It is the distance corresponding to a parallax of one second of arc. An unit of distance defined as the distance at which 1 astronomical unit (AU) subtends an angle of 1 sec of arc. It is equal to 206264.806 AU equal to 3.08568E16 m equal to 3.26166 light years.

It is generated when the current, flowing westward in the night and evening sectors near the geomagnetic equator, is closed by the system of the zone-2 field-aligned currents and by ionospheric currents.

The distance across the ground between two terminals.

The surrounding penumbra is a lighter sunspot’s region, where the magnetic field lines are more inclined.

The point on the orbit of one planet where it is closest from the Sun.

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

Instrument used to measure the airglow integrated intensity.

The Light particles. It is has both, wave and particle characteristics.

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

It is a prefix in the SI system of units denoting a factor of 1E-12.

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

Oscillations on the atmospheric fields, such as, temperature, wind, density and pressure, at periods greater than one day and with horizontal extension of the order of Earth diameter.

An ionized 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.

It is a large-scale ionospheric plasma depletions or bubbles (large-scale Spread-F) typically finding in tropical F-region.

In aeronomy, it is the speed of a plasma parcel in an ionosphere region.

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

The region covering the magnetosphere that extends toward the cusp region that contains de-energized magnetosheath plasma.

A layer of weaker magnetic field and denser plasma that is centered on the equator and separates the two tail lobes.

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.

Zone of the magnetosphere that co-rotates with the Earth.

Signature of ionospheric irregularity observed at RTI plot from radar data, that show the development/evolution of small irregularities trought time and altitude.

Luminous atmospheric phenomenon (boreal in the northern hemisphere, austral in the southern hemisphere) due to the de-energizing of atmospheric gas excited by collisions with solar wind particles.

The ionization 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.

Parallel beyond which the Sun does not rise at least one day per year (or does not set at least one day per year).

Zone of the magnetosphere where the geomagnetic field lines open out onto the magnetosheath and into space.

Satellite orbit with an inclination of 90°, thereby crossing both poles once every orbit.

In an ionized medium in the presence of a magnetic field, a radio wave is split into two circularly polarized 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. Positive Cloud-to-Ground Lightning (CG+). Electrical discharge from cloud to ground, who transfer positive charges from a positively charged region within the cloud to the ground

The electron antiparticle. It has the same electron characteristics (spin, mass, charge magnitude) but its charge is positive.

Force per unit of area exerted on a surface.

Direct ionization of the atmospheric constituents caused by absorption of the solar radiation in the extreme ultra-violet and x-rays spectrum

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

Filaments seen on the limb of the Sun.

The transmission of electromagnetic energy as waves through or along a medium.

It is a material that is used to produce energy to move an object. This will often involve a chemical reaction. It may be a gas, liquid, plasma, or, before the chemical reaction, a solid.

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

It is a sequence of numbers generated by a deterministic algorithm, used as an identifier for satellites.