The F region is the portion of the ionosphere existing between approximately 160 and 400 km above the surface of the Earth, consisting of layers of increased free-electron density caused by the ionizing effect of solar radiation.

Note 1: The F region reflects normal-incident frequencies at or below the critical frequency (approximately 10 MHz) and partially absorbs waves of higher frequency.

Note 2: The F₁ layer exists from about 160 to 250 km above the surface of the Earth and only during daylight hours. Though fairly regular in its characteristics, it is not observable everywhere or on all days. The principal reflecting layer during the summer for paths of 2,000 to 3,500 km is the F₁ layer. The F₁ layer has approximately 5 × 10⁵ e/cm³ (free electrons per cubic centimeter) at noontime and minimum sunspot activity, and increases to roughly 2 × 10⁶ e/cm³ during maximum sunspot activity. The density falls off to below 10⁴ e/cm³ at night.

Note 3: The F₁ layer merges into the F₂ layer at night.

Note 4: The F₂ layer exists from about 250 to 400 km above the surface of the Earth. The F₂ layer is the principal reflecting layer for HF communications during both day and night. The horizon-limited distance for one-hop F₂ propagation is usually around 4,000 km. The F₂ layer has about 10⁶ e/cm³. However, variations are usually large, irregular, and particularly pronounced during magnetic storms.

Source: from Federal Standard 1037C