Space and Solar Definitions - D

Dark Surge On Disk (Dsd) - Dark gaseous ejections visible in h-alpha.

DC - Direct current

DEC - Declination

Declination - The measure of a celestial body's apparent height above or below the celestial equator.

Descending node - The point at which an orbit crosses the ecliptic plane going south.

Differential Rotation - The change in solar rotation rate with latitude. Low latitudes rotate at a faster angular rate (approx. 14 degrees per day) than do high latitudes (approx. 12 degrees per day).

Disappearing Solar Filament (Dsf) - The sudden (timescale of minutes to hours) disappearance of a solar filament (prominence).

Disk - The visible surface of the sun (or any heavenly body) projected against the sky.

DKF - DSN keyword file, also known as KWF.

Doppler effect - The effect on frequency imposed by relative motion between transmitter and receiver.

Downlink - Signal received from a spacecraft.

DSCC - Deep Space Communications Complex, one of three DSN tracking sites at Goldstone, California; Madrid, Spain; and Canberra, Australia; spaced about equally around the Earth for continuous tracking of deep-space vehicles.

DSMS - Deep Space Mission System, the system of computers, software, networks, and procedures that processes data from the DSN at JPL.

DSN - Deep Space Network, NASA's worldwide spacecraft tracking facility managed and operated by JPL.

DSOT - Data System Operations Team, part of the DSMS staff.

DSS - Deep Space Station, the antenna and front-end equipment at DSCCs.

Dst Index - A geomagnetic index describing variations in the equatorial ring current.

Dyne - A unit of force equal to the force required to accelerate a 1-g mass 1 cm per second per second. Compare with Newton.