PCA
See Polar Cap Absorption.

Photoionisation
The production of positive ions and free electrons by the action of energetic radiation (e.g., EUV and X rays) on atoms and molecules.

Photon
Light behaves as a wave in some circumstances and as an energetic particle in others. A particle of light is called a photon.

Photosphere
The surface of the Sun that we see. It lies beneath the corona and the chromosphere. Sunspots are visible on the photosphere.

Plage
Bright areas in the chromosphere overlying sunspots. The source of EUV radiation.

Plasma
A gas in which there are approximately equal numbers of positive particles and negative particles. There may also be many neutral particles, as in the ionosphere.

Plasma Frequency
The maximum frequency of internal oscillation of a plasma. The plasma frequency is proportional to the square root of the electron density.

Plasmapause
The outer boundary of the plasmasphere. The plasmasphere resides in the magnetosphere and consists of ions and electrons - it may be considered an extension of the ionosphere.

Polar Cap Absorption
The ionisation of the D region over the polar latitudes by high energy solar protons causes radio blackouts for trans-polar circuits which can last for several days. PCAs are almost always preceded by a major solar flare on the visible hemisphere of the Sun. The time between the flare event and the onset of the PCA ranges from a few minutes to several hours.

Polarisation
In an ionised medium in the presence of a magnetic field, a radio wave is split into two circularly polarised components, each propagating independently. In the ionosphere a radio wave is split by the Earth's magnetic field into ordinary (o) and extra-ordinary (x) waves. The partitioning of the wave energy between the two depends on the angle the wave makes with the magnetic field. At low frequencies, the x-wave is heavily attenuated relative to the o-wave.

Production
In the ionosphere, it refers to the production of free electrons.

Prominences
Filaments seen on the limb of the Sun.

Proton Flare
A flare that liberates significant amounts of high energy protons. If this stream intercepts the Earth, the protons cause a polar cap absorption (PCA).