Auroral radio emission and absorption of medium frequency radio waves observed in Iceland

Abstract

In order to study the generation and propagation processes of MF auroral radio emissions (referred to as auroral roar and MF burst) in the polar ionosphere, an Auroral Radio Spectrograph (ARS) system was installed at Husafell station in Iceland (invariant latitude: 65.3• ). Data analysis of man-made transmissions also provides useful information for the ionosphere study as well as an investigation of auroral radio emissions since the propagation character of MF radio waves changes depending on electron-neutral collisions in the bottomside ionosphere. Thus, ionospheric absorption is examined in comparison with the solar zenith angle and auroral phenomena. The results indicate that the ARS data can be used to detect ionization occurring at distant regions. In late 2006, the ARS detected one auroral roar and two MF bursts, which were identified as left-handed polarized waves. Results of data analysis, including other auroral data and particle spectra observed by the DMSP satellite, suggest that the MF bursts are generated by electrons with an average energy of several keV associated with auroral breakup. On the other hand, the auroral roar is generated as upper hybrid waves by relatively low-energy electrons over the observation site and propagates downward, being converted into L-O mode electromagnetic waves.

Right‐hand polarized 4fce auroral roar emissions: 2. Nonlinear generation theory

“…The ground‐level detection of the auroral roar is believed to be the result of mode conversion of upper hybrid/ Z mode to left‐hand/ O mode at the density gradient. Indeed, the measured polarization of 2 f c e and 3 f c e is consistent with this interpretation in that the L ‐ O mode polarization is dominant for these emissions [ Shepherd et al , ; Sato et al , ]. Recently, 4 f c e [ Sato et al , ] and 5 f c e [ LaBelle , ; LaBelle and Dundek , ] auroral roars have also been detected near the sunlit side of the ionosphere where the electron density is enhanced via photoionization.…”

First observations of 4fce auroral roar emissions

“…But the narrowband auroral radio emission they detected was only 2f ce roar near 3 MHz. Subsequent measurements with sweeping receivers have been conducted below 5 MHz [e.g., Weatherwax et al, 1993;LaBelle et al, 1997;Sato et al, 2008]. The ARS-S observation, conducted for 24 hours every day, enables us to find a new frequency component of auroral roar near 5.5 MHz.…”

“…Because these frequency bands are considered to be close to the second and third harmonics of the electron cyclotron frequency in emission sources, each type of auroral roar is called 2f ce and 3f ce roars. The polarization character of 2f ce and 3f ce roars was identified as left-handed polarized waves by Shepherd et al [1997] and Sato et al [2008]. Both types are commonly attributed to mode conversion to the L-O mode of upper hybrid waves excited in the bottomside ionosphere by auroral electrons [e.g., LaBelle and Treumann, 2002].…”

Polarization observations of 4fce auroral roar emissions

“…[] and Sato et al . [], respectively. Both harmonics are commonly attributed to conversion of upper hybrid waves to O‐mode electromagnetic waves, excited by auroral electrons under the f UH ~ nf ce ( n = 2 and 3) condition in the bottomside ionosphere (review by LaBelle and Treumann []).…”