Two Pi subtypes were recognized in a study of three years of continuous magnetic tape and chart recordings made at College, Alaska. Impulsive broadband bursts occurring near local geomagnetic midnight are associated with impulsive ionospheric cosmic noise absorption events, and thus with impulsive bursts of charged particles, which seem to come down from the tail region of the magnetosphere. Another Pi subtype is nonimpulsive, narrower in frequency range, and seems to be generated mainly within the auroral electrojets. Simple events of the latter subtype were inaugurated by at least one impulsive Pi burst. ‘Elementary’ Pi storms thus have two distinct components.

A class of events, sweeping down in frequency, occurring between 20 and 200 Hz, were observed for the first time. The observing site is at Poker Flat, Alaska, 65°N. Characteristics include a daytime diurnal maximum, an apparent seasonal minimum in winter, a tendency for maximum occurrences on magnetically quiet days, and a typical event duration of ∼1 minute. The events occur at the same frequencies as a type of hiss observed on OGO‐5 (Kivelson and Russell, 1973) and a class of events observed on Injun 5 called ‘lion's roar’ (Gurnett and Frank, 1972), but the spectral form of the events observed on the ground is quite different from either of those types.

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 I bursts. The intensity of the band is greatest in afternoon at College and has a positive correlation with Kv 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-see 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-see band is due to the principal exospheric resonance. 2763 2764 R. R. HEACOCK rent pearl-type data have been compared to amplitude nighttime bands of micropulsation middle-and low-latitude induction loop data activity at low-latitude sites. The lowest of the by Tepley et al. [1965] with no evidence of bands was frequently in the neighborhood of spurious effects unique to the telluric current 0.25 cps in the evening and midnight hours, system. The telluric current instrumentation tending to rise in frequency in morning hours, has a sufficiently flat response at frequencies and dropping again just before sunrise. The over 0.02 cps for sonagram purposes. The re-bands were sometimes in evidence throughout sponse of the preamplifier, a Tektronix 156a, an entire night. falls off rapidly below 0.02 cps, but this has Benioff's [1960] type B, periods 3-8 sec, oblittle relevance in the present study and in-served in southern California at night during volyes only the lowest several per cent of the times of high K index, may be of the same gensonagram displays that are included in the eral type as the July 1963 examples of Tepley figures contained herein. e• al. [1964]. Since Bentoff recorded continuously for many years, the nighttime occur-I{EVIEW OF PUBLISHED REPORTS OF rence maximum for his type B is assumed to •4-SEC ACTIVITY be accurate for that location. A recent cornpart-The literature on the 4-sec band is rather son of College and Palo Alto sonagrams [Tepmeager. Indeed, it would not be clear, from ley et al., 1965] showed that daytime shortthe literature, that the band exists as a sig-period (T < $ sec) micropulsations are comnificant entity. As will be seen, however, cer-mon at College but are seldom observed at rain consistencies exist in the different reports, Palo Alto. Judging from those studies, together and, when taken together with the College data with the results reported in this paper, it seems reported here, a more unified picture of the 4-likely that Benioff observed only the nightsec band is possible. time portion of a type of activity that is ob-Yanagihara [1963] observed continuous (type served in both day and night at. a6roral-zone Pc) micropulsations in the period range 3-10 sites.

The occurrence of periodically structured Pc 1 (pPc 1) during several intense geomagnetic storms is examined in detail. Unlike unstructured or quasi‐structured Pc 1, pPc 1 strongly tend to occur only during a later epoch of the recovery phase of geomagnetic storms. The available pPc 1 data, together with satellite and ionospheric data, are consistent with the idea that pPc 1 activity originates in the proton electromagnetic cyclotron instability that occurs when the high‐energy tail of the ring current is brought to the stable trapping limit. This occurs in the late recovery phase when the plasmasphere has filled sufficiently from the ionosphere. Some characteristics of the observed pPc 1 activity are found to be dependent on the character of minor substorm activity occurring on the ring current recovery phase.

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.