Direct multiple path magnetospheric propagation: A fundamental property of nonducted VLF waves

Abstract

ISEE 1 satellite observations of nonducted whistler mode signals from the Siple Station VLF transmitter show that all well‐defined pulses (S/N ≈ 20 dB) are elongated by 20 ms to 200 ms. This elongation is attributed to closely spaced multiple paths of propagation between the ground and the satellite. The presence of multiple paths is further confirmed by the observed amplitude fading pattern where fading occurs not only at half the spin period (1.52 s) of the rotating antenna but also at time scales of the order of 1 s. Electric field measurements show a 2 to 10‐dB amplitude variation, which is again consistent with direct multiple path propagation. We illustrate the results with two cases: one on October 29, 1977, inside the plasmapause and the other on May 7, 1979, outside the plasmapause. Our results establish that, in general, at any point in the magnetosphere the direct signals transmitted from the ground arrive almost simultaneously along two or more closely spaced direct ray paths. It is shown that multiple paths can be explained by assuming field‐aligned irregularities of 1 to 10‐km horizontal scale in the ionosphere with a few percent enhancement or depletion in the plasma density. We discuss the implications of our results for nonducted wave‐particle interaction in the earth's magnetosphere. We show that for reasonable parameters of nonducted multiple path propagation, a cyclotron resonant electron will experience a wave doppler broadening of a few tens of hertz to a few hundreds of hertz.