Publication Date:
2015-01-13
Description:
We report on the first polarization measurement of auroral roar emissions near 4 times the ionospheric electron cyclotron frequency (4 f ce ). A ground-based passive receiver that uses orthogonal loop antennas, installed in Iceland, revealed the sense of polarization of 11 events of 4 f ce roar emissions. In 9 of 11cases, 4 f ce roar was left-handed elliptically polarized, namely, O-mode waves. The O-mode 4 f ce roar was observed under both sunlit and dark ionospheric conditions during geomagnetic storms. For O-mode 4 f ce roar generation, satisfaction of the matching condition where upper hybrid frequency ( f UH ) equals 4 f ce requires a high-density, F-region ionosphere, even during darkness, which might be attributed to auroral precipitation or tongue of ionization (TOI). In two cases, right-handed elliptically polarized 4 f 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 4 f ce roar.
Print ISSN:
0094-8276
Electronic ISSN:
1944-8007
Topics:
Geosciences
,
Physics
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