SADS for SADs: SemiAutomatic Detection Software for SupraArcade Downflows Sabrina Savage1, David E'

1
SADS for SADs Semi-Automatic Detection Software
for Supra-Arcade DownflowsSabrina Savage1,
David E. McKenzie1, Letisha McLaughlin2 1Montana
State University, 2University of North Carolina -
Wilmington

• Abstract
• Detectable signatures of magnetic reconnection,
  a possible source of solar coronal heating, aid
  in constraining flare energetics. Supra-arcade
  downflows (SADs), first detected during the
  Yohkoh mission, are an example of an observable
  consequence of magnetic flux tube reconnection.
  These sunward-traveling voids above arcade loops
  are consistent with outflows resulting from
  standard 3D reconnection models of solar flares.
  We have developed semi-automated detection
  software to detect downflows and analyze their
  trajectories, speeds, sizes, and magnetic flux in
  order to constrain parameters for flare modeling.
  We will present these measurements as observed
  primarily by SXT and TRACE and discuss their
  implications. We will also introduce detections
  from the newest solar X-ray telescope aboard
  Hinode.

SXT A total of 39 SADs were detected between
the four flares shown in Figure 4. Due to the
relatively low resolution and high noise in the
SXT images, 19 of the tracks were only partially
measured and are not included in the table below.
The low resolution also means that small-area
SADs, which account for the bulk of the TRACE
detections, cannot be detected in the SXT images.

• 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 during the 20 January 1999 flaring
  event (McKenzie Hudson, 1999), these
  X-ray-dark, blob-shaped features 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.

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
6, 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.
XRT The X-ray telescope aboard the new solar
observatory Hinode provides the opportunity to
observe SADs in environments similar to those
observed by SXT but with resolution closer to
that of TRACE (1 versus 5). The current
quiet state of the sun coupled with the recent
launch of Hinode has not afforded many
opportunities to ideally observe solar flares on
the limb however, downflows have been observed
during flaring events on the disk. While
magnetic flux estimates have yet to be applied
using XRT, we do present detections and
velocity/area measurements for the 13 December
2006 flare (Figure 5).

• 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 TRACE histograms (Figure 3)
  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). Using the
  rough-estimate magnetic fields from the PFSS
  approximation, the flux in each shrinking loop is
  on the order of 2 x 1018 maxwells, (Table 1)
  which is on the same order as the per-loop flux
  estimated by Longcope et al. (4 x 1018 Mx). It
  should be noted, however, that most of the
  quantities derived from our routine are
  conservative underestimates. 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.
• 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 ?WB2 A ?L / 8?, where A
  is the cross-sectional area of the flux tube.
  Using the mean values shown in Table 1, the
  conservative estimate derived from our automated
  software yields shrinking energy on order of 1027
  ergs per 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 SXTs 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, TRACEs 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.
• With increased solar activity, XRT and AIA will
  allow for more detailed SAD observations due to
  wider fields of view, high cadence capability,
  and high resolution the TRACE-like angular
  resolution of XRT and AIA ensures that a wide
  spectrum of SAD sizes will be observable.
  Moreover, AIAs high-temperature
  sensitivity--greater than TRACEs--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 AIAs 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 the flares
  presented here, extending over four contiguous
  images in sequence, then SADs moving as fast as
  1700 km/s should be detectable, if they exist.

Figure 1. An example supra-arcade downflow (SAD)
from the 12 July 2000 flare as seen by
Yohkoh/SXT. The right panel displays the
sharpened version of the prepped data (left
panel). A total of 9 SADs were tracked and
analyzed from this event.
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. Furthermore, combining the magnetic
flux with the displacement 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
semi-automated software for detection and
measurement of SADs. Software development and
testing are described in a paper submitted to the
Astrophysical Journal. Here, we provide the
latest results from the analysis of the famous 21
April 2002 flare as observed by TRACE (Figure 2),
four west limb Yohkoh/SXT flares (including
Figure 1), and one Hinode/XRT flare seen on the
disk. Overview As is suggested by the left
panel of Figure 1, an important first step in
performing our analysis is to sharpen the image
sequences. Our techniques for extracting the
SADs from the noisy, faint region above
post-flare arcades include flattening,
run-mean-differencing, and smoothing. The
software then searches each image for voids using
variable thresholds and then matches void paths
between frames while allowing for small
accelerations. Outputs from analysis on the
subsequently accepted trajectories include
initial and average velocities (head and
centroid), accelerations (from a polynomial fit),
initial and average areas, initial magnetic
fields and fluxes for west limb flares (based on
the potential-field source-surface (PFSS)
approximation), initial height, and total
displacement. TRACE The 21 April 2002 flare
was observed by TRACE (Figure 2), RHESSI, SOHO,
and numerous other observatories (e.g., Wang et
al., 2002 Innes et al., 2003). The TRACE 195Å
images show SADs in high resolution and are
therefore relatively easy to track with our
automated software. Because the flare was
observed on the west limb of the sun, there is
the added bonus that we can implement the PFSS
software to obtain magnetic flux estimates, we
can estimate the height above the arcade, and the
plane-of-sky velocities reported are also closest
to true velocities.
Table 1. Estimates of the mean values for
initial area, average head speed, initial
magnetic flux, loop shrinkage, and energy for
each well-represented downflow detected by our
automated routine. Except for the area, which is
a result of resolution discrepancies, note the
similarities in quantities between the
instruments despite the large difference between
temperature sensitivities.
Figure 6. 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)
This figure has been updated to include
quantifiable information as a result of using the
semi-automated detection software.

• 5. Acknowledgements
• This work is supported by NASA Grant NNG04GB76G.
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