicating that there is not an incursion of cooler plasma. Thetwo large SADs in the upper part of the arcade, contained within the white box of Figure 4(a), also have emission measures that are lower than the surrounding plasma. Thus, theSADs are indeed areas where the plasma density is depleted with respect to the rest of thesupra-arcade plasma. The density depletion of the SADs, along with their sizes and flowspeeds, are characteristic of previously reported events. 11 –
3. Discussion
The increased temporal cadence, continual full disk field of view, and temperature cov-erage of the
SDO /AIA observatory has provided new information upon which to base thepresent re-interpretation of SADs. Namely, SADs should no longer be considered cross-sections of newly-reconnected, large evacuated flux tubes; instead, these descending voidsare apparently density depletions left in the wake of thin flux tubes retracting from a recon-nection site in the supra-arcade region (Figure 5).There is the possibility of a high-beta plasma in the sheet, which is suggested by theobserved vortical eddy-like motions, though concrete supporting evidence for high beta is stilllacking. Alternatively, the density depletions which we observe could be considered to resultfrom the wave-interaction effects modeled by Costa et al. (2009), Schulz et al. (2010), andMaglione et al. (2011); or from plasma flows initiated/accelerated by the passage of shrinkingloops. Both of these effects are found in models of low-beta plasma. The latter, field-alignedflows resulting in rarefaction behind the shrinking loops, is the subject of modeling currentlyunderway.In light of this development, previous parameter estimations of SAD characteristicsrequire amending and re-evaluation. For example, the sizes, fluxes, and shrinkage energyof post-reconnection flux tubes based on SADs measurements (Savage & McKenzie 2011;McKenzie & Savage 2011) are all over-estimates because the leading loop is much smallerin diameter than the void. The relationship of a void area to the size of the associated fluxtube, or to its flux, is not known at present. However, we note that the smaller SADs tendto travel straight down into the arcade while the larger ones approach the arcade from anangle.The presence of apparent wakes, and other complex flows revealed by the AIA data,are unexpected in the magnetically structured plasma of the supra-arcade region, and arenot predicted in any current model of post-CME current sheet formation of which we areaware. They raise questions about current sheet characteristics such as the plasma betathat surely must be investigated, but which are beyond the scope of the present Letter. Asaforementioned, magneto-hydrodynamic simulations of wake creation are underway.While these observations indicate a fundamental difference from the original interpre-tation of SADs, the plasma voids are still inherently associated with reconnection outflowsand thus remain probes of the reconnection process:1) The presence of downflows in the flare decay phase illustrates that reconnectioncontinues for many hours after the initial eruption. Their presence in the impulsive phase,coinciding with non-thermal emissions (hard X-ray and microwave bursts), demonstrates 12 –Fig. 5.— (a) Based on new evidence, the interpretation of SADs as the cross-sections oflarge, evacuated retracting flux tubes is no longer supported. Rather, they appear to be thewakes of much thinner shrinking loops. (b) Only shrinking loops are observed during theearly phase of the eruptive event while SADs become apparent after there is a significantincrease in hot plasma (c), presumably surrounding the current sheet, in the supra-arcaderegion. Note that the viewing orientation does not need to change to observe the two features;however, the increase in supra-arcade plasma is necessary to observe SADs (i.e., the wakes).A color version of this figure is available online.that the SADs are associated with energy release (Khan, Bain, & Fletcher 2007; Asai et al.2004).2) The discreteness of SADs indicates that the reconnection is highly localized (i.e., 13 –“patchy”). Similarly, temporal variation in the appearance of SADs indicates their pro-duction is bursty. The reconnection in each location turns on and off on short timescales,independently of other locations (Linton & Longcope 2006).3) The speeds of SADs and SADLs are approximately 50 −
500 km s − , and are constantor slightly decelerating. For comparison, typically assumed Alfv´en speeds are on the orderof 1000 km s − . Reconnection models typically predict outflow speeds of (0.3 − × v A (Linton & Longcope 2006). Deceleration may be expected, due to buildup of downstreammagnetic pressure, though drag mechanisms may also be considered (Savage & McKenzie2011).The revised relationship between SADs and shrinking loops is depicted in Figure 5. Theonly distinguishing characteristic necessary between the two observational circumstances isthe amount of hot plasma in the supra-arcade region surrounding the current sheet. A changein orientation may indeed occur; however, SADs will only be observed in the presence of thisplasma.The sizes of the shrinking loops ( ∼ − − SDO /AIA, which is cur-rently the highest resolution solar observatory capable of coronal measurements. Therefore,the relevant size scales for post-eruption current sheet reconnection (i.e., the physical size ofreconnection patches) appear to be less than ∼
435 km ( < SOHO /LASCO; analysisof those data, which pertains to aspects of the flare other than the fundamental nature ofthe SADs, will be described separately in a forthcoming paper.
Acknowledgements
S. L. Savage is supported by an appointment to the NASA Postdoctoral Program atGoddard Space Flight Center administered by Oakridge Associated Universities through acontract with NASA and under the mentorship of G. Holman. D.E. McKenzie is supportedunder contract SP02H3901R from Lockheed-Martin to MSU. K. K. Reeves is supportedunder contract SP02H1701R from Lockheed-Martin to SAO. 14 –
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