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] Nose, 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] Bosinger, 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.
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Measurements of low-frequency electric field by double probe technique suffer from a spurious electric field component produced by the spacecraft charging and photoelectron cloud whose center is supposed to be shifted from the center of the orbit of the probes toward the sun. The waveforms of the spurious potential difference between the spacecraft and the probes are nearly sinusoidal, so it is not easy to separate them from the natural electric field. Furthermore, the higher harmonics of the waveform are contaminated by the modulation of the probe potential arising from the interaction of photoelectrons between the probe and the spacecraft when the external magnetic field connects them.

In this study, we analyze data from PWE/EFD onboard Arase, and remove the fast, spin-phase dependent variation of probe potential from the waveform, to obtain sinusoidal curve that is supposed to be the sum of the spurious electric field and natural electric field. On the assumption that the natural electric field is perpendicular to the eternal magnetic field and that the spurious electric field is parallel to the direction toward the sun, we can estimate the spurious component only when the eternal magnetic field is parallel to the orbital plane of the probes (the spin plane of the spacecraft). A possible relationship is examined between the spurious electric field component and the spacecraft electric potential, since the spacecraft potential is related with the spacecraft charging due to photoemission, and the photoelectron yield is determined by the solar irradiance and the surface material of the spacecraft, which do not change so abruptly. If we find the relationship, we can estimate the spurious electric field component and subtract it from observation.
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