We 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.
- auroral roar
- mode conversion
- upper hybrid waves
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)