Abstract
It is found that an impulsive geomagnetic increase, named the preliminary positive impulse (PPI), often precedes the main impulse (MI) of geomagnetic sudden commencement at the dayside middle latitude (Memambetsu). The duration time of the PPI is about 1 min, and the average magnitude is 16 nT. The PPI also appears near the geomagnetic equator (Guam), although the occurrence is less frequent. No equatorial enhancements are present in the PPI, while the MI is enhanced appreciably at Guam. Most of PPI's accompany an electric field with a westward component in the ionospheric F region, as observed with the HF Doppler technique. These observational results lead to a conclusion that the PPI is transmitted from above the ionosphere by a compressional hydromagnetic wave, rather than produced by ionospheric electric currents generated in the polar ionosphere.
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