Whistler duct properties deduced from VLF observations made with the Ogo 3 satellite near the magnetic equator

While the great majority of ground whistlers are interpreted as indirect evidence of magnetospheric ducts, the first direct evidence of ducts was obtained from the Stanford University broadband VLF experiments on Ogo 1 and Ogo 3. Five discrete whistler ducts were encountered by Ogo 3 on the inbound pass of June 15, 1966, between L = 4.7 and 4.1. Each duct was characterized by reception at the satellite of ducted whistlers with a distinct spectral shape, and of the high-frequency portions of whistlers (leakages) that propagated inward from outer ducts. The data were interpreted in detail by ray tracing in a model magnetosphere that includes ducts of enhanced ionization. The following conclusions resulted: (1) the L shell thicknesses of the observed ducts ranged between 0.035 and 0.070 earth radii, and the interduct separations ranged between 0.017 and 0.18 earth radii; (2) the dimension of the ducts in longitude was estimated to be of the order of 4°, or 0.3 earth radii at the equator, which is a factor of ∼4–8 greater than the L shell dimension; (3) the whistler ducts are much more likely to be enhancements than troughs; (4) the minimum enhancement factors needed to trap frequencies up to half the equatorial electron gyrofrequency are on the order of 8%, with smaller values producing upper cutoffs at lower frequencies; (5) the limited spreading of the calculated leaked rays is in general agreement with the corresponding regions of observation and relative signal amplitudes; (6) the low cutoffs of leaked signals are probably due to accessibility; (7) cyclotron and Landau interactions are likely to play a role in upper cutoffs of leaked signals; (8) the upper cutoff of ground whistlers near fH0/2 is a trapping (rather than absorption) effect; (9) the hydrostatic type of distribution of ionization along the field lines is applicable in the plasmasphere; and (10) the travel times (and frequency of minimum delay) of ducted whistlers can be calculated with good accuracy by assuming purely longitudinal propagation.