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Why are ionospheric electron densities highest near noon?

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Diagram of Earth's atmosphere layers-troposphere, stratosphere, mesosphere, and thermosphere-showing how temperature and air pressure change with increasing altitude, with key features like the ozone layer and pressure percentages at various heights. — Ionosphere and magnetosphere The layers of Earth's atmosphere, with a yellow line showing the air temperature at various heights.

Ionospheric electron densities are highest near noon because of the direct relationship between solar radiation and the production of ionization in the atmosphere.1

The ionosphere is created when solar radiation interacts with neutral atoms and molecules. In the early morning, the Sun is low in the sky, forcing radiation to penetrate a larger column of air, which results in lower ionization rates and higher altitudes for the ionized layers.21

As the Sun rises toward its highest point at noon, the solar elevation angle increases, reducing the atmospheric path length for incoming radiation. This leads to more efficient production of electrons and ions. Consequently, the ionized layers reach their lowest altitudes and electron densities peak at midday. Conversely, at night, the absence of solar radiation causes these electrons and ions to recombine into neutral gases, leading to a decrease in electron density.3415