Ionosphere Layers: D, E, F1 and F2 Explained for HF Operators

Every HF contact beyond the horizon depends on the same thing: a region high above the Earth where the air is ionized by the Sun, so that radio waves can be bent back down. That region is the ionosphere, and it is not a single mirror but a stack of ionosphere layers with different heights, different densities and very different behavior. One layer swallows your signal in daytime, another throws it halfway round the world, a third appears at random on a summer afternoon and opens 6 meters. This guide explains what each of the D, E, F1 and F2 layers is, how high it is, what makes it appear and disappear, and what it means for the band that you operate on. The numbers come from the published scientific description of the ionosphere, and we use them to explain what you hear on the air.

What Makes a Layer

The Sun emits ultraviolet light and X-rays, which strike the thin air of the upper atmosphere and knock electrons loose from the atoms and molecules. The result is a mixture of free electrons and ions called a plasma. Radio waves that enter it make the free electrons vibrate. At lower altitudes, the air is dense and the electrons collide with it often, so the wave loses energy; that is absorption. Higher up, the air is thin, collisions are rare and the electrons re-radiate the energy, which bends the wave away from regions of high electron density and, if the conditions are right, returns it to the ground. As the Wikipedia article on the ionosphere puts it, both the angle of incidence and the frequency determine how much the wave is refracted, and refraction decreases as the frequency goes up.

The layers are bands of different electron density at different heights. Each is formed by a different part of the solar spectrum acting on a different gas, which is why their heights and their reaction to day and night are so different. At night the sources of ionization disappear, the electrons and ions recombine, and some layers fade away almost completely. Our guide to what decides whether you are heard on HF gives the operating view; this article gives the physical picture behind it.

The D Layer: Absorber of the Low Bands

The D layer is the lowest, about 60 to 90 km above the surface, with an electron density of only 100 to 10,000 per cubic centimeter. The source of ionization is mainly Lyman-alpha hydrogen radiation at a wavelength of 121.6 nanometers acting on nitric oxide, and solar flares can add hard X-rays that ionize nitrogen and oxygen. Because the air at that height is still fairly dense, electrons collide often, and a radio wave that passes through is attenuated. The absorption is much stronger at low frequencies, and the source describes it as varying with the inverse square of the frequency.

That is the reason for something every low-band operator knows from experience. In daytime, the D layer is fully formed and absorbs signals on 160, 80 and, to a lesser degree, 40 meters, so these bands are used mainly for local contacts. After sunset, the D layer disappears, and the signals can reach the higher layers, so the same bands open for long-distance contacts. Higher frequencies pass through the D layer with much less loss, which is why 20 meters is a daytime DX band.

The D layer also explains one kind of sudden trouble. When a solar flare occurs, X-rays arrive at the speed of light, ionize the D layer more strongly in the whole sunlit hemisphere within minutes, and absorb HF signals on the sunlit side, often for a short time. This is the HF radio blackout described in our article on solar flares, the R-scale and D-RAP. At high and polar latitudes, solar proton events can raise ionization in the D region to unusual levels, producing polar cap absorption, which can last for days, though it is less frequent than auroral events.

The E Layer: A Daytime Mirror for Shorter Hops

The E layer sits in the middle, from about 105 to 160 km, with an electron density of several times 100,000 per cubic centimeter. Ultraviolet radiation at 80 to 103 nanometers ionizes molecular oxygen, and flare X-rays add to it. The ionization is lowest just before sunrise, increases through the morning and peaks at noon. It is also called the Kennelly-Heaviside layer after Arthur Kennelly and Oliver Heaviside, who predicted its existence in 1902 independently and almost simultaneously, and its existence was detected in 1924 by Edward Appleton and Miles Barnett.

For amateurs, the normal E layer gives short-range daytime propagation. Signals at medium frequencies and the lower HF bands can be reflected by it, but the geometry limits the distance of a single hop, and the absorption in the D layer below it is a problem on the lowest bands. At night the layer fades. The more interesting E-layer phenomenon is not the regular layer but a variation of it.

Sporadic E: The Surprise on 10, 6 and 2 Meters

Sporadic E, written Es, is a thin patch with a much higher electron density than the normal E layer. The source says it can reflect radio waves up to 100 MHz, and events can last from a few minutes to many hours. The patches can be up to 2 km thick and hundreds of kilometers long, and they are common in the daytime in the equatorial region, while at mid-latitudes they occur more often in summer daytime and may be associated with metallic ion debris from meteors. For radio amateurs, sporadic E makes VHF operating exciting, because paths that are normally out of reach suddenly open for two-way communication.

The numbers for a single-hop path are large: the skip distance is generally around 1,640 km, and a single hop can cover anywhere from 900 to 2,500 km. Multi-hop paths over 3,500 km are common and sometimes reach 15,000 km or more. This is why summer is the season of 10 meter and 6 meter openings in Europe. We cover the operating techniques in our article on sporadic E and gray-line propagation.

The F Layers: Where Long-Distance HF Lives

The F region is the highest and the most important for DX. In daytime, it separates into two parts. The F1 layer lies from about 160 to 180 km, with an electron density of several times 100,000 to a million per cubic centimeter, and it is formed by extreme ultraviolet radiation ionizing atomic oxygen. It is a weaker feature than the F2 layer, and it merges into the F2 layer at night, and also in winter at solar maximum. The F2 layer extends from about 200 km to over 800 km, with its peak electron density at around 300 km in daytime, several million per cubic centimeter. Above about 700 km, ionized hydrogen is more abundant than ionized oxygen.

The F2 layer is the one that does most of the work, and it persists both day and night. It is the main region responsible for the refraction and reflection of radio waves in the HF range. Because its density is highest, it can bend the highest frequencies, and signals of 14 to 28 MHz and sometimes higher return from it. The ordinary F2 layer is why a signal from Japan can reach Lithuania by a path of more than 7,000 km with two or three hops. A small part of the layer’s behavior is described in our article on MUF maps and foF2, where you can see the highest frequency that the layer reflects at a given time.

How the Day Reshapes the Stack

The behavior of the layers through a day explains most of the standard advice about bands. At night, the F layer is the only layer of significant ionization, and the E and D layers are extremely weak. During the day, the D and E layers become much more heavily ionized, and the F layer develops its additional, weaker F1 region. Appleton showed in the 1920s that solar radiation generates the most ions around noon, which pushes the effective height of the F layer lower, while the low production at midnight lets it rise. This fits the usual operating pattern: the higher bands open for DX in the daytime, when the F2 density is highest and the paths are short, and the lower bands open at night, when absorption has gone.

The year matters as well. The F2 layer shows seasonal changes and anomalies in its daily variation, and at mid-latitudes it does not disappear at night. The sunspot cycle matters the most: the maxima of the E, F1 and F2 layers depend on the number of sunspots, and the 11-year variation in solar activity, measured by the number of sunspots, the rate of flares or the 10 cm radio flux, changes the intensity of the ionizing radiation. In years near solar maximum the F2 layer often supports the 10 meter band for much of the day; in years near minimum it may not. Our article on solar flux, the A index and the K index explains the numbers that are used to follow these changes.

Putting It to Use

You can use the layers to make better decisions about which band to choose. If you want to work a distant station in daytime, choose a band high enough to pass the D layer without much loss and low enough to be reflected by the F2 layer; 20 meters is the classic choice, and 15 and 10 meters work when solar activity is high. If you want to work a close station on a low band in daytime, remember that the D layer will reduce your range, but a steep-angle antenna such as a low dipole can still serve for regional contacts. If you want to use the low bands for DX, wait for darkness along the whole path, since the D layer must have disappeared at both ends.

Finally, use data rather than guesses. Beacons, reverse beacon data, ionosonde maps and the K index all tell you what the layers are doing now, and a few minutes of listening usually tells you more than a forecast. The NOAA Space Weather Prediction Center publishes the current solar and geomagnetic data, and the Wikipedia article on skywave propagation gives the general background.

The Bottom Line

The ionosphere is a stack of layers formed by the Sun at different heights. The D layer, 60 to 90 km up, absorbs low-frequency signals in daytime and disappears at night. The E layer, from about 105 to 160 km, reflects at shorter range and has sporadic patches that can reflect signals as high as 100 MHz. The F1 layer is a weaker daytime feature, and the F2 layer, from 200 km upward, persists day and night and carries most long-distance HF communication. Knowing which layer matters for your band at a given hour helps you to choose a band, a time and an antenna.

Frequently Asked Questions

Which ionosphere layer is the most important for HF DX?

The F2 layer. It extends from about 200 km upward, persists day and night and is the main region responsible for refracting and reflecting HF signals.

Why do 80 and 160 meters work better at night?

Because the D layer, which absorbs low-frequency signals, disappears at night. Signals then reach the higher layers with much less loss.

What is sporadic E?

A thin patch of unusually high electron density in the E region. It can reflect signals up to about 100 MHz and opens long paths on 10 meters, 6 meters and sometimes 2 meters.

How high are the layers?

The D layer is about 60 to 90 km high, the E layer about 105 to 160 km, the F1 layer about 160 to 180 km and the F2 layer from about 200 km to over 800 km.

What happens to the layers at night?

The D layer disappears, the E layer fades, and the F1 layer merges into the F2 layer. The F2 layer remains, though with lower density.

How does the sunspot cycle change the layers?

More solar activity means more ionizing radiation, so the layers become denser. Near solar maximum the F2 layer can support the higher HF bands for much of the day.