The Ionosphere

The ionosphere is a layer of electrons and electrically charged atoms and molecules that surrounds the Earth, stretching from a height of about 50 km to more than 1000 km.   It is created by the Sun, primarily due to ultraviolet and X-Ray radiation from the Sun.  This radiation contains sufficient energy to dislodge electrons from gases in the atmosphere (ionization).   The density of these free electrons can greatly affect radio propagation and therefore the ability to receive radio waves over long distances. 

The ionosphere is comprised of layers that behave differently to the the various types of radiation received from the Sun, location relative to the Sun, altitude, time of day, day of year, sunspots and geomagnetic activity.

Ionospheric Layers

Ionospheric Layers

Sudden Ionospheric Disturbance (SID)


A sudden ionospheric disturbance (SID) is an abnormally high density of free electrons in the ionosphere caused by an occasional sudden solar flare, which often interrupts or interferes with telecommunications systems.


When a solar flare occurs on the Sun a blast of ultraviolet and x-ray radiation hits the dayside of the Earth after 8 minutes. This high energy radiation produces intense increase in the “D" layer ionization. The effects of this sudden increase in the "D" layer ionization on High Frequency (HF) radio waves is that they are absorbed by the increased particles in the low altitude ionosphere causing a complete black out of HF radio communications. This is called a Short Wave Fadeout. These fadeouts last for a few minutes to a few hours and are most severe in the equatorial regions where the Sun is most directly overhead. However, the ionospheric disturbance enhances Very Low Frequency (VLF) radio propagation.

SIDs can be observed and recorded by monitoring the signal strength of a distant VLF transmitter.

Sudden Ionospheric Disturbance (SID) Monitoring


NASA, Stanford University and the Society of Amateur Radio Astronomers (SARA) have undertaken a Worldwide effort to monitor and better understand Sudden Ionospheric Disturbances.  This project is conducted by schools and students around the World, with the resulting data transmitted to Stanford University for study and also forwarded to NASA for monitoring space weather.

Stanford University has partnered with the Society of Amateur Radio Astronomers (SARA) to build and distribute SuperSID monitoring devices.  These very low cost monitoring devices are distributed to teachers and students for investigating the affects of SID in their area of the World.  Grants are available for qualified educators to receive and install this equipment at little or no cost.  Contact SARA for more information regarding these programs.

SuperSID Monitoring Equipment
SuperSID Monitoring Equipment

For a complete lesson plan utilizing the SID monitoring program, click here.