衛星による電場観測にて多く用いられるダブルプローブ法では、 衛星から伸展させた一対のプローブ間の電位差を計測する。 日照中でのプラズマ中にある衛星の電位は、周辺プラズマからの流入電子電流と 衛星からの流出光電子電流とのつり合いで決まり、前者が少ない低密度域では正に定まる。 プローブに電子を注入するバイアス電流を供給すると、プローブの電位はよりプラズマ電位に近く安定するため、 ２つのプローブの電位差は宇宙空間の電場を反映することになる。
この方法で計測された電場は、電子密度の低いときに太陽方向の成分が強まる問題がある。 これは自然電場と考えにくく，計測によって生じた人工的な擬似電場と考えられる。 その原因として、衛星からの光電子雲と正の衛星帯電が考えられている。 光電子雲は衛星から太陽方向に偏って放出され、衛星帯電の中心も衛星表面が等電位になるよう スピン軸から太陽方向にずれると予想される。その結果生じる双極子電場は プローブ間の電位差波形に正弦波をつくり、太陽方向の擬似電場として検出される。 もう一つの原因として、太陽方向のプローブでは光電子雲からの電子電流によって電位が下がり、 反対側のプローブでは日照側から放出された光電子が逃走するためのポテンシャル障壁が衛星のつくる電位のために下がり、 正味の光電子電流が増加するためプローブ電位が上がる結果、 プローブのシース電位の違いが太陽方向の電位差として検出されることも考えられる。
これらの影響を排した電場解析のためには、擬似成分を推定して観測値から差し引くことが求められる。本研究では、 あらせ衛星のプラズマ波動・電場観測器(Plasma Wave Experiment / Electric Field Detector, PWE/EFD) のデータから 擬似電場の推定を試みる。ここで、観測される電場は自然の成分と擬似成分からなり、自然電場は外部磁場と直交し、 擬似電場は太陽方向を指すと仮定する。これらを踏まえると、外部磁場が衛星のスピン面と平行の場合には、 擬似電場の太陽方向成分が推定できる。2017 年4 月から2024 年3 月までのデータを1 か月ごとに調査したところ、 電子密度100cm?3 以下で擬似電場の発生が確認された。 なお、この電子密度はPWE/HFA によってUHR 周波数から求められたものである。 推定ができた月のうち2/3 は、電子密度が10 cm^-3 - 100 cm^-3の範囲で 電子密度の対数と擬似電場の間に線形性がみられた。 線形回帰式から擬似電場を電子密度の関数として試算したところ、 観測された電場の太陽方向成分に近い値となる場合もあった。 試算した擬似成分を観測電場から差し引くと、太陽向き成分の卓越が改善された。しかしこれは いつもうまくいくわけではなく、過剰に差し引いてしまう場合もあった。 光電子雲が背景磁場にガイドされて擬似電場が太陽方向を指さない可能性を考えたが、 背景磁場の方向と電場の差し引き結果に関連性はなかった。 擬似電場の推定値と電子密度の対数との間に線形性がみられないケースは2018 年- 2020 年に集中していた。 太陽紫外線と比較したものの、原因はわかっていない。

The most commonly used spaceborne technique for electric field measurement is the double probe method, which measures the potential difference between a pair of identical probes extending from the spacecraft. The potential of a sunlit probe in a tenuous plasma becomes positive due to the balance of photoelectron current and the ambient electron current. A bias current applied to the probe reduces its potential to be lower than that of the spacecraft potential. In a tenuous plasma, the electric field measurement using double probes can suffer from a spurious sunward electric field due to photoelectrons emitted from the spacecraft and the positive charging of the spacecraft body. The photoelectrons are emitted from the sunlit side of the spacecraft and the center of the photoelectron cloud is thought to be shifted toward the sun, attracting the surface charge of the spacecraft. The resultant pair of charges localized on the sunlit side induce a dipole-like electric field pointing sunward. This local electric field is registered as a sinusoidal curve of the potential difference between the two probes as a function of the phase of the satellite’s spin motion. Imbalance of the sheath potentials between the two probes can also be detected as a sunward spurious field. The sunward-side probe collects more photoelectrons emitted from the spacecraft, decreasing the probe potential, while the anti-sunward probe loses more escaping photoelectrons due to the positively charged spacecraft body, increasing the probe potential.
For a precise analysis of the ambient (natural) electric field, it is desirable to estimate the spurious electric field and then subtract it from a measured electric field. In this study, an attempt is made to estimate the spurious electric field from the potential difference obtained by PlasmaWave Experiment (PWE) / Electric Field Detector (EFD) onboard the Arase satellite. Our estimation of the spurious electric field was based on the following assumptions: 1) an observed electric field is a sum of natural and spurious fields, 2) the natural electric field is perpendicular to the background magnetic field, and 3) the spurious electric field points sunward. For selected cases in which the background magnetic field was parallel to the spin plane, we estimated the x-component (sunward in the spin plane) of the spurious electric field.
Monthly statistics starting from April 2017 to March 2024 revealed the presence of the spurious electric field during periods in which the electron density was less than 100 cm?3. A linear relationship is identified between the x-component of the spurious electric field and the log of electron density (log ne) in the range 10 The relationship between the spurious electric field and log ne disappeared in 2018-2020. The fitting parameters of the regression line were examined to see if there is any correlation with F10.7 intensity as an indicator of the solar UV, but the reason for the loss of the linear relation in 2018-2020 is not yet understood.
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磁気圏電場の観測は、太陽風に対する磁気圏のダイナミックな反応を知るうえで重要である。 人工衛星からの電場観測に最もよく使われるのは 2つのプローブ間の電位差を距離で割って電場を求めるダブルプローブ法であるが、 これにより検出される電場には、衛星からの光電子放出に由来する擬似的な太陽向き成分が含まれてしまうことが多い。 この影響を除いた解析を行うため、擬似電場の推定が必要である。
これまで、あらせ衛星搭載のPWE/EFD による電場観測を用いて、次のような方法で 光電子由来の擬似電場の推定を行ってきた。観測された電場を自然電場と擬似電場の和と考え、 自然電場は外部磁場に直交し、擬似電場は太陽方向を向くと仮定すると、 観測電場と外部磁場の内積は擬似電場によるものと考えることができる。 電場は衛星のスピン面内2成分のみ観測されているが、外部磁場がスピン面に平行な場合を選べば 擬似電場のスピン面内太陽方向の成分を算出することができる。こうして得られた擬似電場成分は -5〜15mV/m の大きさで、衛星電位0 − 5V の範囲では衛星電位につれて疑似電場も大きくなる傾向があった。 電子密度100cm^-3以上では擬似電場は消え、それ以下では 擬似電場と電子密度の対数との間に線形の関係が見られることがあった。 この関係から、電子密度を介して擬似電場を推定できる可能性が示された。
あらせ衛星のスピン軸は太陽方向から15 度程度であったのに対し、GEOAIL 衛星のスピン軸は 太陽方向に対し垂直に近いため、擬似電場の影響がより大きいと考えられる。 そこで、今般、GEOTAIL 衛星の電場観測について、同様の擬似電場推定を行った。使用したのは Data Archives and Transmission System (DARTS) で公開されている1994 年1-2 月の電場(EFD)、磁場(MGF) である。 その結果を低エネルギー粒子LEP によるプラズマ（イオン）密度と合わせて解析した。 擬似電場の強さはおおむね0-10mV/m であったが、あらせ衛星とは異なり、プラズマ密度10 cm^-3個以下では 擬似電場が減少していくことが分かった。これは低密度のために 衛星電位が数十V に上昇し、衛星電位から逃走できる光電子が極めて少なくなり、 日照側プローブへの光電子の影響が小さくなったためと考えられる。擬似電場成分があらせ衛星の推定より小さいのは アンテナ長（50m）があらせの場合(15m) より大きいためと考えられる。

In a tenuous plasma, electric field observations using the double probe technique suffer from spurious sunward electric field arising from photo-electron emissions from spacecraft. An attempt was made to estimate the spurious electric field component assuming that 1) an observed electric field is a sum of natural and spurious fields, 2) the natural electric field is perpendicular to the background magnetic field, and 3) the spurious electric field points the sun. A sunward component of the spurious electric field in the spin plane can be calculated for selected cases in which the background magnetic field is parallel to the spin plane. The spurious electric field components estimated from Arase PWE/EFD data exhibited a relationship with ambient electron density obtained by Arase PWE/HFA.
The spurious electric field arising from the asymmetric photoemission should be more pronounced for spacecraft such as GEOTAIL or Mio, whose spin axes are nearly perpendicular to the sun-spacecraft direction. Estimation of the spurious electric field in the GEOTAIL electric field observation was carried out by using EFD and MGF data from January 1, 1994 to February 28, 1994 in ISAS/ Data Archives and Transmission System (DARTS). The result was investigated with the spacecraft potential observed by EFD and the plasma density observed by LEP also opened in DARTS. Differently from Arase cases, the spurious electric field of GEOTAIL observation decreased in low density plasma (less than 10 cm^-3) due to high spacecraft potential.
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Measurements of low-frequency electric field in tenuous plasmas by double probe technique suffer from a spurious electric field component produced by the spacecraft charging. It is not easy to separate them from the natural electric field. The spurious electric field often points toward the sun and the waveform detected by the probes are very close to sinusoidal curve. Even though the probes are spherical, an imbalance of probe potential occurs depending on the spin phase, when the spacecraft potential affects the behavior of photoelectrons from the probes. This paper reports investigation of the potential difference between the probes and the spacecraft measured by Plasma Wave Experiment(PWE)/ Electric Field Detector(EFD) equipped with 4 spherical probes at the tip of 15m wire antenna extending from Arase spacecraft whose spin axis is typically at 15 degrees from the direction of the sun.

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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