the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Nov 2025
Incoherent Scatter Radar Observations of the F Region of the Ionosphere above the Eastern US during the April 2024 Solar Eclipse
Abstract. We present dynamic mid-latitude ionospheric changes associated with the Apr 8, 2024 solar eclipse, as observed in the eastern to central United States using wide-field scans from the Millstone Hill incoherent scatter radar. The radar’s field of view covers a large portion of the continental US (25–55° N, 75–95° W), and is capable of observing altitude profiles from the E-region up to the topside ionosphere. We find that extensive ionospheric changes triggered by the eclipse along the US east coast were detected before the totality shadow entered the continent, and in particular when the eclipse obscuration percentage became greater than 0 %. During the time interval where the obscuration percentage is significant (>75 %) over the radar scanning region, we find maximum electron density (Ne) drops of up to 50 % (2 ×1011 m−3) in the F1 region (200–250 km) to the north of the totality path, with a large Ne decrease lasting for 1 hour, from 19–20 UT. In the topside ionosphere, a delayed drop in Ne values occurred beginning 30 min–1 hour later, with electron density decreasing by 40–50 % (2.5–3 ×1011 m−3) below background levels, followed by a recovery to typical background values only after 22 UT. The electron temperature (Te) showed a faster response and recovery rate which closely mirrored the eclipse obscuration level, with a visible decrease in Te when the obscuration rate exceeded 0–30 % depending on altitude, with a faster response time at higher altitudes. A decrease in Te of the order of 40 % (850 K) was observed at altitudes from 325–400 km between 18.75–19.25 UT. The ion temperature (Ti), similar to Te, responded rapidly to the obscuration level as it exceeded 30 %, with up to 20 % or 225 K reduction in a narrow geographic area and at two distinct altitude regions: 200–225 km and 300–325 km between 19–20 UT.
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- RC1: 'Comment on egusphere-2025-5389', John Foster, 11 Jan 2026
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RC2: 'Comment on egusphere-2025-5389', Anonymous Referee #2, 21 Jan 2026
The manuscript presents the results of a comprehensive study of a number of ionospheric characteristics over the eastern part of the USA during the total solar eclipse of 8 August 2024. The authors conducted extensive investigations and identified distinctive features of ionospheric behavior over a large portion of the region. This was made possible through the employment of the incoherent scatter technique with wide-field scans. This technique is unique in that it simultaneously provides a suite of ionospheric parameters for specific altitudes and locations. The reported findings have strong potential to improve our understanding of the complex atmospheric and ionospheric processes occurring during solar eclipses and to enhance the predictive capability of numerous ionospheric models.
My main concerns are related to the reliability of the retrieved ionospheric characteristics, especially the electron and ion temperatures. My estimates indicate that the range to the scattering volume is about 1500 – 3500 km for altitudes of 200 – 400 km. Taking into account that the echo signal power is inversely proportional to the square of the range, I have serious concerns about the ability to retrieve these parameters with acceptable accuracy, despite the operation in uncoded, very long transmitted pulse mode. I fully realize that these concerns might be resolved by reading the tens of publications cited by the authors. However, the manuscript itself should provide sufficiently comprehensive methodological information, without requiring the reader to refer the external sources unless absolutely necessary. The description of the applied methods in the manuscript is currently insufficient and requires expansion in order to address potential questions from readers.
The major comments presented below are mostly based on the above concerns and call for stronger emphasis on the key factors necessary to improve the clarity of the reported results.
- The authors do not provide information on the errors (in percentage) associated with the electron density and plasma temperature measurements during the solar eclipse observations. In addition, it would be very useful if the authors could present representative range profiles for the pre-obscuration, obscuration, and post-obscuration intervals. If any measures were taken to reduce these errors, they should be briefly highlighted in the manuscript.
- The ion-acoustic power spectra method does not directly yield absolute electron density values unless calibration coefficient or external data for calibration are applied. It is therefore unclear which calibration methodology was used in this study. I strongly recommend that the authors provide electron density values obtained by independent techniques (e.g., ionosonde and/or Swarm data) for cross-validation.
- The authors provide Equation (1) illustrating the applied Tikhonov regularization. However, the manuscript does not specify the chosen values of the regularization coefficient α, nor does it justify the criteria used for selecting these values.
- Using the 2 ms uncoded transmitted pulse with a 6° elevation angle yields a horizontal resolution of about 280 km. During the motion of the Moon’s shadow, horizontal gradients in electron density and temperature can arise at spatial scales smaller than 300 km. How were such horizontal gradients accounted for in the retrieval of ionospheric characteristics?
- The Discussion section should be extended to address potential sources of distortion in ionospheric characteristics when the incoherent scatter technique is used. In particular, the negative effect of “space debris” at altitudes above 200 km cannot be completely excluded by filtering. Such contamination can introduce significant bias in the retrieval of electron and ion temperatures.
Below are several minor comments.
- The authors claim on the “observed features that challenge current understanding…”. In fact, all results can be physically justified and explained, what the authors did brilliantly in the Discussion section. I suggest to replace the phrases like “present a new challenge…” and similar ones by more emotionally neutral wording.
- Figure 2 does not provide substantial new information, because the specific values of the model errors and their dependence on latitude and longitude strongly depend on the particular composition of the training and testing data sets. I recommend excluding this figure from the manuscript, since the text already contains the necessary information.
- Line 206: the typo. It should be “figures 9 and 10”.
Citation: https://doi.org/10.5194/egusphere-2025-5389-RC2
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- 1
This manuscript addresses ionospheric observations and perturbations during the April 8, 2024 solar eclipse, as observed in the eastern to central United States using wide-field scans from the Millstone Hill incoherent scatter radar. Ionospheric changes begin immediately as obscuration > 0%. Max F1-region electron density reduction of 50% to the north of the totality path. Altitude dependence of the effects and detailed ion and electron temperatures measurements are described. The manuscript is very well written, clearly presenting and discussing extensive observations.
Following comments are referenced to manuscript line numbers
20 Eclipse effects on thermosphere-ionosphere-magnetosphere coupling processes were mentioned, but this topic was not pursued in any detain in the manuscript. There was the mention of geomagnetic activity, but only as it related to non-eclipse induced effects away from the path of totality.
52 The main goals of the study are (1) to accurately identify eclipse-induced variations in Ne, Te, Ti over central to eastern North America, and (2) to discuss observed features that challenge current understanding of ionosphere-thermosphere coupling processes. Goal (1) was extensively addressed in the text, but I did not find a clear statement of the unanswered questions arising from the observations.
59 the IS radar technique is the only available method which directly produces ionospheric plasma temperature as an incisive diagnostic. I strongly agree with this statement.
72 wide field 6◦ elevation scans (more clearly: wide-field azimuth scans at 6◦ elevation).
85-94 This section provides a concise summary of the complex procedure used to derive ionospheric parameters from the radar signal.
102-145 provides a very detailed description of the generation of the background (unperturbed) model used for comparison with the eclipse-day observations.
Section 3 Results: Figures 3 through 13 present extensive detail of the eclipsed-induced variation of ionospheric Ne, Te, and Ti as functions of Latitude, altitude and time deduced from the Millstone Hill scanning data. Parameter percent and absolute variation with respect to the background model are presented providing a very detailed picture of the ionospheric eclipse-induced effects over North America for this event.
271-274 The reviewer agrees that the impressive results presented validate this statement.
Discussion - overall the discussion section is very well written, making clear reference to the observations and extensive comparisons to previously published results.
310-324 I found the discussion of thermospheric winds and the wind convergence point interesting and informative