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[PEM14-10] Occurrence property of spectral resonance structures with frequency separation of 0.2 Hz detected at Kawatabi, Miyagi, Japan
Keywords:spectral resonance structure, ionospheric Alfvén resonator, ELF, induction magnetometer, Kawatabi
Nearly two decades of magnetic field observations made at Kawatabi, Miyagi prefecture Japan, were re-examined to investigate the occurrence properties of spectral resonance structures (SRS) with narrow frequency separation (about 0.2 Hz). The SRS are structured enhancements of magnetic field variation in evenly spaced frequency bands in extremely low frequency range. They are thought to be generated by ionospheric Alfven resonator which is an ionospheric cavity with the minimum Alfven velocity bounded by E layer and a steep gradient of the Alfven velocity above the maximum of F layer [1]. At Kawatabi (magnetic latitude N30, L=1.35), the SRSs with typical frequency separation of 0.65 Hz were found in the record of magnetic field variation obtained by an induction magnetometer placed in North-South direction at a sampling frequency of 128 Hz [2]. In accordance with previous literatures, they were found during nighttime, showing a clear anticorrelation with sunspot number [2].
In response to new findings of SRS with narrow frequency separations in low latitudes [1][3][4], Kawatabi magnetic field data were Fourier transformed every 128 second in order to obtain high-frequency resolution of 7.8 mHz, and there found SRSs with narrow frequency separation of typically 0.2 Hz [5]. Figure 1 shows an example. In this paper, the whole magnetic field data in N-S direction obtained at Kawatabi during the period from December 1, 1998 to June 2, 2016 were examined. The occurrence rate of SRS with narrow frequency separation was less than 1%. Although the data coverage was as small as 40%, the occurrence showed a clear concentration in winter. They did not show an anticorrelation with the sunspot number, differently from the previously reported SRS with wider frequency separation.
[1] Nosé, et al. (2017), J. Geophys. Res., Space, 122, pp.7240-7255, doi:10.1002/2017JA024204.
[2] Nakagawa, et al. (2023), URSI Radio Science Letters, 5, DOI: 10.46620/23-0035.
[3] Bösinger, et al. (2004), Geophys. Res. Lett., 31, L18802, doi:10.1029/2004GL020777.
[4] Adhitya, et al. (2022), Earth Planet. Space, 74,169, doi:10.1186/s40623-022-01730-2.
[5] Konno et al., (2024), PEM10-P10, JpGU 2024.
In response to new findings of SRS with narrow frequency separations in low latitudes [1][3][4], Kawatabi magnetic field data were Fourier transformed every 128 second in order to obtain high-frequency resolution of 7.8 mHz, and there found SRSs with narrow frequency separation of typically 0.2 Hz [5]. Figure 1 shows an example. In this paper, the whole magnetic field data in N-S direction obtained at Kawatabi during the period from December 1, 1998 to June 2, 2016 were examined. The occurrence rate of SRS with narrow frequency separation was less than 1%. Although the data coverage was as small as 40%, the occurrence showed a clear concentration in winter. They did not show an anticorrelation with the sunspot number, differently from the previously reported SRS with wider frequency separation.
[1] Nosé, et al. (2017), J. Geophys. Res., Space, 122, pp.7240-7255, doi:10.1002/2017JA024204.
[2] Nakagawa, et al. (2023), URSI Radio Science Letters, 5, DOI: 10.46620/23-0035.
[3] Bösinger, et al. (2004), Geophys. Res. Lett., 31, L18802, doi:10.1029/2004GL020777.
[4] Adhitya, et al. (2022), Earth Planet. Space, 74,169, doi:10.1186/s40623-022-01730-2.
[5] Konno et al., (2024), PEM10-P10, JpGU 2024.