| 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.
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