Space and Solar Definitions - G

Gal - Unit of gravity field measurement corresponding to a gravitational acceleration of 1 cm/sec2.

Galaxy - One of billions of systems, each composed of numerous stars, nebulae, and dust.

Galilean satellites - The four large satellites of Jupiter so named because Galileo discovered them when he turned his telescope toward Jupiter: Io, Europa, Ganymede, and Callisto.

Gamma - A unit of magnetic field intensity equal to 1 x 10.0e-05 gauss, also equal to 1 nanotesla.

Gamma rays - Electromagnetic radiation in the neighborhood of 100 femtometers wavelength.

Gauss - The unit of magnetic induction in the cgs (centimeter-gram- second) system.

GCF - Ground Communications Facilities, provides data and voice communications between JPL and the three DSCCs.

GDS - Ground Data System, encompasses DSN, GCF, DSMS, and project data processing systems.

GDSCC - DSN's Goldstone Deep Space Communications Complex in California.

GEO - Geosynchronous Earth Orbit.

Geomagnetic Elements - The components of the geomagnetic field at the sur- face of the earth. In sesc use, the northward and eastward components are often called the h and d components, where the d component is expressed in gammas and is derived from d (the declination angle) using the small angle approximation.

Geomagnetic Field - The magnetic field observed in and around the earth. The intensity of the magnetic field at the earth's surface is approximately 0.32 gauss at the equator and 0.62 gauss at the north pole.

Geomagnetic Storm - A worldwide disturbance of the earth's magnetic field, distinct from regular diurnal variations. Minor geomagnetic storm: a storm for which the ap index was greater than 29 and less than 50. Major geomagnetic storm: a storm for which the ap index was greater than 49 and less than 100. Severe geomagnetic storm: a storm for which the ap index was 100 or more. Initial phase: of a geomagnetic storm, that period when there may be an increase of the middle-latitude horizontal intensity (h). Main phase: of a geomagnetic storm, that period when the horizontal magnetic field at middle latitudes is generally decreasing. Recovery phase: of a geomagnetic storm, that period when the depressed northward field component returns to normal levels.

Geophysical Events - Flares (importance two or larger) with centimetric outbursts (maximum of the flux higher than the quiet sun flux, duration longer 10 minutes) and/or strong sid. Sometimes these flares are followed by geomagnetic storms or small pca. (class m flares).

Geostationary - A geosynchronous orbit in which the spacecraft is constrained to a constant latitude.

Geosynchronous - A direct, circular, low inclination orbit about the Earth having a period of 23 hours 56 minutes 4 seconds.

GHz - Gigahertz (109 Hz).

GLL - The Galileo spacecraft.

GMT - Greenwich Mean Time. Obsolete. UT, Universal Time is preferred.

Gradual Commencement - The commencement of a geomagnetic storm that has no well-defined onset.

Granulation - Cellular structure of the photosphere visible at high spatial resolution.

Gravitation - The mutual attraction of all masses in the universe.

Gravitational waves - Einsteinian distortions of the space-time medium predicted by general relativity theory (not yet directly detected as of January 2001).

Gravity assist - Technique whereby a spacecraft takes angular momentum from a planet's solar orbit (or a satellite's orbit) to accelerate the spacecraft, or the reverse.

Gravity waves - Certain dynamical features in a planet's atmosphere (not to be confused with gravitational waves).

Great circle - An imaginary circle on the surface of a sphere whose center is at the center of the sphere.

Green Line - The green line is one of the strongest (and first-recognized) visible coronal lines. It identifies moderate temperature regions of the corona.

Ground-Level Event (Gle) - A sharp increase in ground-level cosmic ray count to at least 10% above background, associated with solar protons of energies greater than 500 mev. Gles are relatively rare, occur- ring only a few times each solar cycle.

GSSR - Goldstone Solar System Radar, a technique which uses very high-power X and S-band transmitters at DSS 14 to illuminate solar system objects for imaging.

GTL - Geotail spacecraft.

GTO -Geostationary (or geosynchronous) Transfer Orbit.