Tsutomu Nagatsuma

Understanding space weather to shield society : A global road map for 2015-2025 commissioned by COSPAR and ILWS

There is a growing appreciation that the environmental conditions that we call space weather impact the technological infrastructure that powers the coupled economies around the world. With that co ...

Equatorial electrodynamics and neutral background in the Asian sector during the 2009 stratospheric sudden warming

Using ground observations of total electron content (TEC) and equatorial electrojet (EEJ) in the Asian sector, along with plasma and neutral densities obtained from the CHAMP satellite, we investigate the ionospheric electrodynamics and neutral background in this longitude sector during the major stratospheric sudden warming (SSW) in January 2009. Our analysis reveals the following prominent features. First, the TEC response in tropical regions is strongly latitude dependent, with monotonic depletion at the dip equator but a semidiurnal perturbation at low latitudes. Second, the TEC semidiurnal perturbation possesses a significant hemispheric asymmetry in terms of onset date and magnitude. It starts on the same day as the SSW peak in the Northern Hemisphere but 2 days later in the Southern Hemisphere. Its magnitude is twice as strong in the north than in the south. Third, strong counter electrojet occurs in the afternoon, following the strengthening of the eastward EEJ in the morning. Fourth, semidiurnal perturbation in both TEC and EEJ possesses a phase shift, at a rate of about 0.7 h/day. Comparisons with results reported in the Peruvian sector reveal clear longitude dependence in the amplitude and hemispheric asymmetry of the semidiurnal perturbation. Finally, thermospheric density undergoes ?25% decrease at low latitudes in the afternoon local time sector during the SSW, indicating significant cooling effects in the tropical upper thermosphere.

Pi2 pulsations observed from the Akebono satellite in the plasmasphere

Magnetic field, electric field, and electron density measurements from the Akebono satellite are used to study the properties of two Pi2 pulsations that occurred in succession on February 13, 1990, when the satellite was in the plasmasphere at L = 2.4–3.8, 24°–40° magnetic latitude, and 22.5 hours magnetic local time. Magnetic pulsations with a nearly identical waveform were observed in the same time interval at three ground stations (Aedey, L ≈ 6.84; York, L ≈ 2.55; and Hermanus, L ≈ 1.83), which were located near midnight, confirming that the pulsations propagated to the ground. At the satellite the pulsations had comparable radial and azimuthal components in both the magnetic and electric fields. In contrast to the observations near the magnetic equator by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer spacecraft [Takahashi et al., 1995], no compressional component was detected in the magnetic field. The orthogonal components of the electric and magnetic fields oscillated either in phase or 180° out of phase, a property of a propagating (rather than standing) wave. The Poynting flux of the Pi2 pulsations was parallel to the ambient magnetic field and directed toward the nearer ionosphere, with little indication of ionospheric reflection. This unidirectional flow of electromagnetic energy is consistent with the strong ionospheric damping of Alfven waves estimated from a numerical calculation. It is significant that the measured Poynting flux could damp a cavity mode oscillation in ∼10 s, assuming that the previously reported equatorial compressional Pi2 pulsations represent the cavity mode. Since the transverse Pi2 pulsations at Akebono lasted ∼400 s, they cannot be due to a gradual energy leakage from a simple cavity mode oscillation. Consequently, if the observed energy flow is a general property of plasmaspheric Pi2 pulsations, a simple cavity mode oscillation excited by an impulsive source is not an appropriate model for Pi2 pulsations.

Main-phase creation of "seed" electrons in the outer radiation belt

During a geomagnetic storm in early November 1993, NOAA satellite observations revealed that a population of energetic electrons appeared in the center of the outer radiation belt during the main phase of the storm. At the beginning of the main phase of the magnetic storm, the number of electrons with energies from 30 keV to 100 keV increased rapidly and contributed to build up of the ring current. At the end of the main phase the flux of electrons with energies greater than 300 keV increased significantly. Akebono satellite observations showed that the flux of electrons with energies ranging from 300 keV to 950 keV increased late of the storm main phase and that the flux of electrons with energies from 950 keV to 2.5 MeV increased during the storm recovery phase. The electron flux increase observed by both NOAA and Akebono took place first in the central part of the outer radiation belt (L~4) and propagated to higher L shells with a significant time delay. We think that the ring current electrons that appeared first and near L~4 during the main phase seeded the subsequent increase in the flux of MeV electrons in the entire outer radiation belt.

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.

The Energization and Radiation in Geospace (ERG) Project

The Energization and Radiation in Geospace (ERG) project for solar cycle 24 will explore how relativistic electrons in the radiation belts are generated during space storms. This geospace exploration project consists of three research teams: the ERG satellite observation team, the ground-based network observation team, and the integrated data analysis/simulation team. Satellite observation will provide in situ measurements of features such as the plasma distribution function, electric and magnetic fields, and plasma waves, whereas remote sensing by ground-based observations using, for example, HF radars, magnetometers, optical instruments, and radio wave receivers will provide the global state of the geospace. Various kinds of data will be integrated and compared with numerical simulations for quantitative understanding. Such a synergetic approach is essential for comprehensive understanding of relativistic electron generation/loss processes through crossenergy and cross-regional coupling in which different plasma populations and regions are dynamically coupled with each other. In addition, the ERG satellite will utilize a new and innovative measurement technique for wave-particle interactions that can directly measure the energy exchange process between particles and plasma waves. In this paper, we briefly review some of the profound problems regarding relativistic electron accelerations and losses that will be solved by the ERG project, and we provide an overview of the project.

Field-aligned currents associated with Alfven waves in the poleward boundary region of the nightside auroral oval

We have examined characteristics of the field-aligned currents in the poleward boundary region of the nightside auroral oval, using magnetic field, electric field, and particle data obtained from the Akebono satellite. We found that large-amplitude, wave-like fluctuations of magnetic and electric fields frequently occur at mid-altitudes confined in this latitudinally narrow region. We examined the relationship between electric and magnetic field fluctuations, examined using the complex impedance function defined as , where and are the X component of the electric field and the Y component of the magnetic field, respectively. The most important result obtained from this analysis is that the features of the impedance functions in the frequency range between 2 × 10−3 and 8 × 10−2 Hz are well explained by a time-dependent magnetosphere-ionosphere coupling model. Further, the calculation of the field-aligned Poynting flux showed that most of the Alfven wave energy is reflected at the ionosphere rather than dissipated in the ionosphere. These facts suggest that the majority of the field-aligned current fluctuations observed at mid-altitudes in the poleward boundary region of the nightside auroral oval are due to the superposition of incident and reflected Alfven waves. It is speculated that the breaking of reflected Alfven waves in the PSBL may cause the heating of plasma sheet electrons and ions.

TEC prediction with neural network for equatorial latitude station in Thailand

This paper describes the neural network (NN) application for the prediction of the total electron content (TEC) over Chumphon, an equatorial latitude station in Thailand. The studied period is based on the available data during the low-solar-activity period from 2005 to 2009. The single hidden layer feed-forward network with a back propagation algorithm is applied in this work. The input space of the NN includes the day number, hour number and sunspot number. An analysis was made by comparing the TEC from the neural network prediction (NN TEC), the TEC from an observation (GPS TEC) and the TEC from the IRI-2007 model (IRI-2007 TEC). To obtain the optimum NN for the TEC prediction, the root-mean-square error (RMSE) is taken into account. In order to measure the effectiveness of the NN, the normalized RMSE of the NN TEC computed from the difference between the NN TEC and the GPS TEC is investigated. The RMSE, and normalized RMSE, comparisons for both the NN model and the IRI-2007 model are described. Even with the constraint of a limited amount of available data, the results show that the proposed NN can predict the GPS TEC quite well over the equatorial latitude station.

Effects of the IMF and substorms on the rapid enhancement of relativistic electrons in the outer radiation belt during storm recovery phase

Abstract It is often the case that the flux of the relativistic electrons in the outer radiation belt decreases substantially once the major magnetic storm takes place. After the disappearance of the relativistic electrons which can last as much as one day, an increase in the flux occurs, often to levels that exceed the pre-storm level. We have investigated a correlation of the increment of the relativistic electron flux at geosynchronous orbit with the interplanetary magnetic field (IMF) properties as well as the magnetic disturbance signature during the magnetic storms. Results demonstrate that a large flux enhancement occurs when the IMF is southward during the storm recovery phase.

Geomagnetically conjugate observation of plasma bubbles and thermospheric neutral winds at low latitudes

This is the first paper that reports simultaneous observations of zonal drift of plasma bubbles and the thermospheric neutral winds at geomagnetically conjugate points in both hemispheres. The plasma bubbles were observed in the 630 nm nighttime airglow images taken by using highly sensitive all-sky airglow imagers at Kototabang, Indonesia (geomagnetic latitude (MLAT): 10.0°S), and Chiang Mai, Thailand (MLAT: 8.9°N), which are nearly geomagnetically conjugate stations, for 7 h from 13 to 20 UT (from 20 to 03 LT) on 5 April 2011. The bubbles continuously propagated eastward with velocities of 100–125 m/s. The 630 nm images at Chiang Mai and those mapped to the conjugate point of Kototabang fit very well, which indicates that the observed plasma bubbles were geomagnetically connected. The eastward thermospheric neutral winds measured by two Fabry-Perot interferometers were 70–130 m/s at Kototabang and 50–90 m/s at Chiang Mai. We compared the observed plasma bubble drift velocity with the velocity calculated from the observed neutral winds and the model conductivity, to investigate the F region dynamo contribution to the bubble drift velocity. The estimated drift velocities were 60–90% of the observed velocities of the plasma bubbles, suggesting that most of the plasma bubble velocity can be explained by the F region dynamo effect.