Polarization observations of 4fce auroral roar emissions

Abstract

AbstractWe report on the first polarization measurement of auroral roar emissions near 4 times the ionospheric electron cyclotron frequency (4fce). A ground‐based passive receiver that uses orthogonal loop antennas, installed in Iceland, revealed the sense of polarization of 11 events of 4fce roar emissions. In 9 of 11 cases, 4fce roar was left‐handed elliptically polarized, namely, O‐mode waves. The O‐mode 4fce roar was observed under both sunlit and dark ionospheric conditions during geomagnetic storms. For O‐mode 4fce roar generation, satisfaction of the matching condition where upper hybrid frequency (fUH) equals 4fce requires a high‐density, F region ionosphere, even during darkness, which might be attributed to auroral precipitation or tongue of ionization. In two cases, right‐handed elliptically polarized 4fce roar was observed during darkness hours and the main phase of a geomagnetic storm. This polarization indicates that nonlinear coupling of two upper hybrid waves may also work to generate X‐mode 4fce roar.