The magnetoionic modes and propagation properties of auroral radio emissions

Abstract

The different magnetoionic wave modes which accompany the aurora are identified using DE 1 not only by their appearance on satellite radio spectrograms, but also by concurrent measurements of their wave polarization and arrival directions, and by ray‐tracing models of their expected propagation behavior. Of the four possible propagation modes, designated O, X, W, and Z for the ordinary, extraordinary, whistler, and Z modes, respectively, all four are found to occur in the auroral zone, as follows: The most intense, of course, is the well‐known auroral kilometric radiation (AKR), which originates primarily in the X mode near the electron cyclotron frequency, but which is frequently also accompanied by a weaker O‐mode component from the same location. The next most prominent auroral emission is the W‐mode auroral hiss originating from altitudes always well below the DE 1 satellite at frequencies below the local cyclotron frequency. For a particular case which was studied in detail, both of these components were found to originate from approximately the same source field lines near the poleward edge of the auroral plasma cavity, with the latter exhibiting a funnel shape on the radio spectrograms, centered on the source field lines but having different shapes inside and outside the cavity because of the differing plasma densities of these two regions. Equatorward of the cavity and at frequencies above the minimum plasma frequency within the cavity, this upcoming W mode exhibits a shadow zone which is attributed to the blockage of W‐mode propagation at the plasma frequency. The previously reported Z‐mode auroral radiation was also detected, but from sources also below the satellite and at the poleward edge of the cavity, and not from the expected AKR source at the cyclotron frequency. A weaker O‐mode component seems to accompany these emissions also, both within the polar cap poleward of the source and inside the cavity, the latter seemingly being guided upward by the cavity's lower plasma densities. Finally, exactly on the source field lines at the poleward edge of the cavity, there also occasionally seems to be localized Z‐mode emissions extending from the Z‐mode cutoff at quite low frequencies up to and above the plasma frequency.