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太陽風プロトンの月面散乱における!散乱角依存性の研究
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上村洸太"#!齋藤義文$#!西野真木$#!横田勝一郎$#!浅村和史$#!田中孝明"#!綱川秀夫%!
"東大・理・地球惑星#!$&'(')*(+(#!%東工大・理・地惑
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月は・・・!• グローバルな固有磁場を持たない!
!但し局所的に磁気異常は存在!• 濃い大気を持たない!• レゴリスに覆われている!
!太陽風中のイオンは月面のレゴリスと相互作用
月について
イントロダクション
しかし「かぐや」衛星以前は・・・!月表面との相互作用はよく分かっていない!
!(月低高度での観測が殆ど無かったため)
3PACE In-flight Performance and Initial Results
Fig. 20 An example of backscattered solar wind ions. Panels (a) and (b) are omni-directional E-t spectro-grams from IEA and IMA. The vertical scale is the energy of ions while the horizontal axis is time. The colorof each bin depicts the ion counts in each energy bin at the time of observations. IMA observed backscat-tered solar wind ions (during the time period indicated by a horizontal blue line between panels (a) and(b)) while IEA observed solar wind ions. The two red arrows indicate solar wind ions observed by IMA inthe polar region. Panels (c) and (d) are altitude of SELENE, magnetic field intensity and direction in theGeocentric Solar Ecliptic (GSE) polar coordinate system. Panels (e) and (f) show the solar zenith angle andlatitudinal/longitudinal position of SELENE in the Mean Earth/Polar Axis (ME) coordinate system
ion populations are solar wind ions backscattered at the lunar surface (Saito et al. 2008b).Instead of being perfectly absorbed by the lunar surface, about 0.1% to 1% of the inci-dent solar wind ions were backscattered. Before showing the data, we will briefly explainthe measurement configuration of the PACE sensors shown schematically in Fig. 19. SinceESA-S1 and IMA were installed on the spacecraft panel facing the Moon surface, ESA-S1and IMA mostly measured electrons and ions propagating away from the Moon. On theother hand, ESA-S2 and IEA which were installed on the opposite spacecraft panel mostlymeasured electrons and ions going towards the Moon. Note that solar wind ions were de-tected by IEA or IMA depending on the position of the spacecraft. While IEA measuredsolar wind ions on the dayside, IMA measured solar wind ions near the day-night termi-nator line. Figure 20 shows an example of the backscattered ions (during the time periodindicated by a horizontal blue line between panels (a) and (b)). The backscattered ions hadlower energy than the incident solar wind ions since part of the energy was lost when so-lar wind ions collided with the Moon. Although the solar wind consists of alpha particlesas a second major component, it was found that the backscattered ions consisted of almostno alpha particles (Saito et al. 2008b). The upper limit of the alpha particle flux was 0.1%of the backscattered proton flux. We also found a relationship between the lowest energyend of the backscattered ions and the latitude where the backscattered solar wind ions wereobserved. While the maximum energy of these ions was constant at slightly lower than thesolar wind proton energy, the width of the energy distribution varied gradually, so that theminimum energy was larger at high latitudes than at the equator. Since the detail of the solar
太陽風プロトン
PACE In-flight Performance and Initial Results
Fig. 20 An example of backscattered solar wind ions. Panels (a) and (b) are omni-directional E-t spectro-grams from IEA and IMA. The vertical scale is the energy of ions while the horizontal axis is time. The colorof each bin depicts the ion counts in each energy bin at the time of observations. IMA observed backscat-tered solar wind ions (during the time period indicated by a horizontal blue line between panels (a) and(b)) while IEA observed solar wind ions. The two red arrows indicate solar wind ions observed by IMA inthe polar region. Panels (c) and (d) are altitude of SELENE, magnetic field intensity and direction in theGeocentric Solar Ecliptic (GSE) polar coordinate system. Panels (e) and (f) show the solar zenith angle andlatitudinal/longitudinal position of SELENE in the Mean Earth/Polar Axis (ME) coordinate system
ion populations are solar wind ions backscattered at the lunar surface (Saito et al. 2008b).Instead of being perfectly absorbed by the lunar surface, about 0.1% to 1% of the inci-dent solar wind ions were backscattered. Before showing the data, we will briefly explainthe measurement configuration of the PACE sensors shown schematically in Fig. 19. SinceESA-S1 and IMA were installed on the spacecraft panel facing the Moon surface, ESA-S1and IMA mostly measured electrons and ions propagating away from the Moon. On theother hand, ESA-S2 and IEA which were installed on the opposite spacecraft panel mostlymeasured electrons and ions going towards the Moon. Note that solar wind ions were de-tected by IEA or IMA depending on the position of the spacecraft. While IEA measuredsolar wind ions on the dayside, IMA measured solar wind ions near the day-night termi-nator line. Figure 20 shows an example of the backscattered ions (during the time periodindicated by a horizontal blue line between panels (a) and (b)). The backscattered ions hadlower energy than the incident solar wind ions since part of the energy was lost when so-lar wind ions collided with the Moon. Although the solar wind consists of alpha particlesas a second major component, it was found that the backscattered ions consisted of almostno alpha particles (Saito et al. 2008b). The upper limit of the alpha particle flux was 0.1%of the backscattered proton flux. We also found a relationship between the lowest energyend of the backscattered ions and the latitude where the backscattered solar wind ions wereobserved. While the maximum energy of these ions was constant at slightly lower than thesolar wind proton energy, the width of the energy distribution varied gradually, so that theminimum energy was larger at high latitudes than at the equator. Since the detail of the solar
378 Y. SAITO et al.: LOW ENERGY CHARGED PARTICLE MEASUREMENT BY PACE
at the entrance of the LEF TOF mass analyzer, which gen-erates start electrons when the ions pass through the carbonfoil. The start electrons are accelerated by the electric fieldinside the mass analyzer and their positions are detectedby one-dimensional circular resistive anode that is placedbehind the MCP. These start electrons also generate startsignals when they pass through a grid anode that is placedbetween the position-sensitive anode and the MCP. Mostof the ions that pass though the carbon foil lose their ini-tial charge state and enter into the mass analyzer as neu-tral particles. These neutral particles are detected by ananode that is in the center of the position-sensitive anode.These signals are used as stop signals. The mass/charge ofthe incident ions can be calculated from its energy/chargeand the time of flight. The relation between the time offlight t and the mass/charge m/q for neutral particles is ex-pressed as m/q = 2/L2(E/q + Vacc)t2, where L is theflight path length of the neutral particles, and Vacc is thepost-acceleration voltage. Some of the incident ions enterthe time-of-flight section as ions. These ions are reflectedby the linear electric field whose intensity is proportionalto the distance from the entrance point. The reflected ionsgenerate secondary electrons when they collide with thetop part of the mass analyzer. These electrons are accel-erated and detected by the center anode, a process whichgenerates stop signals. The relation between the time offlight t and the mass/charge m/q for ions is expressed asm/q = t2/!2a, where a is the linear electric field gradi-ent. Since time of flight of the reflected ions is proportionalto the square root of the ion mass, the mass of the incidentions can be determined precisely without being affected bythe angular scattering and the energy degradation caused bythe ion passage in the carbon foil (McComas and Nordholt,1990; Yokota et al., 2005). For the ions, the time-of-flightsection of IMA acts as the so-called “isochronous time-of-flight”. With this design of LEF-TOF mass spectrometer,the center anode detects both neutrals (direct time-of-flight)and secondary electrons from the top part of the mass an-alyzer, generated by ions (isochronous time-of-flight). Wehave measured the time profile of several ion species byinjecting those ions into the IMA. Each ion species hasits own characteristic TOF profile, including neutral peak,ion peak, and/or negative ion peak (Yokota et al., 2005).Since some of these TOF profile overlap with each other,we will have to use a deconvolution tool made by using cal-ibration data of TOF profiles in order to determine the massof measured ions if a number of species exist simultane-ously (Yokota et al., 2005). Tables 3 and 4 summarize thespecifications of IMA and IEA, respectively.
4. Instrument CalibrationWe have calibrated the PACE sensors by putting the sen-
sors in a vacuum chamber one by one and injecting nitro-gen ions. In order to calibrate IMA, we have also used hy-drogen, helium, carbon, oxygen, and heavier ions such assodium and chlorine to obtain the time profile of the TOFanalyzed ions. Figure 4 shows the schematic configurationof the calibration experiment. The sensor under calibrationis installed on a rotation table that has a rotation axis parallelto the sensor’s axis of rotational symmetry. The rotation ta-
Fig. 2. Cross section of IMA.
10cm
ionsions
IEA-Supper deflector
lower deflector
MCP position sensitive anode
Fig. 3. Cross section of IEA.
ble with the sensor under calibration is installed on anotherrotation table that has a rotation axis perpendicular to theother rotation axis. The ion beam profile is intermittentlymonitored by an MCP with a position-sensitive anode thatis installed on a movable arm. Most of the data are obtainedusing 6-keV ion beams since the beam profile is uniformand stable. Lower energy ions (2 keV) are used to measurethe performance of the geometrical factor controlling elec-trode. An analysis of the obtained data confirmed that thePACE sensors are assembled within the mechanical toler-ance (<0.1 mm) and that their characteristics are quite sim-ilar to the numerically calculated results. The homogene-
[1]Saito, Y., et al. (2008), Low-energy charged particle measurement by MAP-PACE onboard SELENE, Earth Planets Space, 60, 375."[2]Saito, Y., et al. (2008), Solar wind proton reflection at the lunar surface: Low energy ion measurement by MAP-PACE onboard SELENE (KAGUYA), Geophys. Res. Lett., 35, L24205, doi:10.1029/2008GL036077."[3] Saito Y., S. Yokota, K. Asamura, T. Tanaka, M. N. Nishino, T. Yamamoto, Y. Terakawa, M. Fujimoto, H. Hasegawa, H. Hayakawa, M. Hirahara, M. Hoshino, S. Machida, T. Mukai, T. Nagai, T. Nagatsuma, T. Nakagawa, M. Nakamura, K. Oyama, E. Sagawa, S. Sasaki, K. Seki, I. Shinohara, T. Terasawa, H. Tsunakawa, H. Shibuya, M. Matsushima, H. Shimizu, F. Takahashi, In-flight Performance and Initial Results of Plasma Energy Angle and Composition Experiment (PACE) on SELENE (Kaguya), Space Sci. Rev., 154, 1-4, 265-303, 2010, doi:10.1007/s11214-010-9647-x"