To the Editor,
We sincerely thank the referee for his/her detailed comments on our manuscript.
We revised our paper accordingly. Our detailed response is inserted below.
Best Regards,
Yingna Su et al.
---------------------------------------------------------------------- Referee Report Reviewer's
Comments: The article by Y. Su et al on "Observations and magnetic field modeling of
the Flare/CME event on 2010 April 8" is an interesting combination of high quality
observations from different instruments (especially SDO/AIA images) and advanced
magnetic field modeling using the magnetic flux insertion technique. Even if this work is
interesting enough to be published in ApJ, there are confusing parts which need to be
revised and clarified. I will therefore recommend major revisions to the paper.
There are major points in the organisation of the paper:
- the description of the observations is not clear: a section on the structure of the region
and another on the time evolution of the event will be more appropriate for the reader.
Most of my comments below will help covering this issue;
We made the modifications as suggested. The previous Section 2 was split into two
Section 2: Instruments, and Section 3: Observations. The two sections described by
the referee can be found as the subsections of Section 3 in the revised manuscript.
We also add a new figure (Figure 1) to present the structure of the region.
- a large part of the paper is about the two schemes used to relax the magnetic field
configuration. In the last sections we learn that only the Relaxation 1 is relevant for this
event. I thus suggest that the comparison between the two Relaxation 1 & 2 is done in an
Appendix rather than in the body of the article. Once again that will help the reader.
Ok. We moved the parts (text and figure) on comparison of the two relaxation
methods to the Appendix.
Section 2: Observations
- In the conclusion, the authors describe the different structures and events related to
observed flare (CME, dimmings, EIT wave, ...): I would have expected a clear
description of the structure involved in the event (annotated Fig. 1) as well as a
description of the events related to the flare (CME in SOHO/LASCO, image of the
dimmings, ...). [not all people will watch the movie before reading the article, so the
paper has to be self explaining]
A detailed description of this flare and associated events can now be found in the
first two paragraphs of Section 3.2.
- Fig. 1: it is difficult to see the arrows on the printed version. The fact that the AIA and
EUVI images have not the same field of field is confusing: there is an easy way to correct
for the different point of view.
We enhanced the arrows. The AIA and EUVI images are the same in both the figure
and on-line video now.
- the main question is: from the movie provided I clearly see the eruption of the long
filament outside the active region but the filament in the core of the active region seems
to remain at the same location (or to form again) after the flare, could you explain that?
This issue is discussed in the second paragraph of Section 3.2.
A clear eruption of the internal filament can be found in the EUVI-A 195A movie.
It is possible that only part of the internal filament is erupted.
- the authors mention the CME observed in SOHO/LASCO at 01:31 UT which before the
occurrence of the flare (02:30 UT), the timing of the event is not really well explained?
and it will be of interest to know if the LASCO images are consistent with the best fit
model.
The information on the associated CME mentioned above is taken from the SOHO
LASCO CME catalog. The CME started at 03:30 as provided by the Cactus CME
catalog, this information appears to be more reasonable. In the revised manuscript,
we use the information provided by Cactus.
The present magneto-frictional model is unable to follow the evolution of the
erupting flux rope to he height of the LASCO observations.
- the author mentioned several times that the resolution of AIA images was 0.6
arcseconds. Do you mean pixel size instead? It is understood that the spatial resolution is
about 1 arcsecond and also depends on the wavelength observed.
Yes, we do mean that the pixel size of AIA images is about 0.6 arcsecond. We
modified the text in the revised manuscript.
- As SDO/AIA images are relatively new, it will be important to mention the level of the
series used in this paper, the time cadence (justify the 20s cadence as the nominal
cadence is 12s)
We used level 0 data, and this information is added to the text. The observations
were taken during the “commissioning” phase when the cadence had not yet been
lowered to its nominal value. The 20s cadence is also mentioned in the first para. of
Section 3.2.
- quite a lot of data mentioned later in the article are missing at this stage: Hinode/XRT,
description of STEREO-A/SECCHI/EUVI, definition of the PIL with SDO/HMI. The
better description of the HMI magnetograms will be valuable as they are the boundary
conditions of the model (total flux, maybe a movie to see clearly the flux cancellations).
For instance why do you not show the magnetogram after the flare?
Thanks. Most of the required information is added to Section 3.2 and Figure 1.
Video 2 shows the flux cancellations. Figure 5c shows the magnetogram after the
flare. The information on the total flux of the region is added to Section 4.1.
- for the light curves, the choice of a field-of-view of 614x614 arcsecond is not justified
(and confusing compared to the field-of-view used before). It is important to note that the
spikes or oscillations in the 1600A channel are in fact dominated by the 5min
oscillations.
We modified the light curves, and the larger field of view is 300x300 arcsecond as
shown in Figure 1. We also added the information on the 5min oscillations to the
text.
Could you explain the double-peak structure of the flare in most channels? How does it
compare to the work done by Chifor et al. (2007)?
Double-peak or multi-peak structures are common in flares. We don’t think that
there is a physical difference between these different peaks in our target flare. In
Chifor et al. (2007), the first peak generally refers to the precursor of the main flare
(second peak). But in our case, the main flare (i.e., brightenings of two ribbons and
filament eruption) started around 02:30 UT. Therefore, all of the peaks refer to
different peaks of the main flare.
Section 3.2 (last paragraph, last 5 lines): from my understanding, this discussion assumes
that the main trigger of the eruption is the instability of the flux itself and de facto
excludes the possibility of an external trigger such as flare. Does the observation confirm
this discussion?
In this paper, we assumed that the target active region is already close to the
unstable state when the flare occurs. In this case the eruption may be due to the lossof-equilibrium of the magnetic configuration, or may be triggered by some sudden
external event that is not very energetic. We find that the unstable model fits the
observed flare loops quite well, which is consistent with our assumption. There is no
need for a sudden external trigger that is very energetic, and indeed none is
observed.
This information is added to the 5th para. of Section 5.
Section 3.3:
- As mentioned above, I really think that adding the comparison between relaxation
methods 1 & 2 in an Appendix will clarify the purpose of the article
Ok. We moved this comparison to the Appendix.
- p.8 last line: the authors introduce the "X-point". It will be interesting to have a clear
definition of an X-point. From my point of view, an X-point is a null point in 2D (refer to
Priest book) and has little meaning in 3D. It is also of interest to describe how the authors
have determine the location of X-point (visual inspection or more sophisticated
technique). The same will be interesting for the center of the flux rope (algorithm to
search for a minimum of B or just visual inspection).
Good point. We have significantly expanded our description of the X-point and Opoint in the second and third para. of Section 3.4. Basically, a visual inspection of
the magnetic vectors in Figure 7d suggests two points at heights 5 and 14 cells (1 cell
= 1.4 Mm) where the projected magnetic field vanishes; these two points are called
X-point and O-point, respectively. We call them X-line and O-line, when a 3D
configuration is considered. The axis/center of the flux rope refers to the O-point ,
and is found by visual inspection.
The above texts are added to the second and third para. of Section 4.3.
- p.9 second paragraph line 4: "the height of the flux rope increases", could you detail this
statement? This result is indeed used in the last section. Section 3.4:
To clarify this statement, the following sentence has been added to the third para. of
Section 4.3. “Note that these vectors circle around a point where the projected
vectors vanishes (i.e., O-point); this is also defined as the axis of the flux rope. This
figure shows that the height of the axis of the flux rope increases with the iteration
numbers.”
- in Table 3: it is not clear why, in the best-fit model (model 2) the ADs for loop 3 and 4
are considered as relatively good with values 0.7 and 0.3 while it is considered as bad
(0.5, 0.5) for model 3?
We define the ``average deviation" (AD) between an observed loop and a model
field line by measuring the distance between a point on the observed loop and the
closest point on the projected field line in the image plane, and then averaging these
distances for various points along the observed loop. For Loops 3 and 4 in Model 3
the best-fit field lines do not fit the observed loop very well because the field lines
are much longer than the observed loops. The reason is that the AD is measured
from a point on the loop to the nearest point on the projected field line, so parts of
the field line that extend beyond the observed loop are not taken into account in the
AD measurement, but nevertheless indicate disagreement with the observations.
The above texts are added to the first and second para. of Section 4.4.1.
Miscellaneous:
- in Table 1: the temperature of the 193A channel does not match the temperature of
Lemen et al. (2011).
This temperature is modified according to Lemen et al. (2011).
- Fig.5: is the field-of-view of both images the same? I do not think that showing field
lines at this stage of the paper is appropriate and certainly confusing.
The field of view is different. We moved the figure with field lines to Figure 11.
The information on the field of view of the figure is also added.
- Fig. 6: could you explain what are the white arrows on the top left corner of figures d, e,
f?
This is now Figure 7.
In order to show the vectors well, we use black arrows for brighter background, and
white arrows are used for darker background. This information is added to the text
and the annotation of this figure.
- in Table 3: could you justify why the models for Relaxation 1 & 2 are not all the same?
Why do you not include Relation 1 with Phi_axi of 1 or 2 like in models 16 and 17?
The reason why we have two extra models for Relaxation 2 is: the poloidal flux is
preserved better with Relaxation 2, which lowered the threshold value of axial flux.
In order to find the best-fit model, we should have at least three stable models.
This information is added to the last para. of the Appendix.
- in Table 3, I was hoping to see the percentage of free magnetic energy above the
potential field. Especially to check if in some cases, the free energy is above the upper
limit (open magnetic field).
The percentage is easy to calculate, since both the free energy and the potential
energy are provided. We are reluctant to add this information to the table, because
the table is already rather large.
The second sentence in this comment is a very good point. We estimate the free
energy in the open field which is about 3.0$\times10^{31}$ erg. Therefore, all the
models with free energy larger than this value are not considered as valid models.
Moreover, after revisiting our models with relaxation 1, we find that the critical
poloidal flux is 30e10 Mx/cm instead of 50e10 Mx/cm, for a fixed axial flux of 4e20
Mx. Therefore, we removed the models with poloidal flux of 70e10 Mx/cm and
100e10 Mx/cm. Instead, we added the models with poloidal flux of 20e10 Mx/cm
and 40e10 Mx/cm, for axial flux of 4e20 Mx.
The detailed information on the estimation of the free energy in the open field can
be found in the third para. of Section 4.2.
- Fig. 9: it seems that some models are missing? (i.e., (4, 70)). Why not add the
Relaxation 2 on the same plot?
Thanks. We added the models with relaxation 2 on the plot. We also replaced the
models (4,70) and (4, 100) with (4,20) and (4, 40). The reason is listed above.
The models with (4, 0) are left out from the plot. Because it is not easy to make a
nice plot if these two models are included.
- check for typos.
Thanks, we did.
Additional modification to the text based on Dr. Bernhard Kliem’s comments:
1. Point out that significant reconnection occurs during the relaxation process,
which is added to the end of the second para. of Section 4.1.
2. Clarify the definition of ‘marginally unstable state’ as the end of the second para.
of Section 4.2.
3. Add more interpretation on Figure 7 in the middle of the second para. in Section
4.3.
4. In the third para. of Section 5, we added the follow information:
(a) The ratio between the threshold axial flux and the total flux of the active
region.
(b) The estimation of the critical twist in the marginally unstable model due to
increase of poloidal flux. We also compared our results with the ideal
numerical models.