Abstract
In an attempt to resolve some ambiguity in defining geomagnetic sudden impulses (Sis) and storm sudden commencements (SSCs) using the existing phenomenological definition (see, for example, Mayaud and Romana [1977]; Mayaud [1980]), Joselyn and Tsurutani [1990] recently constructed a scheme in which SSCs are a subset of Sis, depending on the magnitude of subsequent geomagnetic activity. For quantitative application, they have proposed that an SI be specified as a sharp change (at least 10 nT in 3 minutes or less) observed nearly simultaneously (within a few minutes) in either component of the horizontal magnetic field at globally spaced observatories near 20° geomagnetic latitude. In addition, SSCs are those Sis followed within 24 hours by an hourly Dst index of at least −50 nT. Because the Dst index is not readily available, the recommended provisional alternative indicators are a 3‐hourly Kp index of 5 or more and a half‐daily a a index of 60 or more. Joselyn and Tsurutani [1990] have recommended these new quantitative definitions of the two terms (Sis and SSCs) for open discussion.
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