Wave propagation experiments at medium frequencies between two ionospheric satellites, 1, General results

Occasionally the ISIS I and ISIS II spacecraft rendezvous in parts of the world where simultaneous telemetry coverage is feasible. Electromagnetic propagation between ISIS I and ISIS II has been studied using their topside sounders at medium and high frequencies. Above the gyrofrequency, the O and upper‐frequency X modes have been identified often during separations of up to a few thousand kilometers. In a few instances, the lower‐frequency X , or Z , mode has been observed with separations less than 1000 km. This mode is continuous below the gyrofrequency where the whistler mode is also seen. Intersatellite ionograms confirm that the O ‐, X ‐, and Z ‐mode pulses propagate largely as expected on the basis of magnetoionic theory applied to a smoothly varying medium, with little distortion except near cutoff. On the other hand, whistler‐mode pulses are significantly elongated by the medium. Their interpretation is the subject of a companion paper, part 2.