Peer review of: Evidence of ubiquitous Alfvén pulses transporting energy from the photosphere to the upper chr (2019)

Reviewers' comments:  

 

Reviewer #1 (Remarks to the Author):  


The authors claim that short-lived swirls observed in the solar phostosphere excite  

Alfvén pulses, which propagate upwards into the upper solar chromosphere. These pulses  

would then be are able to transport sufficient energy to balance energy loss in quiet  

Sun regions, thus suggesting a viable mechanism to heat the quiet regions  

in the entire upper chromosphere.  

 

The topic of the manuscript is certainly of interest and importance. To trace  

and characterise the propagation of energy through the different layers of the  

solar atmosphere is essential to find a solution to the chromospheric and coronal  

heating problems. The authors employ state-of-the-art instrumentation with data of the  

highest available quality. A multi wavelength multi instrument analysis is performed.  

Also, advanced numerical computations were carried out trying to mimic the generation  

and propagation of the disturbances thought to be responsible for the observed swirl  

propagation.  

 

The study shows some unquestionable pieces of evidence:  

 

1) Observations by Hinode/SOT and SST/CRISP show the presence of a large sample of  

photospheric and chromospheric swirls in a large sample. The statistical properties  

of these populations are analysed.  

 

2) By means of correlation and time-lag analyses, the authors find co-spatial and  

co-temporal relationships between both samples. Correlation is found in about 40% of the cases.  

The chromospheric swirls would therefore be caused by the upward propagation of the  

photospheric ones. The time it takes to the swirls to travel upwards is statistically  

calculated to be around 130 sec.  

 

3) Support to this idea was sought by performing numerical simulations of the excitation  

and propagation of Alfvén pulses. The hypothesis is made that rotational motions introduced  

above the tau_500=1 level can propagate upward in the form of Alfvén pulses. The time it takes  

them to travel from photosphere to chromosphere agrees well with the observed estimate.  

 

4) Analytically calculated energy estimates are given, which seem to point to a large  
reservoir of energy flux.  
 

These results are certainly important since they provide convincing evidence about the  
possible importance of this numerous small-scales dynamics on the photosphere-chromosphere  
energetics. Observational evidence about swirls in the solar photosphere is known since decades 
ago.  

The merit of this study is in observationally finding their chromospheric counterparts and in 
providing statistical evidence about their relationship in space and time.  

 
However, these findings do not support the strong claims being made in the title of the manuscript  
and in the concluding paragraph of the manuscript. It is said that these pulse supply enough energy 
to  heat the chromosphere. But the following concerns inmediatlly arise:  

 The average energy flux carried out by the Alfvén pulses is calculated using simple analytical  
relationship for the energy flux of plane Alfvén waves in an infinite and uniform medium and  
by inserting values for the physical parameters in the simulation.  

 

This energy of the pulses could have been computed from the numerical simulations, but these 
simulations  do not even model the process of dissipation of energy nor the heating associated to this dissipation.  

To conclude that any heating can arise from these simulated pulses is therefore an overstatement, 
because  the numerics does not model the dissipation of energy, not the heating associated to this 
dissipation.  

What is the heating mechanism? Also, why do I see rotational motions in the driver, but not 
thereafter  when the disturbance propagate upwards?  

 

The authors show observational evidence for the presence of swirls but do not offer convincing 
evidence about them to be important for chromospheric heating. No dissipation mechanism is 
discussed. No energy calculation is performed in which one could see how much of the energy can 
be dissipated. No series of interconnected events are suggested for the full process that is being 
invoked. For instance, in the concluding paragraph, the authors admit that "future work should be 
focused on to discover the complete energy channeling scenario by swirls in the solar atmosphere". 
It is precisely this type of evidence that I was expecting this paper to show to merit publication in 
Nature Communications.  

 

For this reasons, I do not recommend acceptance.  

---  

Reviewer #2 (Remarks to the Author):  

This is an interesting paper on the long standing puzzle, the heating mechanism of the solar 
chromosphere and corona. The idea to compare swirls in the photosphere and those in the 
chromosphere is very interesting and good. However, there are some fundamental questions on the 
method and results. Hence I cannot recommend publication at the present stage. I would like to 
encourage further study of the problem with this approach and revise the paper. My questions and 
comments are as follows:  


Major Points:  

(1) The authors did not show the relation between the swirls and the small scale vertical 
magnetic flux tubes. However, in order to excite the Alfven pulse along the vertical flux tube, 
photospheric swirls must be present just on the small scale vertical magnetic flux tubes in the 
photosphere. Please study this correlation, and please show such results on the correlation between 
swirls and flux tubes in the revised paper.  
 

(2) How much Poynting energy flux is carried by the Alfven pulse into the chromosphere from
the photosphere ?  

 
Minor points:  

(3)  According to the results, there are 55 swirls in the chromosphere, while there are 21 swirls 
in the photosphere. Why there are more wirls in the chromosphere than in the photosphere ?  

 

(4) In the 4th paragraph of page 2, the author wrote “the azimuthal magnetic field perturbation 
is opposite oriented to the velocity field perturbation”. I cannot understand this sentence. How can I understand the direction of azimuthal velocity and magnetic field vector from Figure 4 ?  

 

(5) Figure 1: Please show the images of photospheric swirls in the photosphere  

 

(6) Figure 4: Is delta Bh the same as delta Ba ?  

 

(7) Reference 19 (De Pontiou et al. 2004) is not the paper on Alfven waves nor Alfvenic waves.  
Instead, the author should cite Kudoh and Shibata (1999) ApJ 514, 494 which is an important paper 
on the Alfven wave model of spicules and chromospheric/coronal heating.  

 
Reviewer #3 (Remarks to the Author):  


The paper aims at providing observational evidence that Alfven wave pulses excited by swirls in the 
solar photosphere are responsible for chromospheric and coronal heating. The authors perform 
correlation analysis between swirls detected at different heights in the solar atmosphere, and 
provide results of 3D MHD numerical simulation of these dynamical structures. However, the 
statistical analysis clearly shows that the time resolution of the study is inadequate, since most swirls 
are not well resolved, and that the correlation between the photospheric and chromospheric swirls 
is weak. The authors suggest that the Alfven pulses are prevalent, and that they propagate upwards 
through the solar atmosphere and reach the chromospheric layers. The authors claim the average 
energy flux carried by the Alfven pulses associated with the swirls are 33 - 131 W m−2 , and suggests 
that these events could transport a considerable amount of non-thermal energy into the upper 
chromosphere to balance the radiative (and other) energy loss. Overall, the radiative energy losses 
in the chromosphere are two order of magnitude larger than the estimates energy flux provided by 
the swirls in this study. While the upper chromosphere has lower radiative losses than the mid-
chromosphere, there is no evidence that the presumed Alfvenic pulses would preferentially deposit 
their energy in the upper chromosphere. While it is plausible that Alfven waves can heat the corona, 
this is not adequately supported by the present analysis. In summary the paper does not provide 
convincing evidence that swirls and the associated waves that may be produced, can supply 
adequate energy flux for chromospheric heating, and the statistical analysis is weak. Therefore, I 
cannot recommend publication for this study.  

 

Minor comments:  


The title implies very firm conclusion of the study. However, the analysis in the paper does not 
support such firm conclusion. Please revise.  

There are minor English grammar issues through the paper that need attention. For example, in the 
abstract: “in stead”=> “instead”; “the enough” => “enough”, etc.  

Please clarify “when assessing the affinity to dissipate these perturbations.”  


Lines 80-88 discuss 3D MHD modeling of the swirls. However, additional details of the modeling, 
such as adequacy of the resolution, numerical dissipation effects, and other simplifications and 
assumptions of the model should be given in the paper or in the supplement.  

The provided numerical simulation results are not entirely new, already discussed in detail by Fedun 
et al. (2011).