Polarization observations of 4f_ce auroral roar emissions

We report on the first polarization measurement of auroral roar emissions near 4 times the ionospheric electron cyclotron frequency (4f_ce). A ground-based passive receiver that uses orthogonal loop antennas, installed in Iceland, revealed the sense of polarization of 11 events of 4f_ce roar emissions. In 9 of 11 cases, 4f_ce roar was left-handed elliptically polarized, namely, O-mode waves. The O-mode 4f_ce roar was observed under both sunlit and dark ionospheric conditions during geomagnetic storms. For O-mode 4f_ce roar generation, satisfaction of the matching condition where upper hybrid frequency (f_UH) equals 4f_ce 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 4f_ce 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 4f_ce roar.