Local time features of frequency deviations of the HF standard signals associated with storm sudden commencements (ssc), SCF's, are examined by using data obtained at Kokubunji and Okinawa, Japan, during the period of March 1981 to August 1983. It is shown that of 30 SCF's 17 cases are type SCF(+ −) and 7 cases type SCF(− +). It is found that SCF(+ −) appears in the daytime and evening sectors (0600–2100 LT), while SCF(− +) appears in the nighttime sector (2100–0600 LT). This result indicates that the preliminary frequency deviations (PFD's) ((+) of SCF(+ −) and (−) of SCF(− +)) are caused by an electric field directed from dusk to dawn, while the main frequency deviations ((−) of SCF(+ −) and (+) of SCF(− +)) by a dawn‐to‐dusk electric field. It is suggested that the nighttime PFD is caused by a polar‐originating electric field associated with ionospheric currents responsible for the preliminary reverse impulse of ssc(− +) (sc*), while the daytime PFD could be caused by an induction electric field associated with the abrupt magnetic increase of ssc.

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