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Updated March 14, 2011
Radio Wave Propagation
Some radio waves can be reflected by objects in their path, even by gases in a certain state.
Lower energy or lower frequency radio waves can be heard at great distances because they reflect off the earth's ionosphere. AM radio waves for example can be heard from great distances across the North American continent. Amateur radio operators use the ionosphere to reflect their shortwave radio signals around the globe.
Not so with higher frequency radio waves such as FM and TV transmissions. These can only be received by your radio or TV when they are in a line of sight with your receiver. They do not reflect off the ionosphere. But they do reflect off gas ionized by meteors entering the earth's atmosphere. And we can detect these transmissions and do such things as count the number of meteors as a result.
What Happens When Meteors Burn in the Atmosphere?
Tens of thousands, some believe billions, of meteoroids hit the earth's atmosphere each day. When they do they are called meteors. They either heat and burn up from friction or they bounce back out into space like a flat stone thrown across water. The ones that enter and burn in earth's atmosphere are sometimes called shooting stars.
A meteoroid travels through space at tens of thousands of kilometers per hour unrestricted in its movement since it is traveling in a vacuum. When the meteor hits the atmosphere, the air in front of it compresses quickly. When a gas is compressed its temperature rises. Meteors entering the atmosphere compress the air to such an extent it becomes superheated. The superheated atmosphere then causes the meteor to become hot by proximity. This is what causes meteors to reach temperatures in excess of 1500 degrees C.
Meteors come in different sizes from dust grains, which burn up completely, to large boulder sized meteors which also burn on entry but are large enough that they don't completely burn up. These hit the ground or land in water and are known as meteorites.
Meteors enter the atmosphere at velocities that range from 11 kilometers per second to 72 kilometers per second. It is friction between the meteor and air molecules that cause the meteor to heat up and give off light. About 10% of the energy of motion in the meteor is converted to heat and light this way. The remaining energy is consumed by stripping electrons from atoms in the air leaving ionized gas in its trail.
The Ionization Trail
There are two types of meteor trail referred to as under-dense and over-dense. Some meteors have enough energy to produce an ionization trail with a high density of free electrons so that radio waves cannot enter the trail and are reflected by it. This plasma acts like a mirror to radio waves and is referred to as an over-dense trail. If you are in the right place, you can receive reflected radio waves from these trails on your receiver from great distances away.
Not all meteors have enough mass or speed to completely ionize a trail capable of reflecting radio waves. These under-dense portions of the trail allow radio waves to enter the trail. The waves are then reflected randomly by the free electrons rather than as if from a smooth surface. The received signal from reflection from an under-dense trail is weaker because the signal power is radiated in many directions.
Reflection from the trail follows the rules of optics. A flashlight shone on a wall mirror will reflect onto a predictable location on an opposite wall. The pattern and brightness on the wall will depend on how smooth the mirror is and may be distorted due to irregularities in the mirror surface. So too is the case with the reflection of radio waves from a meteor ionization trail.
Ionization trails from meteors occur between 50 and 150 km above the earth's surface. The earth curves away from the line of sight at a rate of x km per km of distance along the earth's surface.
A description of the math involved in calculating many of the aspects of radio meteor physics can be found at this American Meteor Society web address http://www.amsmeteors.org/radio/scatter_notes.txt.