AnGeo, Arecibo measurements of D-region electron densities during sunset and sunrise: implications for atmospheric composition

Carsten Baumann et al.

Authors:

Carsten Baumann, Antti Kero, Shikha Raizada, Markus Rapp,5, Michael P. Sulzer, Pekka T. Verronen, and Juha Vierinen 

The paper presents Arecibo ISR D-region measurements from 2016 for sunrise and sunset conditions.

The observed electron densities are compared to SIC and WACCM-D models, discussing the mutual similarities.

The paper is well structured and nicely written which makes it a smooth and pleasant read.

The topic is well introduced, followed by the descriptions of the Arecibo measurements as well as used 1D and GCM models and their comparison. 

The authors highlight the asymmetry of sunrise and sunset ionization conditions mostly caused by the different ionization and recombination rates.

One limiting factor I see is that the observational data only consists of two sunrise and four sunset measurements, which is of course rather sparse, but the high quality makes the results convincing.

specific comments:

1) I couldn't find any statement about the solar and geomagnetic conditions during the measurements, only when it comes to using GCM 2005 data given an equivalent solar condition.

2) L19: electron density measurement techniques are introduced: in situ, VLF radio wave reflections, ISR measurements.

Later on, L33, suddenly MF radar techniques are mentioned if not highlighted as it is in the discussion section.

I'm confident the MF techniques, given the system is well capable of it, is more useful and reliable than inferring VLF radio wave propagations...?

Perhaps MF techniques could be mentioned already in L21?

3a) Fig1: I suggest to adjust the color scales to higher electron densities max. 5e4 or 1e5 to limit the saturation for the E-region peak, even though it's not in the focus of this paper. But it will beautify the plot.

3b) Fig1: I assume the obvious gaps have been excluded for the subsequent statistics, but couldn't find a note?

3c) Fig1: Judging on that plot the noise floor, so the sensitivity, is near 10-100 el/cm^-3. Especially with densities below 10, I'd be very careful near that noise floor... From Fig2 and Fig3 it doesn't look like you applied a kind of SNR selection, do you?

4) L102:  A "25% trimmed mean" is used to explicitely suppress sporadic E layer echoes. How good does this suppression work considering the echoes are pretty intense. Perhaps adding a plot with an example to the manuscript or only as a reply comparing to e.g. median?

Do you apply the same method to suppress the airplane/ship clutter? @ ~L98

5) L110: At 80 altitude... -> At 80 km altitude...

6) L141: I agree the years 2005 and 2016 were quite similar talking about the solar activity. I guess for that purpose that's sufficient, but what about the dynamics? From my impression WACCM-D is nicely reproducing daily means at late summer for these altitudes, not that sure about the time scales you're looking to, though.

7) L152: Nice idea to use multiple longitudes to create a higher SZA resolution... I'd worry about horizontal transport effects (dynamics).?. 1° longitude corresponds to roughly 100km displacement.

8) L240 (, L285 and somewhere earlier): "Both models employ similar ionospheric reaction schemes." I think that's not strictly correct as you pointed out earlier SIC and WACCM-D incoporate different amount of pos./neg. ions, and thus also possible reactions. 

I suppose to relax it by "equivalent", but not similar.

"cosmetics":

- consistent use of value and ° without a space, L106, L108, L109, L111, L112

- L232: ..."altitudes between 90 and 75 km altitude." -> remove the latter

Again, I enjoyed reading the well prepared manuscript and I support its publication after minor corrections and adressing the raised concerns.

This paper reports D-region electron density measurements from the Arecibo incoherent scatter radar being performed during sunset and sunrise conditions at Puerto Rico and the asymmetry of the electron density. The electron density observations are compared to the one-dimensional Sodankylä Ion and Neutral Chemistry (SIC) model andWACCM-D. Authors have made the efforts to explain/discuss asymmetry. I found the subject of the paper of scientific importance and worthy of the publication after adderessing the folloing sugegstions.

1. Page 1 Para 20: The relevant citations be added to rocket borne in situ measurements (citations),

interpretation of VLF radio wave reflections (citations) and its sensing by means of incoherent scatter from free electrons and Faraday rotation. For the VLF following citations are suggested:

• Han, F., & Cummer, S. A. (2010a). Midlatitude daytime D region ionosphere variations measured from radio atmospherics. Journal of Geophysical Research, 115, A10314. https://doi.org/10.1029/2010JA015715
• Kumar, A., & Kumar, S. (2020). Ionospheric D region parameters obtained using VLF measurements in the South Pacific region. Journal of Geophysical Research: Space Physics, 125, e2019JA027536. https://doi.org/10.1029/2019JA027536
• Maurya, A. K., Veenadhari, B., Singh, R., Kumar, S., Cohen, M. B., Selvakumaran, R., et al. (2012). Nighttime D region electron density measurements from ELF‐VLF tweek radio atmospherics recorded at low latitudes. Journal of Geophysical Research, 117, A11308. https://doi.org/10.1029/2012JA017876.
• Thomson, N. R., Clilverd, M. A., & McRae, W. M. (2007). Nighttime D region parameters from VLF amplitude and Phase. Journal of Geophysical Research, 112, A07304. https://doi.org/10.1029/2007JA91227

1. Page 12-13, Para 270: Other studies using MF radar and VLF observations (Coyne and Belrose, 1972; Laštoviˇcka, 1977; Li and Chen, 2014, e.g.). None of the citation is from VFL study. Please check.
2. The VLF is the most coseffective and forms a novel tool to study D-region under the normal and natural Hazards which I think needs to be given bit more emphasis.

The authors analysed an asymmetry of the electron density during sunset than during sunrise. They presend study of electron density measurements from the Arecibo incoherent scatter radar and corresponding velues obtained by modelling. I think that the manuscript is well written and should be published after the following corrections:

The authors should give the full name for WACCM-D and GCM in the abstract. In the text, full names should be given in the first place where the abbreviation appears.

Fig. 1: Is 10 ^ 4 cm ^ - 3} the maximum obtained value of electron density? I ask, because I have the impression that this value is given on large parts of the displayed graphs. I have impression that higher values were obtained but that they are seen as 10 ^ 4 cm ^ {- 3} due to the limitations of the domains in the display.

Lines 122-123: D-region heights is located between 50-60 km and 90 km. For this reason, the part "... the D-region with an altitude range from 20 to 150 km. " should be rewritten.

To my knowledge, the SIC model is primarily used for polar region analyzes. The authors should explain the possibility of applying this model (its original version and the version including meteoric smoke particles) to the area observed in this study. Is it necessary to make some corrections (eg those related to the chemical composition, the influence of the magnetic field, etc.) in these versions of the model to make their application relevant to other areas, or changes depending on observed areas and observation periods can be made in the input files?

Does the model use Eq. (1) for calculations of the effective values of the parameters related to the respective processes, or does it consider the reactions of a single type of particles (and consequently coefficients corresponding to these processes considered in particular)? The authors should explain this in the text. In case the first variant is applied, the names of the corresponding coefficients should be written and it should be explained how the corresponding effective coefficients are changed in accordance with the observed conditions. In case the second variant, Eq. (1) should be rewritten with sums and corresponding indexes and all these quantities should be explained in the text.

line 204: γ and γp are the effective coefficients related to the collisional electron detachment and electron detachment by solar photons, not the collisional electron detachment and electron detachment by solar photons.