小財 正義

Abstract The north–south (NS) anisotropy of galactic cosmic rays (GCRs) is dominated by a diamagnetic drift flow of GCRs in the interplanetary magnetic field (IMF), allowing us to derive key parameters of cosmic-ray propagation, such as the density gradient and diffusion coefficient. We propose a new method to analyze the rigidity spectrum of GCR anisotropy and reveal a solar cycle variation of the NS anisotropy’s spectrum using ground-based muon detectors in Nagoya, Japan, and Hobart, Australia. The physics-based correction method for the atmospheric temperature effect on muons is used to combine the different-site detectors free from local atmospheric effects. NS channel pairs in the multidirectional muon detectors are formed to enhance sensitivity to the NS anisotropy, and in this process, general graph matching in graph theory is introduced to survey optimized pairs. Moreover, Bayesian estimation with the Gaussian process allows us to unfold the rigidity spectrum without supposing any analytical function for the spectral shape. Thanks to these novel approaches, it has been discovered that the rigidity spectrum of the NS anisotropy is dynamically varying with solar activity every year. It is attributed to a rigidity-dependent variation of the radial density gradient of GCRs based on the nature of the diamagnetic drift in the IMF. The diffusion coefficient and mean free path length of GCRs as functions of the rigidity are also derived from the diffusion–convection flow balance. This analysis expands the estimation limit of the mean free path length into the ≤200 GV rigidity region from the <10 GV region achieved by solar energetic particle observations.

The AMIDER, Advanced Multidisciplinary Integrated-Database for Exploring new Research, is a newly developed research data catalog to demonstrate an advanced database application. AMIDER is characterized as a multidisciplinary database equipped with a user-friendly web application. Its catalog view displays diverse research data at once beyond any limitation of each individual discipline. Some useful functions, such as a selectable data download, data format conversion, and display of data visual information, are also implemented. Further advanced functions, such as visualization of dataset mutual relationship, are also implemented as a preliminary trial. These characteristics and functions are expected to enhance the accessibility to individual research data, even from non-expertized users, and be helpful for collaborations among diverse scientific fields beyond individual disciplines. Multidisciplinary data management is also one of AMIDER's uniqueness, where various metadata schemas can be mapped to a uniform metadata table, and standardized and self-describing data formats are adopted. AMIDER website (https://amider.rois.ac.jp/) had been launched in April 2024. As of July 2024, over 15,000 metadata in various research fields of polar science have been registered in the database, and approximately 500 visitors are viewing the website every day on average. Expansion of the database to further multidisciplinary scientific fields, not only polar science, is planned, and advanced attempts, such as applying Natural Language Processing (NLP) to metadata, have also been considered.

This study developed a novel thermal control system to cool detectors of the General AntiParticle Spectrometer (GAPS) before its flights. GAPS is a balloon-borne cosmic-ray observation experiment. In its payload, GAPS contains over 1000 silicon detectors that must be cooled below −40℃. All detectors are thermally coupled to a unique heat-pipe system (HPS) that transfers heat from the detectors to a radiator. The radiator is designed to be cooled below −50℃ during the flight by exposure to space. The pre-flight state of the detectors is checked on the ground at 1 atm and ambient room temperature, but the radiator cannot be similarly cooled. The authors have developed a ground cooling system (GCS) to chill the detectors for ground testing. The GCS consists of a cold plate, a chiller, and insulating foam. The cold plate is designed to be attached to the radiator and cooled by a coolant pumped by the chiller. The payload configuration, including the HPS, can be the same as that of the flight. The GCS design was validated by thermal tests using a scale model. The GCS design is simple and provides a practical guideline, including a simple estimation of appropriate thermal insulation thickness, which can be easily adapted to other applications.

© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International License (CC BY-NC-ND 4.0). We built a large (approximately 4,000 m2) water Cherenkov-type muon detector array under the existing Tibet air shower array at 4,300 m above sea level, to observe 10-1000 TeV gamma rays from cosmic-ray accelerators in our Galaxy with wide field of view at very low background level. A gamma-ray induced air shower has significantly less muons compared with a cosmic-ray induced one. Therefore, we can effectively discriminate between primary gamma rays and cosmic-ray background events by means of counting number of muons in an air shower event by the muon detector array. We make a status report on the experiment.

We analyze the galactic cosmic ray (GCR) density and its spatial gradient in Forbush Decreases (FDs) observed with the Global Muon Detector Network (GMDN) and neutron monitors (NMs). By superposing the GCR density and density gradient observed in FDs following 45 interplanetary shocks (IP-shocks), each associated with an identified eruption on the Sun, we infer the average spatial distribution of GCRs behind IP-shocks. We find two distinct modulations of GCR density in FDs, one in the magnetic sheath and the other in the coronal mass ejection (CME) behind the sheath. The density modulation in the sheath is dominant in the western flank of the shock, while the modulation in the CME ejecta stands out in the eastern flank. This east-west asymmetry is more prominent in GMDN data responding to similar to 60 GV GCRs than in NM data responding to similar to 10 GV GCRs, because of the softer rigidity spectrum of the modulation in the CME ejecta than in the sheath. The geocentric solar ecliptic-y component of the density gradient, G(y), shows a negative (positive) enhancement in FDs caused by the eastern (western) eruptions, while G(z) shows a negative (positive) enhancement in FDs caused by the northern (southern) eruptions. This implies that the GCR density minimum is located behind the central flank of IP-shocks and propagating radially outward from the location of the solar eruption. We also confirmed that the average Gz changes its sign above and below the heliospheric current sheet, in accord with the prediction of the drift model for the large-scale GCR transport in the heliosphere.

From three-dimensional spatial density gradient of galactic cosmic rays (GCRs) observed with the Global Muon Detector Network (GMDN), we derive average features of the GCR depleted region behind the IP (interplanetary) shock. We identify 207 IP-shocks that passed the earth based on the geomagnetic storm sudden commencements (SSCs) and extract 50 events that are associated with solar coronal mass ejections (CMEs) in a period between 2006 and 2014. From the first order GCR anisotropy corrected for the solar wind convection and Compton-Getting effect arising from the earth's orbital motion, we deduce the density gradient on an hourly basis for each event. We then derive the average temporal variation of the density gradient by superposing its variations at the SSC onset timing. We confirm that the density gradient components are clearly enhanced after the shock passage, indicating the existence of GCR depleted region behind the shock which causes the Forbush Decrease in the cosmic ray intensity. The enhancement of the radial gradient shows longer duration when the earth has encountered the western flank of the shock, implying an asymmetric shielding effect of the shock on the GCRs. The longitudinal gradient, on the other hand, shows that the GCR density minimum is located around the longitudinal center behind the shock, which can be ascribed to the centered ejecta driving IP-shock.

We analyze the north-south anisotropy (NSA) of galactic cosmic rays observed with the GMDN on an hourly basis and compare with the anisotropy derived from the GG-component of a large multidirectional muon detector at Nagoya, Japan. The NSA is a component of the three dimensional anisotropy vector parallel to the Earth's rotation axis. We find a significant seasonal variation of the NSA from the GG-component indicating the influence of the anisotropy component in the ecliptic plane to the NSA. We calculate the average and standard deviation of daily mean NSAs in the 'toward' (T) and 'away' (A) IMF sectors separately in each Carrington Rotation between December 1993 and November 2014. It is confirmed that the temporal variations of the NSA observed with the GMDN and GG-component are consistent with each other, as reported earlier. We find the T-A separation between average NSAs in T and A sectors shows a long-term variation with minima (maxima) around the solar activity minima (maxima). The standard deviation in each rotation also shows a similar long-term variation, keeping the ratio of T-A separation to the standard deviation roughly constant thorough out an entire period of analysis. We discuss this in relation with the 'success rate' which is introduced as a parameter indicating to what extent we can infer the IMF sector polarity from the sign of the observed NSA.

We analyze the three-dimensional anisotropy of the galactic cosmic ray (GCR) intensities observed independently with a muon detector at Nagoya in Japan and neutron monitors over four solar activity cycles. We clearly see the phase of the free-space diurnal anisotropy shifting toward earlier hours around solar activity minima in A > 0 epochs, due to the reduced anisotropy component parallel to the mean magnetic field. This component is consistent with a rigidity-independent spectrum, while the perpendicular anisotropy component increases with GCR rigidity. We suggest that this harder spectrum of the perpendicular component is due to contribution from the drift streaming. We find that the bi-directional latitudinal density gradient is positive in theA > 0 epoch, while it is negative in the A < 0 epoch, in agreement with the drift model prediction. The radial density gradient of GCRs, on the other hand, varies with a similar to 11 yr cycle with maxima (minima) in solar maximum (minimum) periods, but we find no significant difference between the radial gradients in the A > 0 and A < 0 epochs. The corresponding parallel mean free path is larger in A < 0 than in A > 0. We also find, however, that the parallel mean free path (radial gradient) appears to persistently increase (decrease) in the last three cycles of weakening solar activity. We suggest that simple differences between these parameters in A > 0 and A < 0 epochs are seriously biased by these long-term trends.

© 2013 Sociedade Brasileira de Fisica. All Rights Reserved. We analyze the long-term variation of the first order anisotropy observed with the Global Muon Detector Network (GMDN) on hourly basis and compare it with one derived from the diurnal variation observed by a single muon detector at Nagoya in Japan. We confirmed that the three dimensional (3D) anisotropies including the NS anisotropy derived from two analyses are fairly consistent with each other as long as the yearly mean value is concerned. From the 3D anisotropy vector corrected for the solar wind convection and the Compton-Getting effect arising from the Earth’s orbital motion around the Sun, we deduce modulation parameters including the spatial density gradient and the parallel mean free path for the pitch angle scattering of GCRs in the turbulent interplanetary magnetic field (IMF). We show the derived density gradient and mean free path varying with the solar activity- and magnetic-cycles. We discuss the physical implication of these variations by comparing with the prediction of the drift model. We also find a close correlation between variations of the anisotropy parallel to the IMF with the solar wind speed.

© 2013 Sociedade Brasileira de Fisica. All Rights Reserved. We analyze the three dimensional (3D) anisotropy of the galactic cosmic ray (GCR) intensity observed with a multi-directional muon detector over four solar activity cycles between 1971 and 2011 and compare them with the anisotropy seen by neutron monitors (NMs) to examine the rigidity dependence of the anisotropy. We clearly see the phase of the free-space diurnal anisotropy shifting toward earlier hours around solar activity minima in A > 0 epochs from ~18 hr local solar time in A < 0 epochs. The magnitude of the phase-shift is much larger in the muon data than in the N M data. In particular, we find that the anisotropy component perpendicular to the magnetic field, (after correction for the solar wind convection and the Compton-Getting effect due to Earth's orbital motion around the Sun) is significantly larger in the muon data than that in N M data, while the parallel components in two data sets are quite similar to each other. This energy dependence of the perpendicular anisotropy, which is seemingly due to the harder energy spectrum of the drift streaming than the diffusion in this energy region, naturally explains the larger phase-shift observed in muon data. We also find a clear correlation of the magnitude of the parallel anisotropy with the solar wind velocity which varies without any clear 11-year or 22-year periodicity.

We deduce the spatial gradient of galactic cosmic rays in three dimensions from the fist order anisotropy observed with the Global Muon Detector Network. The anisotropy vector is first corrected for the solar wind convection and the Compton-Getting effect arising from the solar wind and the Earth's orbital motion around the Sun. We then convert the component anisotropy perpendicular to the interplanetary magnetic field to the spatial density gradient by assuming that the perpendicular anisotropy is mainly due to the diamagnetic streaming. In this paper, we analyze the solar cycle variation of the gradient observed with the GMDN during ten years between 2001 and 2010 and show that the derived density gradient is clearly decreasing with decreasing the solar activity toward the solar activity minimum in 2008-2009. We also find a clear seasonal variation in each of the radial and longitudinal component of the gradient vector in a close correlation with the heliographic latitude of the Earth. The amplitude of this seasonal variation also decreases with decreasing the solar activity. We discuss the origin of this seasonal variation in terms of the global distribution of cosmic-ray density in the heliosphere.

We plan to use the SciCR as a new muon detector and fill a gap remaining in the viewing directions of the present GMDN over the north and middle America. In order to minimize the interference to the solar neutron detection, we trigger the muon measurement by the four-fold coincidence between pulses from the top and bottom pairs of the xand y-layers. We analyze the data recorded by a prototype detector "mini-SciCR" at the observation site for the SciCR and evaluate the observed count rate, zenith- And azimuth-angle distributions and the atmospheric pressure effect by comparing with the numerical expectations from the response of the atmospheric muons to the primary galactic cosmic rays.