Minoru Tsutsui

Electrostatic solitary waves (ESW) in the magnetotail: BEN wave forms observed by GEOTAIL

The authors report on broadband wave features captured by the wave form capture receiver on the GEOTAIL satellite. In the plasma sheet boundary layer, the broadband specta are composed of a series of pulses, which the authors term electrostatic solitary waves. They propose a model to account for these wave events as highly nonlinear instabilities of electron beams.

Generation mechanism of ESW based on GEOTAIL plasma wave observation, plasma observation and particle simulation

Data of electrostatic waves and plasma particles in the deep magnetotail (X ∼ -209R E ) respectively obtained by Plasma Wave Instrument and Comprehensive Plasma Instrument onboard the GEOTAIL spacecraft are presented. When the GEOTAIL spacecraft experienced multiple crossings of the plasma sheet boundary layer, broadband electrostatic noise (BEN) and Langmuir wave were observed alternatively. The dynamic frequency spectra of BEN are very bursty in time, and their waveforms are a series of electrostatic solitary waves (ESW). The LW is observed when an enhancement of an electron flux is found on a high-energy tail of a relatively cold velocity distribution function of the major thermal electrons. The ESW, on the other hand, are observed in the presence of a hot thermal electron distribution function, in which electrons responsible for the ESW are embedded. These plasma conditions are in agreement with the ESW generation model based on particle simulations.

Low Frequency plasma wave Analyzer (LFA) onboard the PLANET-B spacecraft

The Low Frequency plasma wave Analyzer, LFA, on board the PLANET-B spacecraft has been developed to measure the Martian plasma waves. Two orthogonal electric dipole wire antennas, 50 m tip-to-tip, in the spacecraft spin plane are used to measure plasma waves, dc electric fields, and the spacecraft potential relative to the ambient plasma. The LFA has capability to measure the wave spectrum in the band from 10 Hz to 32 kHz, and to capture the signal waveform in the band from dc to 32 kHz by using a 4 MByte memory. The LFA scientific objectives are to explore the following: (1) Macroscopic plasma environment and boundaries from the solar wind to the ionosphere, (2) Microscopic plasma phenomena induced by the interaction between the solar wind and the Martian atmosphere and the moon Phobos, (3) Generation and propagation of electromagnetic waves, (4) Plasma densities and waves in the nightside ionosphere and tail, and (5) Comparison of Martian plasma waves with those of other planets such as non-magnetized Venus and magnetized Earth.

Behaviors of Electromagnetic Waves Directly Excited by Earthquakes

We detected electromagnetic (EM) waves directly excited by earthquakes in a deep borehole and confirmed them by simultaneous capturing of their waveforms and of seismic waves measured at the same observation site. Furthermore, the excitation mechanism of the EM pulse was confirmed as the piezoelectric effect by a laboratory experiment, in which a seismic P-wave was readily generated by a small stress impact, and the EM wave was simultaneously excited basically by the P-wave. Here, we show behaviors of seismic waves and of their excited EM waves when small and large earthquakes occurred. We also found that EM waves excited by seismic waves have leaked out of the ground surface.

Polarization and propagation property of electromagnetic pulses in the earth

In order to confirm the generations of electromagnetic (EM) pulses in the earth when earthquakes occurred, we have been conducting EM measurements using sensor systems composed of tri-axial magnetic search coils installed at the bottom of 100 m-deep borehole and on the seashore ground. We examined polarizations of magnetic field vectors of EM pulses simultaneously detected at the vertical two positions, and confirmed several kinds of propagation modes. One was vertically incident waves with linear polarization. Using their waveforms, we estimated electrical conductivity, skin-depth and effective phase velocity of the EM pulse in the electrically high conductive medium of the sedimentary layer around the borehole. The second was obliquely incident and ellipsoidal polarized mode, which became Zennek wave propagating along the ground surface, and the third was artificial EM pulses.

Development of Poynting vector direction method for electromagnetic pulses in the earth

We detected electromagnetic (EM) pulses in the earth whose waveforms were quite different from those generated by lightning discharges. In order to confirm their propagation directions (up- or down-ward), we conducted comparative measurements of phase and amplitude between waveforms of horizontal magnetic fields of the EM pulses detected on the ground and at the depth of 95 m in a borehole. Clear differences in the phase and the amplitude seemed to suggest that some of them were up propagating modes and others were down propagations. We, however, found that the decision by this method was wrong, because EM pulses measured at the 95 m-depth in the borehole represented ellipsoidal polarizations whereas those simultaneously detected on the ground were linear polarizations. We concluded that EM pulses of this kind were artificial ones having propagated along the ground surface. For determining propagation directions of unknown EM pulses, we have to obtain their propagating directions from analysis of strict Poynting vector of EM pulses using field values detected by tri-axial electric dipole antennas and tri-axial magnetic search coils. In the present paper, we introduce the measuring method of Poynting vector of EM pulses in the earth.

Excitation mechanism and behaviors of co-seismic electromagnetic waves

For clarifying the excitation mechanism of co-seismic electromagnetic (EM) waves, I have been observing earthquake-related EM waves in the deep earth and above the ground together with measurements of seismic waves, and also conducted a laboratory experiment. As the result, I have found that EM waves were easily excited by seismic P-wave oscillations in the earths crust due to piezo-electric effect. The amplitude of the EM wave was enlarged at arrival of seismic S-wave which largely deformed the P-wave amplitude. It has been confirmed, from observed waveforms, that a large amplitude of co-seismic EM wave always appears in the wave-front of the seismic S-wave. Since the EM wave was radiated but rapidly decayed due to a large electrical conductivity of the earths crust, we could imagine a composite wave system, in which a rapidly decaying co-seismic EM wave is antecedent to the seismic S-wave, and the system is moving with the velocity of the seismic S-wave. It has been also confirmed that a co-seismic EM wave detected above the ground showed ellipsoidal polarization although another EM wave simultaneously detected in the earth showed a linear polarization, which is a result of phase shifts of the EM wave in its penetration through a boundary of two media (from the earth medium to the air).

Three dimensional propagation directions of electromagnetic pulses detected in the earth

Completing a tri-axial electromagnetic (EM) sensor system used in a narrow borehole, and inserting it into 150 m deep borehole constructed on a seashore, we began to measure three dimensional arrival directions of EM pulses detected in the earth by means of strict calculation of Poynting vector. Measured arrival directions have shown clear difference in EM environment between on the landside and seaside of the observation point on the seashore. On the landside, many up-propagating lightning EM pulses were detected, which would result from their reflections at deeper layers than the sensor depth after penetrating into the ground. Another is less detections of EM pulses on the seaside, which is due to strong decay of lightning EM pulses penetrated into the sea water. The less detections of lightning EM pulses on the seaside suggests favorable situation for the observation of earth-origin EM pulses.