The 4-second summertime micropulsation band at College

A significant summertime micropulsation band at ∼4-sec period is revealed in the several years of continuous micropulsation recording at College. The band contains mainly Pc-type activity, but is also seen in certain weak Pi 1 bursts. The intensity of the band is greatest in afternoon at College and has a positive correlation with Kp in afternoon. A small diurnal variation in band midperiod was observed, with periods near 4 sec in day and near 3 sec at night. The 4-sec events are usually less well structured than higher frequency pearl-type events, and the frequency-time elements are often irregularly spaced. The possibility exists that the 4-sec band is due to the principal exospheric resonance.