Polarimetry of auroral kilometric radiation with a triaxial nonorthogonal antenna system



[1] The response of a triaxial nonorthogonal radio-polarimeter on board a spinning spacecraft has been analyzed in order to determine effective length vectors of its short electric antennas. In the ideal case all three antennas of a radio-polarimeter would be orthogonal to each other. In practice, their electric axes do not coincide with the physical ones. It is shown that effective length vectors of the antennas can be found by fitting theoretical temporal variations of the Stokes parameters to those determined from a spinning spacecraft. This idea has been applied with the Polrad radio-polarimeter on board the Interball-2 spacecraft, which frequently observed circularly polarized auroral kilometric radiation. Polrad was equipped with one monopole (Y) and one dipole (Z) in the plane perpendicular to the spacecraft spin axis, and one monopole (X) extended along the spacecraft spin axis. For the Y antenna the resulting components of the antenna effective length vector are hy/hz from 0.88 to 0.94 (depending on the source colatitude), colatitude 7.9° ± 0.1°, azimuth 5.5° ± 0.2° measured from its physical axis, and for the X antenna hx/hz from 1.1 to 0.85 and the tilt a negligible. The Z antenna is taken as a reference. These results have been applied to determine the transformation matrix (describing the instrumental polarization), used for rectification of the measured Stokes parameters to the orthogonal antenna system. The method can also be applied to determine complete polarization state and direction of arrival of any radio emission received by a triaxial polarimeter.