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 (N_e) 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 N_e decrease lasting for 1 hour, from 19–20 UT. In the topside ionosphere, a delayed drop in N_e 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 (T_e) showed a faster response and recovery rate which closely mirrored the eclipse obscuration level, with a visible decrease in T_e when the obscuration rate exceeded 0–30 % depending on altitude, with a faster response time at higher altitudes. A decrease in T_e of the order of 40 % (850 K) was observed at altitudes from 325–400 km between 18.75–19.25 UT. The ion temperature (T_i), similar to T_e, 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.