The Ionosphere

The Earth's atmosphere knows no true boundary. Even at the orbit of the Space Shuttle, there is still quite a lot of air there. Satellites can easily detect our atmosphere out to 10,000 kilometers from the Earth's surface. The outer atmosphere actually occupies the same region of space as the plasmasphere, the ring current and the van Allen Belts themselves.

The Earth's atmosphere is far more complex than a simple onion-layered picture would suggest. These layers exchange gas and energy all the way out into the depths of the invisible magnetosphere, and deep into the atmosphere. One of the most interesting of these atmospheric layers is the ionosphere: A layer of charged particles surrounding the Earth at an altitude of about 100 kilometers.

The ionosphere has been the workhorse of communication technology for most of the 20th century, until satellite communications offered another means of relaying radio messages from place to place across the globe. No sooner had Marconi invented the wireless radio in 1909, but scientists used this to prove that there must be a charged layer of gases several hundred miles above the Earth's surface. By 1931, it was also discovered that the charged layer could be upset by solar flares, causing radio blackouts across the globe.

Just as a mirror reflects light, a cloud of charged particles can reflect longer-wavelength radio waves. The denser the cloud, the higher the frequency of the wave that can be reflected. In the ionosphere there are typically 100 charged particles per cubic inch, so this means that radio frequencies in the AM radio band are easily reflected, but it also means that the much higher frequency FM signals pass through it very easily. TV signals are at such high frequencies that the ionosphere is completely transparent. As a system, the ionosphere is electrically connected to the ground through the tops of thunder storm clouds, and it is connected to the rest of the magnetosphere through the magnetic lines of force and currents that flow along them.

The ionosphere is a lumpy, cloudy layer, and radio signals actually 'twinkle' like stars because of the changing transparency and location of these cloudlets, in a phenomenon called radio scintillation. Meteors that disintegrate near these layers also add their charged gases to this layer. Radio amateurs and the military alike use these meteor trails to reflect radio signals at very high frequencies; much higher than what the ionosphere can naturally reflect.

Also in this layer, or near its base, we can occasionally see very high altitude clouds forming, which can be seen well after sunset. They are called noctilucent clouds and scientists still don't know how they form or why. Their appearance seems to have something to do with the level of solar activity.


Teacher Notes:

How is our atmosphere most effected by space weather events?

A system is a collection of things that have some influence on one another. Any part of a system may be considered a system. This is referred to as a sub-system, it has its own interactions.

(K-2) Most things are made of parts.

(3-5) In something that consists of many parts, the parts usually influence one another.

(6-8) Thinking about things as systems means looking for how every part relates to others. The output from one part of a system (which include material, energy or information) can become the input to other parts. Such feedback can serve to control what goes on in the system as a whole.

(9-12) Understanding how things work and designing solutions to problems of almost any kind can be facilitated by systems analysis. In defining a system, it is important to specify its boundaries and subsystems, indicate its relation to other systems, and identify what its input and its output are expected to be.


Science Nuggets:

2001...TIMED explores the ionosphere and provides a global view of its properties and interactions with the other atmospheric layers.

2000...IMAGE detects plumes of gas leaving the Earth's polar regions. The plumes come from a very sharp boundary over the south pole auroral zone where high speed oxygen atoms are ejected from the ionosphere.

1996...Polar detects plumes of ions at the north and south poles which eject over 50 tons of oxygen per day at speeds up to 100,000 kilometers per hour.