Spatial characteristics of the dayside auroral ionosphere observed by Incoherent Scatter Radar

Ingeborg Frøystein, Andres Spicher, and Kjellmar Oksavik

This paper studies the ionospheric structure, including density and temperature in the vicinity of the cusp region, using the EISCAT Svalbard radar.  They show that the ionosphere is structured by its position relative to the open-closed field line boundary, being markedly different equatorwards of it in the closed field line region and polewards of it in the polar cap.  The paper is well written, the results are interesting, and the data is presented clearly.  Hence, I recommend it for publication.  I have only a few minor comments, listed below.

Lines 94-95: “Each experiment … visual inspection.”  It is not clear what is meant by this sentence.

Line 115: Perhaps state that it is IMF B_T that is being referred to.

Line 155: Mis-spelling of “first”.

Lines 204-208: The authors are looking for a relationship between latitudes of regions and PCN or AE.  Perhaps it should be stated that it is expected that the OCB should be in a state of flux when AE or PCN are elevated.

Line 341: Mis-spelling of “assess”.

Line 382: Is “increases” meant here?

Line 412: Should read “beneficial to include”.

Review of “Spatial characteristics of the dayside auroral ionosphere observed by Incoherent Scatter Radar”

This paper uses many observations from the EISCAT Svalbard Radar (ESR) to quantify the ionospheric properties of the dayside auroral region, and present both case study and statistical results for these ionospheric properties separated by magnetic topology. The authors use a recent technique to identify the Open Closed Boundary (OCB) using the electron temperature and solving the 1D electron energy equation to identify the region where there is an energy source. They then define the Polar Cap (PC) as the region +3 degrees latitude poleward of the OCB. Using these definitions they look at two case studies, one with primarily northward IMF, and another with a southward turning IMF. They find that there is more structure to the ionospheric properties and evidence of poleward moving auroral forms in the southward turning case. They also conduct a statistical survey of the ionospheric properties with respect to the defined boundaries. They find that there are clear and persistent gradients in electron temperature across the OCB, and less strong signatures in ion temperature or electron density. In terms of local time variations, they also find less clear variations in ionospheric properties, but some evidence of change in the ratio of electron density in the E region to F region. The results are overall consistent with other studies, and this work demonstrates the effectiveness of ISR and the OCB boundary identification technique, while providing clear benchmarks of ionospheric properties for others to compare against.

Overall comments:

The authors do a great job describing the treatment of the data and the boundary identification techniques, and they make consistent and detailed comparisons of this work to previous studies.

One of the apparent main conclusions appears circular, the increase in electron temperature across the OCB cannot be both the technique used to define the OCB itself, and also the conclusion. By definition, there MUST be increased Te across the OCB because it is defined as a Te threshold. There are still plenty of interesting points that are made, including discussion on the magnitude and width of the gradient, and the fact that it is so strong and persistent to me is one of the most important takeaways.

It appears to me that some of the trends in the results and subsequent discussion are a bit overstated. When the uncertainties are so much larger than the supposed trend and the trend is made of up only a handful of bins on the X axis, it is okay to soften the claims. Indeed in some places the authors do a good job of this, but it is inconsistent.

In the discussion and conclusion there is explanation about the shadow height and ion velocity and how these can help explain some of the results, but I believe this can be sufficiently covered just in the text. This would clear up space from panels c and h for the case study figure, as these panels are otherwise not the focus of the results.

Overall, I think this a well conducted, detailed study of ISR observations of the dayside auroral region and is a worthy contribution to the field. The comments about the existence of the Te gradient across the OCB are the only major comments, the rest can be considered minor.

Detailed Comments:

Line 13: Introduce EISCAT Svalbard Radar before ESR the acronym appears

Line 36: “… the following.” Change to “section 4.1.”

Line 44: change “of the observations covering the cusp region” to “of observations located in the cusp region”

Line 50: need to add a sentence to connect this to the present work. “Observations of Te, Ti, and Ne can be used to identify FTEs and PMAFs” or something

Line 59: list out ESR, this is the first time it appears outside the abstract

Line 60: Can you give explanation of why the open field region is split into OCB and PC here?

Line 68: Maybe a colon after “Section 4 presents the results”

Line 86: Why is 15 minutes chosen as the time delay. Is there a reference that could be pointed to?

Line 101: This paragraph was a bit difficult to understand. My interpretation of the method by reading Frøystein et al. 2024 is that by including a neutral atmosphere model and making some simplifying assumptions one can solve the electron energy equation for the heating rate Q at all locations when ESR gives data. This Q is effectively what tells us where the auroral region is because where Q is large is where energy precipitation is happening. Instead of setting a threshold on Q itself, however, the results are used to adjust a threshold on Te, given the solution to the energy equation. Is this the correct interpretation?

Line 107: change “band is discarded” to “buffer is discarded”

Line 110: how is the averaging done to obtain the expected value required to calculate relative error here?

Line 123: remove the sentence starting with “Then, statistical …”

Figure 2: Great job with the error bars and quality flags, along with the boundary panel (i), this was very easy to understand and a good way to present these results.

Figure 2: Consider removing panels (c) and (h) or moving them to supplementary

Line 150: see general comments, need to focus only on the descriptions of the Te gradient and not the fact that there is an increase in Te across the boundary. Because a threshold on Te is used to define the boundary, there will always be a gradient (except when there’s not and that triggers the quality flag so those points are removed…). It may be worth noting this for the reader here.

Line 153: change to “region of slightly lower density”

Line 157: From looking at the figure, I would describe the Ti enhancements as “patchy”, “intermittent” or something like that, rather than “enhanced in several regions”

Line 161: I don’t think the inclusion of the ion velocity measurements are helpful. In the discussion it’s noted why the property would be useful to have, but that the measurement is difficult and that is sufficient. Could either remove this paragraph or move these to supporting information.

Line 172: Is this 20 minute shift on top of the 15 minute shift that has already been done to the data?

Line 174: the AE signal has a sharp peak which is not reflected in the OCB latitude, overall the enhanced AE lines up well with OCB latitude change. Why not plot AE vs OCB latitude and do a quick correlation calculation?

Line 175: see above, Te must be enhanced by definition

Line 185: The increase in ion velocity occurs before the OCB latitude changes which is noteworthy

Line 209: change “the trend is similar” to “the trend is slightly weaker”

Line 223: see above, there is plenty of great content in this paper without the ion velocity results

Line 234: see above, what’s important here is how the properties of the Te gradient at the OCB, not the fact that there is one

Line 236: same as above

Line 247: I believe it’s “superposed” not “superimposed”

Figure 6: very nice visualization of the data

Line 255: see above, the Te increase poleward of OCB is a given

Line 256: For future studies, perhaps the 3degrees definition for the PC region should be bumped up to 4 degrees based on these results

Line 259: change “def” to “d-f”

Line 261: change to “slightly enhanced”

Line 274: Rather than just provide the reference, could you give a one sentence overview of what this ratio might be able to tell us?

Line 284: To me, the trend in the ratio for all three regions is too small relative to the uncertainties. The most important feature that stands out to me is that the CFL ratio is higher than both the  OCB and PC. If someone wanted to find the OCB and only had access to Ne, maybe this ratio could be useful for that.

All figures: very readable and well formatted figures

Line 302: if the ion velocity is kept in, it’s worth noting that the timing of the velocity reversal is not aligned with the OCB latitude change

Line 316: did you look to see if there are any spacecraft upstream in the solar wind that are closer to Earth for these two case studies?

Line 324: I’m confused about the phrase “a bias is possibly introduced.” It is perfectly acceptable to make the boundary definition based on one of the measured quantities, but when you do so it’s important to note that. It is not a bias, but simply how the boundary is defined. For the magnetopause boundary under southward IMF we sometimes look for rotations in the magnetic field in spacecraft data as the indicator of a magnetopause crossing. It’s not a bias to say that the field rotates across the magnetopause, but we also don’t call it the most striking feature. We would describe how fast it rotates, just how you can describe how sharp and wide the Te gradient across the OCB is.

Line 345: Proving out that this OCB boundary identification holds across this large set of conditions, and that it has advantages over other methods, is one of the most important findings of this study, in my opinion.

Line 373: change “def” to “d-f”

Line 378: consider “slight trends” or revising this sentence

Line 382: remove “The precipitation rather increase the density in the F region.”

Line 398: superposed

Line 400: change “if” to “of”