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SESK: Stuff Everyone Should Know · sesk.info/hame-d-region

Which ionospheric region exists in daytime, from about 40 to 90 km altitude?

The D region.

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  1. The F2 region
  2. The D region ✔
  3. The exosphere

Why: NOAA's glossary defines the D region as a daytime region of the ionosphere from about 40 km to 90 km altitude. Solar flare X-rays enhance it, which can block HF on the sunlit side.

Huh, didn't know that: The ionosphere's D, E and F layers form where solar radiation ionizes the thin upper atmosphere.

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Son: Dad can you tell me what a solar eclipse is? Dad: No sun.
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References

Answer: The D region. NOAA's glossary defines the D region as a daytime region of the ionosphere from about 40 km to 90 km altitude. Solar flare X-rays enhance it, which can block HF on the sunlit side.

  1. NOAA SWPC Space Weather Glossary www.spaceweather.gov/content/space-weather-glossary
    A daytime region of the Earth’s ionosphere
    Checked 2026-10-11.
  2. NOAA SWPC HF Radio Communications www.spaceweather.gov/impacts/hf-radio-communications
    create an enhancement of the D layer of the ionosphere
    Checked 2026-10-11.
  3. NOAA SWPC Ionosphere www.spaceweather.gov/phenomena/ionosphere
    called the D, E, and F-layers
    Checked 2026-10-11.

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