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