Lowest virtual height of the ionospheric F layer. Top

CMEs aimed at Earth are called "halo events" because of the way they look in coronagraph images. As the expanding cloud of an Earth-directed CME looms larger and larger it appears to envelop the Sun, forming a halo around our star. Top

The strongest spectral line in the solar spectrum. The wavelength is 656.3 nm which falls in the red part of the visible spectrum. This absorption line of neutral hydrogen is formed in the chromosphere and its intensity increases during solar flares. It is the spectral line used for world wide surveillance of solar flares. Top

Pascal is the standard unit of pressure and tension in the SI, the hectopascal is 1E-2 Pascal. Top

The variation of a parameter with height. Top

It is the study of the propagation of pressure waves in the Sun. Top

It is a bubble in space "blown" into the interstellar medium by the solar wind. Almost all of the material in the heliosphere emanates from the Sun itself. Top

A current layer that separates adjacent interplanetary magnetic field regions with opposite magnetic polarity. Top

It is a heliocentric orbit where the satellite's period of revolution matches the Sun's period of rotation. The satellite would appear stationary to an observer on the Sun's surface. Top

It is the upper portion of the Earth's atmosphere, in which gases are fractionated according to their molecular weight. It lies above the homosphere, where chemical constituents are well mixed. Top

The virtual height of the F2 layer. At night when the F2 and F1 regions merge to form the F region, h'F is measured. Similar heights are obtained for the E and F1 layers. Top

SEE Solar Energetic Particles. Top

That part of the radio electromagnetic spectrum between 3 and 30 MHz. Top

That geographic region of the Earth that lies between the polar circles and the poles. Top

SEE Fast Solar Wind. Top

It is the height of the electron density peak. Top

Solar flares that occur repetitively in the same active region, with essentially the same position and with a common pattern of development. Top

The homosphere includes the troposphere, stratosphere, and mesosphere and is a layer in which chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence. Top

In HF sky wave communications, this refers to a radio wave travelling up to the ionosphere, being refracted and then returning to Earth. Top

The distance across the ground a radio wave travels with one refraction from the ionosphere. The distance will depend on the antenna elevation angle and the height of the reflecting layer. Top

It is horizontal component of the geomagnetic field. Top

In solar-terrestrial terms, it is a filament-associated two-ribbon flare, often occurring in spotless regions. The flare presumably results from the impact on the chromosphere of infalling filament material. Top