Metadata Catalogue

Solar Indices Database
The National Geophysical Data Center maintains an active database of solar activity indices to further the understanding of Earth magnetism and the Sun-Earth environment. The sun emits radio energy with a slowly varying intensity. This radio flux, which originates from atmospheric layers high in the sun's chromosphere and low in its corona, changes gradually from day-to-day, in response to the number of spot groups on the disk. Radio intensity levels consist of emission from three sources: from the undisturbed solar surface, from developing active regions, and from short-lived enhancements above the daily level. Solar flux density at 2800 megaHertz has been recorded routinely by radio telescope near Ottawa since February 14, 1947. Each day, levels are determined at local noon (1700 GMT) and then corrected to within a few percent for factors such as antenna gain, atmospheric absorption, bursts in progress, and background sky temperature. Beginning in June 1991, the solar flux density measurement source is Penticton, B.C., Canada. The relative sunspot number is an index of the activity of the entire visible disk of the Sun. It is determined each day without reference to preceding days. Each isolated cluster of sunspots is termed a sunspot group, and it may consist of one or a large number of distinct spots whose size can range from 10 or more square degrees of the solar surface down to the limit of resolution (e.g., 1/25 square degree). The relative sunspot number is defined as R = K (10g + s), where g is the number of sunspot groups and s is the total number of distinct spots. The scale factor K (usually less than unity) depends on the observer and is intended to effect the conversion to the scale originated by Wolf. The data contain fluxes from the entire solar disk at a frequency of 2800 megaHertz in units of 10 to the -22 Joules/second/square meter/Hertz. Each number has been multiplied by 10 to suppress the decimal point. Three sets of fluxes - the observed, the adjusted, and the absolute - are summarized. Of the three, the observed numbers are the least refined, since they contain fluctuations as large as 7% that arise from the changing sun-earth distance. In contrast, adjusted fluxes have this variation removed; the numbers in these tables equal the energy flux received by a detector located at the mean distance between sun and earth. Finally, the absolute levels carry the error reduction one step further; here each adjusted value is multiplied by 0.90 to compensate for uncertainties in antenna gain and in waves reflected from the ground. Group Sunspot Numbers (Rg) were derived to provide a homogeneous record of solar activity from 1610 to 1995. Care was taken that the long-term changes are more self-consistent than are the changes using the Wolf Sunspot Numbers. Daily standard deviations of the Group Sunspot Numbers for 1610 to 1995 represent the random errors in the daily means.

Polar Cap Index
Polar Cap Index : The PC index is an index to monitor the polar cap magnetic activity which is mainly caused by changes in the IMF southward component (BZ) and solar wind velocity. Numerous investigations have shwon that the IMF southward component drives ionospheric convection over the polar caps, which is sensed by ground magnetometers responding to the two-cell system of currents flowing in the ionosphere. As the Earth rotates under this two-cell current system, a near-pole station is always located under the sun-aligned part of the system (i.e. a transpolar current). This allows derivation of an index from data obtained at a single near-pole station simply calculating a magnetic horizontal component disturbance along the dawn-dusk meridian. Based on an availability of geomagnetic data from near-pole stations Thule (Greenland, 86.5° geomagnetic latitude) and Vostok (Antarctica, -83.4° ), the new magnetic activity index PC was introduced and studied by Troshichev and Andrezen [Planet. Space Sci., 33, 415, 1985], and Troshichev et al. [Planet. Space Sci., 36, 1095, 1988].