Satellite measurements of field‐aligned auroral electron precipitation have been analyzed using 16 months of data from the Ogo 4 auroral particles experiment. As can be seen from the moving satellite, the anisotropies are of short duration and are most likely to occur when particle fluxes are high. Field‐aligned 2.3‐kev electron precipitation is found in an oval‐shaped region primarily in the nighttime hours; maximum probability is at ∼70° invariant latitude near midnight, congruent to and poleward of the auroral optical emissions in these hours. This precipitation is found to be associated with the high‐latitude boundary of auroral electron precipitation during substorm expansion and is characterized by a slightly harder and considerably more intense energy spectrum than typical isotropic precipitation.

Following a large sudden commencement on June 17, 1972, a large magnetic storm evolved, with a well‐developed main phase and recovery phase. Explorer 45 (S³‐A), with its apogee near 16 hours local time in June, measured the equatorial particle populations and magnetic field throughout this period. By use of data obtained during the symmetric recovery phase it is shown that through a series of self‐consistent calculations, the measured protons, with energies from 1 to 872 keV, can account for almost all of the observed ring current magnetic effects within the limits of experimental uncertainties. This enables us to set an upper limit to the heavy ion contribution to the storm time ring current of a few percent of the proton contribution.

Simultaneous measurements of low‐energy precipitating electrons and magnetic fluctuations from the low‐altitude polar‐orbiting satellite Ogo 4 have been compared. Analysis of the two sets of experimental data for isolated events led to the classification of high‐latitude field‐aligned currents as purely temporal or purely spatial variations. Magnetic field disturbances calculated by using these simple current models and the measured particle fluxes were in good agreement with measured field values. Although fluxes of electrons of greater than 1 keV were detected primarily on the night side, magnetometer disturbances indicative of field‐aligned currents were seen at all local times, in both the visual auroral regions and the day side polar cusp. Thus electrons with energies of less than ∼1 keV are the prime charge carriers in high‐latitude day side field‐aligned currents. The satellite measurements are in good agreement with previously measured field‐aligned current values and with values predicted from several models involving magnetospheric field‐aligned currents.

Recently, Berko and Hoffman [1974] have presented results of an analysis concerning the probability of occurrence of field-aligned 2.3-keV electron fluxes as a function of both the altitude of observation and the season of the year. The most striking of their results was that the probability that the electron precipitation was field-aligned increased dramatically when the Ogo 4 satellite was above 800-km altitude. The probability at these higher altitudes was some 2-4 times larger than that at altitudes b•!ow 800 km.Berko and Hoffman f:omment that this altitude dependence 'implies that the field-aligned portion of the electron beam becomes destroyed as it passes to lower altitudes'The expla-

Data from the Ogo 4 auroral particles experiment were analyzed to determine properties of the higher latitude region of electron precipitation in the dayside hemisphere. From 7 months of data a probability map of occurrence of this structured low‐energy precipitation was compiled. The highest probabilities are concentrated predominantly between 0500 and 1400 hours magnetic local time, and from 75° to 82½° invariant latitude. There is excellent congruity of these high probabilities of occurrence and the high probability of occurrence of discrete auroral optical emissions, which define the auroral oval in these hours. The soft energy spectrums measured and the estimated total energy influx during typical precipitation events are appropriate for producing the type of aurora observed in these hours. It is concluded that the structured low‐energy electron precipitation is the primary energy source for the dayside auroral oval.