[Loops] EXTERNAL: Paper: Periodic spectral line asymmetries in solar coronal structures from slow magnetoacoustic waves

[Tongjiang Wang]

Dear Bart and Scott,

Thank you for your preprint and discussions. I have read your paper in 
detail and have some comments and disagreements as following:

1) The bottom panel of Fig.2 shows the blue-wing asymmentry from R-B 
analysis in range Y=60"-110" (which is consistent with that of my 
skewness analysis), however, the propagating disturbances in intensity and Doppler
shift span from Y=80" up to 200". How do you understand the blue wing
asymmetry is only limited at the footpoint of fan loops? In addition,
the B-R aysemmtry pattern does not show the propagating feature as seen
in intensity and Doppler shift, this suggests that the outflow present
at the loop footpoint and the propagating disturbances may be different
things.

2) You stated that "Most of the line width peaks are associated with 
stronger blueward asymmetries in Fe XIII" and "correlation of variations 
in intensity and Doppler shifts in Fe XIII and Fe XIV are significant".
I don't agree with you. The approximate accounting from your Fig.4 and
appendix Fig.1 and 2, show that the associations are less than 50%.

3) Unfortunately, you did not show results of single-Gaussian fitting for
Fe XIV as a comparison with Fe XIII.
In fact, there is a clear difference in intensity, Doppler shift, line 
width, and R-B analysis of rastering image and time sequence between
Fe XIII and Fe XIV. The FeXIV line has a very strong blue-wing asymmetry, so
you could make a double-Gfit to separate the flow component. Have you also 
made a double-Gfit for Fe XIII image and time sequence? If so, could you
show me your result? I'm very interested in it. Moreover, your double 
gaussian fit to Fe XIV does not directly explain the propagating 
disturbances seen in Fe XII or Fe XIII, because you even did not prove in
your paper that the intensity and Doppler shift in Fe XIV show the 
same propagating pattern as in Fe XIII (at least I did not find that in 
my analysis). In addition, from your Fig.9 (time sequence of 2nd 
component) I hardly see any propagating signature. If this is due to the
quanlity of the plot, I'm sorry about it.

4) I think it is too early to make any conclusion, more detailed analysis 
of this and other data sets are needed.

Best Regards

Tongjiang

-------------------
Tongjiang Wang

NASA GSFC - Code 671
Bldg 21 - RM C121
Greenbelt, MD 20771

Tel. 301-286-6575
Fax. 301-286-1617
tongjiang.wang at nasa.gov

On Sun, 19 Sep 2010, Bart De Pontieu wrote:

> Dear all,
>
> You may also be interested in our paper (De Pontieu & McIntosh, 2010), which was accepted to ApJ  in early August, and which uses detailed modeling and double component fits of EIS observations to show that some widely studied examples of "slow magnetoacoustic waves" are in fact most likely caused by quasi-periodic upflows of order 50 km/s.  Here is the link: http://arxiv.org/abs/1008.5300
>
> It is clear that both flows and sound waves occur in the corona. The point of our paper is that disentangling which scenario dominates specific observations is not straightforward. We find that coronal seismology just based on quasi-periodic changes in intensity (e.g.., using TRACE, STEREO or AIA) is ambiguous at best. Instead, a detailed study of at least intensity, velocity, linewidth and asymmetry of the spectral line (as well as signal-to-noise) is required to distinguish between both scenarios. While Erwin's paper expands in general on one of the interpretations we offered in our paper, our results show that the devil is in the details of the observed amplitudes, periods and correlations of the oscillations of the first 4 moments of the spectral line profile, and their phase/amplitude relationships. Such a detailed comparison with specific observations remains to be done for the waves interpretation. At least for the observations we focused on, the evidence from our double Gaussian fits support the upflow scenario. We are preparing a follow-up paper that shows that several other popular "waves" datasets are compatible with the presence of quasi-periodic upflows.
>
> It is possible that some observations end up being compatible with both flows and waves (although this remains to be shown directly). However it is clear that the difficulties of discriminating between both scenarios in specific observations presents significant challenges to coronal seismology based on sound waves. Especially because the presence of ubiquitous upflows at least are well supported by spectral imaging data from various instruments (Hinode, ground-based observations, SDO). Detailed analysis that looks into S/N, line of sight, the first 4 moments of the spectra and high quality imaging data will be necessary for each dataset to see if both scenarios co-exist (e.g., waves triggered by upflows), or whether one or the other dominates.
>
> Cheers,
> 		Bart & Scott
>
> On Sep 19, 2010, at 9:25 AM, Erwin Verwichte wrote:
>
>> Dear all,
>>
>> You may be interested in our paper, which has just been accepted for publication in ApJL and which adds to the current debate on periodic flows and waves.
>>
>> Periodic spectral line asymmetries in solar coronal structures from slow magnetoacoustic waves
>> Verwichte, E., Marsh, M., Foullon, C., Van Doorsselaere, T., De Moortel, I., Hood, A.W. and Nakariakov, V.M.
>>
>> Abstract: Recent spectral observations of upward moving quasi-periodic intensity perturbations in solar coronal structures have shown evidence of periodic line asymmetries near their footpoints. These observations challenge the established interpretation of the intensity perturbations in terms of propagating slow magnetoacoustic waves. We show that slow waves inherently have a bias towards enhancement of emission in the blue wing of the emission line due to in-phase behaviour of velocity and density perturbations. We demonstrate that slow waves cause line asymmetries when the emission line is averaged over an oscillation period or when a quasi-static plasma component in the line-of-sight is included. Therefore, we conclude that slow magnetoacoustic waves remain a valid explanation for the observed quasi-periodic intensity perturbations.
>>
>> http://go.warwick.ac.uk/erwin_verwichte/publications/verwichte_apjl2010.pdf
>>
>> Best regards,
>>
>> Erwin
>>
>> Dr Erwin Verwichte
>> Centre for Fusion, Space and Astrophysics
>> Department of Physics
>> University of Warwick
>> Coventry CV4 7AL, United Kingdom
>> Office PS1.09
>> tel: +44(0)2476524917
>> fax: +44(0)2476524887
>> http://www.warwick.ac.uk/go/erwin_verwichte
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Bart De Pontieu    --  Lockheed Martin Solar & Astrophysics Lab, Palo Alto
> bdp at lmsal.com   --     3251 Hanover St., Org. ADBS, Bldg. 252, CA 94304
> http://leffe.lmsal.com/bdp -- Phone 1-650-424-3094 / Fax 1-650-424-3994
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
>
>