Abstract
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.
Original language | English |
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Pages (from-to) | 249-255 |
Number of pages | 7 |
Journal | Geophysical Research Letters |
Volume | 42 |
Issue number | 2 |
DOIs | |
Publication status | Published - 2015 Jan 28 |
Keywords
- auroral roar
- mode conversion
- polarization
- upper hybrid waves
ASJC Scopus subject areas
- Geophysics
- Earth and Planetary Sciences(all)