Space and Solar Definitions - H

H-Alpha - This absorption line of neutral hydrogen falls in the red part of > the visible spectrum and is convenient for solar observations. The h-alpha line is universally used for patrol observations of solar flares.

H-Component Of The Geomagnetic Field - See geomagnetic elements.

HA - Hour Angle.

HEF - DSN's high-efficiency 34-m DSS, replaces STD DSSs.

Heliocentric - Sun-centered.

Heliopause - The boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the sun's influence, perhaps 100 AU from the sun.

Heliosphere - The space within the boundary of the heliopause, containing the sun and solar system.

HEMT - High-electron-mobility transistor, a low-noise amplifier used in DSN.

HGA - High-Gain Antenna onboard a spacecraft.

High Energy Event - Flares (class two or more) with outstanding centimetric bursts and sid. High energy protons are reported at the earth in case of most of these events occurring on the western part of solar disk. (class x flares)

High Frequency (HF) - That portion of the radio frequency spectrum between between 3 and 30 mhz.

High Latitudes - With specific reference to zones of geomagnetic activity, "high latitudes" refers to 50o to 80o geomagnetic.

High-Speed Stream - A feature of the solar wind having velocities that are about double average solar wind values.

Hohmann Transfer Orbit - Interplanetary trajectory using the least amount of propulsive energy.

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

Horizon - The line marking the apparent junction of Earth and sky.

Hour Angle - The angular distance of a celestial object measured westward along the celestial equator from the zenith crossing. In effect, HA represents the RA for a particular location and time of day.

Hyder Flare - A filament-associated two-ribbon flare, often occurring in spotless regions. The flare presumably results from the impact on the chromospheres of infalling filament material.

Hz - Hertz, cycles per second.