Geomagnetic Sudden impulses and storm sudden commencements: A note on terminology



The geomagnetic field occasionally exhibits abrupt, worldwide variations that have a morphology similar to that shown in Figure 1. Known as sudden impulses (SIs) or storm sudden commencements (SSCs), these signatures have been successfully explained as a compression of the magnetosphere caused by the passage of a shock or tangential discontinuity in the solar wind [e.g., Nishida, 1978]. Shocks propagate through the solar wind (outward from the Sun for fast forward shocks and inward for reverse shocks, in the solar wind frame of reference) and tangential discontinuities are simply carried with the solar wind. The purpose of this note is to examine the definitions of and distinctions between SSCs and SIs; to modernize present definitions of SIs and SSCs, in which SSCs are a subset of SIs depending on subsequent observed values of Dst or alternate geomagnetic indices; and to recommend quantitative definitions of the two terms for open discussion.