Ducted propagation of chorus waves: Cluster observations

Ducted propagation of whistler waves in the terrestrial magnetosphere-ionosphere system was discussed and studied long before the first in-situ spacecraft measurements. While a number of implicit examples of the existence of ducted propagation have been found, direct observation of ducts has been hampered by the low sampling rates of measurements of the plasma density. The present paper is based on Cluster observations of chorus waves. The ability to use measurements of the spacecraft potential as a proxy for high time resolution electron density measurements is exploited to identify a number of cases when increased chorus wave power, observed within the radiation belts, is observed simultaneously with density enchantments. It is argued that the observation of ducted propagation of chorus implies modification of numerical models for plasma-wave interactions within the radiation belts.

RELATIONSHIP BETWEEN THE AXIAL WAVE, AND FIRST NORMAL MODES FOR DUCTED, PROPAGATION IN A WAVEGUIDE

For ducted propagation in a waveguide when the source and receiver are placed close to the depth of the waveguide axis, there exist cusped caustics repeatedly along the axis. In neighborhoods of these cusped caustics, the interference of the wave fields that correspond to near-axial rays occurs. This results in the formation of a coherent structure (the axial wave) that propagates along the waveguide axis like a wave. In this paper, for the two-dimensional reference point source problem with the parabolic index of refraction squared the axial wave is represented in the form of a sum of the first normal modes and a remainder field. The mathematical framework is provided by two different representations of the acoustic field. The first one was obtained by Grigorieva et al.1 It includes a sum of ray summands and the axial wave. The second representation including ray summands, a sum of the first normal modes, and a remainder field is derived in the present paper. For the remainder a simple formula including a special function is obtained. Numerical simulations are carried out for parameters corresponding to long-range ocean acoustic propagation experiments.

DEPENDENCE OF THE AXIAL WAVE ON RANGE AND SOUND-SPEED PROFILE PROPERTIES IN A RANGE-INDEPENDENT OCEAN

For ducted propagation in a waveguide when the source and receiver are placed closely to the depth of the waveguide axis, there exist cusped caustics repeatedly along the axis. In neighborhoods of these cusped caustics, the interference of the wave fields that correspond to near-axial rays occurs. This results in the formation of a coherent structure (the axial wave) that propagates along the waveguide axis like a wave. In this paper the integral representation of the axial wave obtained before for an arbitrary waveguide in a two-dimensional range-independent medium is generalized to a three-dimensional range-independent medium. Through numerical simulation, the dependencies of the axial wave on range, sound-speed profile properties, and geometry of the experiment are studied for two sound-speed profiles: the average profile from the AET experiment and the Munk canonical profile. The sound source frequency is taken equal to 75 Hz; the propagation range is up to 3250 km. The strong difference between shapes of the axial wave for the average profile from the AET experiment and the Munk canonical profile is shown for all the examined models.