Incoherent Scatter Radar Observations of the F Region of the Ionosphere above the Eastern US during the April 2024 Solar Eclipse

We present dynamic mid-latitude ionospheric changes associated with the 8 April 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 began as early as when the penumbra shadow makes contact with the scanning region or when the eclipse obscuration percentage exceeds 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 × 10 ¹¹  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 h, from 19:00–20:00 UT. In the topside ionosphere, a delayed drop in N _e values occurred beginning 30 min–1 h later, with electron density decreasing by 40 %–50 % (2.5–3 × 10 ¹¹  m ⁻³ ) below background levels, followed by a recovery to typical background values only after 22:00 UT. The electron temperature ( T _e ) showed a faster response time 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 the altitude, with a faster response time with increasing altitude below 300 km, but an approximately flat response time above 300 km altitude. 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 and 300–325 km between 19:00–20:00 UT.