5:15 PM - 6:45 PM
[PEM10-P12] Wave mode of broadband ELF waves based on Akebono satellite VLF/MCA observations
Keywords:Broadband ELF waves, Akebono satellite, polar region, ion outflow
Broadband ELF (BBELF) waves are plasma waves observed in the polar region of the Earth’s ionosphere/magnetosphere. The spectral properties of BBELF waves are characterized by their electric field amplitude following a power law from DC to several kHz, while the wave magnetic field component appears in the frequency range below the ion cyclotron frequency. Although previous studies suggested a close relationship between BBELF waves and the ion acceleration process contributing to the outflow of heavy ions from the polar region, the wave mode of BBELF waves has not been fully understood.
In the present study, we analyzed the properties of plasma waves observed by the Akebono satellite during intense ion acceleration events reported by Ishigaya (2017). We focused on BBELF waves during the intense heating event observed on 11 February 1990, 18:00-18:15 UT, in the northern cusp region at an altitude of 5255-6368 km. We used a 0.5-sec resolution dataset of the wave spectra in the frequency range from 3.16 Hz to 17.8 kHz measured by VLF/MCA and DC magnetic field observed by MGF onboard the Akebono satellite. By utilizing the change of the antenna direction during one spin period of 8 sec, we analyzed the most significant spin angle of the wave electromagnetic field oscillation (θEmax and θBmax) with respect to the ambient magnetic field in each frequency range. We identified during the event that the distributions of θEmax and θBmax respectively maximized at 150-160 degrees and 30-50 degrees at 3.16 Hz, corresponding to 0.37-0.43 fcO, where fcO represents the oxygen cyclotron frequency. Also, θEmax and θBmax at 100 Hz, corresponding to 0.72-0.84 fcO, are distributed broadly but sometimes localized at 70-150 degrees and 20-30 degrees, respectively. We compared the observed θEmax and θBmax with the polarization characteristics derived from the dispersion relation in cold plasma. We simulated the wave electromagnetic oscillations as a function of the wave normal angle and the azimuth angle of the wave vector. We examined the wave normal and azimuth angles consistent with the MCA observation at the corresponding spin phase. We found that the properties observed at 3.16 Hz are consistent with R-mode waves whose wave normal angle is 70 degrees or 110 degrees. The properties observed at 100 Hz can be explained by either L-mode (40-60 or 120-140 degree wave normal angle) or R-mode waves (30-80 or 100-150 degree wave normal angle). The wavelength of the identified R-mode and L-mode ranges from 113 km to 2701 km, suggesting the spatial scale of the wave enhancement during the event.
In the present study, we analyzed the properties of plasma waves observed by the Akebono satellite during intense ion acceleration events reported by Ishigaya (2017). We focused on BBELF waves during the intense heating event observed on 11 February 1990, 18:00-18:15 UT, in the northern cusp region at an altitude of 5255-6368 km. We used a 0.5-sec resolution dataset of the wave spectra in the frequency range from 3.16 Hz to 17.8 kHz measured by VLF/MCA and DC magnetic field observed by MGF onboard the Akebono satellite. By utilizing the change of the antenna direction during one spin period of 8 sec, we analyzed the most significant spin angle of the wave electromagnetic field oscillation (θEmax and θBmax) with respect to the ambient magnetic field in each frequency range. We identified during the event that the distributions of θEmax and θBmax respectively maximized at 150-160 degrees and 30-50 degrees at 3.16 Hz, corresponding to 0.37-0.43 fcO, where fcO represents the oxygen cyclotron frequency. Also, θEmax and θBmax at 100 Hz, corresponding to 0.72-0.84 fcO, are distributed broadly but sometimes localized at 70-150 degrees and 20-30 degrees, respectively. We compared the observed θEmax and θBmax with the polarization characteristics derived from the dispersion relation in cold plasma. We simulated the wave electromagnetic oscillations as a function of the wave normal angle and the azimuth angle of the wave vector. We examined the wave normal and azimuth angles consistent with the MCA observation at the corresponding spin phase. We found that the properties observed at 3.16 Hz are consistent with R-mode waves whose wave normal angle is 70 degrees or 110 degrees. The properties observed at 100 Hz can be explained by either L-mode (40-60 or 120-140 degree wave normal angle) or R-mode waves (30-80 or 100-150 degree wave normal angle). The wavelength of the identified R-mode and L-mode ranges from 113 km to 2701 km, suggesting the spatial scale of the wave enhancement during the event.