T. Terasawa

Solar wind proton reflection at the lunar surface: Low energy ion measurement by MAP-PACE onboard SELENE (KAGUYA)

[1]xa0Interaction 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 Moons 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%∼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.

Solar‐wind proton access deep into the near‐Moon wake

[1]xa0We study solar wind (SW) entry deep into the near-Moon wake using SELENE (KAGUYA) data. It has been known that SW protons flowing around the Moon access the central region of the distant lunar wake, while their intrusion deep into the near-Moon wake has never been expected. We show that SW protons sneak into the deepest lunar wake (anti-subsolar region at ∼100 km altitude), and that the entry yields strong asymmetry of the near-Moon wake environment. Particle trajectory calculations demonstrate that these SW protons are once scattered at the lunar dayside surface, picked-up by the SW motional electric field, and finally sneak into the deepest wake. Our results mean that the SW protons scattered at the lunar dayside surface and coming into the night side region are crucial for plasma environment in the wake, suggesting absorption of ambient SW electrons into the wake to maintain quasi-neutrality.

First direct detection of ions originating from the Moon by MAP‐PACE IMA onboard SELENE (KAGUYA)

[1] The Moon has no global intrinsic magnetic field and only has a very thin atmosphere. Ion measurements made from lunar orbit provide us with information regarding interactions between the solar wind and planetary surface, the surface composition through secondary ion mass spectrometry and the source and loss mechanisms of planetary tenuous atmosphere. An ion energy mass spectrometer MAP-PACE IMA onboard a lunar orbiter SELENE (KAGUYA) has detected low-energy ions at 100-km altitude. The MAP-PACE measurements have elucidated that the ions originate from the lunar surface and exosphere and that the ions are at least composed of He + , C + , O + , Na + and K + . Following the discovery of the lunar Na and K exospheres by the ground-based observation, MAP-PACE IMA have found the He, C and O exospheres around the Moon.

First in situ observation of the Moon‐originating ions in the Earth's Magnetosphere by MAP‐PACE on SELENE (KAGUYA)

[1]xa0In contrast to many ground-based optical observations of the thin lunar alkali exosphere, in situ observations of the exospheric ions by satellite-borne plasma instruments have been quite rare. MAP-PACE-IMA onboard Japanese lunar orbiter SELENE (KAGUYA) succeeded in detecting Moon originating ions at 100 km altitude. Here we make the first report of the ion detection during intervals when the Moon was embedded in the Earths magnetotail lobe. In the absence of plasma effects on the source process, ion species of H+, He++, He+, C+, O+, Na+, K+ and Ar+ are definitively identified. The ion fluxes were higher when the solar zenith angle was smaller, which is consistent with the idea that the solar photon driven processes dominates in supplying exospheric components.

Repeated injections of energy in the first 600?ms of the giant flare of SGR?1806 - 20

The massive flare of 27 December 2004 from the soft γ-ray repeater SGRu20091806–20, a possible magnetar, saturated almost all γ-ray detectors, meaning that the profile of the pulse was poorly characterized. An accurate profile is essential to determine physically what was happening at the source. Here we report the unsaturated γ-ray profile for the first 600u2009ms of the flare, with a time resolution of 5.48u2009ms. The peak of the profile (of the order of 107u2009photonsu2009cm-2u2009s-1) was reached ∼50u2009ms after the onset of the flare, and was then followed by a gradual decrease with superposed oscillatory modulations possibly representing repeated energy injections with ∼60-ms intervals. The implied total energy is comparable to the stored magnetic energy in a magnetar (∼ 1047u2009erg) based on the dipole magnetic field intensity (∼ 1015u2009G), suggesting either that the energy release mechanism was extremely efficient or that the interior magnetic field is much stronger than the external dipole field.On December 27, 2004, plasma particle detectors on the GEOTAIL spacecraft detected an extremely strong signal of hard X-ray photons from the giant flare of SGR1806-20, a magnetar candidate. While practically all gamma-ray detectors on any satellites were saturated during the first ~500 ms interval after the onset, one of the particle detectors on GEOTAIL was not saturated and provided unique measurements of the hard X-ray intensity and the profile for the first 600 ms interval with 5.48 ms time resolution. After ~50 ms from the initial rapid onset, the peak photon flux (integrated above ~50 keV) reached the order of 10^7 photons sec^{-1} cm^{-2}. Assuming a blackbody spectrum with kT=175 keV, we estimate the peak energy flux to be 21 erg sec^{-1} cm^{-2} and the fluence (for 0-600 ms) to be 2.4 erg cm^{-2}. The implied energy release comparable to the magnetic energy stored in a magnetar (~10^{47} erg) suggests an extremely efficient energy release mechanism.

Pairwise energy gain‐loss feature of solar wind protons in the near‐Moon wake

[1]xa0We study solar wind (SW) intrusion into the near-Moon wake using SELENE (KAGUYA) data. It has been known that SW protons are gradually accelerated toward the wake center along magnetic field in the distant lunar wake, while SW intrusion into the near-Moon wake has never been measured. We show that the SW protons come into the lunar wake at ∼100 km altitude in the direction perpendicular to the magnetic field, as they gain kinetic energy in one hemisphere while lose in the other hemisphere. Particle trajectory calculations and theoretical treatment demonstrate that proton Larmor motions and inward electric field around the wake boundary result in energy gain and loss of the SW protons. Our result shows emergence of proton particle dynamics around the near-Moon space, and suggests that the SW protons may relatively easily access the low-latitude and low-altitude region on the lunar night side.

Low-energy charged particle measurement by MAP-PACE onboard SELENE

MAP-PACE (MAgnetic field and Plasma experiment-Plasma energy Angle and Composition Experiment) is one of the scientific instruments onboard the SELENE (SELenological and ENgineering Explorer) satellite. PACE consists of four sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). ESA-S1 and S2 measure the distribution function of low-energy electrons below 15 keV, while IMA and IEA measure the distribution function of low energy ions below 28 keV/q. Each sensor has a hemispherical field of view. Since SELENE is a three-axis stabilized spacecraft, a pair of electron sensors (ESA-S1 and S2) and a pair of ion sensors (IMA and IEA) are necessary for obtaining a three-dimensional distribution function of electrons and ions. The scientific objectives of PACE are (1) to measure the ions sputtered from the lunar surface and the lunar atmosphere, (2) to measure the magnetic anomaly on the lunar surface using two ESAs and a magnetometer onboard SELENE simultaneously as an electron reflectometer, (3) to resolve the Moon-solar wind interaction, (4) to resolve the Moon-Earth’s magnetosphere interaction, and (5) to observe the Earth’s magnetotail.

Whistler critical Mach number and electron acceleration at the bow shock: Geotail observation

[1]xa0The ‘whistler critical Mach number’, Mcritw, is one of the dimensionless parameters that characterizes collisionless shocks. Originally, it was introduced to indicate the critical point above which whistler waves do not propagate upstream. Indeed our analysis of Geotail data at the Earths bow shock shows intense whistler waves in the sub-critical regime, MA < Mcritw, but not in the super-critical regime. In this paper, we further report that Mcritw seems to regulate the electron acceleration efficiency at the shocks. At the shock transition layer it is found that the spectral index Γ of electron energy spectra defined by f(E) ∝ E−Γ is distributed between 3.5 and 5.0 in the sub-critical regime, while the hardest energy spectra with Γ = 3–3.5 are detected in the super-critical regime. We discuss a possible relationship between Mcritw and the electron acceleration.

Ionospheric disturbances caused by SGR 1900+14 giant gamma ray flare in 1998: Constraints on the energy spectrum of the flare

[1]xa0On 27 August 1998, the Soft Gamma Ray Repeater (SGR) 1900+14, which is an exotic neutron star located near the Galactic Center, produced a giant flare. Gamma rays from the giant flare unusually ionized the lower ionosphere, and the ionospheric disturbance was detected as a large-amplitude change of the VLF signal whose propagation distance is relatively short (870 km). The peak flux of the flare was so huge that it saturated all the gamma ray detectors on the spacecraft; consequently, the flux and the spectrum during the most intense period was poorly determined. Tanaka et al. (2007) have recently derived the accurate peak flux of the flare from the Geotail data. By means of model calculations based on this accurate estimation and their comparisons with the short-distance VLF data, we have found that the spectrum during the most intense period was one temperature (kT = 240 keV) optically thin thermal Bremsstrahlung (OTTB). This result provides us with a clue to reveal the emission and triggering mechanisms of the giant flare.

Field‐aligned beam observations at the quasi‐perpendicular bow shock: Generation and shock angle dependence

[1]xa0On 19 October 1995 the Geotail satellite skimmed along the quasi-perpendicular bow shock for more than 3 hours, and field-aligned ion beams (FABs) were continuously observed in the foreshock region. Also observed were 11 crossings with the bow shock where these FABs are thought to be generated. The upstream condition of the bow shock was in the state of low Mach number (MA ∼ 2.9) and low beta (β ∼ 0.02). By a detailed study of the evolution of the ion distribution across one of the crossings, we have found that leakage as origin of these ions from the magnetosheath is unlikely. Comparison of the observations with simulations suggests that FAB ions are generated by multiple interaction of incoming solar wind ions with the bow shock. We have then compared the FAB flux normalized by the solar wind flux, , with the shock angle θBn and have found that falls off rapidly just above θBn > 60°, but it maintains significant level ( ∼ 0.01%) up to ∼75°.