Ion
An atom or molecule with one or more electrons removed (positive ion)or attached (negative ion).

Ionisation
The process of removal or attachment of electrons to atoms or molecules to form positive or negative ions, respectively.

Ionogram
A picture or image of frequency versus the time delay (virtual height) of HF echoes from the ionosphere. Recorded by an ionosonde.

Ionosonde
A swept frequency HF pulsed radar used to monitor the ionosphere. Pulses are transmitted and the ionosonde records the time delay of the returning echoes. Vertical ionosondes normally sweep in frequency from about 1 to 20 MHz.

Ionosphere
That part of the Earth's atmosphere that is ionised by the Sun's radiation and forms into regions with differing chemical composition. Extends upwards from about 60 km. The free electrons in the ionosphere support radio wave propagation.