Enhancement of Electron Density in the Ionospheric F2 Layer Near the First Contact of the Total Solar Eclipse on 21 August 2017

It can be expected that a moon shadow of solar eclipse can largely reduce the ionospheric electron density (Ne) during the obscuration. However, in this study, the total electron content (TEC) from the ground‐based Global Navigation Satellite System receivers recorded a significant Ne enhancement over the continental United States (CONUS) near the first contact of the total solar eclipse on 21 August 2017. The ionosonde observations of the F2‐layer peak density (NmF2) and its height (hmF2) over Idaho National Lab (43.81°N, 112.68°W, 99.6% obscuration), ID, and Boulder (40°N, 105.3°W, 93.1% obscuration), CO, verified the unexpected early enhancement. The ionospheric Ne starts to enhance at ∼1520 UT (before the obscuration) and increases by ∼25% to ∼80% at ∼1600 UT (near the first contact) in the F2 layer. The early enhancement reveals the ascension of the F2 layer that tilted east over the CONUS. Plain Language Summary The moon shadow of a solar eclipse can largely reduce the atmospheric temperature and the ionospheric electron density as the umbra and penumbra cones are perpendicular to the Earth's surface. However, it is surprising to see that the ionospheric electron density increases significantly over the continental United States near the first contact of the total solar eclipse on 21 August 2017. The ground‐based Global Navigation Satellite System receivers and ionosondes comprehensively observed the pre‐ascension of the ionospheric F2 layer that could be attributed to the upward motion of plasma at midlatitude. Key Points An unexpected Ne enhancement occurred near the first contact of the total solar eclipse on 21 August 2017 by comparing the data on 20th August The total electron content and NmF2 increase by ∼10% and ∼25%–80%, respectively, near the first contact over the north and central area of CONUS The early enhancement occurred mainly in the F2 layer