PPARC セミナー (2025/06/09)

PPARC セミナー (2025/06/09)

(1)

[Name]

Keiya Kawagata

[Title]

Arase衛星の衛星電位を用いた地球磁気圏・電離圏における電子密度・温度導出の検討

Examination of electron density and temperature in the Earth’s magnetosphere and ionosphere using the satellite potential observed by Arase

[Abstract]

　Arase衛星は、2017年3月から約7年間に渡りジオスペースの電子密度・温度決定に資するデータを取得し続けている。密度・温度は、電離圏・プラズマ圏・磁気圏の構造を決める基本情報である。密度・温度は、分散関係への影響を通して波動の成長・減衰・伝搬にも支配的である。

　Arase衛星では、PWE/HFA（プラズマ波動計測器/高調波受信部）の電場スペクトル（10kHz～10 MHz）からUHR（高域ハイブリッド共鳴波動）周波数を自動判定と目視の組み合わせで同定し（時間分解能：1 min）、背景磁場強度と併せて電子密度を決定している。この方法は、10eV以下の低温成分を含む電子密度を高精度で決定できる。しかし、UHR周波数近傍に他の強い波動が見られる領域やUHR波動が弱い低密度領域で、電子密度が決定されない領域が多く生じる。他衛星では低エネルギーイオン・電子データが用いられてきたが、Araseの静電プラズマ分析器LEPe・LEPiは〜20 eV以下の計測が難しい。粒子計測からの密度導出では、地球に近いArase軌道で多い低温プラズマの分布関数に仮定が必要となる。

　衛星電位も、電子密度の指標となりえる。プラズマ中における衛星の浮動電位は、太陽紫外線による光電子流出と周辺プラズマからの電子流入のバランスで決定される。衛星表面材料に依存する光電子・二次電子放出効率、衛星の形状・姿勢、および周辺電子温度に影響され、衛星電位から演繹される電子密度の精度は低い。しかし、目視や特定の仮定に依存することなく1-spin（8sec程度）の分解能で観測量を取得できるため、光電子放出が無い日陰時を除き、ある程度の信頼性を持った電子密度を常時導出することが可能である。

しかし、Geotail衛星やCluster衛星におけるプラズマ密度（イオン直接計測で決定）- 衛星電位関係と比べると、Arase衛星におけるUHR由来電子密度 – 衛星電位関係は特に1 /cc以下で精度が低い。これは、低密度域でUHR周波数由来電子密度の精度が低いことに加え、アンテナ長が15 mと短く（Geotail・Clusterは50 m）「Debye長＞アンテナ長」となりやすい低密度域ではプローブ電位が衛星電位の影響下にありうることなどに起因しうる。

これらの検証のため、Arase衛星の軌道全域にわたり、複数の手法を用いたプラズマ密度導出の比較とそれらの精度の検証を行ってきた。UHR周波数に基づく電子密度情報が提供されている2017年4月から2022年4月を対象としている。軌道位置への依存性の評価も行い、領域によって異なるプラズマ温度環境が与える密度推定への影響評価も目指している。これまでの解析結果では、低密度域で明らかにUHR由来電子密度が過小評価されている例を見出しており、具体的な事例について検討を加えつつある。また、磁気擾乱時と比較して磁気静穏時の方が、特に低密度領域において良好な相関を示す。プラズマ温度に影響ありえるため、この視点でさらなる検討を行う。

上記に基づき、GeotailやClusterで行われた先行事例、および理論的に予測される「電子密度 ∝ exp(衛星電位)」の関係から逸脱するデータの原因を探る。具体的には、LEPe・LEPiによる低エネルギー電子・イオンの分布関数を参照し、プラズマ粒子情報との関係を明確化する。さらに、LEPeが提供する暫定的な低温電子密度・温度データと比較し、異なるプラズマ密度推定手法の一貫性・信頼性を検証へ向かう予定である。

プローブに与えるバイアス電流量との比較や太陽UV fluxとの関連も確認し、プローブ電位の安定度の検証も含む。この安定度は、磁気圏・電離圏電場および低周波波動の精度決定要因でもある。本研究は、喧々諤々の議論が続くArase電場計測の精度追求への一助であり、また2026年から周回観測を開始するBepiColombo/Mio水星探査機における同型プローブを用いた電子密度・電場計測の精度保証の基盤となる。

Since March 2017, the Arase satellite has continuously acquired data that contributes to determining the electron density and temperature in the geospace over a period of about seven years. Density and temperature are fundamental information which defines the structure of the ionosphere, plasmasphere, and magnetosphere. These parameters also play a dominant role in the growth, decay, and propagation of waves through their influence on the dispersion relation. 

On the Arase satellite, the electron density is determined using the electric field spectrum (10 kHz to 10 MHz) from the PWE/HFA (Plasma Wave Experiment/High Frequency Analyzer), where the UHR (Upper Hybrid Resonance) frequency is identified through a combination of automatic detection and manual inspection (temporal resolution: 1 min), in conjunction with the background magnetic field strength. This method can determine the electron density with high accuracy, including low temperature components below 10 eV. However, there are many regions where electron density cannot be determined, such as in areas where other strong waves are observed near the UHR frequency or in low-density regions where UHR waves are weak. While low-energy ion and electron data have been used on other satellites, the electrostatic plasma analyzers LEPe and LEPi on Arase have difficulty measuring energies below ～20 eV. In deriving density from particle measurements, assumptions are required for the distribution function of low-temperature plasma, which is abundant in the orbit of Arase near Earth.

Satellite potential can also serve as an indicator of electron density. The floating potential of a satellite in plasma is determined by the balance between photoelectron emission due to solar ultraviolet radiation and electron influx from the surrounding plasma. This potential is influenced by factors such as the efficiency of photoelectron and secondary electron emission depending on the satellite’s surface material, the shape of satellite  and orientation, and the surrounding electron temperature, which results in low accuracy for electron density inferred from satellite potential. However, since observational data can be obtained with a resolution of 1-spin (about 8 seconds) without relying on manual inspection or specific assumptions, it is possible to consistently derive electron density with a certain degree of reliability, except during shadow periods when there is no photoelectron emission.

Compared to the plasma density (determined by direct ion measurement) versus satellite potential relationship observed with the Geotail and Cluster satellites, the UHR-derived electron density versus satellite potential relationship on Arase shows particularly low accuracy below 1 /cc. This may be due to the low accuracy of UHR-derived electron density in low-density regions and the short antenna length of 15 m (Geotail and Cluster have 50 m), which in low-density regions where ‘Debye length > antenna length’ may cause the probe potential to be influenced by the satellite potential.

To verify these findings, we have conducted a comparison of plasma density derivation using multiple methods across the entire orbit of the Arase satellite and verified their accuracy. The study covers the period from April 2017 to April 2022, during which electron density information based on UHR frequencies is provided. We also evaluate the dependence on orbital position and aim to assess the impact of different plasma temperature environments on density estimation depending on the region. In our analysis so far, we have found cases where UHR-derived electron density is clearly underestimated in low-density regions, and we are examining specific cases. Additionally, there is better correlation during magnetically quiet periods compared to magnetically disturbed periods, particularly in low-density regions. Further investigation is needed from this perspective due to its potential impact on plasma temperature.

Based on the above, we will explore the causes of data that deviate from the theoretically predicted ‘electron density ∝ exp(satellite potential)’ relationship and from prior cases observed with Geotail and Cluster. Specifically, we will refer to the distribution functions of low-energy electrons and ions provided by LEPe and LEPi to clarify their relationship with plasma particle information. Furthermore, we plan to compare with the provisional low-temperature electron density and temperature data provided by LEPe to verify the consistency and reliability of different plasma density estimation methods.

We will also examine the comparison with the amount of bias current applied to the probe and its relation to solar UV flux, including verification of the stability of probe potential. This stability is also a factor in the accurate determination of electric fields in the magnetosphere and ionosphere as well as low-frequency waves. This research contributes to the pursuit of accuracy in Arase’s electric field measurements, which have been the subject of heated debate, and also lays the foundation for ensuring the accuracy of electron density and electric field measurements using similar probes on the BepiColombo/Mio Mercury exploration mission, which will begin orbiting in 2026.

(2)

[Name]

Takeshi Sakanoi

[Title]

Super plasma bubble observed at Iitate during a magnetic storm on Jan. 1, 2025
2025年1月1日磁気嵐時に飯舘で観測されたスーパープラズマバブル

[Abstract]

We report a northern most optical emission of plasma bubble extending to the northern Tohoku areaobserved at the Iitate observaroty during a magnetic storm event on Jan. 1, 2025. We developedthe monochromatic all-sky camera at a wavelength of OI 630nm and its operation system, which isthe same all-sky camera system as installed atthe Syowa, Davis, Casey, Dumont d’Urville, and Concordia (Dome-C). A major magnetic storm occurred on Jan. 1, 2025 with the Dst index reaching to -221 nT at 17UT. There are reports that low-latitude auroras appeared at several locations in Hokkaido and Iwate prefectures. During this night, the weather of Iitate is partially cloudy, and we could sometimes see stars and 630nm airglow emission with clear sky conditions.

In this presentation, we fucus on two emission bands extending from north to south in the all-sky field-of-view during the period from ~19:20 UT to dawn twilight (~20:40 UT). The emission bands moved almost correlated with the rotation of Earth, and also drifted to the westward direction. At almost the same timing from 19:10 UT, the enhanced Rate of TEC change Index (ROTI) area expanded from the Kanto area (36 deg Glon.) to the north of Tohoku (42 deg Glon) obtained from GNSS Earth Observation Network (GEONET).  From the ROTI data, we suggest the north-south enhancement of ROTI and optical emission structure were caused by plasma bubble caused by strong equatorial electric field during the major magnetic storm. The plasma bubble was also observed by the OMTI-all sky imagers at Shigaraki and Rikubetsu.

The event that the plasma bubble extended to the north Tohoku during the major magnetic storm indicates that ionospheric disturbances directly above the Kanto metropolitan area could cause a degradation of pointing accuracy..