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Helicon waves were excited by a Nagoya type 111 antenna in a steady state argon magnetized plasma column with DC magnetic field 6 = 400-500 G. electron density 10'2-10'3 ~m -~ and neutral species pressure about 20 mTorr. Wavelength measurements made with 6-dot probes at the edge of the plasma COIumn show the presence of multimode standing helicon waves. Azimuthal mode numbers m = +1, 0 and -1 were detected with m = +1 dominant. Radial density profiles were measured with Langmuir probes. The wavelength measurements are shown to agree with theoretical calculations of helicon waves in radially non-uniform plasma.

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As a result of the presence of two or more ions with different height profiles in the upper F region of the ionosphere, propagation ducts that are associated with the lower‐hybrid‐resonance frequency may form. Such ducts are generally oriented along a horizontal direction. These ducts were originally suggested as a mechanism for the explanation of certain VLF hiss bands observed by a number of satellites with electric antennas. They were questioned, however, as a result of detections of VLF noise components at frequencies well above their trapping bands. The conditions in a diffusive equilibrium region under which such ducts may form are investigated. It is found that the ducts are essentially narrow‐band guides with maximum possible bandwidths of the order of 2.5 to 3.0 kHz; the bandwidths vary with altitude. Duct quality is found to depend on the parameters at the ion density crossover level and at the F region peak. Duct formation is favored by low electron density conditions, thus implying that their existence at high midlatitudes in or near the ionospheric trough is favored. Preliminary examination of data from the VLF receiver connected to the long electric antenna on Alouette 1 revealed a number of hiss events for the year 1963 with bandwidths that fit the duct properties.