Space and Solar Definitions - F

F Corona - Of the white-light corona (that is, the corona seen by the eye at a total solar (eclipse), that portion which is caused by sunlight scattered or reflected by solid particles (dust) in inter- planetary space.

F Region - The upper layer of the ionosphere, approximately 120 to 1500 km in altitude. The f region is subdivided into the f1 and f2 regions. The f2 region is the most dense and peaks at altitudes between 200 and 600 km. The f1 region is a smaller peak in electron density, which forms at lower altitudes in the daytime.

Facula - A bright region of the photosphere seen in white light, seldom visible except near the solar limb.

FDS - Flight Data Subsystem.

FE - Far Encounter phase of mission operations.

Femtometer - 10-15 meter.

Fibril - A linear pattern in the h-alpha chromosphere of the sun, as seen through an h-alpha filter, occurring near strong sunspots and plage or in filament channels.

Filament - A mass of gas suspended over the photosphere by magnetic fields and seen as dark lines threaded over the solar disk. A filament on the limb of the sun seen in emission against the dark sky is called a prominence.

Filament Channel - A broad pattern of fibrils in the chromosphere, marking where a filament may soon form or where a filament recently disappeared.

Flare - A sudden eruption of energy on the solar disk lasting minutes to hours, from which radiation and particles are emitted.

Fluence - Time integrated flux

Fluorescence - The phenomenon of emitting light upon absorbing radiation of an invisible wavelength.

Flux - The rate of flow of a physical quantity through a reference surface.

fm - Femtometer (10-15 m)

Fmin - The lowest radio wave frequency that can be reflected from the ionosphere

Foes - The maximum ordinary mode radiowave frequency capable of reflection from the sporadic e region of the ionosphere.

Fof2 - The maximum ordinary mode radiowave frequency capable of reflection from the f2 region of the ionosphere.

Forbush Decrease - An abrupt decrease, of at least 10%, of the background galactic cosmic ray intensity as observed by neutron monitors.

FTS - DSN Frequency and Timing System. Also, frequency and timing data.

FY - Fiscal year.