Absorption Limiting Frequency
This is a guide to the lowest frequency for reliable radio communications by the ionosphere. The ALF is significant only on circuits with daylight sectors.

Active
When referring to the Sun, the term means "changing". Solar activity is the changing appearance of the Sun.

Active Region
A region on the Sun which is active. Usually an active region incorporates sunspots, plage and filaments. Active regions contain strong magnetic fields. Flares occur within active regions.

A Index
Linear index for measuring the disturbance level in the Earth's magnetic field. The index is defined over a period of one day. An A index can be defined for any location on Earth and also for the entire globe. A very useful planetary index is the Ap index. Levels of A index are described as follows: A < 8 - "quiet"; 8 <= A <= 15 - "unsettled"; 16 <= A <= 24 - "active"; 25 <= A <= 35 - "minor storm"; A >= 36 upwards - "major storm".

ALF
See Absorption Limiting Frequency.

Ap Index
The planetary index for measuring the strength of a disturbance in the Earth's magnetic field. The index is defined over a period of one day from a set of standard stations around the world.

Atom
See Molecule.

Attachment
The collision of an electron with a neutral molecule or an atom which causes the formation of a negative ion. Later the negative charges disappear due to recombination between positive and negative ions. Attachment depends on the density of the oxygen atoms, the greater this density, the faster the ionisation will disappear.

Aurora
Excitation of particles from the Sun spiralling in the geomagnetic field near the poles resulting in the release of energy in different forms, including light.

Auroral Oval
An annular ring around each geomagnetic pole where aurora are most likely to occur.