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Critical frequency
Short answer
Critical frequency is the highest frequency a layer reflects at vertical incidence. It is fc = 9 sqrt(Nmax) Hz, with Nmax the maximum electron density per m^3. Frequencies above fc penetrate the layer at vertical incidence.
Define critical frequency for ionospheric region.
The critical frequency (fc) of an ionospheric layer is the highest frequency that is reflected back to earth by that layer when the wave is sent vertically upward (normal incidence). Any frequency above fc penetrates the layer and escapes at vertical incidence.
It depends on the maximum electron density Nmax of the layer:
fc = 9 √Nmax (fc in Hz, Nmax in electrons per m³)
This comes from the refractive index n = √(1 − 81N/f²); at vertical incidence the wave turns back where n = 0, i.e. f² = 81Nmax.
- Typical values: E layer ≈ 3–4 MHz, F2 layer ≈ 5–12 MHz.
- It is higher by day and in high sunspot years.
- For oblique incidence, the highest returnable frequency is MUF = fc sec θi.
Example: Nmax = 1.24 × 10¹² /m³ gives fc = 9 × √(1.24 × 10¹²) ≈ 10 MHz.
✓ Answer checked by Claude Opus 5.5 (high effort)
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