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

[Bart De Pontieu]

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 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



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