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We present a case study of Geotail, Interball-1, IMP-8, and Wind observations of density and magnetic field strength cavities excavated by the enhanced pressures associated with bursts of energetic ions in the foreshock. Consistent with theoretical predictions, the pressure of the energetic ions diminishes rapidly with upstream distance due to a decrease in the flux of energetic ions and a transition from near-isotropic to streaming pitch angle distributions. Consequently, the cavities can only be observed immediately upstream from the bow shock. A comparison of conditions upstream from the pre- and post-noon bow shock demonstrates that foreshock cavities introduce perturbations into the oncoming solar wind flow with dimensions smaller than those of the magnetosphere. Dayside geosynchronous magnetic field strength variations observed by GOES-8 do not track the density variations seen by any of the spacecraft upstream from the bow shock in a one-to-one manner, indicating that none of these spacecraft observed the precise sequence of density variations that actually struck the subsolar magnetopause. Key words. Interplanetary physics (energetic particles; planetary bow shocks) – Magnetospheric physics (solar wind-magnetosphere interactions)

著者人数: 16名, 資料番号: SA1150052000

著者人数: 13名, Accepted: 2013-08-30, 資料番号: SA1004891000

[1] The ion-electron decoupling region where electron outflow speed differs from ion outflow speed is formed in the magnetic reconnection site. Ion and electron dynamics in the ion-electron decoupling region is derived with magnetic field and plasma observations by the spacecraft Geotail in near-Earth magnetotail magnetic reconnection. The ion-electron decoupling region has a spatial extent of approximately 11 λi (ion inertial length) along the GSM x direction, and the dawn-dusk current sheet with main current carriers of electrons exists over this region. An intense electron current layer with a spatial extent of 0.5–1 λi occupies in its center around the X line. High-speed electron outflow jets are formed just outside the central intense electron current layer. They are decelerated and become non-jet outflows with speed slightly higher than ion outflow speed. Electrons have flattop distribution functions indicating heating and acceleration in both the outflow jets and the non-jet outflows; however, heating and acceleration are weak in the central intense current layer. Inflowing ions enter the central intense electron current layer, and these ions are accelerated up to 10 keV inside the electron outflow jet regions. Ion acceleration beyond 10 keV and thermalization operate mostly in the non-jet electron outflow regions. Electrons show thermal distributions without any heating/acceleration signatures immediately beyond the edge of the ion-electron decoupling region, while higher-energy ions pervade even beyond the edge and hot MHD plasma flows are produced.

– The Planetary Material Sample Curation Facility of JAXA (PMSCF/JAXA) was established in Sagamihara, Kanagawa, Japan, to curate planetary material samples returned from space in conditions of minimum terrestrial contaminants. The performances for the curation of Hayabusa-returned samples had been checked with a series of comprehensive tests and rehearsals. After the Hayabusa spacecraft had accomplished a round-trip flight to asteroid 25143 Itokawa and returned its reentry capsule to the Earth in June 2010, the reentry capsule was brought back to the PMSCF/JAXA and was put to a series of processes to extract recovered samples from Itokawa. The particles recovered from the sample catcher were analyzed by electron microscope, given their ID, grouped into four categories, and preserved in dimples on quartz slide glasses. Some fraction of them has been distributed for initial analyses at NASA, and will be distributed for international announcement of opportunity (AO), but a certain fraction of them will be preserved in vacuum for future analyses.

著者人数: 57名, Accepted: 2016-03-22, 資料番号: SA1160037000

1Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 812-8581, Japan 2Department of Life, Environment and Materials Science, Fukuoka Institute of Technology, Fukuoka 811-0295, Japan 3Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka 812-8582, Japan 4Frontier Science Div., Shiseido Co. Ltd., Minato-ku, Tokyo 105-0021, Japan 5Department of Earth and Space Sciences, Osaka University, Toyonaka, Osaka 560-0043, Japan 6Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, Japan 7Astrophysics Branch, NASA-Ames Research Center, Moffett Field, CA 94035, U.S.A. 8Department of Earth and Planetary Material Sciences, Tohoku University, Sendai, Miyagi 980-8578, Japan 9College of Science, Ibaraki University, Mito, Ibaraki 310-8512, Japan 10Geochemical Research Center, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan 11Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan 12Department of Earth and Planetary Sciences, Hokkaido University, Sapporo 060-0810, Japan 13Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (ISAS/JAXA), Sagamihara, Kanagawa 229-8510, Japan

宇宙航空研究開発機構（JAXA）はやぶさ探査機が，近地球型小惑星のイトカワから持ち帰った微小試料についてラマン分光分析を行い，樹脂に包埋された直径30から150 μmの試料中の鉱物の同定を試みた．ラマン分光分析によって同定されたケイ酸塩鉱物及び酸化鉱物は，カンラン石，輝石，斜長石，クロム鉄鉱である．それらは，エネルギー分散型スペクトロメーター付走査電子顕微鏡観察で同じ研磨断面試料から同定されたケイ酸塩鉱物・酸化鉱物と一致した．また，斜長石が衝撃により固相状態で非晶質化したマスケリナイトはラマン分光法で同定可能であるが，完全にマスケリナイト化した斜長石は見いだされなかった．このことは，本研究で分析したイトカワ微小試料が経験した衝撃ステージは，S4には到達していないことを示唆しており，他の初期分析の結果と整合的である．

著者人数: 29名, Accepted: 2010-03-15, 資料番号: SA1002803000

The plasma number density in the near-Earth plasma sheet depends on the solar wind number density and the north-south component of interplanetary magnetic field (IMF Bz) with time lag and duration of several hours. We examined the three-dimensional structure of such dependences by fitting observations of plasma sheet and solar wind to an empirical model equation. Analyses were conducted separately for northward and southward IMF conditions. Effects of solar wind speed and IMF orientation were also examined by further subdivision of the dataset. Based on obtained results, we discuss (i) the relative contribution of the ionosphere and solar wind to plasma sheet mass supply, (ii) the entry mechanisms for magnetosheath particles, and (iii) the plasma transport in the plasma sheet. We found that solar wind number density dependence is weaker and IMF Bz dependence is stronger for faster solar wind with southward IMF, which suggests the contribution of ionospheric particles. Further from the Earth, different interplanetary conditions result in different structures of solar wind dependence, which indicate different solar wind entry mechanisms: (1) southward IMF results in a strong dependence on solar wind number density in the flank high-latitude region, (2) slow solar wind with northward IMF leads to lower-latitude peaks of solar wind number density dependence in the flank region, (3) fast solar wind with northward IMF results in a strong dependence on solar wind number density at the down-tail dusk flank equator, and (4) solar wind number density dependence is stronger in the downstream of quasi-parallel bow shock. These features are attributable to (1) low-latitude dayside reconnection entry, (2) high-latitude dayside reconnection entry, (3) entry due to decay of Kelvin-Helmholtz vortices, and (4) diffusive entry mediated by kinetic Alfven waves, respectively. Effect of IMF Bz and its time lags show plasma sheet reconfiguration associated with enhanced convective transport under southward IMF. Duration of IMF Bz effect under northward IMF is interpreted in terms of turbulent diffusive transport.

We examine photoelectron distributions detected by the low-energy-particle (LEP) instrument onboard the GEOTAIL spacecraft by means of both data analysis and numerical simulations. Statistical data analysis shows asymmetries in the photoelectron distributions. For photoelectrons incident normal to the spacecraft spin axis, a higher flux is observed in the dawnward than in the duskward sector of the LEP. The distribution significantly depends on the ratio of the photoelectron energy to the spacecraft potential. Our numerical simulations reveal that the asymmetry is caused by the electrostatic potential around the thin antenna located at +18deg anticlockwise (viewed from the top) relative to the LEP. Photoelectrons in the dawnward sector are preferentially carried from the sunlit surface by this potential. For upward/downward incident photoelectrons, a higher flux of upward photoelectrons is observed in the antisunward than in the sunward sector, whereas downward photoelectrons show a weak asymmetry. Our numerical simulations demonstrate that the greater flux of upward photoelectrons is caused by the electrons emitted from the sunlit surface; they are attracted to the antisunward sector. Based on these results, the asymmetries in the photoelectron distribution measured around GEOTAIL are found to be caused by the asymmetric positioning of the thin antennas relative to the LEP.

This paper summarizes the current status of the BepiColombo Mercury magnetospheric orbiter (MMO) spacecraft design. The MMO is a spinning spacecraft of 223 kg mass whose spin axis is nearly perpendicular to the Mercury orbital plane. The current status of the overall MMO system and subsystems such as thermal control, communication, power, etc., are described. The critical technologies are also outlined. Furthermore, the outline of the international cooperation between Japan Aerospace Exploration Agency and European Space Agency is also presented.

With two electron beam sources, we have tested two new Hamamatsu [Hamamatsu Photonics K.K., Shizuoka, Japan 〈 http://www.hamamatsu.com/ 〉 ] avalanche photodiodes (APDs) of spl 3988 and spl 6098 to detect electron beams up to 100 keV. Though our previous results showed the effectiveness and the advantage of an APD to measure 2–40 keV electrons, its upper limit was not high enough to detect so-called medium-energy electrons. In addition to the limitation of its detectable range, the response at different energies was also not linear. These newly developed APDs, which have thicker depletion-layers, provide full coverage of this missing range along with a good linearity. The depletion-layer thickness was increased to 140 μ m for both APDs, the dead-layer of spl 3988 became 10 times thicker than that of spl 6098. The thin-surface dead-layer and thick depletion-layer of spl 6098 allows the detection of electrons from 3 keV up to 100 keV with a good linearity and with an excellent energy resolution of 4 keV at 100-keV electrons. The wide dynamic range from 3 keV to 100 keV of those APDs will increase their appeal in detecting electrons for space plasma research.

Accepted: 2007-06-20, 資料番号: SA1000113000

Geotail plasma wave observations show the existence of intense electrostatic quasi-monochromatic (EQM) waves in the downstream region of the Earth’s bow shock. They oscillate parallel to the ambient magnetic field and appear at frequencies between the electron plasma and ion plasma frequencies. Although these waves have been believed to be Doppler-shifted ion acoustic waves, the typical plasma parameters observed in the downstream region do not support the generation conditions for ion acoustic waves. In this paper, the existence of cold electron beam-like components accompanying EQM waves is considered based on waveform and statistical analyses. Linear dispersion analyses using realistic plasma parameters revealed that the cold electron beams cause destabilization of electron acoustic waves at frequencies consistent with those of observed EQM waves. The results of observations and linear analyses suggest that EQM waves are generated by the destabilization of the electron acoustic mode.

We have successfully detected solar neutrons at ground level in association with the X17.0 solar flare that occurred on 2005 September 7. Observations were made with the solar neutron telescopes and neutron monitors located in Bolivia and Mexico. In this flare, large fluxes of hard X-rays and γ-rays were observed by the GEOTAIL and the INTEGRAL satellites. The INTEGRAL observations include the 4.4 MeV line γ-rays of 12 C. The data suggest that solar neutrons were produced at the same time as these hard electromagnetic radiations. We have however found an apparent discrepancy between the observed and the expected time profiles. This fact suggests a possible extended neutron emission.

Accepted: 2006-06-06, 資料番号: SA1000438000

Accepted: 2006-03-08, 資料番号: SA1000591000

Accepted: 2005-09-27, 資料番号: SA1000608000

We investigated the relationship between low-frequency (0.2-4.0 Hz) electric-field fluctuations (LEFs) and ion conics around the dayside cusp/cleft region in the altitude range from 5000 to 10000km from observations made by the Akebono satellite. Ion conics were generally associated with intense LEFs. We found a significant correlation between the power spectral density of LEFs at any frequency and the energy of simultaneously observed ion conics. Ion conics with a conic angle near 90 deg and those more aligned with magnetic field lines both had an equivalent correlation with the local intensity of the LEFs. The LEFs associated with near-perpendicular ion conics were, however, generally more intense than those associated with folded conics. The difference was clearer for low-energy conics. These results are in agreement with a scenario of height-integrated heating of ions and energization of ions by electromagnetic energy supplied by local LEFs. Ions generally stay in the energization region during their upward motion along the field line, so that more folded ion conics with weak energization reach the same energy level as near-perpendicular conics with strong energization, due to the difference in integration time. The limit on residence time in the intense heating region causes the clearer difference for low-energy conics. We set up a simple model to examine the relationship between the energization rate and the evolution of ion conics along the field lines, and obtained good agreement with the observation results.

We summarize the characteristics of the DC electric field measurement by the double probe system, EFD-P, aboard Geotail. The accuracy and correction factors for the gain (effective length) and off-set, which depends on ambient plasma conditions, are provided., 資料番号: AA0049206028, レポート番号: JAXA-SP-05-001E

Accepted: 2005-02-17, 資料番号: SA1002568000

Accepted: 2005-04-25, 資料番号: SA1003445000

The magnetic field and plasma structures during two successive crossings of the subsolar magnetospheric boundary layers (i.e. MagnetoPause Current Layer (MPCL) and Low-Latitude Boundary Layer (LLBL)) under the southward-dawnward IMF are examined on the basis of the data obtained by the GEOTAIL spacecraft. A significant and interesting feature is found, that is, Flux Transfer Events (FTEs) occur in association with the formation of the tilted X-line. During the first inbound MPCL/LLBL crossing, the ion velocity enhancement (in particular, the Vl component negatively increases) can be observed in association with simultaneous typical bipolar signature (positive followed by negative) in the Bn component. In addition, a clear D-shaped ion distribution whose origin is the magnetosheath can also be found in the dawnward direction. A few minutes later, the satellite experiences outbound MPCL crossing. The negative enhancement of the Vm component can be found as well as the positive enhancement of the Vl component. Simultaneously, a typical bipolar signature with the polarity (negative followed by positive) opposite that observed in the first encounter can also be observed. The ions from the magnetosheath flow predominantly in the duskward direction, although the D-shaped ion distribution cannot be observed. These results indicate that the satellite initially observes one part of a reconnected flux tube formed by FTEs whose magnetospheric side is anchored to the Southern Hemisphere. The ions confined in this partial flux tube are flowing in the south-dawnward direction. Then, the satellite observes the other part of the reconnected flux tube whose magnetospheric side is anchored to the Northern Hemisphere. The ions confined in this flux tube flow dominantly in the north-duskward direction. Furthermore, it can be considered that the second MPCL crossing is a direct cut through the diffusion region of FTEs because the LLBL is absent in the vicinity of the MPCL. On the basis of these results, it can be concluded that the satellite was passing near the tilted X-line. The information obtained through this study is expected to be of great use in discriminating between the anti-parallel (steady-state) reconnection and tilted X-line models on the dayside MPCL.

Mariner 10 has been the only spacecraft to visit the innermost planet Mercury. Its three flybys, more than 25 years ago, yielded the first view of this little-understood world. With advances in spacecraft technology and a growing realization of how important Mercury is to our understanding of the solar system and its formation, two missions are now in development for more intensive Mercury exploration. The first is the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission, competitively selected under the NASA Discovery Program, that will send a spacecraft to fly by Mercury in 2007 and 2008 and to orbit Mercury for one Earth year beginning in April 2009. The second is the more comprehensive BepiColombo mission, consisting of three elements: the Mercury Planetary Orbiter (MPO), the Mercury Magnetospheric Orbiter (MMO), and the Mercury Surface Element (MSE). Still in final definition stage, BepiColombo is a partnership between the European Space Agency (ESA) and the Japanese Institute of Space and Astronautical Science (ISAS). With one or two launches (depending upon the final mission architecture) BepiColombo will use solar electric propulsion to send two orbiters (MPO from ESA and MMO from ISAS) and a lander (MSE) to Mercury as early as 2011. The BepiColombo orbiters, in orbits complementary to that of MESSENGER, will extend geochemical, spectral, and photometric mapping of the planet. With its factor-of-ten larger downlink, BepiColombo will complete the intensive study of Mercury begun with the exploration by MESSENGER. Synergistic strategies of exploration will enable efficient use of BepiColombo resources in a more detailed study of the planet than can be accomplished by MESSENGER alone. For example, the earlier MESSENGER mission can help identify appropriate landing sites for the MSE, while BepiColombo can provide complementary orbital measurements of surface features from different phase angles and exploit MESSENGER observations to target high-resolution measurements. Possible joint activities include cooperative use of ground stations and extension of the temporal baseline for geophysical measurements by combining data from all spacecraft. Were MESSENGER to be approved for an extended orbital phase, there is also the possibility of simultaneous dual-spacecraft measurements in the magnetosphere.

Accepted: 2003-02-04, 資料番号: SA1001214000

Accepted: 2003-04-22, 資料番号: SA1002461000

Accepted: 2003-04-24, 資料番号: SA1001213000

Accepted: 2003-02-18, 資料番号: SA1001286000

We present an analysis of sporadic and recurrent injections of magnetospheric ions in the midnight auroral oval during substorms and of the associated ionospheric ion outflows. The source of plasma sheet precipitating ions is determined using a simple method, based on the measured relation between the ion inverse velocity and time ( l = v × t ). This method is applied here to two typical passes of the Interball-Auroral (IA) satellite at distances of ∼ 3 R E above the auroral regions. Substorm related ion injections are shown to be mainly due to time of flight effects. In contrast with particle trajectory computations (Sauvaud et al. , 1999), the inverse velocity method does not require magnetic and electric field models and can thus be used systematically for the detection of time of flight dispersed ion structures (TDIS). This allowed us to build a large database of TDIS events and to perform a statistical analysis of their spatial distribution. For the cases presented here the source region of the injected ions is found at radial distances from 18 to 30 R E near the equatorial magnetosphere. At Interball altitudes (∼ 3 R E ), ion injections detected at the poleward boundary of the nighside auroral oval are associated with shear Alfven waves superimposed over large-scale quasi-static current structures. We show that the most poleward TDIS are collocated with a large outflow of ionospheric H + and O + displaying pitch-angle distributions peaked in the pitch-angle range 90°–120°. These ions are thus accelerated perpendicularly to the magnetic field not only in the main auroral acceleration region but also up to at least 3 R E . The expanding auroral bulge thus constitutes a significant source of H + and O + ions for the mid-tail magnetosphere.

A detailed analysis of Geotail observations on June 1, 1997 provides evidence for the appearance of the plasma mantle at low-latitudes, ZGSM ∼3 RE for southward interplanetary magnetic field (IMF), and clarifies the characteristics of the plasma flow in the low-latitude mantle. The tailward flow speed in the mantle is 80 ∼ 160 km/s, smaller than the flow speed (≳300 km/s) in the adjacent magnetosheath. The component of the velocity perpendicular to the magnetic field lines is predominantly poleward at speeds up to 60 km/s, and the convection tends to be stronger for |;BY| |BZ|;. This polewardconvection can be interpreted as being driven by a J × B force operating on the field lines.

Counterstreaming ions embedded in hot isotropic ions are found at the front of fast earthward plasma flows in the recovery phase of substorms in the Earth’s magnetotail. The counterstreaming ions are present only when the northward component of the magnetic field increases in the equatorial plane. Hybrid simulations of magnetic reconnection have been carried out. It is found that counterstreaming ions form in the leading edge of jetting plasmas produced with magnetic reconnection, where the magnetic field lines pile up due to the pre-existing stationary plasmas. These counterstreaming ions originate from cold ions on the northern and southern tail lobe field lines, and earthward transport of the reconnected field lines makes these cold ions flow into the equatorial plane. The present observations provide strong evidence of magnetic reconnection in the recovery phase of substorms at the kinetic level.

This paper shows the latest results of the feasibility study of the BepiColombo MMO (Mercury Magnetospheric Orbiter) Spacecraft, which was conducted by the ISAS (Institute of Space and Astronautical Science) Mercury Exploration Working Group.

Data measured in the magnetosheath by the Geotail satellite are used to test for the existence of a proportional relation between the temporal changes in magnetic field, ΔB, and the temporal changes in plasma velocity, ΔV. We have compared the ratio |ΔV/ΔB| with that which would be anticipated from magneto-hydrodynamic wave theory. When the plasma's β exceeds 1, the ratio often significantly differs from that expected from any wave mode in isotropic plasmas. The existence of anisotropies in plasma distribution functions and consequent modifications of |ΔV/ΔB| is strongly suggested. Under the assumption that the modifications of |ΔV/ΔB| are predominantly caused by such temperature anisotropies, β || / β ⊥ is evaluated over the range from 1 to 3. These anisotropies of plasma temperature may be caused by the adiabatic expansion of magnetic flux tubes in the magnetosheath.

We performed a multi-instrument ground-based and satellite study of substorm-related phenomena during auroral breakup. Our study is limited to the short (a few minutes) time interval around the breakup onset. A mapping made on the basis of event-oriented mangnetospheric model places the pre-breakup arc into the near-Earth region ( r ≈ 10 R E ) where thin and intense current sheet develops. Disruption of the current sheet was observed in the same region. The plasmoid was observed in the mid-tail. Backtracing the plasmoid onset time and location confirmed that reconnection related to the substorm onset could operate in the near-Earth thin current sheet. These observations are in agreement with the idea that the reconnection and current disruption signatures, as well as auroral breakup are different manifestations of the one process.

We present the first conjugate observations of subauroral ion drifts (SAID) in the magnetosphere (∼9000 km altitude) and ionosphere and coincident measurements by four ionospheric satellites. The parameters measured include ion drifts, energetic precipitating electrons and ions, and the magnetic field perturbations associated with field-aligned currents. Observations indicate that SAID are very coherent features that occur simultaneously over a large magnetic local time (MLT) range, from at least 1600 to 2400 MLT. The equatorward extent of SAID, the ion precipitation, and the region 2 field-aligned currents (FAC) flowing into the ionosphere are all shown to be coincident at all MLT locations where SAID are observed. They also appear to be closely related to the conductivity distribution in the subauroral ionosphere and the midlatitude trough. This is interpreted as an indication that their latitudinal distribution is a consequence of the subauroral conductivity structure and the movement of the plasma sheet ion and electron boundaries. Conjugate measurements at diverse altitudes when mapped along field lines are nearly identical, indicating the absence of significant field-aligned potential drops. Temporally separated SAID measurements in similar MLT regions show a reduction with time in the field-aligned current densities with little reduction in the potential drop across the SAID. We interpret the results as an indication that the magnetosphere acts as a current generator in which large FAC are initially required to support the electric field gradient in a SAID event. Subsequent evolution in the E and F regions produces large conductivity gradients that are in the right sense to remove the intense FAC requirement but maintain the large subauroral electric fields. The reported potential drops in the subauroral region can be a significant fraction of the total, up to 60 kV or more, and must be taken into account when deriving any magnetospheric convection pattern.

In the auroral dawn and dusk magnetosphere at altitudes of ∼2-3 R E , three distinct zones of ion and electron precipitation are commonly detected onboard Interball 2, near the polar edge of the auroral oval. From high to low latitudes the satellite encounters (1) magnetosheath/low-latitude boundary layer (LLBL) like plasma (Zone 1), (2) a mixing region with plasma characteristics between LLBL and plasma sheet (Zone 2), and (3) the auroral plasma sheet precipitation (Zone 3). Further equatorward, the satellite crosses the inner plasma sheet characterized by ion gaps in the morning sector. Inside Zones 1 and 2 impulsive ion injections are often detected. They consist of overlapping energy dispersed structures from about 10 keV down to several hundreds of eV with temperature close to that of the magnetosheath. Using trajectory computations backward in time, these dispersions are shown to be cause by time-of-flight effect from a distant source located close to the equatorial magnetopause. Whereas Zone 1 is located mainly poleward of region 1 (downward field-aligned currents at dawn and upward field-aligned currents at dusk), Zone 2 generally coincides with region 1. It is mainly located on closed field lines, as evidenced from the local detection of bouncing ion clusters. Finally, Zone 3 corresponds with region 2 of upward field-aligned currents. A statistical study of Zone 1 which is present in about 12% of the satellite passes at dawn and dusk reveals that its probability of occurrence seems to be controlled by the interplanetary magnetic field (IMF). That is, it is formed when the IMF has a northward component and tends to be radially directed. Moreover, it is more frequently encountered during periods of enhanced solar wind pressure. Detailed case studies uncover a remarkable correlation between the onsets of individual injections inside Zone 1 and those of pressure pulses in the magnetosheath. Both have a characteristic period of ∼ 200- 250 s, similar to that of Pc5 events associated with these injections and detected on board Interball 2. Altogether, these observations indicate that injections and related Alfven waves are driven by magnetosheath pressure pulses associated with a quasi-parallel bow shock. The plasma penetration mechanism remains to be understood.

We examine the spacecraft potential (SP) variations near the magnetopause. In the magnetosphere the Geotail satellite often observed transient SP increase accompanied by magnetic field enhancements. This signature (SP increase and magnetic field increase) was also observed inside the magnetopause during the outbound (from the magnetosphere to the magnetosheath) and/or inbound (from the magnetosheath to the magnetosphere) magnetopause crossings. For the interval of the SP increase, the plasma density and temperature were intermediate between those of the magnetosheath and the magnetosphere, and strong enhancements of the field-aligned bidirectional electron fluxes were observed mainly in the medium energy (∼300–450 eV) range. These observations are consistent with previous studies in the inner part of the low-latitude boundary layer (LLBL). Thus we suggest that the transient SP increase in the magnetosphere may be a good indicator of the entry into the inner LLBL.

Accepted: 2001-05-17, 資料番号: SA1002543000

We illustrate the structure of the distant (X < -60 R e ) plasma sheet and separatrix layers in terms of low energy ion distributions observed by Geotail. In a statistical survey, ion distributions are classified as single or multi-component. Multi-component populations are clearly observed in 68 % of tailward flows. In two crossings only multi-component populations are observed and in two crossings no multi-component populations are identified. Although multi-component plasmas occur throughout the plasma sheet, they are observed more often in the outer region. Previous plasma sheet formation models do not predict multi-component distributions in the centre of the plasma sheet. However, this data supports the Owen and Mist [2001] model that predicts a similar plasma sheet structure.

Particle acceleration processes at quasi-parallel shocks have been widely discussed; however, the very initial injection from thermal to suprathermal energies is still controversial. Here we show that a nonlinear wave-particle interaction at quasi-parallel shocks results in quick injection and quick further acceleration of ions to nonthermal (NT) energies. Instead of an ensemble of small-amplitude random waves, a large-amplitude, monochromatic, upstream wave is set to propagate into the shock transition layer and test-particle orbits are deterministically calculated. First, we superimpose a purely right-handed, circularly polarized, monochromatic upstream wave whose amplitude is as large as those observed in the upstream of Earth's bow shock. The conversion of the wave at the shock front brings about quick acceleration of selected ions into NT energies. Although the observed orbit is similar to previous results, we propose that the process can be better understood in terms of a nonlinear wave-particle interaction in which the phase angle between an ion's velocity and the upstream wave field is playing a key role. Next, we add a left-handed, circularly polarized wave of the same wavelength to make up an elliptically/linearly polarized upstream wave, which is also observed in the bow shock upstream. Some of the NT injected ions that are leaving the shock front are seen to be quickly scattered back to the shock by the wave. These ions experience repeated acceleration within the limited time available for acceleration in the upstream of Earth's bow shock. The resultant energy spectrum has the exponential slope extending up to ∼ 70E 0 (where E 0 is the upstream bulk flow energy), with the characteristic energy ∼ 7E 0 . We have compared this energy spectrum with self-consistent hybrid simulation results and with Geotail satellite observations in the upstream region to find reasonably good agreement.

During the onset of an auroral substorm at about 0406 UT on August 14, 1996, the position of the Geotail spacecraft was located very near the corresponding region of instability in the equatorial plane as determined with a global model of the geomagnetic fields which provided the mapping from the auroral ionosphere. The Earth Camera on board the Polar spacecraft was used to acquire the global auroral images. The magnetic fields in the ring current were sufficiently stretched by the equatorial current sheet such that the radial position of the Geotail spacecraft at 10.2 RE was earthward of the onset region. Earthward flows at speeds in the range of 200 km/s for a period of several minutes were coincident with the initial brightening of a localized region of the previously existing auroral oval. This auroral luminosity subsequently declined within several minutes without a well-developed expansive phase; that is, a “pseudo-onset” occurred. Partial dipolarization of the magnetic fields occurred with this onset as well as large-scale magnetic fluctuations which continued through the brightening of another spot on the auroral oval at 0413 UT. These fluctuations ceased with the end of a well-developed expansive phase at about 0424 UT, when the auroral luminosities entered their recovery phase. At the beginning of the short expansive phase at 0420 UT the magnetometer on GOES 8 at a radial distance of 6.6 RE recorded a major dipolarization of the magnetic fields as an earthward moving injection front of hot plasma passed across the Geotail position. The present observations of the onset phenomena earthward of the instability region are combined with previously reported measurements of plasmas and magnetic fields at distances tailward of the instability in the ring current in order to begin sketches of the relationship among several dynamical features of the onset region.

Flow burst events with a flux transfer rate exceeding 2 mV/m and with a duration of less than 10 min observed by Geotail are compared with auroral signatures obtained from the Polar ultraviolet imager. It is shown that all the flow bursts correspond either to localized auroral intensifications associated with small poleward expansions and pseudobreakups or to an activation starting at the poleward edge of the expanded auroral oval that develop equatorward toward the foot point of the satellite, including auroral streamers. Earthward flow bursts related to pseudobreakups and small expansions are observed mainly in the region earthward of 15 RE, more inward than those flows related to high-latitude auroral activations and auroral streamers. Although most of these auroral activations precede the observations of the flow bursts by a few minutes, the activations that break up near the foot point of the satellite start typically within ±1 min of the onset of flow burst observation.

On the basis of 3 1/2 years of Geotail data we examine typical plasma sheet reconfigurations that are observed during rapid flux transport events (RFTs) in the central plasma sheet. RFTs are bursts of rapid earthward or tailward plasma flow with a large flux transport rate, EC = [(VXBZ)2 + (VYBZ)2]1/2 > 2 mV m−1. A superposed epoch analysis shows that earthward RFTs are related to nonadiabatic heating, dipolarization, and thickening of the plasma sheet, features typically seen during substorm expansion phase. The average earthward velocity component of the RFTs decreases toward Earth, whereas the average convection electric field, VX BZ, is practically independent of radial distance. Earthward RFTs show characteristics of bubbles, i.e., flux tubes with lower ion density and slightly higher magnetic field strength than the surrounding medium. Tailward RFTs beyond a radial distance of ∼20 RE can be associated either with a northward or a southward magnetic field, and their signatures show that they are probably related to the leading and trailing edges of tailward ejected plasmoids. Inside of 20 RE, yet another type of tailward RFTs with BZ > 0 can be observed. These events are possibly signatures of vortices or rebouncing flows in the near-Earth plasma sheet.

We present observations of transient events on April 13, 1996, using simultaneous measurements in the solar wind, near the magnetopause, at geosynchronous orbit, and on the ground at high latitudes. On this day, Geotail passed outbound through the morningside magnetopause. When Geotail was in the magnetosphere, a clear bipolar perturbation in the magnetic field component normal to the nominal magnetopause and transient outbound (from the magnetosphere to the magnetosheath)/inbound (from the magnetosheath to the magnetosphere) magnetopause crossings were observed. When Geotail was in the magnetosheath, inbound/outbound crossings were observed. During the bipolar magnetic field signature, Geotail observed asymmetric bipolar (less inward/more outward) flow normal to the nominal magnetopause accompanied by sunward flow. Inward flow was observed when the total magnetic field of the event was enhanced, whereas outward flow was observed when the total magnetic field was depressed. We observed that some of the transient outbound/inbound crossings show significantly different signatures in the plasma flow and magnetic field, that is, weak inward flow and enhanced magnetic field before the outbound crossing and strong outward flow and decreased magnetic field after the inbound crossing. The field and flow variations during the transient outbound/inbound crossings are qualitatively similar to those during the bipolar magnetic field signature. These observations suggest that the bipolar field signature and the transient magnetopause crossings would be similar phenomena on the magnetopause. We discuss whether the observed field and flow variations during the transient events are caused by inward/outward magnetopause motion due to compression/expansion of the magnetosphere by comparing the data sets obtained from different regions of the solar wind-magnetosphere system.

On January 8, 1996, the Geotail satellite skimmed along the dayside magnetopause from the dawnside to the duskside and observed a series of quasi-periodically recurring pulsed boundary layers in the prenoon (∼0900–1000 LT) and the postnoon (∼1400–1500 LT) sectors. For both intervals the interplanetary magnetic field observed by the Wind satellite upstream of the bow shock was inferred to be southward from an estimated convection time between Wind and Geotail. During the pulsed events, Geotail observed bipolar signatures in the magnetic field component normal to the nominal magnetopause. In the prenoon sector the bipolar variation had the inward-then-outward polarity, while in the postnoon sector the polarity was opposite (i.e., outward-then-inward). The plasma flows normal to the nominal magnetopause showed bipolar signatures with the inward-then-outward polarity in the prenoon and postnoon sectors. They were accompanied by sunward flows for the lower-density pulsed events and by tailward flows for the higher-density pulsed events. We have interpreted the sunward flow as the local plasma response to a tailward moving flux tube. This may imply that the sunward plasma flow near the magnetopause does not always reflect the direction of the motion of the flux tube. On the basis of these observations, we suggest that the observed pulsed boundary layers are attributed to the passage of a train of flux transfer events propagating tailward.