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Geomagnetic And Solar Indices Data Description

Geomagnetic indices constitute data series aiming at describing at a planetary scale the magnetic activity, or some of its components.
The data series are homogeneous since 1932 for Kp and Ap, 1957 for Dst.

Three-Hour-Range Index K

K indices isolate solar particle effects on the earth's magnetic field; over a 3-hour period, they classify into disturbance levels the range of variation of the more unsettled horizontal field component. Each activity level relates almost logarithmically to its corresponding disturbance amplitude. Three-hour indices discriminate conservatively between true magnetic field perturbations and the quiet-day variations produced by ionospheric currents.

K indices range in 28 steps from 0 (quiet) to 9 (greatly disturbed) with fractional parts expressed in thirds of a unit. A K-value equal to 27, for example, means 2 and 2/3 or 3-; a K-value equal to 30 means 3 and 0/3 or 3 exactly; and a K-value equal to 33 means 3 and 1/3 or 3+. The arithmetic mean of the K values scaled at the 13 observatories gives Kp.( Lerwick (UK), Eskdalemuir (UK), Hartland (UK), Ottawa (Canada), Fredericksburg (USA), Meannook (Canada), Sitka (USA), Eyrewell (New Zealand), Canberra (Aus- tralia), Lovo (Sweden), Brorfelde (Denmark), Wingst (Germany), and Niemegk (Germany).)

Equivalent Amplitudes

a index: A 3-hourly "equivalent amplitude" index of local geomagnetic activity; "a" is related to the 3-hourly K index according to the following scale:

K = 0 1 2 3  4  5  6   7   8   9 
a = 0 3 7 15 27 48 80 140 240 400

A index: A daily index of geomagnetic activity derived as the average of the eight 3-hourly a indices.
Ap index: An averaged planetary A index based on data from a set of specific Kp stations.

DST (Disturbance Storm Time)

Dst (Disturbance Storm Time) equivalent equatorial magnetic disturbance indices are derived from hourly scalings of low-latitude horizontal magnetic variation. They show the effect of the globally symmetrical westward flowing high altitude equatorial ring current, which causes the "main phase" depression worldwide in the H-component field during large magnetic storms. Unit - nT.


A qualitative estimate of overall level of magnetic activity for the day determined from the sum of the eight Ap amplitudes. Cp ranges, in steps of one-tenth, from 0 (quiet) to 2.5 (highly disturbed).


A conversion of the 0-to-2.5 range of the Cp index to one digit between 0 and 9

International Sunspot Number

Records contain the Zurich number through December 31, 1980, and the International Brussels number thereafter.

Relative Sunspot Numbers

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.

Ottawa 10.7-Cm 2800 MHz Solar Radio Flux

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 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.

Missing data will be represented as ---. The data are displayed as the flux value multiplied by ten (same as the table format). For example, 1324 converts to 132.4 solar flux units for Jan 1, 1950.

Ottawa 10.7-Cm Solar Radio Flux Adjusted To 1 Au measured at 1700 UT daily and expressed in units of 10 to the -22 Watts/ meter sq/hertz. Observations began on February 14, 1947. From that date through December 31, 1973, the fluxes given here don't reflect the revisions Ottawa made in 1966.