Major Comments:
1) The possible scenario of the eruption presented by the authors seems
logical based on the magnetic field configuration. The eruption started in
the northwestern region and reconnection signature i.e. shrinking loops
were seen in the western region and moved southeastward along the
inclined polarity inversion line. (This phenomenon has been called an
"asymmetric eruption" and presented by Tripathi et al. 2006, A&A, v. 453, p
1111. I recommend the author to check that). In this scenario, many
different current sheets would form as the eruption progresses from the
northwestern region to the southeastern region.
Accordingly, that many downflowing and outflowing structures should be
seen (in an ideal case of course, depending on the geometry). The
asymmetric eruption (from northwestern to southeastern region) would
explain the apparent rotation of the current sheet observed. From my point
of view, this would not be a single current sheet rotating from northwest to
southeast. Instead the event will involve very many current sheets. They all
would appear to be emanating from a single region in XRT images,
possibly due to the fact that the eruption region is behind the limb. This
would also explain the fan-like and spiky appearance if seen from different
viewing angle as presented in the right panel of Fig. 4.
Yes. We completely agree. This is consistent with the interpretation we offered
in Section 4 (Figure 20) where we do make a claim very similar to this comment’s
suggestion that it merely appears to rotate. We recognize that the term “rotating
current sheet” may give the wrong impression, so we have replaced it
everywhere in the text with an appropriate variant of “apparent southward
progression.”
While we are not prepared to state that there are multiple current sheets in the
region, we do say that only a portion of the current sheet region is actually active
enough to produce X-rays as reconnection progresses from the northwest to the
southeast. The active portion (where reconnection is occuring) would be bright
and thin in soft X-rays due to heated, turbulent plasma associated with the
reconnection site. This interpretation (without the need for multiple current
sheets although not disproving them either) also explains the fan-like appearance
because the fanning occurs in the middle of the image sequence when the
reconnection appears to be less orderly in the prescribed direction.
The relevant paragraph in the paper is now on page 23 paragraph 2 which
describes it and has been slightly altered for clarity:
"XRT observes high temperature plasma. The Al/poly filter is sensitive to plasma
at several millions of degrees K. It is unlikely that the current sheet shown in
Figure 17 would be emitting at such high temperatures except in regions of active
reconnection. We noted above that the shrinking loops appear to begin in the
western region and move southeastward along the inclined PIL. Figure 19
shows how the current sheet would look at different stages considering this
southeast motion. If only the active portion of the current sheet (where
reconnection is occurring) were to be emitting at high temperatures, then a
bright, thin linear feature would be observed by XRT and appear to move
southward. This phenomenon is exactly what is observed; therefore, we propose
that the current sheet is not being physically rotated. As noted in Section 3.1,
near the middle of the XRT image sequence, the CCS appears fan-like (see
Figure 4). This could indicate multiple regions of patchy reconnection....”
As for labeling the type of eruption as asymmetric, I have included it in the
paragraph describing the mechanics of the eruption now on page 21 paragraph
3:
"Based on the magnetic field topology of the active region........ The excess
open flux resulting from this shift would not be energetically favorable; therefore,
reconnection began to occur (likely in the right to left direction) to replenish the
field lines in the CD region and to counteract the dome shrinkage. This
reconnection is observed as shrinking loops. The SECCHI images strongly
suggest that the shrinking of loops begins in the west and progresses
southeastward along the PIL. The lifting of the dome to its initial configuration is
observed as the rising of the post-flare arcade. Examples of an arcade
brightening from end-to-end, progressively along the length of the PIL, are not
new. See, e.g., Hanaoka et al. (1994) and the more recent analysis by Tripathi
et al. (2006) referring to them as “asymmetric eruptions.””
2) As shown in Figure 3, the associated CME deflects towards the north,
however the current sheet is deflected towards the south. What could be the
possible scenario of this? It would be good to trace the erupting structure
from XRT all the way up to LASCO and find out how much deflection there is in
the CME from the start of the eruption till the CME reaches the end of the
LASCO/C2 FOV. In addition, it would be good to check for other structures
around the eruption region which could possibly be affecting the CME
trajectory. I think explaining the CME deflection would be crucial to establish
the relation between the reconnection events and CMEs.
It appears that the path of the CME is affected by the Open field region just to the
southeast of the active region. In the SECCHI movies, the flux rope appears to be
released, move eastward, encounter some open field (a small coronal hole), and
then move outward. Therefore, the CME motion is quite complicated by projection
effects. Its movement forward (from Earth's perspective) and then to the right may
account for its apparent upward drift. It may also be slightly interacting with the
HCS to the north of its path but much less so.
The apparent motion of the current sheet to the southeast is due to the tilt of the PIL
and the progression of the flare in the west to southeast direction (as described in
the previous comment). We do not feel that the there is a connection between the
small deflected path of the CME and the apparent motion of the current sheet. This
scenario is depicted schematically in Figures 18 and 20 (previously 17 and 19).
The drift of the CME was primarily included in this text to explain why the
extrapolated paths of the XRT upflows remained to the south of their corresponding
LASCO features. However, in light of this discussion, an additional figure has been
produced to provide insight into the CME's trajectory which includes an image
depicting the path of the eruption within the SECCHI FOV and the CME path through
LASCO C2 with respect to the radial (new Figure 2).
Please see the responses to Minor comments #8, 9, and 10 for additional
information.
Minor comments:
Introduction:
1) Page 2: ... and are theorized to be... I would suggest different terminology
for this.. e.g. interpreted. All we know from theory is that it could be a possible
interpretation.
Now reads: "Supra-arcade downflows (SADs) have been observed in several flares and
interpreted as the cross-sections of these shrinking loops as they retract through a
bright fan..."
Observations:
2) Page 3, first sentence: The authors say: This flare is also known as the
Cartwheel CME flare because the “flux rope” appears to rotate as it erupts.
The authors assumes that the structure is a flux rope without providing any
evidence. I would prefer the sentence like... ... flare because the structure
rotates appears to rotate.... Also, this is the first time for me to hear the
terminology Cartwheel CME - did the authors name it?
Now reads (new page 3 last sentence): "This flare has colloquially been nicknamed
the "Cartwheel CME" flare because the observed structure, which we interpret as a flux
rope in Section 4, appears to rotate as it is ejected from the Sun."
Note: This flare has been called the "Cartwheel CME" flare during several
conferences in posters and presentations -- similar to how the April 21, 2002 event
has been nicknamed the "tadpole" flare.
3) Page 3, Para 2: A CME was observed in XRT’s FOV... I would suggest
the authors to write... An eruption was observed.... CMEs by definition are
observed in white-light coronagraphs...
Now reads: "A large body of EUV- and X-ray-emitting mass was observed by XRT from
09:16 UT to 10:11 UT (Figure 1). The flare is obscured by the limb up to about 45 Mm
in the XRT FOV. The speed of the structure as observed by XRT increases from ~80 to
~180 km s^-1. A white-light CME enters the SoHO/LASCO FOV at 11:06 UT and
proceeds through the LASCO C2 with an average speed of ~350 km s^-1."
Note: Some speed information has been added to this paragraph as well for
completeness.
Analysis:
4) Para 1: The authors say that sharpening the images improves the
visibility of individual loops... I would recommend the authors provide an
animation of these sharpened images along with the paper to facilitate its
readability.
We had considered providing this and are very happy to do so. We would also
like to provide a movie of just the “Disconnection Event”.
3.1 Candidate Current Sheet:
5) Page 4, Para 1: ... (henceforth labeled downflows and upflows
respectively)... Where is it labeled? Possibly this should be shown in a
figure...
Now reads (new page 5 paragraph 2): “… (henceforth referred to in the text as
downflows and upflows respectively) …”
Note: The wording has changed from “labeled” to “referred to in the text” so as to
not be misleading. Upflows and downflows are in fact referred to repeatedly within
the text. Also, the caption of Figure 10 states “The arrows represent upflow
trajectories.”
6) The authors again mention I quote “All of these flows... Do the outflows
also follow the southward rotation of the candidate current sheet?
Now reads (new page 5 paragraph 2): "All of these flows both toward and away from
the Sun, including the apexes of the shrinking loops, follow the direction of a bright,
thin linear feature which extends from the apex of the arcade region."
Note: The above alteration to the wording should make it more clear that the
upflows do follow the progressively southward successive brightenings that look
like rotation. This is mentioned again in the caption for Figure 3 (previously 2).
(“All flows track along this feature even as it progresses southward.”) This is also
supported by the trajectories with arrows in Figure 11 (previously 10). On new
page 14 paragraph 3 it is stated: "Note that the [magenta] flow in the southwest
region represents the track of a very faint, diffuse upflow [corrected from outflow] that
occurred after the large data gap. Despite its discrepant path compared to the other
flows, it still tracks along the CCS which had [progressed southward] by the time of this
flow."
7) Page 4 and also on Page 2: The comment regarding the radiation from
the current sheet, This is difficult to comprehend. The authors claim that
current sheet structures can have surrounding areas of high temperatures
due to conduction front making the observation of current sheet radiate in
X-rays.
I agree with the argument that the temperature would be high due to
conduction front. However, it is important to note that the density would be
very low because the region is being evacuated in forms of reconnection
outflows (towards the sun and away from the Sun). This would be make the
total emission measure very small and therefore difficult to observe. I
would recommend some elaboration along these lines.
I think Kathy would be a more appropriate source for responding to this comment. I
would botch it for sure.
8) Page 5: para 1: ... but drifts away from the CME path during ... What was
the height of the CME when the CCS started to drift away from the CME?
Please mention.
Note: There is a 40 minute data gap preceding the XRT CCS observations which I
have now noted in the text. However, its initial appearance lines up with the initial
appearance of the CME in LASCO at 11UT within ~7 degrees (southward). At this
time the CME is at a height of ~2.5 solar radii.
The first paragraph in section 3.1 (new page 5) now reads: “The sharpened image
set… …follow the direction of a bright, thin linear feature which extends from the apex
of the arcade region. This feature becomes apparent at 11:00 UT following a nearly 40
minute data gap in the image sequence and slowly rotates southward…”
2 paragraphs later now reads: “This feature (henceforth referred to as the candidate
current sheet or CCS) is initially detected at ~7 degrees southward of the CME when it
first appears in the LASCO FOV at ~2.5 R_sun around 11 UT. It then immediately
begins slowly drifting away from the LASCO CME path during its aforementioned
apparent southward progression. …”
9) Was the erupting structure started to drift from the very beginning or it
propagated radially first and later it drifted away? Page 5 last sentence,
and Fig. 3: The CME in LASCO/C2 FOV does not propagate radially, rather
showing deflection. Deflection in CMEs have been seen earlier using
LASCO data see e.g Cremades & Bothmer (2004, A&A, v.422, p.307) and
Tripathi et al. (2006, A&A, v.449, pp.369).
A new figure 2 was added to more explicitly show the path of the eruption.
Observations Section, 2nd paragraph now reads: “A large body of EUV … The
onset of the filament eruption is observed by SECCHI beginning at 08:53 UT.
Figure 2 (left) depicts the curved path of the eruption within the SECCHI FOV.
Figure 2 (right) shows the CME as it passes through the LASCO C2 FOV. The
dashed white line indicates the radial direction extending from the active region
projected onto the plane of the sky. These observations indicate that the erupted
structure initially moves in a non-radial direction toward the southeast with its
path becoming more radial as it approaches ~2.5 R_sun. The deflection may be
the result of interaction with an open field region to the southeast of the active
region (Section 4)...”
Section 3.1 (new page 6 paragraph 3) last 2 sentences now read: “This
conclusion is also supported by the CME path as seen within the XRT FOV as
well as by the SECCHI images near the base of the flare (see Figure 2). The
defection occurs prior to 3 R_sun.
Discussion Section 4 (new page 22 paragraph 3) now reads: “…The field
opened by the CME then temporarily joined the flux from the large-scale “Open
Field” region which extends to the southeast of the active region. This
conclusion is supported by SECCHI observations which show that the filament
initially moves southeastward towards the “Open Field” (see Figure 2).”
***Not sure what to do about the suggested reference….
10) Fig.3: From the figure it seems that the CME is being deflected away
towards north from the original CME direction however, the CCS is
deflecting towards the south? It would be good to mention here explicitly. It
would also be good if the authors provide any interpretation to this.
Our interpretation of the apparent southward propagation of the CCS is
described in depth in the Discussion section (Figure 20). We do not interpret the
CCS as being deflected southward; rather, successive brightenings along the PIL
in the north to south direction make it appear to move southward. As shown in
the answer to the question above, the CME appears to be deflected outward by
the open field (i.e. a small coronal hole) to the east/southeast of the active
region. After that, it moves close to radially.
11) Page 7, last paragraph: The authors are measuring the current sheet
using the XRT images. However, other authors have used images recorded
in at different temperatures. I guess that the thickness of the current sheet
would have a temperature bias. For example, the thickness measured using
XRT images would be different than those measured using white-light
images as XRT images are sensitive to high temperature plasma where as
white-light images are not temperature sensitive. As mentioned by the
authors, spatial resolution is another factor. So, I would suggest the
authors to explicitly mention that what wavelength observations were used
by different authors? Did they all use LASCO white-light?
Note: While the paper did include a list of the instruments used by the other
authors to measure the CS thicknesses "(LASCO, EIT, UVCS)", I took the opportunity
to expand upon what was in the paper and in the process, came across some
interesting extra information to add from the Ciaravella & Raymond 08 paper. John
Raymond also presented us with another possibility that is worth including here.
Now reads (new page 8):
“Ciaravella & Raymond (2008) obtained a CS thickness range of 30-60x10^3
km for the 2003 November 4 flare using UVCS and geomtrical arguments. They claim
that the broadened line profiles obtained from UVCS measurements “must result from
either bulk flows or turbulence.” Furthermore, they predict the thickness due to
turbulence to be ≤ 4x10^3 km. The similarity between this prediction and the
measurements obtained with XRT for the “Cartwheel CME” flare suggest that
turbulence may play a role in broadening the CS thickness.
Thickness estimates have been made… The discrepancy in thickness values may
arise from the fact that we observe very hot plasma with XRT’s Al/poly filter (1 – at
least 10 MK ) using much finer resolution (1 arcsec/pixel) than is observed with the
various instruments used by these authors (i.e. LASCO (temperature-insensitive white
light; C1: 5.6 arcsec/pix; C2: 11.4 arcsec/pix), EIT (0.6-3 MK; 2.6 arcsec/pix),
UVCS (2-8 MK; > 40 arcsec/bin for high temperatures)). Also, the current
observations appear to be closer to the diffusion region (refer to the “Disconnection
Event” in Section 3.2), and the Petschek model predictions expect the current sheet
width to increase linearly with distance from the diffusion region (J. Raymond, private
communication).”
Section 3.2:
12) Page 11: A possible reconnection outflow pair episode.... I would like
to draw the attention of the author towards Tripathi et al. (2006, A&A, v.449,
pp.369), Tripathi et al. (2007, A&A, v.472, pp633)
Note: One of the main conclusions of this reference is stated in the final
paragraph: “…the question remains open as to why such bright downflows are
so rare that none was seen prior to the one reported here.” However, bright
downflows had been previously reported in the D. McKenzie (2000) paper
(Abstract, Section 3.10, and Discussion). In our studies, we have found that
bright downflows are not uncommon, but the dark ones have been focused on
because studying their lack of plasma rather than cool/hot plasma is more
indicative of being a magnetic feature versus “coronal rain.” We do report seeing
bright downflows in this flare as well and pose a possible correlation with time for
the dark versus bright flows.
In response to this comment, however, the flows from this reference do not
appear to occur in the same conditions as the outflow pair that we observe nor is
it as clearly defined in space, time, and resolution and in concurence with other
obvious reconnection signatures. To be more precise though, the statement in
question now reads (new page 13 last sentence):
“To our knowledge, a possible reconnection outflow pair episode such as this
(i.e. appearing in a region where retracting loops and upflows have been
observed and along a directly observable SXR current sheet) has not been
observed this close to the solar surface (at nearly 180 Mm). Outflow pairs much
higher in the corona have been previously observed primarily with LASCO
[references]. The large data gap unfortunately occurs immediately following this
event hindering possible observations of continued reconnection occurring in the
region.”
13) Page 14: Figure 11 shows typical ...”observed in this study”. Please
add at the end of the sentence.
This additional wording is a good suggestion. Also added to new page 15 paragraph
1.
Now reads (new page 16 figure 12 caption): "De-projected height-time profiles
represented by (a) a typical downflow and (b) a typical upflow observed in this study. "
14) 11: I would recommend the authors to compare the downflow profile
with h-t profile of a free-falling body from the height where the downflow
seem to emanate. If the speed of the downflow is larger than the free-fall
speed then this would give another evidence in support of reconnection
outflows.
We know that, from observations of filament eruptions, lots of material
drains down to the Sun’s surface during a prominence eruption. So, to be
sure that this is reconnection outflow, this test is important.
Note: Figure 12 (previously 11) was enhanced to include gravitational profiles.
Two typical flows from this study are still being used. Panel (a) was swapped
with a different flow simply due to the aesthetics of the graph itself (in particular
the axes ranges), but there is no information discrepancy. Panel (b) is the same
upflow as before. The gravitational profile with a constant drag was added to the
downflow profile for this figure. All of the profiles (except for one which was only
tracked through 3 frames anyway) had slower trajectories than a freely-falling
body. Adding drag more closely matches the profiles. While providing these
profiles does not provide evidence in support of reconnection outflows, it also
does not rule it out. Keep in mind that several of the flows exhibit clearly-defined
loop shapes. It does, however, shed some light on the kinematical dynamics
above the flaring region – in particular that some sort of drag likely plays a role in
slowing the flows which may help answer why the outflows are so much slower
than expected.
The caption for Figure 12 (previously 11) now reads: “De-projected height-time
profiles represented by (a) a typical downflow and (b) a typical upflow observed in this
study. The vertical dotted lines mark the mid-exposure time of each image in the
sequence. (Note that Flow 16 does not have a contiguously-detected track due to the
very low signal-to-noise ratio at the heights through which it travels.) The solid line is
the 2-D polynomial fit applied to the profile to obtain the initial velocity and
acceleration. The calculated fit parameters are given in the legend for each flow. The
thick dashed line represents the gravitational profile for a body in free-fall given the
initial height and velocity of the flow. The thin dashed profile line in (a) represents the
gravitational profile for a body in free-fall experiencing a constant drag coefficient of
3.5x10^-3 s^-1.”
Note: Page 15 paragraph 1 now reads:
“Figure 12 shows typical…. This fit is represented by the solid profile line in Figure 12.
The gravitational profile for a body in free-fall given the initial height and velocity of
the flow is shown as the thick dashed line. The left panel includes a thin dashed profile
line outlining the trajectory of a body in free-fall experiencing a constant drag force.
All of the downflow speeds were slower than their corresponding free-fall speeds
especially as they near the limb, except for one flow that was only tracked through
three frames. Faster downflow speeds would have supported the reconnection outflow
hypothesis; however, it is not excluded by this opposite result either considering that
other flow characteristics provide significant support. Namely, several of the flows
exhibit a clearly-defined cusped or rounded loop structure and one of the regions is
observed to disconnect into an upflow and a downflow which are expected results from
reconnection. Also, the flow profiles diverge the most from the gravitational profiles as
they near the limb and, presumably, the top of the arcade where the loops are expected
to settle to a potential configuration. If these flows are indeed reconnection outflows,
then this result may indicate that a source of drag (e.g. mass build up in front of the
flows due to density in the current sheet, shocks, magnetic field entanglement, etc) has
a significant effect on the flow speeds.”
15) Page 21: Para 3, line 10: The STEREO observations... along the PIL.
This basically is what has been called "asymmetric eruption", which has
been discussed earlier (Tripathi et al. 2006, A&A, v. 453, pp1111).
Added to the end of the paragraph 3 page 22: "Examples of an arcade brightening
from end-to-end, progressively along the length of the PIL, are not new. See, e.g.,
Hanaoka et al (1994) and the more recent analysis by Tripathi et al (2006) referring
to them as "asymmetric eruptions." "
Note: (References added to bibliography.)
16) Page 26: para 1, line 2: ... released by some means (typically assumed
to be reconnection)... I suggest the authors to remove what is written in the
bracket.)
Note: Fair suggestion.
Now reads (new page 27 paragraph 3): "We interpret the basic standard picture of
this eruptive flare as being initiated by the release of a flux rope by some means. As
the flux rope escapes into the outer corona, ..."
Note: I also added the word "cusped" to the following sentence in the same
paragraph:
"This reconnection results in the formation of pairs of cusped, looped field lines, each
moving in opposite directions along the current sheet."
17) Page 26, para 2, line 4: The CME is observed... I would suggest, The
event was observed...
Now reads (new page 27 paragraph 4): "The event is observed by several
instruments..."
Additional Comments:
1. “STEREO” was changed to “STEREO A” or “SECCHI” where appropriate to
distinguish between the instrument and the spacecraft (just like with Hinode and
XRT).
2. An additional downflow was tracked and added to the statistics and figures of
Section 3.2. Only slight quantitative modifications. No qualitative changes.
3. Upon further review of the SECCHI images, it was determined that while the
filament began to show morphological changes at 08:15 UT, it was not actually
ejected until closer to 08:53 UT. This was corrected in Section 2 and in Figure 9.
4. The eruption front was tracked as an upflow in the XRT FOV and its position
extrapolated into the LASCO C2 FOV in order to compare the results with those of
the other upflow extrapolations discussed in Section 3.3. The correspondence was
very close to that of the CME front within an expected angular separation. It was
decided that the left-hand panel of the previous Figure 15 was unnecessary and that
showing the XRT eruption front associated with the LASCO CME front was more
revealing.
Figure 16 [previously 15] and its caption have been changed.
Section 3.3 now reads (new page 19 paragraph 4): “The XRT upflow positions were
extrapolated by using their final fit velocity within the XRT FOV as an initial velocity.
The corresponding LASCO flow accelerations were then applied to determine
successive flow positions. The XRT upflow paths were assumed to be straight although
the CME path itself initially veers northward (see Figure 2). This results in the upflows
tracking just to the south of the LASCO flows. As a check on this procedure, the
eruption front position, which is expected to be observed in LASCO as a white-light
CME, was extrapolated in the same manner using a measured acceleration of 0.03 km
s^-2. If allowing for some angular separation due to the aforementioned CME path
deflection, the resulting extrapolated positions correspond precisely to the CME front
in the LASCO FOV (Figure 16, top panel).”
5. It was noted that the XRT sun-center coordinates were not accurate and were
centered too far to the West. (Refer to the previous Figure 10. This is also
noticeable in the solarmonitor.org XRT images for this time period up until January
2009.) This was fixed in the images and the flow analysis was redone. Because the
coordinate change was very slight, there were only minor quantitative differences in
the results and were included in the changes noted in Additional Comment #2
above. The figure of tracks [new 11] looks better now though. Note that the
background image was changed to an image from just after the eruption rather than
several hours into the flare because the previous background image was confusing.
6. The previous Figure 12 (now 13) was updated to include the eruption front. The
eruption front and the disconnection event are now labeled in the figure. It was also
changed to black & white since the noteable flows were labeled and color did not
add more information.
7. Some additional information was included in the Summary & Conclusions
section:
“Reports of impulsive-phase RHESSI double coronal sources have been made
by other authors (e.g. Liu et al. 2008, Sui, Holman, & Dennis 2004, & Sui &
Holman 2003) with the lower source corresponding to the top of the rising arcade
and the upper source possibly corresponding to an ascending reconnection
outflow. We note that these upper coronal sources have paths, speeds, and
placements relative to the arcade similar to the bright plasmoid structure tracked
with the eruption front for the “Cartwheel CME” flare (Figure 2). This is consistent
with the extrapolated position of one coronal source from the April 15, 2002 flare
(Sui, Holman, & Dennis 2004) which was calculated to roughly track with a
coronal loop observed by LASCO (similar to Figure 16 (top)). Sui, Holman, &
Dennis conjecture that “the outward-moving coronal source is part of an ejected
plasmoid (or a large-scale, helically twisted loop) with two ends anchored on the
Sun...”. This description matches our interpretation of the April 9, 2008 eruption
(Figure 18).
Liu et al. (2008) also report that their source closer to the solar surface has a
larger emission measure than the higher one. These results are consistent with
our “Disconnection Event” observations (see Section 3.2) where the upflow
portion is much dimmer and more diffuse than its downflowing counterpart.
Indeed, all of the upflows for this flare are dim compared to the bright downflows.
It is also worth noting that a non-radial, southward evolution of the loop-top
source is reported for the flares in Sui, Holman, & Dennis (2004) (Figure 10
therein). The source of this divergence may have a similar mechanism as that
proposed for the apparent southward drift of the CCS for this flare (Figure 20).”