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Schematic illustration of the generation of K and a K indices. Illustrative variations of the two orthogonal horizontal field components measured at one site are shown, X (toward geographic north, in blue) and Y (toward geographic east, in orange). These variations are after the regular diurnal variation has been subtracted from the observations. In the fixed 3-hour UT windows (00–03 UT, or 03–06 UT, and so on up to 21–24 UT), the range of variation of both components between their maximum and minimum values is taken, ΔX and ΔY. The larger value of the two is kept and scaled according to a standard, quasi-logarithmic scale (illustrated by the black and mauve bands to the right) for which all K-band thresholds are set for the site in question by L, the threshold range value for the K = 9 band. The value of L for the site is assigned according to the minimum distance between the site and a nominal (fixed) auroral oval position. The K value is then converted into the relevant quantised value of a K (in nT) using the standard “mid-class amplitudes” (K2aK) scale. In the schematic shown, ΔX > ΔY, thus the X component gives a K value of 8 (whereas the Y component would have given a K of 5). Thus for this 3-hour interval, a K value would be 415 nT. In the case of the classic aa indices, the hemispheric index (aa N or aa S, for the observatory in the northern or southern hemisphere, respectively) is f × a K , where f is a factor that is assumed constant for the observing site.
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