Yoshifumi Saito

The goal of this research is to develop a new detector system for in-situ measurements of hot plasmas with a high time resolution to study electron dynamics in the magnetosphere. The new system consists of microchannel plates (MCP) and a position sensitive multi-anode detector,based on ASIC(Application specific integrated circuit) techniques.The combination of the multianode and the ASIC techniques is expected to make the fastest position signal processing with small size, light weight and low power consumption, compared to other position detection techniques that have ever been used.The key technology is to accommodate the ASIC with the rear side of the anode (ceramic) plate, in which a multiple discrete anode pattern is printed on the front side. Capacitive coupling between the anode pattern on the front side anode and the signal pickup pattern on the rear side is used instead of discrete capacitors, which are usually required to insulate a high voltage applied to the anode.The anode plate is made of Alumina with thickness of 1mm, and the capacitance for each anode is about 3 pF, which is smaller by two orders of magnitude than those of discrete condensers conventionally used. This anode condenser of low capacitancehad been considered to be critical to whether the ASIC is applicable to the electron detector.However, our experimental result showed that the attenuation of signals due to the low capacitance was about 50 % at most, and hence our new concept is useful.Next, effects of electrostatic coupling between the discrete anodes have been measured.This is important, since our new detector consists of many adjacent anodes with small gaps to increase the detection areas. Our experimental results show about the coupling effect of about 10% from the adjacent anodes for the anode condenser used, whereas the coupling effect without using the anode condenser is negligible.This is also understood in terms of the effect of low capacitance of the anode plate.Although the effect of 10% coupling can be effectively avoided with a suitable discrimination level in the signal processing circuit, it is highly preferable to increase the capacitance of the anode plate in future developments. Finally we present the performance of the test model of ASIC on board MCP multianode, though the ASIC has been developed for an application with SSD by Lawrence national Berkley Laboratory.The ASIC chip includes preamplifiers followed by its discriminator and counter and the size is12mm × 12mm. Based on the experiment with the use of ion beam as incident on the MCP of the test model, we conclude that our new multi-anode detector system is applicable (though further studies are still necessary) to future missions for high-time resolution measurements of hot plasmas in the magnetosphere.

The spacecraft potential has been used to derive the electron density surrounding the spacecraft in the magnetosphere and solar wind. The previous studies have examined the relationship between the spacecraft potential and the electron density in the distant tail regions and obtained an empirical formula to show their relation. However the electron density obtained by the empirical formula is often overestimated in the near tail regions with high electron temperature, In this study, we investigate the relationship between the Geotail spacecraft potential and the electron density/temperature in the near tail regions during the period from November 1994 to February 1997, and improve the empirical formula considering the electron temperature. Then we discuss on the investigation of distribution of low energy plasma in the near tail region by comparing the electron density obtained by the improved empirical formula with that measured by the low-energy particle instrument onboard the Geotail spacecraft.

In the Earth's magnetosphere where the spacecraft potential is usually positive, photoelectrons emitted from the spacecraft surface are attracted back to the spacecraft and some of them are detected by electron analyzer onboard. Then photoelectrons are unnecessary contamination. By analyzing such a component detected by LEP/EA-e onboard GEOTAIL spacecraft, we examined velocity/energy distribution functions of photoelectrons, and their relationship to the spacecraft potential. We found that the ratio of the duskward photoelectron flux to the dawnward flux increases when the spacecraft potential is large, and decreases when it is small. Further analysis revealed, by plotting the ratio as a function of the photoelectron energy normalized by the spacecraft potential (E/V(sub sc)), that it is the largest when E/V(sub sec) is about one third. This result implies the existence of azimuthal component of electric field as well as the radial component which is regarded to be dominant in an ordinary case around the spacecraft surface, although it is not clear why such an electric field is generated around GEOTAIL.

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.

Spatial distributions of the charge cloud on the anode are examined both experimentally and theoretically. The initial electron cloud at the MCP output has angular divergence and energies, therefore the size of the charge cloud on the anode becomes broader than the original size at the MCP output. Furthermore, space charge effects may broaden the size of the charge cloud. The size of the charge cloud has been examined with various conditions of the acceleration voltage between the MCP output and the anode(anode voltage) and the MCP voltage to control the gain and hence the amount of output electrons from the MCP. The experimental results are well explained in terms of these effects in comparison with the model calculation of the spreading electron cloud in which a Gaussian distribution is assumed for the spatial distribution so that its standard deviation is taken as a charge cloud radius. The radius was measured to be 0.4mm 〜1mm under the condition that the gap between the MCPoutput and the anode is 2.5mm with accelerated anode voltage of 70 〜 300V. The radius is approximately inverse proportional to the square root of the anode voltage. The radius has also a dependence of MCP output charge. Smaller MCP output charge exhibits smaller charge cloud radius. Spatial distribution of charge cloud are calculated from the model in which pulsed current broad along the way to an anode with space charge effects. The model results are compared with the experimental results of Edgar[11] and show good agreements.

The Moon has no global dipole magnetic field. According to the observations so far made by lunar orbiting spacecrafts, there are crustal magnetic anomalies on the lunar surface. If the locations of the magnetic anomalies with the size of a few km to 10km coincide with the rills on the lunar surface, the magnetic anomalies are probably due to the thermo-remnant magnetism. Furthermore, there is a possibility that the direction and the estimated age of the anomalies are consistent with the existence of the global dipole magnetic field in ancient times. We have developed an electro-static analyzer ESA (Electron Spectrum Analyzer) onboard the SELENE satellite in order to observe the magnetic anomalies by electron reflection method. With the existence of the remnant magnetic field on the lunar surface, the electrons moving toward the moon with large pitch angle around the ambient magnetic field will be reflected back to the satellite by a magnetic mirror. By measuring the pitch angle distribution of the reflected electrons, we can deduce the remnant magnetic field on the lunar surface. Since SELENE satellite is a three-axis attitude controlled satellite, we need two sensors mounted on the moonward and the anti-moonward spacecraft panels in order to obtain three-dimensional electron distribution function. The ESA sensor basically employs a method of a top-hat type electrostatic analyzer placing angular scanning deflectors at the entrance and toroidal deflectors inside. The Field Of View (FOV) is electrically scanned between +/-45 degrees around the center of the FOV that is 45 degrees inclined from the axis of symmetry. In order to decide the sensitivity of ESA, we have estimated the averaged count rate of ESA using the electron flux data obtained by Electron Reflectometer (ER) onboard the Lunar Prospector (LP) satellite. LP is a lunar orbiter launched on Jan 6, 1998 by NASA. In order to evaluate the manufacturing and assembling error of ESA, we have measured the characteristics of ESA and compared the results with the characteristics obtained by numerical calculation. We confirmed the good performance of ESA including the newly developed angular scanning deflectors. In order to observe the lunar magnetic anomalies with the size of a few km to 10km, we must suitably select the angular resolution and time resolution of ESA. We have carried out a test-particle simulation of the electrons around a magnetic anomaly to decide the suitable operation mode of ESA. As a result, we have confirmed that ESA can observe lunar magnetic anomalies with the size of the order of 10km in one second.

Information on energy spectra of 1-100 keV electrons is expected to provide an important clue to understand heating and acceleration processes of magnetospheric plasmas. However, the distribution functions in the transition range of thermal (several keV) energies are not well known observationally owing to problems in the measurement technique to detect these electrons with high reliability by using a solid-state APD (Avalanche PhotoDiode) detector instead of the conventional ones. The APD is a kind of o-n junction semiconductor with an internal gain due to the avalanche amplification of electron-hole pairs in the strong electric field within its depletion region, which is usually applied for photoelectronic devices. To begin with, we have measured characteristic X-rays from Fe 55(5.9keV) in order to calibrate the APD. Secondly, for a fundamental experiment to detect electrons, we have set up an electron gun, which can generate 1-20keV electron beams impinging onto the APD (Type Z7966-20, Hamamatsu Photonics Co.Ltd.) in vacuum chamber. The experimental result shows that the pulse height distribution from the APD signal exhibits a significant peak for the electrons with energies above 8keV, and positions of the peaks shows a good linearity, with which incident electron energies can be highly resolved. The energy resolution slightly depends on the incident electron energy. Tor low-energy electrons (lower than 10keV), it has a characteristic tail on the low energy side, while for incident electrons at higher energies (near 20keV) the energy resolution gets a little worse and the position of the peak appears at slightly lower channel than expected. Qualitatively, the low-energy tail features can be caused by the dead-layer on the surface of the device and its inhomogeneity. Nonlinearity and worse resolution of high-energy peaks may have caused by a space-charge effect of created e-h pairs. For the quantitative understanding, we have developed a Monte Carlo particle simulation of charge transport and collection inside the APD. We have also done a comparative experiment between APD and CEM (Burle industry Co.Ltd.) in terms of detection efficiency. Promisingly, this APD shows a better efficiency, compared to CEM, at higher energies than 5keV and nearly three times higher efficiency at 20keV. Finally we discuss countermeasures for applications of the APD in Space, such as the temperature response and radiation damage, as well as extension of the energy range and enlargement of the detectable area.

記事種別: 会議・学会報告・シンポジウム

A strong electron acceleration event is observed around the interplanetary shock on Feb. 21,1994 on the GEOTAIL satellite. Behavior of the energetic electrons is consistent with the standard Fermi shock acceleration theory, if we include a modification due to the expansion effect of the downstream plasma flow. As far as we know, there has been no observational report which clealy indicates stochastic Fermi shock acceleration mechanism working on electrons at 1AU. We report here details of the electron acceleration phenomenon and related topics.

Magnetic reconnection has been known as one of the most important physical mechanisms intervening in the energy transfer processes in various plasma phenomena. In this article, we shall give a brief review on the observational evidence on magnetic reconnection occuring in the earth's magnetotail.