A index: A daily index of geomagnetic activity derived as the average of the eight 3-hourly a- indices in a UTC day.

D region: A daytime region of the ionosphere ranging in height from approximately 30-50 miles. Radio wave absorption in this region can significantly increase in response to increased ionization associated with changes in solar electromagnetic emissions, e.g., x-ray flares. Under normal conditions, D region absorption drops off dramatically above about 9 MHz but, following an x-ray flare, signals up to and beyond 30 MHz can be significantly degraded. The D region is the lowest region of the ionosphere.

E region: A daytime region of the ionosphere, forming roughly between heights of 70 and 100 miles. Ionization in the E region is caused mainly by x-rays. Other sources of E region ionization include trails from incoming meteors and electrically charged sub-atomic particles from the sun. Transient patches of ionization in the E-region, known as "sporadic E" or Es, can often refract HF and VHF signals over great distances (500-1500 miles).

F region: The upper region of the ionosphere, above approximately 100 miles. F region ionization is highly variable, depending upon the local time, solar activity, season, and geomagnetic activity. The F region contains the F1 and F2 layers. The F2 layer, predominantly responsible for long-distance HF radio propagation, is more dense and peaks in ionization at altitudes between 125 and 375 miles. The F1 layer, which forms at lower altitudes in the daytime, usually possesses less ionization. On the night side of the earth, the F1 rises and the F2 falls to form a single region capable of refracting HF radio signals.

Flare: A sudden release of energy in the solar atmosphere, lasting from minutes to hours, from which radiation and particles are emitted.

Geomagnetic activity: Natural variations in the geomagnetic field, classified qualitatively into quiet, unsettled, active, and geomagnetic storm levels based on the A index and the range of K indices observed.

Geomagnetic storm: A worldwide temporal disturbance of the earth's magnetic field. A storm is defined as occurring when the A index exceeds 29.

K index: a 3-hourly, quasi-logarithmic local index of geomagnetic activity relative to an assumed quiet-day condition. The K index ranges from 0 to 9.

Major solar flare: A class M5 or greater x-ray flare.

Quiet: a descriptive word specifically meaning (1) a probability of <50% for a C-Class x-ray flare in a sunspot region; (2) geomagnetic activity levels such that the A index is < 8.

Short wave fade (SWF): An abrupt decrease of HF radio signal strength, lasting form minutes to hours, caused by increased day-side ionization from solar flares, usually M or X x-ray class.

Solar activity: Transient perturbations of he solar atmosphere as measured by enhanced x-ray emissions, typically associated with flares. Five standard terms are used, based on the size and number of x-ray flares observed.

Ultraviolet (UV): That part of the electromagnetic spectrum with wavelengths between 5 and 400 nanometers (nm). Along with x-rays, solar radiation in these wavelengths produces the majority of ionization in the D, E, and F regions.

Unsettled: With regard to geomagnetic activity, a descriptive word defining conditions between quiet and active, specifically meaning that the A index is between 8 and 15.

X-ray: Electromagnetic radiation in the wavelength range of 0.1-10 nanometers (nm).

X-ray flare class: Ranking of a flare based on its x-ray output. Flares are classified according to the order of magnitude of the peak burst intensity (I) measured at the earth in watts/square meter in the 0.1-0.8 nm wavelength region.

Note: A multiplier is used to indicate the level within each class. For example: An M6 flare is equal to 6x10-5 watts/square meter.