Coronal Field Geometry and Solar Wind Speed

Round 1

Reviewer 1 Report

This paper reports on a work about the relationship between solar wind speed and coronal field geometry parameters in addition to and as distinguished from the well-known WSA model parameters. The authors used three different sets of solar magnetic field observations to determine the correlations among various coronal magnetic parameters and eventually relations to SW speed. The motivation for this work is interesting, the work itself is worth of study. However, the manuscript needs to be improved after considering the following issues/questions are addressed if appropriate.

1.     Line 82-92: About the newly considered five parameters: It should be much helpful if a short comment is given on what intuitive reasoning motivated the authors to consider each of them as a potential cause-and-effect parameter for SW speed. Only a simple blind parametric test among these parameters and SW speed is neither very physical nor intuitive.

2.     Line 105-106: What do you specifically mean by the data with a ~4.5 hr periodicity? The periodicity of what data? Also, does this line mean that all the data (both SW and coronal field parameters) are CR-averaged?

3.     Correlation coefficients in Table 1: First, what specific correlation coefficient do you mean? Is it the usual linear Pearson correlation coefficient? Second, why are there large differences in correlation coefficients for each parameter among different Obs./Instruments? The differences are quite notable. This should be explained.

4.     If I understood correctly, the relationship was tested between the time-shifted SW speed (measured at 1 AU) and the 2.5 Rs source surface parameters. Considering that the SW speed can evolve and fluctuate as it propagates outward, naturally I would hardly expect finding a too strong correlation coefficient between the simply time-shifted SW speed and the relatively detailed solar source surface parameters. It should be commented further to what extent the possible uncertainty in time-shifting SW causes errors and/or uncertainties in determining the correlations with coronal source surface parameters. 

5. The paper emphasizes the role of the coronal field geometry only. In contrast, other processes such as reconnection and wave-particle interactions can affect SW acceleration and evolution. This should be discussed, in particular, to what extent these other mechanisms can affect the conclusion in this manuscript.  

Author Response

We thank the Reviewer for reviewing our work and providing valuable comments and suggestions. We have made appropriate additions and clarifications to our work (indicated in blue).

1. Line 82-92: About the newly considered five parameters: It should be much helpful if a short comment is given on what intuitive reasoning motivated the authors to consider each of them as a potential cause-and-effect parameter for SW speed. Only a simple blind parametric test among these parameters and SW speed is neither very physical nor intuitive.

We have added some explanations to the studied parameters (lines 84-104).

 

2. Line 105-106: What do you specifically mean by the data with a ~4.5 hr periodicity? The periodicity of what data? Also, does this line mean that all the data (both SW and coronal field parameters) are CR-averaged?

We make calculations on a grid with steps of 2.5 degrees of longitude, which corresponds to about 4.5 hours. In the present work, these data with a step of 4.5 hours are used only to illustrate the dependence of SW speed on various parameters. For the quantitative analysis, we average the magnetic-field parameters and the SW speed by CR (lines 117-121).

 

3. Correlation coefficients in Table 1: First, what specific correlation coefficient do you mean? Is it the usual linear Pearson correlation coefficient? Second, why are there large differences in correlation coefficients for each parameter among different Obs./Instruments? The differences are quite notable. This should be explained.

Yes, we calculate the Pearson linear correlation coefficient. We have now indicated this in the title to Table 1. The differences in the correlation for different instruments, in our opinion, are due to the different sensitivity of the instruments. We indicated this in the Discussion (lines 169-172)

 

4. If I understood correctly, the relationship was tested between the time-shifted SW speed (measured at 1 AU) and the 2.5 Rs source surface parameters. Considering that the SW speed can evolve and fluctuate as it propagates outward, naturally I would hardly expect finding a too strong correlation coefficient between the simply time-shifted SW speed and the relatively detailed solar source surface parameters. It should be commented further to what extent the possible uncertainty in time-shifting SW causes errors and/or uncertainties in determining the correlations with coronal source surface parameters.

 

Yes, it is. But, as we said in our response to your second comment, the correlation is estimated numerically only by the CR-averaged magnetic field parameters and, consequently, the CR-averaged solar wind speed. This allows us to neglect the fluctuations in the solar wind propagation. We pointed out this circumstance more clearly (lines 120-122).

 

 

5. The paper emphasizes the role of the coronal field geometry only. In contrast, other processes such as reconnection and wave-particle interactions can affect SW acceleration and evolution. This should be discussed, in particular, to what extent these other mechanisms can affect the conclusion in this manuscript.

The approach to solar wind speed modeling presented in this paper is analogous to the WSA model. Like the WSA model, it considers only the geometry of the solar corona and can be based on PFSS or PFSS-like models. Therefore, we do not have any possibility to evaluate the role of reconnection or wave-particle interactions within this approach.

We also corrected a typo in the placement of brackets in formulas 2 and 5.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript “Coronal field geometry and solar wind speed”  identified correlations between the observed solar wind (SW) velocity and the coronal magnetic field parameters, including the force lines length, the latitude of the force lines footpoints and the great circle distance from the base of the open magnetic field line in the photosphere to the boundary of the corresponding coronal hole (DCHB),. The authors proposed a formula for calculating the SW velocity based on these parameters, and showed a better correlation with observations compared to the WSA model that is based on the factor of "Flux Tube Expansion factor" (FTE).

 

The paper is well written, and there is only some minor suggestions.

 

First, the usage of abbreviations. They must be followed by phrases first appeared in the text. For the example of  ‘PFSS’,  ‘STOP’, ‘SDO/HMI’, and ‘WSO’ in Abstraction.

In addition, the accuration of full names for the abbreviations. For example in Abstracion, "Flux Tube Expansion factor" (FTE)---> "Flux Tube Expansion" (FTE), becasue there is   ‘parameter’ at the begining of the sentence; the boundary of the corresponding coronal hole (DCHB), what is ‘D’ for?

 

Secondly, the tense of some verbs. For example, shows ---> showed in the last sentence of Abstraction.

 

Finally, Line 25, ‘Coronal holes (CHs), which correspond to regions in the photosphere with an open configuration of magnetic field lines’ -->

‘Coronal holes (CHs), which correspond to regions with an open configuration of magnetic field lines rooting in the photosphere’.

 

 

 

Author Response

We thank the Reviewer for reviewing our work and providing valuable comments and suggestions. We have made all the corrections you suggested (indicated in red). However, we are not the only ones who use the abbreviation "DCHB"; it is found, for example, in (Wallace et al., 2020). The letter “D” corresponds to the word “Distance”.

We also corrected a typo in the placement of brackets in formulas 2 and 5.

Round 2

Reviewer 1 Report

The authors responded to my review comments properly and the revison is appropriate and ready for publication in the present form.