Copyright WDC C1 for STP at RAL.
Hourly H-component magnetic variations are analyzed to remove annual secular change trends from records of a worldwide array of low- latitude observatories. A cosine factor of the site latitude transforms residual variations to their equatorial equivalents and harmonic analysis isolates the term used as the Dst index. Sugiura originally described Dst derivation in "IAGA Bulletin 27 in 1969, page 123, res. 2."