Unusual Propagation Modes
Other effects can generate regions of ionization to bounce signals off. The visible aurora is certainly one of these, and the picture (above, left) shows the electron density during an auroral event. While this causes high losses in high latitude HF paths (for instance the dotted blue path between Washington state and northern Europe in the picture above) it does provide an opportunity for VHF signals to be bounced off the ionized region to provide an opportunity for contacts at widely separated distances. The propagation is quite "fluttery" at times, and SSB may not be undestandable, where CW might.
Another place CW (especially high speed bursts) shines is in bouncing signals off of the short lived ionized trails of meteorites as they enter the earth's atmosphere. During meteor showers thoushands of such events take place and VHF/UHF stations can communicate by pointing their antennas at the common region of the sky the ionized trails are in.
In many cases it is possible for VHF stations that do not have direct line-of-site to carry on communications by what is called knife-edge refraction. Refraction in radio waves is the same as in optics, where a small amount of the incoming wave is scattered over a (relatively) sharp obstacle. This, in fact, is one of the methods that supplies cellular phones with coverage in heavily developed urban areas. At lower frequencies the obstacles can be natural features such as mountain ridges.
Transequatorial scatter is a VHF phenomenon typically observed between 28-144 MHz and is associated with the earth's geomagnetic equator. Ionization is more concentrated near the equator due to the larger daytime exposures and this supports propagation at frequencies in excess of what would normally be possible late in the day. Stations too close to the GM equator will have incident angles that are still too high for refraction, so there tends to be a region where no propagation is supported, and the path limit seems to be about 2500 miles on either side of the GM equator. While this makes for great opportunities for contacts between southern Europe and Africa, only the furthest southern extent of the United States is involved in this mode, leaving most of the fun to Mexico and the Caribbean.
As mentioned earlier, it is also possible to deflect radio waves in the troposhere, especially in the VHF through microwave frequency regions. Tropospheric scatter is a high power mode where small amounts of power are reflected from dust particles or water droplets to a sensitive receiver at the destination. Both stations aim high gain antennas at a common point in the sky and depend on the probability that eventually a signal path will exist for some period of time. An aknowledgement/resend approach can ensure that traffic will get though. Signal distortion tends to be high, due to the relative motion of the reflecting media with respect to both stations, so data rates can be low.
The variability of the atmosphere density in the region can also lead to a gentle bending of a wave so that it follows along the gradient providing low-loss signal propagation for hundreds of miles at frequencies up to 432 MHz. This mode can produce contacts between relatively low power stations and is often the bane of repeater operators along the coasts, where the weather patterns are often conducive to the formation of these ducts, which can also be formed by hurricanes or large cold fronts. The effect is typically more pronounced on 2 meters than on 6 meters and a single mobile transmitter might activate a half dozen repeaters. Such ducts can provide reliable communications between Long Island and Maryland for hours at a time and both FM and SSB 2 meter simplex operators will watch their weather maps for clues that lead to duct formation.
