Evening micropulsation events with a rising midfrequency characteristic

Many examples of evening micropulsation events with rising midfrequencies were found on the several years of continuous College micropulsation records. The midfrequency typically rises from 0.1 to 0.5 cps over a ∼1-hour event duration. There is a pronounced diurnal variation in occurrence, with maximum near 2000 local time, and there is a seasonal variation with most occurrences in summer. Three of the possible source mechanisms are briefly discussed: (1) systematic variations in the characteristics of the lower exosphere resonance cavity, (2) excitation via a growing wave mechanism with the additional assumption of a time-decreasing midenergy in the electron or proton beam, and (3) excitation via proton cyclotron instabilities near the equatorial plane with inward diffusion of the protons.