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 preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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Status: open (until 31 Aug 2025)
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RC1: 'Comment on egusphere-2025-2323', Joel Younger, 14 Jul 2025
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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
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