In general, space borne measurement of the magnetospheric electric field employing the double probe technique suffers from a spurious pseudo-sunward electric field component induced by spacecraft charging and photoelectrons. The Electric Field Detector (EFD) of the Plasma Wave Experiment (PWE) instrument onboard the Arase satellite observed a distorted waveform of spin modulation in the electric potential difference between the probes and the spacecraft. The distorted waveform suggests that the spurious electric field can be represented by a combined electric potential applied by two model charges each representing the photoelectron cloud and spacecraft charging. An attempt was made to separate the spurious electric field component from the observed field to deduce the natural magnetospheric electric field by fitting some parameters of the two charges to the observed waveforms. The resultant fitted parameters successfully reproduced the observed distortion in the waveforms of the potential difference. However, in some cases, the fitting procedure overestimated the spurious component, resulting in an over-subtraction of the sunward component and thereby an erroneous electric field. That is because the spurious electric field component has a sinusoidal component with the spacecraft spin period. To prevent the over-subtraction, a higher harmonic component was then employed to estimate the model charges. The new method works in a model calculation, but does not work well for Arase observations, due to inaccuracy of the positions of the model charges, and the spin modulation in the sunlit area of the spacecraft, from which photoelectrons are emitted.

Spectral resonance structures with narrow frequency separations of 0.2 Hz were found in the ELF magnetic field data obtained at Kawatabi, Osaki, Miyagi prefecture Japan (magnetic latitude N30, L=1.35), in addition to those with wider frequency separation around 0.625 Hz [1]. The data were obtained by an induction magnetometer placed in North-South direction at a sampling frequency of 128 Hz. The data were Fourier transformed every 128 second in order to obtain high-frequency resolution spectra. Figure 1 shows an example. We can see structured enhancements during the period from 20:00 JST to 05:30 JST. At 23:30 JST, there are 18 stripes in the frequency range between 0.5 Hz and 4 Hz, thus the separation between the harmonics is 0.2 Hz. In the previous literatures, the spectral resonance structures with large frequency separation (df=0.5 - 1 Hz) were found at high latitudes, while those with small frequency separation were found in low latitudes: df=0.12 Hz at the island of Crete (L=1.3) [2], 0.2 - 0.275 Hz at Muroto (L=1.206) [3], or 0.3 - 0.4 Hz Shillong (L=1.08) [4]. At Kawatabi, the spectral resonance structures with narrow frequency separation was found in 59 days out from 963 days in 2001, 2006, and 2010 - 2012 examined. The occurrence rate 6% is nearly the same or slightly higher than those of wider frequency separation events observed at the same site. they did not always coexist with wider separation structures. The occurrence rate was higher (8.6%) in 2006 (near solar minimum) while lower (5.6%) in 2001, near the solar maximum. They were detected only on nighttime (Local Time 18 - 05) in accordance with previous literatures.
[1] T. Sato, et al.(2023), PEM09-P21, JpGU 2023.
[2] T. Bosinger, et al.(2004), Geophys. Res. Lett., 31, L18802, doi:10.1029/2004GL020777.
[3] M. Nose, et al.(2017), J. Geophys. Res., Space, 122, pp.7240-7255, doi:10.1002/2017JA024204.
[4] P. Adhitya, et al.(2022), Earth Planet. Space, 74,169, doi:10.1186/s40623-022-01730-2

磁気圏の電場を観測することは、磁気圏対流など、グローバルな磁気圏のダイナミクスをとらえるうえで重要である。磁気圏のDC電場を観測するには、人工衛星から伸展したプローブ間の電位差を計測するダブルプローブ法が多く使われてきた。衛星の自転（スピン）周期の間で一定とみなせるようなDCないし低周波の電場であれば、そのスピン面内成分は、スピン周期の正弦波振動として検出される。これによりDC的なオフセット成分を分離し、低周波電場を得るというデータ処理が従来行われてきた。 このような電場計測は、衛星からの光電子放出により大きな影響を受けることが知られている。過去においては、衛星からの光電子がプローブに流入することにより日照側プローブの電位が下がり、偽の太陽向き電場として検出されると考えられた。ジオスペース探査衛星「あらせ」のプラズマ波動・電場観測器(Plasma Wave Experiment / Electric Field Detector, PWE/EFD)においても、偽の太陽向き電場が観測されている。このような時、あらせ衛星の高時間分解能の衛星・プローブ間電位差データは、ピークに凹みのある特徴的な波形を示していた。

This paper reports spectral resonance structures in 0.5 - 8 Hz magnetic field variations found in the ELF magnetic field data obtained by an induction magnetometer EL-12 constructed by Tierra Technica Co. Ltd, placed in NorthSouth direction at Kawatabi, Osaki, Miyagi prefecture Japan. The magnetic latitude of the observation site is N30 and the L value is about 1.3. The magnetometer detects magnetic field variations from 0.1 Hz to 44 Hz at a sampling frequency of 128 Hz. The observation started on December 11, 1997 and continued until February 12, 2020 although the data coverage was limited to 51%. The data were Fourier transformed every 8 second and averaged for 2 minutes to be displayed in the form of dynamic spectra at https://www.ice.tohtech.ac.jp/nakagawa/elfdata/index.html. Figure 1 shows an example.

Measurement of electric fields is a key to understand the magnetospheric dynamics as a response to solar wind. For this objective, double probe technique with long wire antennas extending from a spinning spacecraft has been employed. It is more difficult in a tenuous magnetospheric plasma due to a long Debye shielding distance, dominance of photoemission in current balance of the probes, positive charging of spacecraft body, unstable electric potential of the spacecraft, and so on. In usual, a bias current is fed to the probes to reduce resistance of the probes to the ambient plasma and stabilize the probe potential.

References
[1] Kasahara et al., Earth, Planets and Space (2018) 70:86 https://doi.org/10.1186/s40623-018-0842-4.
[2] Kasaba et al., Earth, Planets and Space (2017) 69:174 https://doi.org/10.1186/s40623-017-0760-x.
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Spectral resonance structures in a frequency range 0.5 - 8 Hz were found in the ELF magnetic field data obtained at Kawatabi, Osaki, Miyagi prefecture Japan (magnetic latitude N30) during the period from December 11, 1997, to June 2, 2016. The data were obtained by an induction magnetometer placed in North-South direction at a sampling frequency of 128 Hz. The data were Fourier transformed every 8 second and averaged for 2 minutes to be displayed in the form of dynamic spectra [1]. Figure 1 shows an example. We can see structured enhancements approximately at 1 Hz, 2 Hz, 2.5 Hz, 3.5 Hz, 4.5 Hz. They are thought to be spectral resonance structures (SRS) 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 [2]. In accordance with previous literatures, the occurrence of the spectral resonance structure in Kawatabi was restricted within the nighttime from17 LT to 06 LT. The frequency rose from the evening toward the midnight. They were detected in rather quiet periods of geomagnetic activity. Although the data coverage was limited as low as 51%, nearly two decades of observations show a clear anticorrelation between the occurrence of the spectral resonance structures and the sunspot number, consistently with the scenario of ionospheric cavity with minimum Alfven velocity.
[1] https://www.ice.tohtech.ac.jp/nakagawa/elfdata/index.html.
[2] M. Nose, et al.(2017), J. Geophys. Res., Space, 122, pp.7240-7255, doi:10.1002/2017JA024204.

Measurement of the electric field is key to understanding the magnetospheric dynamics as a response to the solar wind. For this objective, many spacecraft have employed the double probe technique with long wire antennas extending from a spinning spacecraft body. Despite improvements of the technique achieved by past satellites, accurate measurement of the electric field is still difficult and challenging particularly in a tenuous magnetospheric plasma for several reasons, such as a long Debye shielding distance, dominance of photoemission in the current balance of probes, positive charging of a spacecraft body, unstable electric potential of the spacecraft. On most of the recent satellites, a bias current is fed to the probes to reduce the resistance of probes to the ambient plasma and thereby stabilize the probe potential.

In the Arase satellite case, the Wire Probe Antenna (WPT) connected to the Electric Field Detector (EFD) of the Plasma Wave Experiment (PWE) is responsible for the electric field measurement. The WPT is two pairs of double probes comprising 60-mm-diameter spheres on tips of 15-m wire antennas. Although the antenna length is limited due to an issue of the development cost, Arase attempted to minimize the effects of asymmetric emission of photoelectrons from the spacecraft body by setting its spin axis within 15 degrees from the sun direction. Nevertheless, in a tenuous plasma, the measurement suffers from a fairly persistent, apparent sunward electric field with a strange, non-sinusoidal waveform of potential difference between the probes and the spacecraft.

In order to understand how the spurious sunward field appears, we examined the observed potential waveform data, and modeled the photoelectron cloud and the spacecraft charging by assuming a single negative charge outside the spacecraft and a positive charge on the spacecraft body, respectively. The photoelectrons are emitted from the sunlit side of the spacecraft, and there will be a sunward concentration of photoelectrons. Also, there will be a sunward shift of positive charge on the spacecraft however high the skin conductivity is. We set the model charges shifted toward the sun at different distances from the center of the spin of the wire antennas. Even with small angle of the spin axis with respect to the Sun, the model charges are off the spin axis, and the distance of a probe from each electric charge varies depending on the spin phase. Separation of positive and negative charges causes difference in electric potential arising from them, producing a non-sinusoidal waveform of electric potential at the probe. For each spin of Arase we calculate the electric charges and the best-fit position of the negative charge that can well reproduce the observed waveforms of potential difference between the probes and the spacecraft. With this simple model, the apparent sunward electric field is partly reproduced although an effect of spin-modulation of photoemissions arising from variation of the illuminated cross-section of the spacecraft has not yet been considered.