Space and Solar Definitions - A

A Index - A daily index of geomagnetic activity derived as the average of the eight 3-hourly a indices.

AAAS - American Association for the Advancement of Science.

AACS - Attitude and Articulation Control Subsystem onboard a spacecraft.

AAS - American Astronomical Society.

AC - Alternating current.

Active

(1)Geomagnetic levels such that 15 is less than ap which is less than 29.

(2) Solar activity levels with at least one geophysical event or several larger radio events (10cm) per day (class m flares)

Active Dark Filament (Adf) - An active prominence seen on the disk.

Active Longitude - The approximate center of a range of heliographic longitudes in which active regions are more numerous and more flare-active than the average.

Active Prominence - A prominence displaying material motion and changes in appearance over a few minutes of time.

Active Prominence Region (Apr) - A portion of the solar limb displaying active prominences.

Active Region (Ar) - A localized, transient volume of the solar atmosphere in which plages, sunspots, faculae, flares, etc. May be observed.

Active Surge Region (Asr) - An active region that exhibits a group or series of spike-like surges that rise above the limb.

Afred - Abbreviation for the a index for Fredericksburg.

ALT - Altitude.

AM - Ante meridiem (Latin: before midday), morning.

AMMOS - Advanced Multimission Operations System.

Amor - A class of Earth-crossing asteroid.

Angstrom - A unit of length = 1.0e-08cm.

AO - Announcement of Opportunity.

AOS - Acquisition Of Signal, used in DSN operations.

Ap Index - An averaged planetary a index based on data from a set of specific stations.

Aphelion - Apoapsis in solar orbit.

Apoapsis - The farthest point in an orbit from the body being orbited

Apogee - Apoapsis in Earth orbit.

Apojove - Apoapsis in Jupiter orbit.

Apollo - A class of Earth-crossing asteroid.

Apolune - Apoapsis in lunar orbit.

Apselene - Apoapsis in lunar orbit.

Arch Filament System (Afs) - A bright, compact plage crossed by a system of small, arched filaments, which is often a sign of rapid or continued growth in an active region.

Argument - Angular distance.

Argument of periapsis - The argument (angular distance) of periapsis from the ascending node.

Ascending node - The point at which an orbit crosses the ecliptic plane going north.

Asteroids - Small bodies composed of rock and metal in orbit about the sun.

Astronomical Unit (Au) - The mean earth-sun distance, equal to 1.496e+13cm or 214.94 solar radii.

Aten - A class of Earth-crossing asteroid.

Attometer - 10-18 meter.

AU - Astronomical Unit, mean Earth-to-sun distance, approximately 150,000,000 km.

Aurora - A faint visual phenomenon associated with geomagnetic activity, which occurs mainly in the high-latitude night sky; typical auroras are 100 to 250 km above the ground.

Auroral Oval - An oval band around each geomagnetic pole which is the locus of structured aurora.

Autumnal Equinox - The equinox that occurs in September.

AZ - Azimuth.

Å - Angstrom (0.0001 micrometer, 0.1 nm).