The ionosphere

What it is

The ionosphere is a region of the upper atmosphere, starting roughly 50 km up and extending several hundred, where the air is thin and sunlight has knocked electrons off the gas atoms. The free electrons and charged atoms that result can bend radio waves back toward Earth. This is what makes worldwide HF communication possible.

Layers, from the ground up

The layers are regions where the density of free electrons peaks, and each behaves differently. Heights are typical and vary:

Layer	Typical height	Daytime	Night	What it does to radio
D	about 50 to 90 km	present	disappears	Absorbs HF, most strongly the lowest frequencies
E	about 90 to 150 km	present	much weaker	Bends lower HF; hops of up to roughly 1,200 miles
F1	about 150 to 220 km	present (summer especially)	merges into F2	Adds ionization by day
F2	above about 220 km	strongest	weaker, one F layer	The main layer for long-distance HF

The higher the layer, the farther a signal returns to Earth: see Skywave, hops and the skip zone.

Why it bends HF but passes VHF

A radio wave pushes the free electrons back and forth, and the electrons re-radiate. In a region with enough of them the wave's path curves back toward the ground. Lower frequencies bend more easily; as the frequency rises, the wave needs more and more electron density to turn, until at some point it simply passes through. HF is turned back; VHF and UHF usually escape to space (which is why they work to satellites). The highest frequency that still comes back is the subject of MUF, LUF and the best frequency.

Where the air is thicker, as in the D layer, electrons that are set moving collide with atoms and lose the energy as heat. The wave is not bent back so much as absorbed, and lower frequencies suffer most.

Day, night and the Sun

Ionization needs sunlight, so it follows the Sun. After sunset the D layer fades quickly because its dense air lets electrons and ions recombine fast, and the E layer weakens. The F layer lives on, because the air up there is so thin that electrons rarely meet an ion. That produces the familiar rhythm:

• Low bands (160 m and 80 m, and 40 m to a lesser degree): absorbed by the D layer by day, so mostly short range; they open for long distances at night.
• High bands (15 m, 12 m, 10 m): need strong F-layer ionization, so they work best by day and when the Sun is active; see The solar cycle and propagation indices.

Ionization also varies with season, latitude and the solar cycle, and bursts of solar activity can disrupt it; see Space weather.

Related

• Skywave, hops and the skip zoneHow HF signals return to Earth hundreds or thousands of miles away.
• MUF, LUF and the best frequencyThe highest and lowest usable frequencies for a path.
• The solar cycle and propagation indicesSunspots, solar flux, and the A and K indices.
• Space weatherFlares, CMEs and storms, and how they help or wreck HF.
• Grey line and long pathDawn-and-dusk enhancement, and signals that go the long way around.
• Sporadic EShort-lived patches of ionization that open VHF over long distances.