the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Precision measurement of the index of refraction of deep glacial ice at radio frequencies at Summit Station, Greenland
Christoph Welling
Abstract. Glacial ice is used as a target material for the detection of ultra-high energy neutrinos, by measuring the radio signals that are emitted when those neutrinos interact in the ice. Thanks to the large attenuation length at radio frequencies, these signals can be detected over distances of several kilometers. One experiment taking advantage of this is the Radio Neutrino Observatory Greenland (RNO-G), currently under construction at Summit Station, near the apex of the Greenland ice sheet. These experiments require a thorough understanding of the dielectric properties of ice at radio frequencies. Towards this goal, calibration campaigns have been undertaken at Summit, during which we recorded radio reflections off internal layers in the ice sheet. Using data from the nearby GISP2 and GRIP ice cores, we show that these reflectors can be associated with features in the ice conductivity profiles; we use this connection to determine the index of refraction of the bulk ice as n = 1.778 ± 0.006.
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Christoph Welling and the RNO-G Collaboration
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-745', TJ Young, 31 Jul 2023
Please see attached PDF. I enjoyed reviewing your manuscript!
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AC1: 'Reply on RC1', Christoph Welling, 13 Aug 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-745/egusphere-2023-745-AC1-supplement.pdf
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AC1: 'Reply on RC1', Christoph Welling, 13 Aug 2023
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RC2: 'Comment on egusphere-2023-745', Anonymous Referee #2, 08 Aug 2023
Welling et al estimate bulk index of refraction (n) of glacier ice at the GISP2 location. They use existing conductivity measurements from an ice core and find best cross-correlation between those and radar-detected internal layers, which yields the bulk index of refraction estimate for this particular site.
The paper is very focused on estimating n at this particular location, motivated by neutrino detection, and as such the results might be only relevant to the RNO-G collaboration.
To make this paper relevant to other communities it could include discussion on existing techniques on estimating n, and the values and errors that have been derived elsewhere and from different techniques. This would put the result here in some broader context and make it clear how novel this paper is. At the moment there is a claim of the estimate of n here being the most precise for Greenland at the moment (line 45) but no support is given to this claim.As far as I can tell, the approach for estimating n does not differ from that of Winter et al. If that is the case. If that is not the case, and I apologize if I missed something, it would be good to highlight the improvements/differences.
Uncertainty - supposedly there is some error that comes in during the cross-correlation that comes from the assumption that peaks in conductivity change correspond to radar-detected internal layers. This often holds, but sometimes it does not, potentially affecting the error estimate. Is that something you can quantify?
There is no discussion of the location of how the location of the firn/ice boundary was assessed, past which n is assumed constant. Did you have density data available?
Related to this, some discussion on the assumption of constant n below firn layer seems important given the particular application of neutrino detection in mind. As stated in the introduction the accurate knowledge of n is absolutely key, and I wonder how small variation of n below the firn layer matter in this case.The authors motivate their work by assessing the error on 1% of n, but don't discuss what is the typical error on n, indeed their result is much less than 1% away from other values (e.g. Winter et al). So I wonder if using 1% error as motivation isn't just overstating the need for more precise knowledge of n.
I don't think the authors actually compare the radar-detected internal reflections to a quantity that is equivalent to the rate of change of conductivity with depth. More on that below.
I would like to know how much this estimate of n, and in particular the estimated error, helps reduce the area of sky that needs to be monitored, as opposed to using known values of n and the respective range/errors. I think including this would make it clear whether/how relevant is this paper even to the RNO-G collaboration itself.
In line and Minor comments:
Fig 1 and 5 - It would be better to make all lines thin for better visibility of detail
Fig 1 - How did you determine the noise level? In the green transparent curve (attenuator) it seems that there are still peaks present at the same locations as in the red curve (no attenuator) where red is not shaded.
45 - do you mean precise or accurate? How did you assess that? There is no discussion/overview of existing techniques and results and uncertainties.
60-64 - I don't understand this part, can you give a range for how much the area of the error ellipse increases for 1% error on n, instead of "significantly"?
115- incomplete sentence
124- method of
130-136 - I don't see how the procedure described here, taking a difference between raw and smoothed signal and calculating a rms over some window is equivalent to taking a derivative. I might have missed something in the text (providing an actual formula would be much clearer) but it would be to clarify how it is that the authors are actually comparing differences of conductivity rather than conductivity itself.
147 - past 1500m the signals seem to decorrelate
176 - what is meant by "this"?
184 - What is meant by "this measurement?"
Fig 4 - the dashing obscures detail, better make blue line solid too, same for Fig 2a and orange dashed line - make it thin and solid
Citation: https://doi.org/10.5194/egusphere-2023-745-RC2
Christoph Welling and the RNO-G Collaboration
Christoph Welling and the RNO-G Collaboration
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