The Ionosphere
The ionosphere is created by solar radiation, mostly in the extreme ultraviolet spectrum. Molecules of oxygen, nitrogen and other gases are ionized by losing electrons, knocked out of orbit by energetic solar radiation. While we speak of the ionosphere, it is not the ions that permit long-distance high-frequency communication, but the free electrons.
An electromagnetic wave impinging on a free electron raises its energy level. Because electrons like to be at a certain energy level, they re-radiate the wave. If the layer of free electrons is of sufficient density, the wave is bent away from the area of greater density. If enough bending occurs the wave is directed back to Earth. Fortunately, earth's surface and water will reflect electromagnetic waves.
Under the right conditions, it's possible to bend the wave several times in the ionosphere, while also reflecting it several times off the earth, causing it to end up tens of thousands of miles from the transmitter. The wave loses energy at each hop, so we'd like to get it to the destination with as few hops as possible.
Ionospheric layers. Solar radiation comes from upper right. As the earth turns away from the sun, free electrons in the D region and E layer recombine, and the F1 and F2 layers merge into one layer at lower height. (Not to scale.)
Parts Of The Ionosphere
We believe the ionosphere has three distinct layers or regions. Nearest earth's surface is the D region, extending from 40 to 60 miles. Above the D region is the E layer at from 60 to 75 miles. At the top we find the F layer, its daytime height ranging from 200 miles in Winter to 300 miles in Summer. We refer to one area as a region and the others as layers because of their extent. While the E and F layers are thinner than the D region, they do not start and stop abruptly. Free-electron density is thinner at their upper and lower edges. Their heights and thicknesses depend on the intensity of solar radiation and the angle at which solar radiation strikes them. These characteristics vary, depending on time of day, from season to season, and throughout the sunspot cycle.
The F Layer
The F layer is of most interest to HF operators, because it does most of the work in propagating our signals. The F layer divides in two during daylight hours, creating the F1 and F2 layers. At night the layers recombine into one, and maximum height drops to about 250 miles. We'll come back to the F layer after we look at the rest of the ionosphere.
The E Layer
The E layer is the most-enigmatic of all. At 60 to 75 miles, it sits between the popular F layer and the D region. The E layer may be ionized by solar radiation, and also terrestrial phenomena. Sometimes the solar-created E layer has enough free electrons to refract HF signals. It may also serve as the bottom of a duct, in which 160-M signals are propagated in chords across the ionosophere, the upper edge of the duct being formed by the F layer. Existence of this type of propagation is hard to prove, but the theory matches observations.
Another type of propagation using the E layer is called sporadic E, abbreviated Es. Es causes extemely high ionization, capable of refracting signals into the vHF range. On HF it's most-often used to make contacts out to 1500 or more miles on 10 M. Longer contacts, caused by signal propagation by multiple Es hops, are also possible but less common. The ionizing mechanism is thought to be high-altitude wind shear, so Es openings can occur at any time of day, regardless of solar activity. Good Es openings often accompany large thunderstorm and tornado outbreaks, but smaller openings are frequent, especially in the spring and fall.
The D Region
Though closest to earth ( to miles), the D region was the last to be discovered and named. As far as HF propagation is concerned, there isn't much good to say about the D region. Because it's closer to earth, free-electron is high, and radio waves have a tough time penetrating it. That's why our lower frequency bands, 160 through 40 M, are useless for all but short-range work during the daytime. Fortunately for us, the D region recombines quickly after sunset.
Sunspots
We've been watching sunspots for centuries (Galileo drew pictures of them in 1613), but their effect on our ionosphere -- in fact, the very existence of the ionosphere -- had to await the invention of radio. Sunspots are cool areas of high magnetic intensity on the sun's surface. The number of sunspots varies over an 11-year period, but the latitude at which they appear changes every 22 years. Still, we refer to an 11-year sunspot cycle. When there are many sunspots, there is also very high ionization of our ionosphere. Few or no sunspots doesn't stop some ionization from occurring, but the amount is much less than during peak sunspot years.
While we can predict within a year or two when a new sunspot cycle will begin or end, we can't accurately predict how many spots will appear at its peak. There have been two high-peak years in my lifetime, 1957, which I missed, and 1989, which, while not as productive as 1957, was still a very good peak. QRP was gaining in popularity then, and that cycle no doubt helped increase interest. As I write this in April 2008, we are just beginning a new cycle. There is much speculation about whether it will be good or indifferent, but however it turns out, it has to be better than right now!
Solar radiation at UV wavelengths. (NASA)
A convenient way of judging the effect of solar radiation on the ionosphere is to count sunspots. Because the effects are not instantaneous, the most-accurate way to use sunspot numbers is to average them over a long period. Usually that is 13 months, and that's called the R12 Smoothed Sunspot Number. Another prediction method involves measuring the strength of solar radiation at a wavelength of 10.7 cm (2800 MHz). This frequency has nothing to do with ionization, but was chosen because it penetrates the ionosphere with predictable levels of absorption. Called the Solar Flux, this measurement can be taken accurately at any point in a sunspot cycle.
Propagation-prediction programs, about which more later, require you enter either a smoothed sunspot number or solar flux. We use the smoothed sunspot number because it consistenly provides more accurate predictions.
The Angry Sun
Every morning the sun rises right on time. If our weather cooperates, the sun warms us and lights our way. We tend to take the sun for granted, and think its energy output never varies. In the visible-light spectrum the sun's output is constant; for HF radio users, the sun can be a boon or a curse! We need solar radiation at extreme ultraviolet wavelengths to replenish the ionosphere. It's the other stuff that sometimes comes with it that causes problems.
Solar Flare (NASA)
The sun is constantly throwing off charged particles, which make up the solar wind. These particles press against earth's magnetic field. Occasionally, a massive discharge of particles from the sun roars into our magnetic field, wreaking havoc on HF communications. There are two mechanisms at work here. To some extent, the D region on earth's sunlit side receives more radiation, increasing its absorption. More serious is the distortion caused to earth's magnetic field. Free electroncs in the ionosphere aren't completely free. Depending on the direction they are traveling when knocked free, they may circle a magnetic field line, or travel along it in a spiral path. As long as the magnetic field remains relatively still, all is well for us. When the magnetic field is pushed and nearly torn apart, the ionosphere is no longer an area of smoothly varying electron density.
Aurora Borealis (Andreassen/NASA)
The effects of this blackout occur first on the daylight side. If the disturbance is large enough, it will travel around with the planet, until all parts of the world are in blackout. Poor conditions may last only a few hours, or they may persist for several days. Thanks to obervatory satellites in high orbit, because the charged particles that cause blackouts travel at less-than-light speed, we can predict blackouts, but we can't prevent them. Veteran 160-M DXers sometimes observe enhanced propagation just prior to a blackout, and the increase in highly-charged particles in our polar regions create beautiful aurorae. Otherwise, they are disheartening, and have probably led thousands of hams to wonder what happened to their receivers or antennas all of a sudden.
There are two indices in common use to describe the state of earth's magnetic field (properly called the geomagnetic field. The K index is taken every three hours, and the A index is calculated by averaging K indices for the previous 24 hours. Measurements are taken with sensitive magnetometers, and we'll look at them more closely when we discuss propagation-prediction programs.