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Takahashi, K.

Publications and source records attributed to Takahashi, K..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

The structure of the Birkeland current system in the post-midnight plasma sheet

Simultaneous magnetic field and particle measurements by AMPTE/CCE with magnetic field measurements by GOES 5 and GOES 6 clarify the structure of the field-aligned Birkeland current system in the post-midnight region of the near-earth magnetotail. The current system, consisting of current sheets flowing in opposite directions, exists near the boundary region of the plasma sheet; its magnetic effects are confined mainly to the current region because of its structure. During the expansion phase of substorms, a single-sheet current system is formed in the boundary region of the plasma sheet and net current is probably present in one meridian plane. This type of current system produces the substorm-related transient variations in the azimuthal (D) component normally observed at synchronous orbit.

Nagai, Tsugunobu↗
NASA NTRS1990

Ion flux oscillations associated with a radially polarized transverse Pc 5 magnetic pulsation

The AMPTE CCE spacecraft observed a transverse Pc 5 magnetic pulsation (period of about 200 s) at 2155-2310 UT on November 20, 1985, at a radial distance of 5.7 - 7.0 earth radii, at a magnetic latitude of 1.2 - 19 deg, and near 1300 magnetic local time. The magnetic pulsation exhibits properties consistent with a standing Alfven wave with a second-harmonic standing structure along the ambient magnetic field. The amplitude and the phase of the flux pulsation are found to be a function of the particle detector look direction and the particle energy. The observed energy dependence of the shift is interpreted as the result of a drift-bounce resonance of the ions with the wave. From this interpretation it follows that the wave propagated westward with an azimuthal wave number of approximately 100. Thus the study demonstrates that particle data can be useful for determining the spatial structure of some types of ULF waves.

Takahashi, K.↗
NASA NTRS1990

Observation and theory of Pc 5 waves with harmonically related transverse and compressional components

The properties of 23 magnetic pulsation events observed by the AMPTE CCE spacecraft are studied. These events are selected on the basis of the field magnitude which oscillated at the second harmonic of a simultaneously present transverse oscillation. The events have a second harmonic period of 80-600 s (roughly the Pc 5 range), are observed in cluster in the dawn (0300-0800 magnetic local time, MLT) and dusk (1600-2100 MLT) sectors, and are localized near the magnetic equator. Although the azimuthal wave number estimated from an ion finite Larmor radius effect, is generally large (about 50), there is a marked difference between the events observed in the dawn and dusk sectors. In the dawn sector the waves have low frequencies (1-5 mHz), indicate left-hand polarization with respect to the ambient magnetic field, and propagate eastward with respect to the spacecraft. In the dusk sector the waves have high frequencies (5-15 mHz), indicate right-hand polarization, and propagate westward. It is suggested that the waves are all westward propagating in the plasma rest frame and that local-time-dependent Doppler shift is the reason for the local time dependence of the wave properties.

Takahashi, K.↗
NASA NTRS1989

On the relationship between the energetic particle flux morphology and the change in the magnetic field magnitude during substorms

The relationship between the morphology of energetic particle substorm injections and the change in the magnetic field magnitude over the course of the event is examined. Using the statistical relationships between the magnetic field during the growth phase and the change in the field magnitude during substorms calculated by Lopez et al. (1988), a limited number of dispersionless ion injections observed by AMPTE CCE are selected. It is argued that this limited set is representative of a large set of events and that the conclusions drawn from examining those events are valid for substorms in general in the inner magnetosphere. It is demonstrated that in an event when CCE directly observed the disruption of the current sheet, the particle and field data show that the region of particle acceleration was highly turbulent and was temporally, and perhaps spatially, limited and that the high fluxes of energetic particles are qualitatively associated with intense inductive electric fields.

Lopez, R. E.↗
NASA NTRS1989

PROMIS series. Volume 7: GOES 5 and GOES 6 geosynchronous magnetic field data for March - June 1986

This is the seventh in a series of volumes pertaining to the Polar Region Outer Magnetosphere International Study (PROMIS). This volume contains 24 hour plots of approximately 1-minute average magnetic fields from the GOES 5 and GOES 6 spacecraft for the period March 10 through June 16, 1986. Data are displayed in a VDH coordinate system based on a centered dipole with northern hemisphere geographic coordinates of the pole at 78.80 deg latitude and 289.24 deg longitude.

Fairfield, D. H.↗
NASA NTRS1989

Multisatellite and ground-based observations of transient ULF waves

Transient ULF pulsations associated with variations in solar wind plasma density observed by the IMP 8 satellite are presently studied in light of observations obtained during a fortuitous alignment of the AMPTE and Viking satellites with respect to the EISCAT Magnetometer Cross. It is found that the isolated 10-min oscillation in solar wind plasma density produced magnetic field compression oscillations within the magnetosphere at the same frequency, thereby enhancing resonant oscillations at approximately twice the frequency which were already present. Support is seen for the periodic solar wind density variation's exciting of a tailward-traveling large-scale magnetosphere wave train which excites local field line resonant oscillations.

Potemra, T. A.↗
NASA NTRS1989

SC- and SI-associated ULF and HF-Doppler oscillations during the great magnetic storm of February 1986

Results are presented of an investigation of SC- and SI-associated ULF and HF-Doppler pulsations observed during the great geomagnetic storm of February 1986, which began with a sudden commmencement on February 6 at about 13:12 UT, developed slowly over the next two days, and, after a rapid intensification late on February 8, reached a minimum. It is shown that these ULF and geomagnetic pulsations can be explained by the dynamo-motor mechanism of ionospheric electric fields and by global compressional oscillations in the magnetosphere and ionosphere, respectively.

Yumoto, K.↗
NASA NTRS1989

Relationships between SC- and SI-associated ULF waves and ionospheric HF Doppler oscillations during the great geomagnetic storm of February 1986

The SC- and SI-associated ionospheric Doppler velocity oscillations and geomagnetic pulsations during the great geomagnetic storm of February 1986 are interpreted. This is done by considering the 'dynamo-motor' mechanism of ionospheric E-field and the global compressional oscillations of the magnetosphere and the ionosphere, respectively.

Yumoto, K.↗
NASA NTRS1988

Magnetospheric ULF waves observed during the major magnetospheric compression of November 1, 1984

The magnetospheric ULF waves observed during the magnetospheric compression event of November 1, 1984, were investigated using the magnetic field and medium-energy ion data obtained by the AMPTE Charge Composition Explorer. The electric field of ULF waves was inferred from the particle data, using a method developed on the basis of the gyration acceleration mechanism. Three types of waves were found: (1) a 10-15-min wave with perturbations in the magnetic field intensity and in the flux of ions; (2) a 3-5-min standing Alfven wave with perturbations in the azimuthal magnetic field component and in the radial component of the electric field; and (3) a 2-5-min irregular disturbance near the magnetopause, which involves all components of the magnetic field and the intensity of the ion flux. The origins of these waves are discussed.

Takahashi, K.↗
NASA NTRS1988

Substorm variations in the magnitude of the magnetic field - AMPTE/CCE observations

Using energetic-particle data taken in the near-earth tail by the AMPTE/Charge Composition Explorer (CCE) satellite, 167 ion injection events, that were essentially dispersionless over a 25-285 keV energy range, were identified, and the variations in the total magnetic field strength over the course of these events were examined in order to determine the dependence of the magnetic field strength on dipole latitude. Results indicate that, during periods of substorm activity, the latitudinal position of the current sheet varied significantly within the 32-deg wedge centered on the dipole equator traversed by CCE. Results also suggest that, even in the near-earth magnetotail out to 8.8 R(E) (CCE apogee), the local field measurements are a better guide to the determination of satellite's position relative to the current shield during a substorm, than is the magnetic latitude.

Lopez, R. E.↗
NASA NTRS1988

AMPTE/CCE observations of substorm-associated standing Alfven waves in the midnight sector

Magnetic-field and medium-energy particle data from the AMPTE/CCE spacecraft are used to study substorm-associated ULF pulsations in the midnight sector at a radial distance of 8 to 9 earth radii. The particle data are used to identify ion injections and to detect the electric field of ULF waves. A case study of the events on May 23, 1985 shows that the waves have the properties of a fundamental-mode standing Alfven wave. It is suggested that these observations are evidence of substorm-associated standing Alfven waves in the nightside magnetosphere.

Takahashi, K.↗
NASA NTRS1987

An eastward propagating compressional Pc 5 wave observed by AMPTE/CCE in the postmidnight sector

This paper presents a detailed analysis of a compressional Pc 5 wave observed in the postmidnight sector on July 21, 1986, using data from the magnetometer, the charge-energy-mass spectrometer, and the medium-energy particle analyzer aboard the AMPTE/Charge Composition Explorer (CCE) spacecraft. The Pc 5 wave exhibited harmonically related transverse and compressional magnetic oscillations, modulation of the flux of medium energy protons, and a large azimuthal wave number, i.e., properties that are similar to those of compressional Pc5 waves observed previously at geostationary orbit. The unique observations recorded by the AMPTE/CCE included the occurrence of the wave in the postmidnight sector, its sunward propagation with respect to the spacecraft, and the left-handed polarization of the perturbed magnetic field. In spite of the morphological uniqueness observed, the excitation of the July 21 event is considered to be due to the same type of instability as operates at geostationary orbit.

Takahashi, K.↗
NASA NTRS1987

Disruption of the magnetotail current sheet observed by AMPTE/CCE

An unusual large-amplitude (from less than 10 nT to greater than 40 nT) magnetic oscillation characterized by about-13-sec periodicity and southward turnings of the field was observed by AMPTE/CCE on August 28, 1986. The magnetic field was often stronger southward, with some southward components exceeding 20 nT being noted. The level of the high frequency perturbations was also seen to be enhanced. It is suggested that these observations may be due to the formation of an X-type neutral line and its motion near the spacecraft.

Takahashi, K.↗
NASA NTRS1987

A model for the harmonic of compressional Pc 5 waves

Compressional Pc 5 magnetic waves in the magnetosphere are a unique phenomenon showing a nonsinusoidal waveform in spite of a well-defined period. Although the waveform can be Fourier-decomposed into the fundamental and the second harmonics, the phase between the two is kept constant from event to event, implying that the waveform is not the result of a chance superposition of two magnetospheric eigenmodes. A phenomenological explanation to this waveform is offered using a field-line configuration model that is a modified version of a previously proposed antisymmetric standing wave. In this model, the location of the equatorial node of field-line displacement is assumed to oscillate with the wave, with a peak-to-peak amplitude greater than 10 percent of the wavelength of the standing wave. The predicted waveform at various magnetic latitudes is found to be in excellent agreement with an observation taken near the magnetic equator by the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer spacecraft.

Takahashi, K.↗
NASA NTRS1985

ISEE 1 and 2 observation of the spatial structure of a compressional Pc5 wave

A compressional Pc5 was observed on an ISEE 1 and 2 outbound path on September 28, 1981 at L = 5.6-7.3 near the magnetic equator at about 10 hr local time during the recovery phase of a geomagnetic storm. The wave propagated westward with a large azimuthal wave number of about 30 and exhibited in-phase oscillations of plasma density and magnetic field magnitude. During this event, component-dependent variations in phase and amplitude were observed for the magnetic field oscillations. The radial and compressional components had a constant phase and their amplitude was finite. In contrast, the azimuthal component changed its phase by 180 deg and its amplitude became zero during the middle of the wave event. The observation as indicating the spatial structure of the Pc5 wave is interpreted. The polarization reversal is likely to be caused by a crossing of a node of a standing wave located 4 deg above the geomagnetic equator.

Takahashi, K.↗
NASA NTRS1985

Search for solar neutrons using NM-64 equipment

Two years (1980 to 1982) neutron monitor data from the Chacaltaya (geographic coordinates: N16.32 deg W68. 15 deg; cutoff rigidity: 13.1 GV; altitude: 5,300 m a.s.l.) station has been scanned; the sampling time of the 12NM-64 neutron monitor is 5 min. The nucleonic component increases have been correlated with 66 hard X-, gamma rays satellite data from solar origin, as reported by several groups. Typical neutron monitor time profiles of the events are presented. Chree-analysis was performed discriminating the events according to its solar coordinates. Ground data from solar limb locii are more enhanced at the time of the onset than other geometrically visible flares. Chree histograms of neutron monitor output profiles are also presented from geometrically invisible events from the Chacaltaya station.

Martinic, N. J.↗
NASA NTRS1985

Diurnal variations from muon data at Takeyama underground station

An underground station, Takeyama, is introduced, and some results of the solar diurnal and semi-diurnal variations for the period between 1967 and 1984 are presented. There are clear tendencies of double and single solar cycle variations in the daily variations which are in good accord with those detected by other underground and neutron monitor observations.

Takahashi, K.↗