Polarization observations of 4fce auroral roar emissions

Yuka Sato, Akira Kadokura, Yasunobu Ogawa, Atsushi Kumamoto, Yuto Katoh

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)


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 languageEnglish
Pages (from-to)249-255
Number of pages7
JournalGeophysical Research Letters
Issue number2
Publication statusPublished - 2015 Jan 28


  • auroral roar
  • mode conversion
  • polarization
  • upper hybrid waves


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