the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Jun 2025
Monitoring of Lower Thermospheric Neutral Density Variations Using Meteor Head Echoes
Abstract. Observations of neutral density in the mesosphere and lower thermosphere (MLT) region of the terrestrial atmosphere are important for understanding lower atmospheric, geomagnetic, and anthropogenic forcing. This study introduces a statistical method for measuring neutral density variations using an extensive dataset of meteor head echoes that were observed using the MAARSY high-power large-aperture (HPLA) mesosphere–stratosphere–troposphere (MST) radar. The method relies on observing the mean geocentric velocity of meteor head echoes as a function of initial detection altitude and day-of-year. The meteor head echo catalog used contains 1.4 million meteor head echoes between 2016–2023. Neutral density variations are observed with a 3 day time and 2 km altitude resolution between 85–115 km. The measurements show variations in neutral density potentially due to geomagnetic and atmospheric events. Variations of 20–40 % are common in the dataset, and agree with the magnitude of atmospheric neutral density fluctuations from an Upper-Atmosphere ICOsahedral Non-hydrostatic (UA-ICON) atmosphere model run.
Competing interests: One of the authors is a member of the editorial board of journal AMT.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
(1602 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2025-2323', Joel Younger, 14 Jul 2025
This paper describe the use of a large data set of radar echoes from meteor head ionization to infer density fluctuations at altitudes around 90-110 km. The work is well explained and presented and adequately cited. Of particular interest is the authors’ use of velocity cubed as a proportional proxy for atmospheric density. The writing is of an overall high quality and figures are both easy to read and complimentary to the text. The paper provides a useful introduction to the authors’ methodology and will serve as a useful foundation for their future publications on the topic. It is recommended for publication with the following minor changes.
General: This is perhaps pedantic, but “bulk density” may be more appropriate than “neutral density” While the atmosphere is relatively lightly ionized in the meteor ablation region, collisional heating at entry speeds does not distinguish between neutral molecules and those missing a full complement of electrons. It is recognized that “neutral density” is commonly used in literature, but this may be an opportunity to use more precise language.
Line 20-21: It would also be worth mentioning Yi et al. 2018 (doi: 10.1002/2017JA025059)
Line 27 change “signal” to “line of sight vector” or similar
Should define MST acronym at first use in main text.
Line 54: remove “A”, use commas
Line 84-85: It would be good to also cite the work of Campbell-Brown in generating precise maps and models of the sporadic background e.g. Campbell-Brown et al. 2008 (doi: 10.1016/j.icarus.2008.02.022)
Line 85: Maybe use autumn instead of fall to avoid confusion.
Line 103/figure 2: Is this mean detection altitude or mean initial detection altitude?
Line 110a/section 4: This assumes that the size distribution and composition of meteoroids remains constant throughout the year. While this may be a reasonable assumption for the sporadic sources, it is less so for shower sources. This raises the possibility of transient contamination of the results during strong shower activity.
Line 110b: Equation 1 describes the energy balance of collisional heating, radiation, and vaporization, but does not describe the amount of plasma generated or the reflectivity near the meteoroid. Readers would benefit from an expression describing meteor head plasma density and echo strength at the wavelengths considered. It also seems important to mention the aspect sensitivity of meteor head echoes, which may affect the average seasonal results. This only affects the absolute terms in figures 1 and 2, but not the later figures, which portray relative fluctuations between years.
Line 114: Would substitute “plasma density” for “ablation rate”. The latter refers to material loss rate, not specifically generation rate of detectable plasma
Figure 2: These are well constructed and easy to read plots. The correlation in the top panel is clear, but the deviation of the 60 and 40 km/s heights from associated iso-density contours in the bottom panel for the first half of the year goes unremarked in the text, except the disclaimer in lines 108-109. Do the authors think that this could be a shortcoming of the MSIS model, dynamical features of the atmosphere, or something to do with temporal changes in head echoes?
Line 120: “…panel of the background…”
Line 129: incomplete sentence ending in “…to compare with.”
Section 4.4: The summer/autumn reduction in density above 100 km is not obvious to me in figure 3. Is there some other way of presenting the data to make this claimed feature stand out?
Section 4.5: This section could benefit from the inclusion of a wavelet spectrogram that should clearly show the presence of planetary waves. Alternatively, a line plot showing the velocity cubed ratio variation at a fixed height may provide readers with a clearer depiction of oscillations.
Citation: https://doi.org/10.5194/egusphere-2025-2323-RC1 -
RC2: 'Comment on egusphere-2025-2323', Anonymous Referee #2, 22 Aug 2025
This manuscript describes a statistical method for deriving neutral density variations in the mesosphere and lower thermosphere from 1.4 million meteor head echoes observed by the MAARSY HPLA radar between 2016 and 2023. The authors report fluctuations of 20–40% with a 3-day temporal and 2 km altitude resolution, consistent with atmospheric model predictions and influenced by geomagnetic and atmospheric events.
A fundamental concern, however, is that the work falls outside the scope of Atmospheric Measurement Techniques. The journal is explicitly dedicated to advances in measurement methodologies, including the development, intercomparison, validation, or simulation of remote sensing, in situ, and laboratory techniques. This manuscript does not present any such advancement. Rather, it applies an established radar technique to derive neutral density variations, without contributing innovation in measurement methodology, error analysis, or instrument simulation. While the scientific topic may be of interest, the absence of methodological novelty or development renders the work misaligned with the stated aims and objectives of the journal.
Equally serious is the manuscript’s reliance on an overly simplistic assumption that meteor detection altitude depends solely on V^3. The authors themselves acknowledge this limitation in the introduction:
“With this technique the height variations are determined as an average quantity with a neutral density isocontour assumed to follow this altitude variation. This provides a general overview of atmospheric neutral density variations, but provides minimal information about differences between altitudes for the same time.”
Such an assumption is physically unsound. A more rigorous treatment would account for kinetic energy, since the ablation profile—and thus detection altitude—of a large, slow particle may closely resemble that of a small, fast one. Furthermore, the astronomical origin of the meteoroids, and therefore the entry angle, exerts a significant influence on detection altitude, as do local atmospheric conditions (e.g., Dawkins et al., 2024). Equally, the physical composition of the meteoroids has been shown to play a decisive role in ablation behaviour in optical studies (e.g., Kikwaya et al., 2011a,b). That the authors chose not to employ a comprehensive ablation model, despite the ready availability of such tools, is a serious shortcoming that undermines the robustness of the presented analysis.
In addition to these two fundamental flaws, there are numerous further issues which must be addressed:
- Throughout the manuscript the authors refer to “measuring the neutral density”. This is incorrect; the study concerns variability in neutral density, which is conceptually distinct.
- Page 3, line 34: The claim that “Measurements of the meteor head plasma provide more details on the meteor ablation and trajectory, but require HPLA radars” is outdated. This has not been true for some time (see Janches et al., 2014; Panka et al., 2021).
- Page 3, line 77: The statement “This suggests that meteoroids with higher velocity are, on average, detected at higher altitudes” is not a suggestion—it has long been established (e.g., Janches & ReVelle, 2005; Vondrak et al., 2008). Moreover, as noted earlier, detection altitude depends on several other parameters, particularly in the context the authors are attempting to present.
- Page 3, line 84: The authors state that “Throughout the year, the radiant distribution of meteors observed by a radar changes (e.g., Janches et al., 2006; Kero et al., 2012).” This is correct but incomplete. The variability is highly location dependent; at equatorial sites, for example, such changes are minimal.
- Page 5, lines 95–100: The discussion presented is already well established in the literature, and the variability again depends strongly on geographical location. For example, such variability has not been measured at equatorial latitudes (see Sparks & Janches, 2009a,b). The authors should clarify and reference the prior work properly.
- Page 5, line 102: The repeated reference to the “background model” is ambiguous. It is not clear what background the authors are referring to, and this requires clarification.
In its present form, the manuscript suffers from serious conceptual, methodological, and contextual shortcomings. Most importantly, it does not offer an advance in measurement methodology and therefore lacks relevance to Atmospheric Measurement Techniques. The paper may be more appropriately considered by a journal focused on atmospheric dynamics or variability, rather than one dedicated to the advancement of measurement techniques.
References
Dawkins E. C., D. Janches, G. Stober, et al. 2024. "Seasonal and Local Time Variation in the Observed Peak of the Meteor Altitude Distributions by Meteor Radars." Journal of Geophysical Research: Atmospheres 129 (21): [10.1029/2024jd040978] [Journal Article/Letter]
Janches D. and D. O. ReVelle. 2005. "The Initial Altitude of the Micrometeor Phenomenon: Comparison between Arecibo radar observations and theory." Journal of Geophysical Research 110 A08307 [10.1029/2005JA011022] [Journal Article/Letter]
Janches D., W. Hocking, S. Pifko, et al. 2014. "Interferometric meteor head echo observations using the Southern Argentina Agile Meteor Radar." Journal of Geophysical Research - Space Physics 119 [10.1002/2013JA019241] [Journal Article/Letter]
Kikwaya, J.-B., Campbell-Brown, M., & Brown, P. 2011,406 Astronomy & Astrophysics, 530, A113,407 doi: 10.1051/0004-6361/201116431408
Kikwaya, J. B., Campbell-Brown, M., & Brown, P. G. 2011, A&A, 530, A113, doi: 10.1051/0004-6361/201116431
Panka P. A., R. J. Weryk, J. S. Bruzzone, et al. 2021. "An Improved Method to Measure Head Echoes Using a Meteor Radar." The Planetary Science Journal 2 (5): 197 [10.3847/psj/ac22b2] [Journal Article/Letter]
Sparks J. J. and D. Janches. 2009a. "Latitudinal dependence of the variability of the micrometeor altitude distribution." Geophysical Research Letters 36 L12105 [10.1029/2009GL038485] [Journal Article/Letter]
Sparks J. J. and D. Janches. 2009b. "Correction to ‘‘Latitudinal dependence of the variability of the micrometeor altitude distribution’’." Geophysical Research Letters 36 L17101 [10.1029/2009GL039987] [Journal Article/Letter]
Citation: https://doi.org/10.5194/egusphere-2025-2323-RC2 -
AC1: 'Reply on RC2', Devin Huyghebaert, 26 Aug 2025
Dear Reviewer,
Thank you for your time in reviewing the manuscript. We wish to respond to your comment that this work is not within the scope of AMT. We respectfully disagree.
While the measurement of meteor head echoes is established, here we leverage the extensive dataset available to showcase a novel analysis technique applied to meteor head echoes to derive atmospheric neutral density variations between years for the same day-of-year. To quote the AMT landing page (https://www.atmospheric-measurement-techniques.net/), "The main subject areas comprise the development, intercomparison, and validation of measurement instruments and techniques of data processing and information retrieval for gases, aerosols, and clouds." The work we present falls well within the subject of techniques of data processing for information retrieval for the atmosphere.
We also wish to emphasize that the methodology presented has not been applied previously to meteor head echo measurements. We are able to perform this analysis due to the extensive dataset of greater than 1 million meteor head echo detections made with the MAARSY radar system on a consistent basis between the years of 2016-2023. To the authors knowledge, no other meteor head echo dataset of this magnitude is available globally.
The remaining comments will be addressed during the response phase.
Best regards,
Devin HuyghebaertCitation: https://doi.org/10.5194/egusphere-2025-2323-AC1
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 405 | 40 | 16 | 461 | 11 | 22 |
- HTML: 405
- PDF: 40
- XML: 16
- Total: 461
- BibTeX: 11
- EndNote: 22
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1