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Peer-Review Record

Practical Modeling of Large-Scale Galactic Magnetic Fields: Status and Prospects

Reviewer 1: Anonymous
Reviewer 2: Anonymous
Received: 30 January 2019 / Revised: 19 April 2019 / Accepted: 23 April 2019 / Published: 28 April 2019
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)

Round  1

Reviewer 1 Report

This is an excellent and timely review of recent work on Galactic magnetic field modeling, congratulations to the author. It is written clearly, with relatively few concrete examples and references, but from an overarching view. I recommend the manuscript for publication after taking into account a few relatively minor comments.

I do recognize that a review cannot mention every relevant paper, and in general I appreciate the sparseness of references, but in a number of places I do think that important contributions are missing. I suggest some additions below.


line 66: intuitively it is not clear why EoR 21 cm (line) emission would be contamination by Galactic synchrotron (continuum) emission. This needs one more line of explanation to make it understandable, possibly with a reference to Asad et al (2015).

Section 2.1: I think that the series of papers by Pavel (Clemens, et al) deserve mention here, since they were the first to my knowledge to connect polarized starlight properties to 3D GMF modeling.

In Table1, I would recommend separating radio synchrotron emission from its polarized part (fractional polarization), as they probe different GMF properties.

l97: polarized --> partially linearly polarized

l121: intergalactic medium AND Faraday rotation intrinsic to the source.

l122,123: references [8,9] the newly accepted Schnitzeler et al (2019) MNRAS paper has enough new sources to warrant a place in this list as well.

l142: it would be good to mention a recent update on polarization horizons by Alex Hill (http://adsabs.harvard.edu/abs/2018Galax...6..129H), in addition to the old discussion by Uyaniker.

l164: ... cosmic-ray electron (CRE)...

section 2.7: if you discuss GMF models from supernova remnant morphology, you should also briefly discuss similar attempts using Zeeman splitting in masers in star-forming regions, confirming the nearby GMF reversal (Fish et al 2003, Han & Zhang 2007).

section 2:  there are a number of other (proposed) methods to probe Galactic magnetic fields in the literature. It would be good to mention their existence, even if they are not discussed in more detail, such as ground state alignment by Huirong Yan and co-workers or HI velocity gradients (Gonzalez-Casanova & Lazarian 2017).

l226: '...warm and hot ionized gas...'

section 3.1.3: a third way to probe the isotropic random field is using wavelength-dependent depolarization, as e.g. in Gaensler et al 2001.

l372: "more recent work shows": please give a reference.

l426: "Unger et al below": do you mean "Unger and Farrar below"?

l479: "the fields appear to be strongest in spiral arm structures that may or may not be coincident with the material spiral arms": are you sure you mean that 'the fields are strongest' in these magnetic spiral arms? I would think that magnetic arms are the places where the fields are most regular, i.e. where the ratio of (regular + ordered random) to (isotropic random) field is the highest. The isotropic random component is not constrained by the measurements of magnetic arms, I think, so that one cannot make any statements about the strength of the (assumed total) magnetic field there.

l486: similarly, "strongest arms": arms with the highest (regular + ordered random) field strength?

page 15: there are a number of undefined references: Han17, O'Dea11

l636: supernovae --> supernova remnants

section 5.2.1. In this context, it would be good to mention a possible direct influence of the North Polar Spur/ Loop I region on large-scale GMF modeling: the possibility that this region causes (part of) the butterfly RM pattern on the sky. This pattern has been interpreted as global, which would be evidence of an A0 dynamo (eg Han 2006). However, Wolleben et al (2010) discussed the possibility of this pattern being caused by local loops.

l666: define GMIMS and/or give a reference

section 5: In the "challenges" section, I miss a mention of the broad Galactic Center region, which definitely counts as a challenge for  large-scale GMF modeling. It is of course related to the Galactic outflow discussion and could be included there.

l732: "detect every pulsar": please give reference

l747: can you define "local"?

l755: see my earlier remark abt section 2.7


Author Response


Thanks very  much for the detailed and useful comments.  All are addressed below with my response preceded by "****".

line 66: intuitively it is not clear why EoR 21 cm (line) emission would be contamination by Galactic synchrotron (continuum) emission. This needs one more line of explanation to make it understandable, possibly with a reference to Asad et al (2015).

****  Added ref to Zaroubi 2013

Section 2.1: I think that the series of papers by Pavel (Clemens, et al) deserve mention here, since they were the first to my knowledge to connect polarized starlight properties to 3D GMF modeling.

**** Done (Pavel 2011)

In Table1, I would recommend separating radio synchrotron emission from its polarized part (fractional polarization), as they probe different GMF properties.

****  I thought about several ways to do this but decided that I prefer to leave it as is.  The distinction between the information in total and polarized intensities is made in the text.  Here, I'm summarizing more generally and don't wish to split the table across pages to fit more information.  

l97: polarized --> partially linearly polarized

**** Done

l121: intergalactic medium AND Faraday rotation intrinsic to the source.

**** Done

l122,123: references [8,9] the newly accepted Schnitzeler et al (2019) MNRAS paper has enough new sources to warrant a place in this list as well.

**** Done

l142: it would be good to mention a recent update on polarization horizons by Alex Hill (http://adsabs.harvard.edu/abs/2018Galax...6..129H), in addition to the old discussion by Uyaniker.

****   Indeed, an important one I somehow missed!  Added.  Thanks.  

l164: ... cosmic-ray electron (CRE)...

**** Done.  

section 2.7: if you discuss GMF models from supernova remnant morphology, you should also briefly discuss similar attempts using Zeeman splitting in masers in star-forming regions, confirming the nearby GMF reversal (Fish et al 2003, Han & Zhang 2007).

****  I've added mention of Zeeman as well as HI velocity gradients in a couple of places in Sections 2 (observables), 4.2.3 (reversals), 6 (Prospects).

section 2:  there are a number of other (proposed) methods to probe Galactic magnetic fields in the literature. It would be good to mention their existence, even if they are not discussed in more detail, such as ground state alignment by Huirong Yan and co-workers or HI velocity gradients (Gonzalez-Casanova & Lazarian 2017).

****  Added refs.  

l226: '...warm and hot ionized gas...'

****  Done

section 3.1.3: a third way to probe the isotropic random field is using wavelength-dependent depolarization, as e.g. in Gaensler et al 2001.

****  I prefer to keep this section simply discussing ways that the isotropic random field can be distinguished from an anisotropic random field.  

l372: "more recent work shows": please give a reference.

****  Added

l426: "Unger et al below": do you mean "Unger and Farrar below"?

****  Yes thanks, fixed.  

l479: "the fields appear to be strongest in spiral arm structures that may or may not be coincident with the material spiral arms": are you sure you mean that 'the fields are strongest' in these magnetic spiral arms? I would think that magnetic arms are the places where the fields are most regular, i.e. where the ratio of (regular + ordered random) to (isotropic random) field is the highest. The isotropic random component is not constrained by the measurements of magnetic arms, I think, so that one cannot make any statements about the strength of the (assumed total) magnetic field there.

****  You are correct, thanks.  Sloppy language fixed.  

l486: similarly, "strongest arms": arms with the highest (regular + ordered random) field strength? 

**** Clarified as "arms with the strongest coherent fields"  

page 15: there are a number of undefined references: Han17, O'Dea11

****  Fixed.  

l636: supernovae --> supernova remnants

**** Fixed

section 5.2.1. In this context, it would be good to mention a possible direct influence of the North Polar Spur/ Loop I region on large-scale GMF modeling: the possibility that this region causes (part of) the butterfly RM pattern on the sky. This pattern has been interpreted as global, which would be evidence of an A0 dynamo (eg Han 2006). However, Wolleben et al (2010) discussed the possibility of this pattern being caused by local loops.

**** Added.

l666: define GMIMS and/or give a reference

****  Added

section 5: In the "challenges" section, I miss a mention of the broad Galactic Center region, which definitely counts as a challenge for  large-scale GMF modeling. It is of course related to the Galactic outflow discussion and could be included there. 

****  Paragraph added in outflow section.

l732: "detect every pulsar": please give reference

****  Added Keane et al. 2014

l747: can you define "local"?

****  Clarified that this depends on the particle rigidity.

l755: see my earlier remark abt section 2.7

****  Likewise.


Reviewer 2 Report

First suggestion - to give reference to first original 

book about CR   by Ginsburg and Syrovatskii

http://adsabs.harvard.edu/abs/1964ocr..book.....G

It contained  the general equations for  synchrotron

radiotion.

Second one  - to  show a  figure illustrated

the modern vision of the GMF.


Small typos: 183: therefor

258: but but 

Author Response


Thanks very  much for the useful comments.  All are addressed below with my response preceded by "****".

First suggestion - to give reference to first original  book about CR   by Ginsburg and Syrovatskii

http://adsabs.harvard.edu/abs/1964ocr..book.....G

It contained  the general equations for  synchrotron radiotion.


****  There are many fundamental texts in the many astrophysical topics that involve

Galactic magnetic fields.  I have generally chosen to cite the more

recent works and leave the reader to pursue older foundational work if

they wish.  This may be the original book about CR, but the fact is that I

have not read it myself and have learned about CRs from later sources that

are cited.  


Second one - to show a figure illustrated the modern vision of the

GMF.  

**** I have added this figure with the following caveat.   The problem

is that there are many models in many different publications all

plotted differently (and with different journals whose permission

would be needed to reproduce their figures).  I have codes that can

reproduce some of these in a common way, but it's a selection

therefore biased by my own interests.  


Small typos: 183: therefor

**** Fixed


258: but but  

**** Fixed


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