Main Menu
Star Resources
With A
6: Overview
Main Menu > 6: Space Weather > Sun-Earth Connection > Ionosphere

Noctilucent Clouds
Nightime noctilucent clouds.

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.

A Complex System
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 Earth's Ionosphere. (Click for a larger image.)
© UC Regents

Relaying Communications
No sooner had Marconi invented the wireless radio in 1909, than scientists used this to prove that there must be a charged layer of gases several hundred miles above the Earth's surface. This was the ionosphere. By 1931, it was also discovered that the charged layer could be upset by solar flares, causing radio blackouts across the globe. The ionosphere was the workhorse of communication technology for most of the 20th century, until satellite communications offered another means of relaying radio signals from place to place 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 5 charged particles per cubic centimeter, 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.

Noctilucent Clouds
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.

globe icon Find out more about the Sun-Earth Connection at the Sun-Earth Connection Education Forum Web site.

Text adapted from the Sun-Earth Connection Tutorial courtesy of NASA, originally written by Dr. Sten Odenwald. Images and videos courtesy of NASA unless otherwised noted.

Related to chapter 6 in the print guide.
Related Materials

Visit other pages in this section that focus on aspects of the Sun-Earth Connection.

The Solar Interior
Solar Cycle
Solar Flares
The Solar Wind
Plasma Clouds
Glossary Terms

Click for the definitions of the following words that are used on this page: (Definitions appear in a pop-up window.)

coronal mass ejection
geomagnetic field
geomagnetic storm
radiation belts
solar cycle
solar maximum
solar minimum
solar wind

View the full, printable version of the glossary.

Top of Page
Last Page Next Page
Main Menu | Resources | 6: Overview

©2002 UC Regents