A burst of X rays was observed at balloon altitude over Alaska with the onset of a sudden commencement geomagnetic storm at 0146 UT on June 27, 1960. The electron bombardment of the upper atmosphere that gave rise to the X rays occurred on a large scale, ionospheric absorption coincident with the X‐ray burst being observed by riometers in Alaska, Sweden, and Norway.

Cosmic noise absorption coinciding with the sudden commencement of geomagnetic storms has been studied for 71 SC events from the data recorded at 25 riometer stations at or near the auroral zone during the period July 1958‐December 1960. The greatest absorption was registered at stations near the central line of the auroral zone. The effect was observed simultaneously on the day and on the night side of the earth. This type of absorption was earlier found to be associated with bremsstrahlung X rays created by electrons entering the lower ionosphere. The source of these electrons has not been definitely established by the present study. Although the data tends to favor a source outside the earth's magnetosphere, no definite conclusion is possible because of the present uncertainties concerning the trapped radiation belts.

Simultaneous balloon observations of X‐ray pulsations in the 5–10 second range made from Fairbanks, Alaska, and Macquarie Island, Australia, showed no detailed correlation in time or amplitude. Observations in the northern hemisphere, with two balloon instruments separated by about 150 km in the east‐west direction, showed no obvious correlation, but when the separation was reduced to about 100 km in the north‐south direction, X‐ray pulsations showed in‐phase variations. Observations with two balloon instruments separated by 150 km in the east‐west direction showed time coincidences for about one‐third of the microbursts; the other two‐thirds were observed on one balloon or the other. From the small scale size of microburst electron precipitation it is suggested that magnetospheric plasma instabilities are responsible for the electron bombardment of the auroral zone atmosphere. Similar considerations are suggested for pulsating electron precipitation.

Fast pulsations, characterized by half-widths of 2-10 seconds and temporal spacings of 4-30 seconds, in the intensity of auroral-zone X rays have been investigated using simultaneous pairs of balloon-borne detectors with spatial separations of approximately 100-200 km. Such events, usually of an hour's duration, occur almost exclusively in the midnight to dawn sector (0200-0• local hours) and appear preferentially on the equatorward side of the auroral zone. Spectral analysis shows the phenomena to be nonstationary but suggests that quasi-periods of 5-10 seconds ought to be considered a significant feature of the process responsible for such variations. Cross-c. orrelation studies confirm the limited spatial extent of individual pulsations, on the average, and further indicate the possibility of longitudinal motions. Microburst events, characterized by scores of transient bursts each of about 200-msec half-width, occur most frequently around local noon, favoring the late morning and early afternoon. There appears to be little average latitude dependence across the auroral zone. Individual events, however, may be quite restricted in latitude. On one occasion, microbursts were observed for the better part of 3 hours at L --5J in the absence of similar activity at L --6.5, suggesting a rather localized magnetospheric origin. [Anger et al., 1963a; Evans, 1963; Bareus and Rosenberg, 1965; Brown, Barco, Parsons, and Reid, 1965; and Barcus and Christensen, 1965]; and more rapid fluctuations in which the intensity changes occur on a time scale shorter than 30 seconds [Winckler et al., 1962; Anger et al., 1963b, Anderson and Milton, 1964; and Brown et al., 1965b]. Generally, the slower variations have been interpreted in terms of hydromagnetic wave mod•ation or some sort of interaction with fundamental modes of oscillation of geomagnetic field lines of force. On the other hand, the more rapid variations have been attributed to the dynamics of trapped particle or magnetospheric plasma instabilities, but at present this does not represent an essential difference between the two classes. For the more rapid fluctuations, the temporal range of interest here, a further distinction is drawn between fast pulsations (P), for which the average spacing between individual burst lies in the range 4-30 seconds [Anger et al., 1963a; and Brown et al., 1965b], and microbursts (M) for which the full width of each burst at halfmaximum is about 200 msec [Anderson and Milton, 1964]. Brown et al. [1965a, b] suggest the spatial scale size for coherent electron precipitation to be essentially the same for both types, namely, around 100 lrm in extent. How-125 IN AURORAL-ZONE X RAYS 127

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