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.

here are mainly relativistic electrons and bremsstrahlung X rays. Each of these sources is found to dominate ionization in a separate altitude region, with the electrons usually controlling ionization in the upper heights above 55-60 km and the X rays below to about 40 km, where cosmic rays take over. The relativistic electron ionizing radiation source has not usually been considered in modeling studies for the ion chemistry of the lower mesosphere (55-70 km), yet this radiation is capable of increasing ion concentration and electrical conductivity in excess of a factor of 10. Moreover, the persistence of relativistic electrons as an important source in all events studied here strongly suggests that this deficiency be corrected.

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

Low‐frequency hydromagnetic disturbances which are generated in the sunlit part of the distant magnetosphere and propagate as transverse waves to auroral latitudes are accompanied by pulsating electron precipitation and ionospheric absorption of cosmic radio noise with a similar temporal behavior. The region over which this pulsating activity occurs is belt‐like, ≳1000 km long and ∼100–400 km across. Sometimes the instantaneous pattern of precipitation exhibits spatial structure over these dimensions, but at least on one occasion the pulsations were in phase over a large region near the noon meridian. A unique acceleration process does not appear to be required, as the bremsstrahlung X‐ray spectrums obtained for the pulsations are not essentially different from those observed during activity preceding or following these events. The statically trapped radiation is eliminated as a possible source, and some mechanisms involving local energization are briefly discussed.

Balloon observations of X rays produced by energetic (~50-250 kev) electron precipitation into the auroral zone suggest a rather consistent diurnal pattern of occurrence for certain types of bremsstrahlung activity. Distinguishing spectral characteristics and variability, represented by approximate e-folding energies in the range 10-50 kev, are emphasized. Whereas the harder spectrums (Eo • 25-50 kev) may be precipitated at any local hour, the softer spectrums (Eo • 10-25 kev) appear primarily between the midnight and noon meridians. The softest spectrums (Eo • 10-15 kev) observed in this energy range occur preferentially around midnight and into the early morning in association with pronounced negative magnetic bays and post-breakup auroral activity. The results suggest the existence of an intermediate spectrum (Eo • 10-20 kev), in addition to the very soft (Eo • 5 kev) precipitation characteristic of the luminous aurora and the rather harder precipitation (Eo • 25 kev) characteristic of the 'average' auroral-zone X-ray event. 8O3 Hultqvist, B., Aurora and the lower ionqsphere in relation to satellite observations of particle precipitation, Goddard Energetic Particles preprint, X-611-64-107, Goddard Space Flight Center, Greenbelt, Maryland, 1964. Kremser, G., On the relationship between auroral zone X-ray bursts and polar magnetic substorms, Tech. Note, A summary of results from the Imp 1 magnetic field experiment, Goddard preprint X-61•-65-180, Goddard Space Flight Center, Greenbelt, Maryland, 1965. O'Brien, B. J., High-latitude studies with satellite Injun 3, 3, Precipitation of electrons into the atmosphere, J. Geophys. Res., 69, 13-43, 1964. O'Brien, B. J., and H. Taylor, High-latitude geophysical studies with satellite Injun 3, 4, Auroras and their excitation, J. Geophys. Res., 69, 45-63, 1964.