Measurements of the angular distribution of auroral kilometric radiation (AKR) are presented by using observations from the Hawkeye 1, Imp 6, and Imp 8 satellites. The University of Iowa plasma wave experiments on Hawkeye 1 and Imp 6 provide electric field measurements of AKR in narrow frequency bands centered at 178, 100, and 56.2 kHz, and the Imp 8 experiment provides measurements at 50 kHz. From a frequency of occurrence survey, at radial distances greater than 7Re (earth radii) it is shown that AKR is preferentially and instantaneously beamed into solid angles of approximately 3.5 sr at 178 kHz, 1.8 sr at 100 kHz, and 1.1 sr at 56.2 kHz, directed upward from the nighttime auroral/zones. Simultaneous multiple satellite observations of AKR in the northern hemisphere show that the radiation occurs simultaneously throughout these solid angles and that the plasmapause acts as an abrupt propagation cutoff on the nightside of the earth. No comparable cutoff is observed at the plasmapause on the dayside of the earth.

The results of computer ray tracing calculations for both the right-hand (R-X) and left-hand (L-O) polarized modes are also presented in an attempt to understand the propagation characteristics of the AKR. These calculations assume that a small source emits radition at various frequencies along a magnetic field line at 70¿ invariant latitude near local midnight. The approximate altitude of the source can be determined for each of the two modes of propagation by adjusting the source altitude to give the best fit to the observed angular distributions. The R-X mode is found to give the best agreement with the observed angular distributions.