Kazushi Asamura

Remote energetic neutral atom imaging of electric potential over a lunar magnetic anomaly

The formation of electric potential over lunar magnetized regions is essential for understanding fundamental lunar science, for understanding the lunar environment, and for planning human explorati ...

Relation between fine structure of energy spectra for pulsating aurora electrons and frequency spectra of whistler mode chorus waves

We investigate the origin of the fine structure of the energy spectrum of precipitating electrons for the pulsating aurora (PsA) observed by the low-altitude Reimei satellite. The Reimei satellite achieved simultaneous observations of the optical images and precipitating electrons of the PsA from satellite altitude (~620 km) with resolution of 40 ms. The main modulation of precipitation, with a few seconds, and the internal modulations, with a few hertz, that are embedded inside the main modulations are identified above ~3 keV. Moreover, stable precipitations at ~1 keV are found for the PsA. A “precipitation gap” is discovered between two energy bands. We identify the origin of the fine structure of the energy spectrum for the precipitating electrons using the computer simulation on the wave-particle interaction between electrons and chorus waves. The lower band chorus (LBC) bursts cause the main modulation of energetic electrons, and the generation and collapse of the LBC bursts determines on-off switching of the PsA. A train of rising tone elements embedded in the LBC bursts drives the internal modulations. A close set of upper band chorus (UBC) waves causes the stable precipitations at ~1 keV. We show that a wave power gap around the half gyrofrequency at the equatorial plane in the magnetosphere between LBC and UBC reduces the loss rate of electrons at the intermediate energy range, forming a gap of precipitating electrons in the ionosphere.

First direct observation of sputtered lunar oxygen

We present the first direct measurement of neutral oxygen in the lunar exosphere, detected by the Chandrayaan-1 Energetic Neutral Analyzer (CENA). With the lunar surface consisting of about 60% of oxygen in number, the neutral oxygen detected in CENAs energy range (11 eV−3.3 keV) is attributed to have originated from the lunar surface, where it was released through solar wind ion sputtering. Fitting of CENAs mass spectra with calibration spectra from ground and in-flight data resulted in the detection of a robust oxygen signal, with a flux of 0.2 to 0.4 times the flux of backscattered hydrogen, depending on the solar wind helium content and particle velocity. For the two solar wind types observed, we derive subsolar surface oxygen atom densities of N0= (1.1 ± 0.3) · 107m−3 and (1.4 ± 0.4) · 107m−3, respectively, which agree well with earlier model predictions and measured upper limits. From these surface densities, we derive column densities of NC= (1.5 ± 0.5) · 1013 m−2and (1.6 ± 0.5) · 1013 m−2. In addition, we identified for the first time a helium component. This helium is attributed to backscattering of solar wind helium (alpha particles) from the lunar surface as neutral energetic helium atoms, which has also been observed for the first time. This identification is supported by the characteristic energy of the measured helium atoms, which is roughly 4 times the energy of reflected solar wind hydrogen, and the correlation with solar wind helium content.

Electron properties in inverted-V structures and their vicinities based on Reimei observations

This paper reports two event studies on characteristic pitch angle variations of inverted-V electrons observed by the Reimei satellite. At edges of the inverted-V structures, the electron pitch angles are collimated between 0° and 20° while the electric field of the potential distribution is commonly depicted perpendicular to the local magnetic field. As Reimei moved toward the center of the inverted-V regions, the electron pitch angles broadened up to ∼120°, and their energies rapidly increased and continuously changed to those of the energetic inverted-V component. At the higher latitudes of the inverted-V structure, diffuse electrons with the isotropic distribution were also observed. Estimations of the electron density and temperature indicate that the source region of the beam electrons is the topside ionosphere by comparison with those of the diffuse electrons and the energetic inverted-V electrons. For the auroral emissions, in the first event, some horizontal auroral motions were observed which may be accompanied by the horizontal drift motion of the whole structure of the parallel potential drop. This motion could supply electrons in the topside ionosphere into the potential structure, and then the beam electrons are continuously formed at the low altitudes. In the second event, on the other hand, at both edges of the auroral band, the auroral emissions did not expand while the beam electrons were observed. One of the probable reasons to produce the beam electrons is the inertial Alfven waves although they are inconsistent with previous studies because the velocity dispersions were not observed.

Transport of solar wind plasma onto the lunar nightside surface

We present first measurements of energetic neutral atoms that originate from solar wind plasma having interacted with the lunar nightside surface. We observe two distinct ENA distributions parallel to the terminator, the spectral shape and the intensity of both of which indicate that the particles originate from the bulk solar wind flow. The first distribution modifies the dayside ENA flux to reach ∼6∘ into the nightside and is well explained by the kinetic temperature of the solar wind protons. The second distribution, which was not predicted, reaches from the terminator to up to 30∘ beyond the terminator, with a maximum at ∼102∘ in solar zenith angle. As most likely wake transport processes for this second distribution we identify acceleration by the ambi-polar electric field and by the negatively charged lunar nightside surface. In addition, our data provide the first observation indicative of a global solar zenith angle dependence of positive dayside surface potentials.

Spin-Axis Tilt Estimation for Spinning Spacecraft

The spin-axis tilt, which is also known as dynamic imbalance or coning error, is one of the most significant bias errors deteriorating the attitude determination accuracy for spinning spacecrafts. Although it is a common practical issue for spin spacecraft missions, estimation algorithms for the dynamic imbalance have not been studied and issued well. This paper proposes a simple algorithm for spin-axis tilt estimation. The algorithm is based on the Singular Value Decomposition (SVD) and makes use of the attitude rates estimated by an Unscented Kalman Filter (UKF), along with the star scanner measurements. Its accuracy is demonstrated using the models for the Exploration of Energization and Radiation in Geospace (ERG) spacecraft. Other bias errors’ effects on the estimation accuracy are examined and the results are compared with a straightforward averaging approach for dynamic imbalance calculation.