Satoshi Yagitani

The Numerical Simulation of VLF Chorus and Discrete Emissions Observed on the Geotail Satellite using a Vlasov Code

The work consdiers VLF chorus elements observed omn the Geotail satellite, which passes the equator at 10 earth radii. We have used a VHS Vlasov simulation code to model these emissions, employing all the ambient plasma data observed on Geotail. Excellent agreement with observation results, with steep risers and slow fallers being reproduced. The results confirm the overall validity of the non linear trapping theory of VLF emissions, and also confirm the efficacity of the Vlasov VHS code.

Statistical nature of impulsive electric fields associated with fast ion flow in the near-Earth plasma sheet

Statistical characteristics of impulsive electric field (IEFD) associated with fast earthward ion flows in the inner central plasma sheet were studied by using electric field data obtained from the double-probe instrument on board the Geotail satellite. It is shown that the strongest electric field is produced in the region between X AGSM = -15 R E and -9 R E where the fast earthward flow is decelerated. The IEFDs are short-lived with a timescale of ∼2 min on the average, compatible with the timescale of other signatures of substorm fine structures. This strong cross-tail electric field, coincident with the braking of the fast flow and correlated with the magnetic field dipolarization, has important implications for the particle acceleration in the near-Earth plasma sheet.

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.

Displays of paternal mouse pup retrieval following communicative interaction with maternal mates

Compared with the knowledge of maternal care, much less is known about the factors required for paternal parental care. Here we report that new sires of laboratory mice, though not spontaneously parental, can be induced to show maternal-like parental care (pup retrieval) using signals from dams separated from their pups. During this interaction, the maternal mates emit 38-kHz ultrasonic vocalizations to their male partners, which are equivalent to vocalizations that occur following pheromone stimulation. Without these signals or in the absence of maternal mates, the sires do not retrieve their pups within 5 min. These results show that, in mice, the maternal parent communicates to the paternal parent to encourage pup care. This new paradigm may be useful in the analysis of the parental brain during paternal care induced by interactive communication.

Full wave calculation of ELF/VLF propagation from a dipole source located in the lower ionosphere

A full wave technique has been developed to calculate the field intensities both in the upper ionosphere and on the ground when a dipole source immersed in the lower ionosphere radiates ELF/VLF waves. The radiated wave is divided into a large number of elementary plane waves, for each of which the propagation in the horizontally stratified model consisting of the ionosphere, the free space and the ground is calculated by the full wave technique. Then the plane waves are summed up to give a horizontal distribution of the radiated wave intensities at any altitudes. This technique is applied to investigate the propagation characteristics of radiation from a polar electrojet (PEJ) antenna modulated at an ELF/VLF frequency, which was created by the Tromso heating facility. Comparison between the results of experiments and the calculation is given in the companion paper [Kimura et al., this issue]. We discuss several propagation characteristics, such as the frequency dependence of the radiated wave propagation up to the upper ionosphere and down to the ground. Under the conditions that the frequency is 2.5 kHz and the ionospheric dc electric field is 5 mV/m, the calculated results are roughly consistent with the experimental results and the radiation efficiency of the PEJ antenna is found to be extremely small, 2.5×10−6 upward and 3×10−8 downward.

Geotail observation of upper band and lower band chorus elements in the outer magnetosphere

Using the Geotail observation of upper band and lower band (dual-band) rising tone chorus emissions in the dayside outer magnetosphere, we evaluate the mechanism of creating their frequency gap bandwidths, on the basis of a nonlinear damping mechanism at half the local gyrofrequency proposed by Omura et al. (2009). For ELF dual-band chorus elements observed in the dayside of the equatorial outer magnetosphere (L ∼ 10.5), the lower cutoff frequency of the upper band elements follows half the local gyrofrequency. On the other hand, the upper cutoff of the lower band elements is almost consistent with half the gyrofrequency at a minimum-B pocket (a possible chorus generation region) along the field lines connecting Geotail according to the Tsyganenko geomagnetic TS05 model. This is consistent with the scenario that the rising tone wave packet initially excited in a wide frequency range suffers from damping at half the local gyrofrequencies during quasi-parallel propagation. Since the local gyrofrequency gradually increases away from the generation region, the upper cutoff of a lower band element should represent half the gyrofrequency at the generation region, whereas the lower cutoff of an upper band element should follow half the local gyrofrequency. We confirm that the frequency sweep rates and amplitudes of the observed chorus wave packets are consistent with those predicted by the nonlinear growth theory of chorus emissions, except for the frequency gap. This indicates an observational evidence of nonlinear chorus generation in the minimum-B pocket in the outer magnetosphere and creation of dual-band elements due to nonlinear damping through quasi-parallel wave propagation.

Properties of dayside nonlinear rising tone chorus emissions at large L observed by GEOTAIL

This paper studies some wave properties of nonlinear rising tone chorus emissions in the outer regions of the dayside equatorial magnetosphere at ∼(6.3,–4.7, 0.7RE) in GSE coordinates. We analyze data obtained with the PWI and WFC receivers on GEOTAIL associated with a substorm on April 29, 1993. Fine structure of the chorus elements and inter-element spacings are shown. Directions of propagation of the chorus elements relative to the local magnetic field lines are analyzed. Wave polarizations, intensities and spectral properties of chorus in the equatorial Earth’s magnetosphere are discussed.

Measurements and analysis of antenna impedance aboard the Geotail spacecraft

A practical method of estimating the effective impedance of electric dipole antennas aboard the scientific spacecraft Geotail has been developed. The principle of the newly proposed method is to derive the complex impedance of the dipole antennas immersed in the space plasma by means of the analysis of signal waveforms captured when a calibration signal was imposed directly onto the antennas. The derived complex antenna impedance is expressed as frequency responses of its real and imaginary parts. During the flight of Geotail in the Earths magnetosphere, the impedance measurements were conducted in various regions of the magnetosphere. It has been confirmed, from the analysis of the measured data, that the impedance of the Geotail antenna forms an equivalent electric circuit consisting of a resistance and a capacitance connected in parallel. It has been found that the resistance value is easily changed by the extremely rare ambient plasma density in the magnetosphere. The validity of the measured values have been examined along a theory of probes in a sheath region, taking into account antenna potentials which are lifted up due to the photoelectron emissions from the antenna surface. The analyzed result shows that the measured values of the antenna resistance are consistent with those within the sheath calculated from the theory. Correlation plots of the capacitance values obtained from both the measurements and the calculations have shown that the major parts of the measured capacitance represent the “sheath capacitance,” but other parts might result from a combination of the sheath impedance and the plasma impedance, in addition to the free-space capacitance.

Wave form analysis of the continuum radiation observed by GEOTAIL

The Plasma Wave Instrument (PWI) onboard the GEOTAIL spacecraft has frequently observed Continuum Radiation (CR) throughout the geomagnetic tail region, including the Magnetosheath (MS), the Lobe, the Plasma Sheet Boundary Layer (PSBL), and the Plasma Sheet (PS). In addition to the usual Escaping CR (from 10 kHz to 30 kHz) and the Trapped CR (from about 5 kHz to 10 kHz), CR with the frequency structure extending down to 1 kHz (the local plasma frequency) has often been observed in the Lobe region. Waveforms of the electric field in the frequency range less than 4 kHz, which is acquired by Wave Form Capture in the PWI, are used to analyze in detail the frequency-time structure of such low-frequency CR near the lower cutoff. Two distinct cutoff frequencies modulated by the antenna spinning indicates that the CR in the Lobe region propagates both in the O mode and in the X mode almost perpendicular to the earthward geomagnetic field line. The CR in the Lobe region, especially in the vicinity of the PSBL, is sometimes accompanied by intense electrostatic electron-cyclotron-harmonic (ECH) (n + 1/2) waves. These suggest that such a low-frequency CR in the distant tail region is most likely to be generated from the ECH waves near the PSBL, and trapped within the Lobe between the PSBL and the MS.

Geospace exploration project ERG

The Exploration of energization and Radiation in Geospace (ERG) project explores the acceleration, transport, and loss of relativistic electrons in the radiation belts and the dynamics for geospace storms. This project consists of three research teams for satellite observation, ground-based network observation, and integrated data analysis/simulation. This synergetic approach is essential for obtaining a comprehensive understanding of the relativistic electron generation/loss processes of the radiation belts as well as geospace storms through cross-energy/cross-regional couplings, in which different plasma/particle populations and regions are strongly coupled with each other. This paper gives an overview of the ERG project and presents the initial results from the ERG (Arase) satellite.