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Yumoto, K.

Publications and source records attributed to Yumoto, K..

13 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2014

A Tailward Moving Current Sheet Normal Magnetic Field Front Followed by an Earthward Moving Dipolarization Front

A case study is presented using measurements from the Cluster spacecraft and ground-based magnetometers that show a substorm onset propagating from the inner to outer plasma sheet. On 3 October 2005, Cluster, traversing an ion-scale current sheet at the near-Earth plasma sheet, detected a sudden enhancement of Bz, which was immediately followed by a series of flux rope structures. Both the local Bz enhancement and flux ropes propagated tailward. Approximately 5 min later, another Bz enhancement, followed by a large density decrease, was observed to rapidly propagate earthward. Between the two Bz enhancements, a significant removal of magnetic flux occurred, possibly resulting from the tailward moving Bz enhancement and flux ropes. In our scenario, this flux removal caused the magnetotail to be globally stretched so that the thinnest sheet formed tailward of Cluster. The thinned current sheet facilitated magnetic reconnection that quickly evolved from plasma sheet to lobe and generated the later earthward moving dipolarization front (DF) followed by a reduction in density and entropy. Ground magnetograms located near the meridian of Cluster's magnetic foot points show two-step bay enhancements. The positive bay associated with the first Bz enhancement indicates that the substorm onset signatures propagated from the inner to the outer plasma sheet, consistent with the Cluster observation. The more intense bay features associated with the later DF are consistent with the earthward motion of the front. The event suggests that current disruption signatures that originated in the near-Earth current sheet propagated tailward, triggering or facilitating midtail reconnection, thereby preconditioning the magnetosphere for a later strong substorm enhancement.

Dipolarization Front↗
NASA NTRS2010

THEMIS Observations of Directly-Driven Pi2 Pulsations

The THEMIS tail seasons have provided an unprecedented opportunity to examine the causal relationship between midtail plasma flows and low latitude Pi2 pulsations. We present several events where multiple THEMIS spacecraft observed magnetotail flow bursts which were followed up to several minutes later by ground Pi2 pulsations. We find good agreement with the waveforms of the flow bursts and flank Pi2, in agreement with the hypothesis that Pi2 at low-latitude on the flank are directly-driven by periodic variations in the flow bursts. For at least I event we are able to follow the Pi2 impulses from the periodic flow bursts on the nightside. to ground Pi2 at the flanks, and finally through the dayside magnetosphere as observed by GOES. We further place the physical mechanism generating these Pi2 into the context of sub storm onset. We conclude by discussing the sequence and coupling of events that are necessary to explain the correlation, and the constraints this places on models of transient magnetospheric transport.

Kepko, L.↗
NASA NTRS1995

Observation and modeling of compressional Pi 3 magnetic pulsations

Compressional magnetic pulsations with irregular waveforms and periods longer than 150 s (here termed Pi 3) have been studied by using data from Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE/CCE) and GOES 5 and 6 in the dayside magnetosphere and compared with signatures on the ground at low latitudes by using data from Kakioka station (L = 1.25). On the ground, the pulsations appear in the horizontal component. A study of 17 such concurrent events during a 2-month period in 1986 reveals the following pulsation characteristics. (1) The peak-to-peak amplitudes in space (delta B(sub T)) and on the ground (delta H) are comparable and are in the range of 0.5-7 nT. (2) On the ground the pulsations can be seen at all local times, even at midnight, while at geostationary orbit they are observed only on the dayside with a clear amplitude maximum at noon. (3) The pulsations on the ground lag those observed by CCE near local noon, and the lag increases as the local time separation between CCE and the ground station increases. The time lag is 1-2 min longer when the ground station is on the nightside than when it is on the dayside. (4) The time lag between pulsations observed at geostationary orbit and near noon by CCE varies systematically with local time and is about 2 min per 6 hours of local time separation. These observations indicate that some nightside pulsations in the Pi 3 band have dayside origins. The position dependence of the pulsation amplitude can be explained well by changes in the magnetopause current, which are in turn presumably caused by changes in the solar wind dynamic pressure. The time lags observed in space are consistent with signal propagation in the MHD fast mode, but the variation in space-ground time lags with ground station local time must be attributed to another mechanism.

Matsuoka, Hitoshi↗
NASA NTRS1994

A flux rope followed by recurring encounters with traveling compression regions: GEOTAIL observations

This paper reports a peculiar series of events observed with Geotail on October 24, 1992 at X approximately equals -80 R(sub E). On this day a combination of a bipolar perturbation in B(sub y) and a double-peaked perturbation in B(sub z), signature of a flux rope, was observed. Following this flux rope, bipolar B(sub z) events, identifiable as Traveling Compression Regions (TCRs), were recurrently observed. The axis of the flux rope was deduced to be field-aligned. Energetic ions (electrons) streamed earthward (tailward) in the flux rope. During the TCRs, the E x B flow showed a bipolar north-then-south perturbation, consistent with expected lobe plasma motion. The TCRs corresponded to weak ground magnetic disturbances and Pi2 waves. The observations can be explained by a model in which quasi-stagnant plasmoids are recurrently generated.

Kawano, H.↗
NASA NTRS1990

Multiple ground-based and satellite observations of global Pi 2 magnetic pulsations

Four Pi 2 events are examined using data obtained concurrently at the Syowa-Iceland conjugate stations, the EISCAT-Magnetometer Cross, low-latitude stations HER on the nightside and KAK on the dayside, and at AMPTE/CCE in the outer magnetosphere during the period from 2300 UT on May 22 to 0300 UT on May 23, 1985. Several characteristics of Pi 2 are established on the basis of high time resolution magnetic field data. First of all, horizontal components, H and D, of the Pi 2 oscillate nearly antiphase and in-phase, respectively, between the high- and low-altitude stations in the midnight southern hemisphere. Both H and D components of the Pi 2 have nearly in-phase relationships between the nightside and the dayside stations at low altitude. The Pi 2 amplitude is larger at the high-latitude station and decreases toward low latitudes. The dominant periods of the Pi 2 are nearly identical at all stations.

Yumoto, K.↗
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 NTRS1987

Structure and dynamics of the plasma tail of comet P/Halley. I - Knot event on December 31, 1985

On the basis of about 500 photographs of comet Halley taken by many observers, Saito et al. (1986) classified the detected disturbances of the plasma tail as outstanding rays, streamer, helix, kink, arcade, and disconnection event (DE). In this paper, the interaction of the solar wind with the plasma tail of Comet P/Halley is examined by using results of observations by the Sakigake spacecraft of the December 31, 1985 event, which included various disturbances and one DE-like knot. On the basis of twenty photographs taken on December 31 by Japanese astronomers, the dynamic pressure model proposed by Saito et al. (1986) is examined, and the mechnism of the knot event that appeared in the plasma tail of the comet on December 31, is explained.

Saito, T.↗
NASA NTRS1986

A disturbance of the ion tail of Comet Halley and the heliospheric structure as observed by Sakigake

In order to study the interaction between the solar wind measured by Sakigake and ion tail disturbances of comet Halley, more than 500 photographs of the comet taken on the ground during this apparition are surveyed. The focus of the present study is the December 31, 1985, event, when various types of disturbances occurred, including an outstanding disconnection event (DE)-like knot. Analysis of the Sakigake/IMF data reveals that comet Halley did not encounter the heliospheric neutral sheet on that day, demanding a new explanation for the DE-like event, different from the Niedner-Brandt model. During this event the comet encountered a high-speed solar wind stream from a coronal hole tongue of the sun. The event can be explained by a dynamic pressure model, according to which the DE-like plasmoid was caused by a sudden increase in the dynamic pressure of the solar wind. A result of the simulation work by Ogino is found to support this interpretation.

Saito, T.↗
NASA NTRS1986

Three-dimensional structure of the heliosphere as inferred from observations with a Japanese Halley spacecraft

A sinusoidal neutral line with a pair of giant regions appeared on the sun about one year before the launch of 'Sakigake', the first of two Japanese Comet Halley spacecraft. The Sakigake magnetometer data during the early part of the mission (February-March 1985) are well interpreted by an eastward shift of the tilting neutral sheet. The shift is further explained by an effect of a new giant region appearing at about 10 deg heliolatitude and about 50 deg Carrington longitude in August 1984. The toward polarity ratio of IMF observed by Sakigake changed from about 22 percent in February to about 62 percent in early June and then increased rapidly up to 98 percent. This ratio is interpreted as a decrease of the tilt angle of the sheet down to only about 4 deg. It is the first spacecraft observation of 'the disappearing sector structure' with such small tilt angle.

Saito, T.↗
NASA NTRS1985

Propagation mechanism of daytime Pc 3-4 pulsations observed at synchronous orbit and multiple ground-based stations

Observational data obtained during the last two decades show that the amplitude of daytime Pc 3-4 magnetic pulsations is controlled by the solar wind conditions. The high degree of correlation between the solar wind parameters and Pc 3-4 pulsations in the dayside magnetosphere suggests that the ultimate cause of the daytime Pc 3-4 pulsations must be the interaction of the solar wind with the earth's magnetosphere. The present paper is concerned with details regarding the control of the properties of the Pc 3-4 pulsations by the solar wind parameters, taking into account observations made at multiple ground-based stations. It is attempted to establish the relation between the daytime Pc 3-4 pulsations at the ground stations and the compressional Pc 3-4 waves in the magnetosphere. Attention is given to the most probable propagation mechanism of the daytime Pc 3-4 pulsations in the magnetosphere.

Yumoto, K.↗
NASA NTRS1984

Relationship between the IMF magnitude and Pc 3 magnetic pulsations in the magnetosphere

The relationships between the IMF magnitude and pulsation frequencies in the Pc 3-4 range simultaneously observed both at synchronous orbit and at low latitudes on the ground are statistically described. A theoretical discussion is given on how these observations can be interpreted in terms of the characteristic frequency of compressional Pc 3-4 magnetic pulsations in the magnetosphere, based on the well-established ion cyclotron resonance mechanism between magnetosonic mode of low-frequency upstream waves and narrowly reflected ion beams in the earth's foreshock.

Yumoto, K.↗