Quantitative Examination of Supra-Arcade Downflows withHigh-Resolution Imagers

David E. McKenzie, Sabrina SavageMontana State University

1. Abstract

Downward motions above post-CME flare arcades are an exciting discovery of the Yohkoh mission, and have subsequently been detected with TRACE, SOHO/LASCO, SOHO/SUMER, and GOES SXI. These supra-arcade downflows have been interpreted as outflows from flux tube reconnection, consistent with a 3D generalization of the standard reconnection model of solar flares. We will present results from our observational analyses of downflows, which include automated schemes for detection and measurement of speeds, sizes, and other aspects related to 3D reconnection. We will also indicate the limitations of present observations, and motivations for utilizing SDO/AIA for measurements of these reconnection signatures.

5. Acknowledgements

• This work is supported by NASA Grant NNG04GB76G.

6. References

• “Downflow Motions Associated with Impulsive Nonthermal Emissions Observed in the 2002 July 23 Solar Flare”, by Asai, Ayumi; Yokoyama, Takaaki; Shimojo, Masumi; Shibata, Kazunari 2004, ApJ, 605, L77.

• “SUMER Spectral Observations of Postflare Supra-Arcade Inflows”, by D.E. Innes, D.E. McKenzie, & T. Wang 2003, Solar Physics, 217, 247.

• “A Model for Patchy Reconnection in Three Dimensions”, by M. Linton and D. Longcope 2006 (in press).

• “Observations of Separator Reconnection to an Emerging Active Region”, by D. Longcope, D. McKenzie, J. Cirtain, and J. Scott 2005, ApJ, 630, 596.

• “X-Ray Observations of Motions and Structure Above a Solar Flare Arcade”, by D. E. McKenzie and H. S. Hudson 1999, ApJ, 519, L93.

• “Supra-arcade Downflows in Long-Duration Solar Flare Events”, by D. E. McKenzie 2000, Solar Physics, 195, 381.

• “Signatures of Reconnection in Eruptive Flares”, Invited Review, by McKenzie, D.E., in Multi-Wavelength Observations of Coronal Structure and Dynamics, P.C.H. Martens and D.P. Cauffman, eds., COSPAR Colloquia Series, Elsevier Science Ltd. pub. (2002), 13, 155.

• “Characteristics of Coronal Inflows”, by Sheeley, N. R., Jr.; Wang, Y.-M. 2002, ApJ, 579, 874.

• “The Origin of Postflare Loops”, by Sheeley, N. R., Jr.; Warren, H. P.; Wang, Y.-M. 2004, ApJ, 616, 1224.

• “Initial features of an X-class flare observed with SUMER and TRACE”, by Wang, T. J.; Solanki, S. K.; Innes, D. E.; Curdt, W. 2002, in SOLMAG 2002. Proceedings of the Magnetic Coupling of the Solar Atmosphere Euroconference and IAU Colloquium 188, 11 - 15 June 2002, Santorini, Greece. Ed. H. Sawaya-Lacoste. ESA SP-505. Noordwijk, Netherlands: ESA Publications Division, ISBN 92-9092-815-8, 2002, p. 607 - 610.

2. Introduction

Supra-arcade downflows (SADs), as the name implies, are downward-moving features observed in the hot, low-density region above post-eruption flare arcades. Initially detected with Yohkoh SXT as X-ray-dark, blob-shaped features (McKenzie & Hudson, 1999), downflows have since been observed with TRACE (e.g., Innes et al., 2003; Asai et al., 2004), SOHO/SUMER (Innes et al., 2003), and SOHO/LASCO (Sheeley & Wang, 2002). The darkness in X-ray and EUV images is due to very low plasma densities, i.e., plasma voids (Innes et al., 2003). [As an aside, we note that many X-ray-emitting SADs are also known. McKenzie (2000) reported faint X-ray-emitting shrinking features in several flares; and in our catalog of 40 SAD flares observed by SXT and TRACE, approximately half display such bright shrinking features alongside dark SADs. See the box inset below.]

The downflows are traced by discrete X-ray features with a characteristic size. The present interpretation states that the downflows represent the outflow of magnetic flux from a reconnection site, in keeping with the standard reconnection model of eruptive flares (see Figure 2, from McKenzie, 2002; see also Sheeley et al., 2004). If they are reconnection outflows, then these tracers strongly suggest that the reconnection takes place between discrete collections of magnetic flux, i.e., flux tubes. This conclusion indicates “patchy” 3D reconnection. While the observed speeds of tens-to-hundreds of km/s were slower than initially expected (i.e., slower than the 1000 km/s which is often assumed to be the Alfvén speed), the recent model of 3D patchy reconnection by Linton & Longcope (2006) indicates the presence of drag forces working against the outflow. In their model, reconnection was allowed to happen in a localized region of slightly enhanced resistivity, and the evolution of the reconnected magnetic field was studied. As expected, the reconnected field retracted away from the reconnection site, accelerated by slow-mode shocks: “This accelerated field formed a pair of three-dimensional, arched flux tubes whose cross sections had a distinct teardrop shape. The velocities of the flux tubes was smaller than the reconnection Alfvén speed predicted by the theory, indicating that some drag force is slowing them down.” These drag forces, which seem to appear only in a genuine 3D simulation, result in outflow speeds at only a fraction of the Alfvén speed. Linton & Longcope considered field line tangling--a truly 3D effect--or added mass, perhaps due to snowplowing, as possible sources of drag.

Quantitative measurements of downflows yield useful constraints for such models. For example, measurements of the characteristic sizes of SADs can be directly applied to the model as a means of limiting the duration of each magnetic reconnection episode, or the size of a resistive patch. Combined with estimates of the magnetic field in the supra-arcade region, measurements of the sizes of reconnected flux tubes yield estimates of the magnetic flux in individual flux tubes, and therefore the characteristic amount of flux that participates in a magnetic reconnection episode. In an example shown below, determining the path of the downward motion yields an estimate of the energy released by the shrinkage. In the model of Linton & Longcope (2006), this shrinkage energy can account for as much as half the total energy converted by an individual reconnection episode.

3 Automated Detection and Analysis

At Montana State University, we are developing automated software for objective detection and measurement of SADs. Testing of the software will be described in a future paper; in this poster we describe only the application of the software to image sequences from two well-known SAD flares. The 21 April 2002 flare was observed by TRACE (Figure 3a), RHESSI, SOHO, and numerous other observatories (e.g., Wang et al., 2002; Innes et al., 2003). The 20 January 1999 flare was observed by Yohkoh/SXT, and was the

discovery event for downflows (Figure 1).

4. Discussion

Supra-arcade downflows are important signatures of reconnection in flares (McKenzie, 2002; Asai et al. 2004). As tracers of reconnection outflow, their characteristics are indicative of the parameters of 3D patchy reconnection, including the size of participating flux tubes, and, by extension, the characteristic size of the localized diffusion region. The application of automated software to real flares, as shown here, demonstrates that it is possible to derive quantitative data from images of these velocity fields. The histograms above demonstrate a range of sizes, with a smooth dropoff towards larger voids. This is directly relevant to models of 3D reconnection, by revealing the distribution of reconnection “patches”. It is worth noting that the areas observed in the 21-Apr flare are similar to the cross-sections of reconnecting loops observed in TRACE by Longcope et al (2005). If we assume a magnetic field strength of, say, 20 G above the arcades, then the flux in each shrinking loop is on the order of 4 x 1018 maxwells, which is strikingly similar to the per-loop flux estimated by Longcope et al. (4 x 1018 Mx). Similarly, the observed speeds indicate outflow that is slower than the nominal Alfvén speed: this is consistent with previous reports of downflow speeds, and with the simulated outflows of Linton & Longcope (2006), although no attempt has been made to estimate the drag forces necessary to produce these speeds. The distributions in Figures 3c and 4c also indicate smooth dropoffs toward higher speeds.

As a further example of the utility of quantitative measurement, consider that as a flux tube undergoes shrinkage by an amount ∆L, the energy lost is given by ∆W=B2 A ∆L / 8, where A is the cross-sectional area of the flux tube. From the measurements shown here, we estimate some typical values as ∆L ~ 1--5 x 109 cm; A ~ 2--12 x 1017 cm2. With B on the order of 20 G, the energy liberated is on the order of 1027--29 ergs per shrinkage event.

To date, SADs have been observed with SXT more often than in the TRACE data. This is no doubt due to the full-Sun field of view of SXT as well as its sensitivity to hotter plasmas--the supra-arcade region is very hot, and the dark SADs are easier to see against a bright background. However, most of the SADs were observed with SXT’s half-resolution (5 arcseconds per pixel), so that the smaller features were not detected. This is borne out by the histograms above--TRACE observes plasma voids much smaller than those seen by SXT. While TRACE offers much higher angular resolution, the cadence of TRACE images is slow enough to allow some flows--particularly the faster ones--to go undetected. In some cases, the cadence of SXT images was even too slow, so that motions faster than 700 km/s may have been unobservable (McKenzie, 2000). Moreover, TRACE’s smaller field of view means that some flares are not observed: in a study of 12 SAD flares observed by SXT, McKenzie (2000) found TRACE data for only two events. While AIA’s full-Sun field of view avoids this problem, the TRACE-like angular resolution ensures that a wide spectrum of SAD sizes will be observable. Moreover, AIA’s high-temperature sensitivity--greater than TRACE’s--is expected to reveal the fan-like structure above eruptive flare arcades much more often than TRACE, so that SADs will be observed more often, and at greater heights. And AIA’s fast cadence of 1 image in each passband per 10 seconds is significantly faster than typical TRACE sequences, and faster even than most flare sequences in SXT, where 1 image per passband per resolution often required as much as 20 seconds. This higher rate of sampling may result in faster SADs being detected by AIA, contributing still further to the observational database. For instance, if AIA observes the same ∆L as found in these two flares, extending over four contiguous images in sequence, then SADs moving as fast as 1700 km/s should be detectable, if they exist.

Reminder: Not All Downflows Are Dark

While the most familiar SADs are dark because of the lack of hot plasma within them, this is believed to be due to loop shrinkage proceeding faster than the rate at which chromospheric evaporation can fill the loop with plasma. How often is the opposite situation observed? Consider the possibility that in some hypothetical flare, the chromospheric evaporation might be especially vigorous. (We don’t speculate why---maybe the energetic particles impacting the chromosphere are more energetic, or more populous, than in other flares.) Suppose that at the same time, the field line shrinkage is slower than “normal” --- this could be due to weak field strength, or greater than normal drag. In this hypothesized situation, it might be possible to see the evaporated plasma reaching up into the coronal loop as the loop is still shrinking. Alternatively, if the loop gains mass through a snowplow effect, the plasma within the loop may become heated as the loop shrinks. The loop gets brighter and brighter in the X-ray images, as it gets shorter and eventually merges into the top of the arcade.

With that idea in mind, consider the images below, from a flare on 16 August 1998. The solar limb is the gentle curve starting in the upper right, and the vertical smear is due to pixel saturation. Next to the arrow, there are two features shrinking into the top of the arcade. They appear to brighten as they get lower. The enhancement to the emission can be seen more clearly in a light curve: In the boxed area shown at right, the X-ray signal was summed for each frame in the movie. The evolution of this sum is shown in the light curve. The two humps in the light curve are due to the two shrinking features. As the shrinking loops get brighter, the light curve intensity increases; as the loops shrink out of the little box, the light curve decreases again. We reiterate that this behavior is seen often: Of 40 flares known to be associated with supra-arcade downflows, half exhibit this kind of bright loop shrinkage, including the famous 21-April-2002 flare observed by TRACE.

Figure 2. Cartoon depiction of supra-arcade downflows resulting from patchy reconnection. Discrete flux tubes are created, which then individually shrink, dipolarizing to form the post-eruption arcade. (From McKenzie, 2002)

Figure 1. Late stages of the 20-Jan-1999 flare in which supra-arcades were first discovered. The white arrow is fixed at the location of a dark void at 11:48:11 UT. The black arrow follows the same feature to its location at later times.

In the TRACE data from 21-Apr-2002, the automated detection routine tracked 34 downflows. All have been manually verified by visual inspection. The trajectories of the detected downflows are plotted in Figure 3b; SADs were detected all along the arcade. In the north, downflows took the form of shrinking cusped loops; while in the south, primarily plasma voids were observed. Speeds of the downward motions, inferred from polynomial fits to the X- and Y-components of the trajectories, range from 10 to 310 km/s (Figure 3c). However, it is known that the speeds found in this way are underestimates: the algorithm fits a polynomial to the trajectory of the centroid of each SAD, which does not coincide with the “head” of the SAD when a long tail extends behind the SAD. The error may be as much as a factor of two. A forthcoming

version of the program will compensate for this.

An additional product of the analysis is automated determination of the size of each plasma void. Not surprisingly, Figure 3c suggests that smaller voids are more numerous. The median area is 20 x 106 km2 (i.e., roughly 40% the Earth’s diameter, if circular).

Figure 3a. The famous flare of 21-April-2002 revealed downflows in TRACE and SUMER for the first time, and represents one of the sharpest observations of downflows.

Figure 3b. Downflows detected in the 21-Apr flare, tracked by the automated routine.

Figure 3c. Distribution of areas and speeds of the tracked downflows: 21-Apr-2002.

In the SXT data from 20-Jan-1999, 16 downflows were found above the arcade (Figure 4a). The trajectories of the detected downflows are plotted in Figure 4a. The downward speeds in this flare are generally higher than those in the 21-Apr flare (Figure 4b). Due to the coarser angular resolution of SXT compared to TRACE, none of the smaller SADs are detected: the median area among these SADs is 125 x 106 km2 (roughly the same as the Earth’s diameter, for comparison).

Nearly 40 observations of downflows have been catalogued with SXT, dating back to 1991. Since the end of the Yohkoh mission, TRACE has detected SADs in several flares, LASCO sees very similar flows much higher in the corona, and SXI frequently detects motions which resemble downflows, although the sharpness and contrast of the SXI images are not optimal for analyzing these features. It is certain that Solar-B/XRT and SDO/AIA will also observe downflows in eruptive flares. To the extent that these flow patterns can help us understand the characteristics of reconnected flux tubes, the capabilities of AIA will provide a rich data set for further exploration of the parameters of 3D reconnection. As we demonstrate below, the observations of SXT and TRACE already provide empirical constraints for reconnection modeling. The angular resolution of AIA (equal to TRACE), its temporal resolution (in effect, faster than SXT and TRACE), and the full-Sun field of view (ensuring that no flare goes unobserved), combined with the high-temperature sensitivity, will greatly improve the observational database of downflows. In this poster we discuss some of the first results from quantitative studies of SADs, produced by an automated algorithm for detection and characterization of plasma voids. While the results are immediately available as inputs to 3D reconnection modeling, the technique for studying downflows will also be intrinsically valuable in the AIA era.

Figure 4a. Downflows detected in the 20-Jan flare, tracked by the automated routine.

Figure 4b. Distribution of areas and speeds of the tracked downflows: 20-Jan-1999.