西野 真木

Interaction between the solar wind and objects in the solar system varies largely according to the settings, such as the existence of a global intrinsic magnetic field and/or thick atmosphere. The Moon's case is characterized by the absence of both of them. Low energy ion measurements on the lunar orbit is realized more than 30 years after the Apollo period by low energy charged particle analyzers MAP-PACE on board SELENE(KAGUYA). MAP-PACE ion sensors have found that 0.1%similar to 1% of the solar wind protons are reflected back from the Moon instead of being absorbed by the lunar surface. Some of the reflected ions are accelerated above solar wind energy as they are picked-up by the solar wind convection electric field. The proton reflection that we have newly discovered around the Moon should be a universal process that characterizes the environment of an airless body. Citation: 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.

月は8割以上の時間を太陽風中で過ごすが,その際に夜側にウェイクと呼ばれる真空に近い領域が形成される.太陽風の電子は一部が夜側の月面に到達できるが,イオンは熱速度が太陽風速度と比較して圧倒的に遅いためウェイク側の月面には到達できないと考えられている.ところが今回,月周回衛星「かぐや」に搭載されたプラズマ・磁場観測装置(MAP)は,月の真夜中側100km高度のほぼ中央部分(太陽直下点の反対側付近の低緯度領域)で太陽風起源とみられるイオンを観測した.これらのイオンは一部が夜側の月面から飛来しており,夜側へと加速された太陽風が月面に衝突して反射したものを観測したと考えられる.また,ウェイク境界で太陽風イオンが夜側へ向かって加速される現場も観測された.これらの現象は何らかの理由でウェイク境界付近の電場が強まった結果として起きたものと考えられる.

During a storm recovery phase on 15 May 2005, the Geotail spacecraft repeatedly observed high-energy (>180 keV) oxygen ions in the dayside magnetosheath near the equatorial plane. We focused on the time period from 11: 20 UT to 13:00UT, when Geotail observed the oxygen ions and the interplanetary magnetic field (IMF) was constantly northward. The magnetic reconnection occurrence northward and duskward of Geotail is indicated by the Walen analysis and convective flows in the magnetopause boundary layer. Anisotropic pitch angle distributions of ions suggest that high-energy oxygen ions escaped from the northward of Geotail along the reconnected magnetic field lines. From the low-energy particle precipitation in the polar cap observed by DMSP, which is consistent with magnetic reconnection occurring between the magnetosheath field lines and the magnetospheric closed field lines, we conclude that these oxygen ions are of ring current origin. Our results thus suggest a new escape route of oxygen ions during northward IMF. In the present event, this escape mechanism is more dominant than the leakage via the finite Larmor radius effect across the dayside equatorial magnetopause.

Abstract. Through the effort to obtain clues toward understanding of transport of cold plasma in the near-Earth magnetotail under northward IMF, we find that two-component protons are observed in the midnight plasma sheet (−10>XGSM>−30 RE, |YGSM| <10 RE) under northward IMF by the Geotail spacecraft. Since the two-component protons are frequently observed on the duskside during northward IMF intervals but hardly on the dawnside, those found in the midnight plasma sheet are thought to come from the dusk flank. The cold proton component in the midnight region occasionally has a parallel anisotropy, which resembles that in the tail flank on the duskside. The flows in the plasma sheet with two-component protons were quite stagnant or slightly going dawnward, which supports the idea that the observed two-component protons in the midnight region are of duskside origin. Because the two-component protons in the midnight plasma sheet emerge under strongly northward IMF with the latitudinal angle larger than 45 degrees, and because the lag from the strongly northward IMF to the emergence can be as short as a few hours, we suggest that prompt plasma transport from the dusk to midnight region occurs under strongly northward IMF. We propose that the dawnward flows result from viscous interaction between the high-latitude portion of the plasma sheet and the lobe cell. Another candidate for plasma transport process from the dusk to the midnight region is turbulent flow due to vortical structures of the Kelvin-Helmholtz instability that developed around the dusk low-latitude boundary under strongly northward IMF. In addition, we also suggest that gradual cooling of hot protons under northward IMF is a global phenomenon in the near-Earth magnetotail.

Abstract. To further our understanding of the solar wind entry across the magnetopause under northward IMF, we perform a case study of a duskside Kelvin-Helmholtz (KH) vortex event on 24 March 1995. We have found that the protons consist of two separate (cold and hot) components in the magnetosphere-like region inside the KH vortical structure. The cold proton component occasionally consisted of counter-streaming beams near the current layer in the KH vortical structure. Low-energy bidirectional electron beams or flat-topped electron distribution functions in the direction along the local magnetic field were apparent on the magnetosphere side of the current layer. We discuss that the bidirectionality of the electrons and the cold proton component implies magnetic reconnection inside the KH vortical structure. In addition, we suggest selective heating of electrons inside the vortical structure via wave-particle interactions. Comparing temperatures in the magnetosphere-like region inside the vortical structure with those in the cold plasma sheet, we show that further heating of both the electrons and the cold proton component is taking place in the cold plasma sheet or on the way from the vortices to the cold plasma sheet.

Abstract. To investigate the cold plasma sheet formation under northward IMF, we study the temperature anisotropies of electrons and two-component protons observed by the Geotail spacecraft. The two-component protons, which are occasionally observed in the dusk plasma sheet near the low-latitude boundary, are the result of spatial mixing of the hot protons of the magnetosphere proper and the cold protons from the solar wind. Recent research focusing on the two-component protons reported that the cold proton component at times has a strong anisotropy, and that the sense of the anisotropy depends on the observed locations. Since electrons have been known to possess a strong parallel anisotropy around the low-latitude boundary layer, we compare anisotropies of electrons and protons to find that the strengths of parallel anisotropies of electrons and the cold proton component are in good correlation in the tail flank. The parallel anisotropy of electrons is stronger than that of the cold proton component, which is attributed to selective heating of electrons. We further find that the strengths of the parallel anisotropies in the tail flank depend on the latitudinal angle of the IMF; strong parallel anisotropies occur under strongly northward IMF. We discuss that the Kelvin-Helmholtz vortices, which developed under strongly northward IMF, and the resultant magnetic reconnection therein may lead to the strong parallel anisotropies observed in the tail flank.

Abstract. In search for clues towards the understanding of the cold plasma sheet formation under northward IMF, we study the temperature anisotropy of the two-component protons in the plasma sheet near the dusk low-latitude boundary observed by the Geotail spacecraft. The two-component protons result from mixing of the cold component from the solar wind and the hot component of the magnetospheric origin, and may be the most eloquent evidence for the transport process across the magnetopause. The cold component occasionally has a strong anisotropy in the dusk flank, and the sense of the anisotropy depends on the observed locations: the parallel temperature is enhanced in the tail flank while the perpendicular temperature is enhanced on the dayside. The hot component is nearly isotropic in the tail while the perpendicular temperature is enhanced on the dayside. We discuss possible mechanism that can lead to the observed temperature anisotropies.

We have studied signatures of the cold plasma sheet in the duskside near-earth magnetotail with the Geotail data. In the duskside plasma sheet, cold plasmas are found 3-4 hours after the northward turning of the interplanetary magnetic field. The cold plasma sheet with two-temperature ions is often found on the duskside, while cold plasma sheet with one-temperature are also found on the duskside when northward interplanetary magnetic field continues for a very long interval (more than several hours). We discuss the development and evolution of the cold plasma sheet on the duskside.

[1] We have analyzed the structure of the magnetotail current sheet during the northward interplanetary magnetic field (IMF) intervals, using the method devised by Sergeev et al. [1998]. Case studies suggest that systematic variations of the current sheet thickness are seen not only during substorm activities but also during a building up phase of the cold dense plasma sheet when the IMF is northward and the magnetosphere is geomagnetically quiet. It is further found that during such northward IMF intervals the plasma "vertical content," namely the product N(0)lambda with the plasma density N-0 at the plasma sheet center and the characteristic thickness lambda, showed an order-of-magnitude increase within several hours. We estimate plasma transport rate during the northward IMF to be as much as 10(26)/s, which cannot be neglected in comparison with that during the southward IMF (similar to10(27) s(-1)).

It has been recognized that during extended periods of the northward interplanetary magnetic field the tail plasma sheet becomes cold and dense, showing a positive density correlation with the solar wind plasma. Recently it has been also recognized that the plasma density integrated along the Z (north-south) direction across the plasma sheet becomes also high during the northward IMF periods, which suggests a fairly high plasma supply rate of ~ 1026 protons/sec amounting nearly 10% of the enhanced supply rate during the southward interplanetary magnetic field periods. While the latter rate is considered to be caused by the efficient dayside magnetopause reconnection, it is not yet known how the plasma transport occurs during the northward interplanetary magnetic field periods. Since the highly evolved LLBL is also observed during such periods, we expect some causal relation between the plasma transport to the plasma sheet and the evolution of the LLBL. We review the key observations and discuss possible physical mechanisms of the plasma transportation.