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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

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.

On the basis of several years of Geotail data we performed a comprehensive statistical analysis of rapid convective transport in the neartail and midtail central plasma sheet. We chose a new approach by using flux transport and not ion bulk velocity as the threshold parameter for the identification of rapid flows. This criterion for rapid convection is independent of the radial distance from the Earth. We found that the occurrence rate of earthward rapid flux transport events was constant at radial distances > 15 R E and that it started to drop only earthward of 15 R E . Tailward rapid flux transport events with B z 0 could be seen at all distances. Their occurrence rate has a minimum at radial distances between 20 and 30 R E and increases earthward and tailward of that region. They are likely to have a different nature and different origins in the near-Earth region and in the midtail beyond about 25-30 R E . In close analogy to bursty bulk flows we defined rapid convection events by using the flux transport criterion instead of a velocity criterion. We found that rapid convection events transport about the same amount of mass, energy, and magnetic flux and have about the same duration at all radial distances between 10 and 50 R E . We found that rapid convection was responsible for 30-50% of the observed total transport of mass, energy, and magnetic flux past Geotail at all observed distances in the central plasma sheet.

Plasma waves observed by the Akebono satellite in the region of ion heating/acceleration transverse to the geomagnetic field line are studied. Especially, electrostatic, broadband low-frequency noise is closely correlated with transversely accelerated ions. Simultaneous electron precipitation in the energy ranges from a few tens to hundreds eV is also usually observed. Our waveform analysis revealed that the electrostatic broadband noise is classified into two types of noise: one is continuous noise with upper cutoff around a few kilohertz, and the other is an intermittent impulsive waveform extended more than 10 kHz. A possible explanation to the dominant part of continuous broadband noise is that they are ion acoustic waves generated by precipitating electrons (approximately a few hundreds eV) and that the wave is the possible main energy source of ion heating/acceleration. The other mechanisms may, however, be sometimes important for generating the broadband noise. Using long-period observation data sets of Akebono, which is orbiting in the altitude range between 270 and 10,000 km, statistical studies on the spatial and temporal distribution of the continuous broadband noise are made. Several kinds of statistical features are clarified: (1) The occurrence region is distributed in the cusp and along the auroral oval, (2) the region is extended toward the lower latitude while the geomagnetic activity is higher, (3) the intensity is larger in the cusp than it is in the nightside, and (4) it is largest in winter and weakest in summer.

Accepted: 2001-03-14, 資料番号: SA1002545000

Accepted: 2000-06-26, 資料番号: SA1002547000

Accepted: 2000-06-26, 資料番号: SA1002549000

We use a global model of Earth's magnetosphere and ionosphere to simulate the Geospace Environment Modeling (GEM) substorm challenge event of November 24, 1996. We compare our results to International Monitor for Auroral Geomagnetic Effects (IMAGE) ground magnetometer data, assimilative mapping of ionospheric electrodynamics (AMIE) polar cap potential and field aligned current patterns, Polar Visible Imaging System (VIS) estimates of the polar cap magnetic flux, GOES 8 geosynchronous magnetometer data, IMP 8 magnetometer data, and Geotail plasma and magnetic field data. We find generally good agreement between the simulation and the data. The modeled evolution of this substorm generally follows the phenomenological near-Earth neutral line model. However, reconnection in the tail is very localized, which makes establishing a causal relation between tail dynamics and auroral dynamics difficult, if not impossible. We also find that the model results critically depend on the parameterization of auroral Hall and Pedersen conductances and anomalous resistivity in the magnetosphere. For many combinations of parameters that enter these parameterizations, no substorm develops in the model, but instead the magnetosphere enters a steady convection mode. The main deviation of the model from the data is excessive convection, which leads to a strong, driven westward electrojet in the growth phase, only partial tail loading, and a reduced recovery phase. Possible remedies are a better model for auroral conductances, an improved anomalous resistivity model, and a more realistic treatment of the ring current.

At close to 2345 UT on September 24, 1998, the magnetosphere was suddenly compressed by the passage of an interplanetary shock. In order to properly interpret the magnetospheric events triggered by the arrival of this shock, we calculate the orientation of the shock, its velocity, and its estimated time of arrival at the nose of the magnetosphere. Our best fit shock normal has an orientation of (−0.981 −0.157 −0.112) in solar ecliptic coordinates, a speed of 769 km/s, and an arrival time of 2344:19 at the magnetopause at 10 RE. Since measurements of the solar wind and interplanetary magnetic field are available from multiple spacecraft, we can compare several different techniques of shock-normal determination. Of the single spacecraft techniques the magnetic coplanarity solution is most accurate and the mixed mode solution is of lesser accuracy. Uncertainty in the timing and location of the IMP 8 spacecraft limits the accuracy of solutions using the time of arrival at the position of IMP 8.

We present a multipoint observational study of the magnetosheath response to the interplanetary magnetic field tangential discontinuities which form hot flow anomaly-like (HFAs) structures. We identify these structures in the vicinity of the bow shock as well as deeper in the magnetosheath. Two or more points of simultaneous observations allow us to describe the gradual evolution and propagation of these HFAs through the magnetosheath. From tens of events recorded by INTERBALL-1, we present two cases. In the first, GEOTAIL identified HFAs in the solar wind near the bow shock, and INTERBALL-1 and MAGION-4 observed related events in the magnetosheath. During the second interval, all three spacecraft observed the HFA features in the magnetosheath. Our analysis of reported events suggests the negligible evolution of these structures in the magnetosheath. A survey of the INTERBALL-1 data has shown that magnetosheath HFAs are observed predominantly during periods of fast solar wind.

Ion conics are frequently observed around the dayside cusp/cleft region and are supposed to be energized by an input of energy from the dayside magnetospheric boundary region. We examined possible influence of the interplanetary magnetic field (IMF) on dayside ion conics observed by the polar-orbiting Exos D (Akebono) satellite. We found the effect of B y and B z components on the energy flux and occurrence frequency of ion conics, ion conics are more energetic and more frequently observed when B z is negative and the magnitude of B y is large. The B z effect is likely to be attributed to the change in energy input to the ion energization region. The B y effect may be also due to the increased energy input to the cusp/cleft region during large B y period, suggesting that a reconnection-related energy input is responsible for dayside ion energization. An alternative interpretation of the B y effect is that the convection pattern may affect the residence time in the ion energization region and therefore the efficiency of ion energization. We found that the polarity of B y controls asymmetrical magnetic local time distribution of ion conics around the local noon, which is attributed to a possible effect of convection pattern on dayside ion energization.

We investigated properties of 43 small magnetospheric substorms. Their general signatures were found to be consistent with the so-called contracted oval or northern Bz substorms. Small but clear pressure changes in the tail corresponding to growth and expansion phases detected in about a half of cases testify that these substorms follow the same loading-unloading scheme as the larger ones. However, rate of the solar wind energy accumulation in the magnetosphere was low due to azimuthal IMF orientation with dominating IMF By and small fluctuating IMF Bz. Plasma sheet signatures could be very strong and likely were localized in their cross-tail size. Negative bays in auroral X magnetograms were of order of 100–300 nT, with maxima at Bear Island station (71°geomagnetic latitude) and in few cases were delayed after magnetotail onsets by tens of minutes. Small substorms probably differ from their larger counterparts in a way that coherency of the magnetotail reconfiguration in the inner and middle-tail regions and across the tail is lost in smaller substorms.

Using Geotail data from the months of December 1994 and January 1995, we have investigated transverse fluctuations in the magnetic field and in the plasma velocity in the magnetosheath. In the frequency range below about 10 mHz, a change in the magnetic field is often linearly correlated with a change in plasma velocity, satisfying the Walen relation expected from the theory of MHD wave propagation. We found well-correlated events in a wide range of radial distances of the magnetosheath, −42 RE < GSM-X < 6 RE, where RE is the radius of the Earth. The correlation is sometimes positive and sometimes negative, depending upon the angle between the magnetic field and the plasma flow direction. The results indicate that, in a frame moving with the plasma, the waves propagate unidirectionally in the same direction as the plasma flow in a draping field. In other words, the wave energy is always propagating simultaneously antisunward and outward. Moreover, our results suggest two possibilities for the origin of the signals in the magnetosheath: the magnetopause or the nose region of the bow shock.

Multiple and sporadic time-of-flight velocity dispersed ion structures (TDIS) are systematically observed above the ionosphere at ∼3 Re altitude by Interball/Auroral spacecraft near the poleward edge of the auroral bulge. These events represent direct snapshots of the impulsive ion acceleration process in the equatorial plasma sheet which allow us to study the details of the connection between ionospheric and plasma sheet manifestations of the magnetospheric substorm. Two events are analyzed during which the spacecraft footpoints passed over the Scandinavian ground network. We found that the TDIS correlate with the intensifications of westward current and auroral activations at the poleward edge of the bulge, which confirms the association of these dispersed ion beams with the temporal evolution of impulsive reconnection in the tail. Furthermore, we present direct evidence of an active neutral line in the magnetotail during one of the events using plasma sheet measurements made concurrently by the Interball/Tail and Geotail spacecraft. The 2–3 min repetition period of these ∼1 min long activations indicates a fundamental time constant of the substorm instability. On the other hand, the estimated injection distances of the energy-dispersed ions were inferred to be smaller than the estimated position of the reconnection region in the tail. We also found that the TDIS ion beams are released within the closed plasma flux tubes deep inside the plasma sheet, and yet they are synchronized with auroral activations at the poleward boundary. These facts imply that the ion beams are formed in a spatially extended region of the plasma sheet rather than in the close vicinity of the neutral line. We argue that braking of the reconnection-induced fast flow bursts when they interact with the closed plasma flux tubes and the earthward propagating fast wave electric field generated in the braking region may be important in forming the observed multiple, sporadic, energy-dispersed ion beams.

The Geotail spacecraft observed high-energy (∼3–10 keV) cold O+ beams (COBs) streaming tail ward together with protons entering from the magnetosheath in the northern dusk lobe/mantle when the IMF (interplanetary magnetic field) By and Bz are steadily negative. He+ beams were also observed intermittently. During the same period the FAST satellite passed across the dayside northern polar regions from dawn to dusk at low altitudes (1200–3400 km) and observed O+ precipitation on both closed and open field lines. There are regions where the magnetosheath and dayside plasma sheet/ring current components coexist and are precipitating together. In the open field line regions the precipitating O+ seem continuous with the precipitation in the closed regions, while the H+ and He++ precipitations are denser and typically have lower energy than O+. The phase space density (PSD) of the precipitating ions is highly isotropic except for the loss cone in the upward direction. Utilizing Liouville's theorem, we have compared the PSD of locally mirroring O+ at FAST with the PSD of COBs observed at Geotail. This comparison shows that the PSD in the high-energy precipitation region on closed field lines is comparable to that of the COBs. In regions where the magnetosheath and dayside magnetosphere ions coexist, the O+ PSD is typically smaller than that of the COBs, but at low latitudes it sometimes reaches values comparable to that of the COBs. These results suggest that the high-energy O+ ions in the dayside magnetosphere are a promising candidate for the source of COBs in the lobe/mantle. The ion dynamics on reconnected flux tubes needs to be examined further to clarify the possible energization mechanisms and their effect on the O+ ions.

Accepted: 2000-02-28, 資料番号: SA1002538000

Upgoing, counterstreaming, and downgoing field-aligned electron beams observed simultaneously with upward ion beams in the auroral acceleration region on Akebono (Exos-D) satellite are studied statistically. These electron beams are narrowly collimated within the local loss cone, generally have lower energies per charge than the potential drop above the satellite, and are associated only with low-energy (

Measurements from the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft were used to investigate dynamics of O+ ions of ionospheric origin at energies of 9 keV to 210 keV in the near-Earth plasma sheet during the substorm expansion phase. Substorm signatures were clearly observed on the ground at 1850 UT on May 17, 1995. In the expansion phase of this substorm, Geotail stayed in the plasma sheet at X∼−10.5 RE and observed a local dipolarization signature accompanied by strong disturbances of the magnetic field. From the energetic ion flux data of EPIC/STICS, we obtained the following results: (1) energetic flux enhancement was more pronounced for O+ than for H+; (2) the flux was enhanced almost simultaneously with local dipolarization; (3) the enhancement factor of O+ ions (EO+), which represents the enhancement of the O+ flux ratio (after and before substorm onset) relative to the H+ flux ratio, was as large as 1.31; and (4) thermal energy increased from 8.9 keV to 42.8 keV for O+ ions and from 9.4 keV to 15.9 keV for H+ ions. We also performed statistical analysis for 35 events of local dipolarization found in the near-Earth region (X∼−6 to −16 RE). We found that EO+ is larger than unity in all ranges of radial distance and that the average value of EO+ is 1.37. These results suggest that O+ ions are commonly more energized than H+ ions during the substorm expansion phase. To interpret these observational results, we propose a mechanism in which ions are accelerated in a non-adiabatic way during substorm-associated field reconfiguration.

Accepted: 2000-02-08, 資料番号: SA1001271000

Accepted: 2000-04-26, 資料番号: SA1002541000

Accepted: 1999-09-13, 資料番号: SA1002537000

We present case and statistical studies of magnetohydrodynamic (MHD) fluctuations observed by the Wind and Geotail spacecraft in the Earth's dawn and dusk magnetosheath. A case study with simultaneous IMP 8 interplanetary magnetic field observations confirms theoretical predictions that most of the fluctuations originate in the solar wind rather than at the magnetopause and that the fluctuations can be almost absent from the outer magnetosheath. A second case study indicates that northward plasma velocity perturbations observed in the prenoon magnetosheath correspond to southward velocity perturbations in the postnoon magnetosheath, implying that the MHD perturbations apply a torque to the magnetosphere. A statistical survey reveals that the fluctuations almost invariably propagate antisunward in the magnetosheath, independent of their propagation direction relative to the interplanetary magnetic field.

Multispacecraft observations are analyzed to examine the dynamics of thin current sheet evolution during the early phases of a substorm during the period 0630–0800 UT on December 10, 1996. The ground signatures showed two activations, one latitudinally localized at 0731 UT and another, larger expansion around 0800 UT. A local model consisting of two current sheets (a thin current sheet and a thicker plasma sheet) was developed utilizing two-spacecraft magnetic field measurements within the magnetotail. Extension of the thin current sheet model to the inner magnetotail shows that during most of the growth phase, the thin current sheet intensified simultaneously and at the same rate in the mid-magnetotail (∼30 RE) and near geosynchronous orbit. These results show that a thin current sheet extended from the near-geostationary region to the midtail, and that the entire current sheet responded to the driving in a coherent way. Furthermore, timings suggest that the onset-associated disturbances were observable in the midtail before the ground or geosynchronous signatures.

We report on a new feature of auroral substorms, namely, the sporadic though recurrent injections of magnetospheric ions throughout the auroral bulge. These injections are interpreted as time of flight dispersed ion structures (TDIS). Our analysis builds on a combination of measurements from Interball-Auroral, from UV imagery onboard Polar, from ground magnetometers, and also from observations on Geotail and from geostationary spacecraft. Backward tracing of ion trajectories from Interball-Auroral orbit using realistic three-dimensional magnetic and electric field models indicates that the injection region can extend over a wide range of radial distances, from ∼7–40 RE in the nearly equatorial magnetosphere. Both hydrogen and oxygen ions are shown to be injected toward the Earth's upper ionosphere. At Interball altitudes we find that ion injections are associated with two types of low-frequency torsional oscillations of the magnetic field: (1) shear Alfven waves with a period of a few minutes with the highest amplitude near the bulge front and decreasing amplitude at lower latitudes and (2) higher-frequency shear Alfven waves of the PlB type, strictly restricted to the poleward boundary of the surge, with a characteristic period of ∼40 s. The systematic observation of sporadic TDIS during the auroral bulge expansion leads us to conclude that the same physical process is at work throughout the midtail. We also show that ion injections are detected well inside the bulge, which suggests that the injection fronts propagate from the outer to the inner magnetosphere over large distances. This topic is more extensively studied by Sergeev et al. [1999]. We also show that the poleward boundary of the surge is associated with a prominent outflow of ionospheric H+ and O+. These ions in the hundred of eV to the keV range are heated perpendicularly to the local magnetic field and subsequently transported into the magnetotail. The expanding auroral bulge thus forms a significant source of ionospheric ions for the midtail magnetosphere.

Cold dense plasma (CDP) with energies less than 32 eV in the outer magnetosphere, L ∼ 10, is reported using data obtained by Geotail. We have found unexpectedly recurrent CDP in afternoon magnetic local times. CDP is often accompanied by westward flows, indicating that the regions with CDP are located outside the corotation region and the plasma is presumably convected toward the magnetopause. CDP is observed in the cases of both small and moderate geomagnetic activity. We have calculated the location of the stagnation point as a function of cross-tail potential by using a Volland-Stern-type [Volland, 1973; Stern, 1975] convection model. Stagnation point locations are variable even if the cross-tail potential or geomagnetic activity is small, which could lead to a high rate of the observation of plasmaspheric material at the Geotail orbit. However, part of the cold plasma is directly supplied by the ionosphere without passing through the plasmasphere. The number of supplied particles could be larger on the drift orbits close to the stagnation points. Finally, an average quantity of cold plasma flowing through the Geotail orbit with quasi-stable westward convection is estimated to be 3.6 × 10 25 ions/s with a standard deviation of 3.7 × 10 25 ions/s, as compared to 9.4 - 14 × 10 25 ions/s for the polar wind toward the plasma sheet.

On the basis of ∼2 years of Geotail data, we use a superposed epoch approach to study the average behavior of plasma and magnetic fields at different radial distances, between 11 and 31 RE, during 66 substorms in the premidnight sector. Magnetic field dipolarization is first seen in the innermost region (11–16 RE) around substorm onset and subsequently moves tailward at a rate of 35 km/s. Fast earthward and tailward ion bulk flows in the central plasma sheet indicate that during substorm expansion the near-Earth neutral line is located between 21 and 26 RE, with a tendency to be closer to 21 RE near substorm onset. About 45 min after onset, the tailward moving dipolarization front reaches the distance range where the near-Earth neutral line is located. Thereafter the near-Earth neutral line disappears beyond 31 RE. This is the classical signature of the start of the recovery phase. We conclude that substorm recovery sets in when the tailward moving dipolarization front reaches the near-Earth neutral line, because the near-Earth neutral line cannot operate in a dipolar field geometry.

We study the response of midtail plasma and field parameters to enhanced solar wind electric field input for two substorm intervals on November 22, 1995. The solar wind input signatures were quite different for these two substorms, which had major Pi2 onsets at 1108 and 1502 UT. The solar wind input for the 1108 UT substorm had a short timescale (∼0.5 hour) electric field enhancement up to 1.5 mV/m, whereas the solar wind electric field for the 1502 UT substorm continuously exceeded 1.5 mV/m for ∼2 hours. In association with the expansion phase onsets of both substorms, the electric field fluctuation in the midtail plasma sheet commenced. The electric field disturbances lasted for a time interval close to the length of time, in which the solar wind electric field was enhanced. The midtail plasma sheet electric field responded well to the enhanced solar wind input with a time delay of 45–80 min. The pressure decrease, which started at the onset of both substorms, ended quite differently for these two substorms: the pressure decreased until it approached the quiet time level in the course of the 1108 UT substorm, whereas the pressure dropped below the quiet time level during the expansion phase of the 1502 UT substorm and stayed at this low level until late in the recovery phase. Using polar cap potential drop and the pressure profile in the midtail, we estimate the relationship between the dayside and nightside potential drop. We suggest that the difference in the nightside flux transport rate could control the configuration of the midtail and could explain why the tail pressure responded differently during the expansion and recovery phase of the two substorms.

We study waveforms of electrostatic solitary waves (ESW) and corresponding electron velocity distribution functions observed by the Geotail spacecraft. When we observe a series of ESW in the plasma sheet boundary layer of the Earth's magnetotail, we find enhanced fluxes of high-energy electrons flowing along the ambient magnetic field. We find good correlation between the propagation direction of the ESW and the direction of the enhanced high-energy electron flux. This supports the model proposed by computer simulations that ESW are formed by the electrons modulated through the bump-on-tail instability. The enhanced electron flux is regarded as the diffused electron beam after the saturation of the instability. From comparison between the variation of waveforms and the velocity of the electron beams, the spatial scales and depths of the ESW potentials and their propagation directions are estimated. We found both tailward and earthward ESW in the plasma sheet boundary layer, even in the deep magnetotail.

In the lobe/mantle regions of Earth's magnetotail (

We present a comprehensive observational study of the magnetospheric response to an interplanetary magnetic field (IMF) tangential discontinuity, which first struck the postnoon bow shock and magnetopause and then swept past the prenoon bow shock and magnetopause on July 24, 1996. Although unaccompanied by any significant plasma variation, the discontinuity interacted with the bow shock to form a hot flow anomaly (HFA), which was observed by Interball-1 just upstream from the prenoon bow shock. Pressures within and Earthward of the HFA were depressed by an order of magnitude, which allowed the magnetopause to briefly (∼7 min) move outward some 5 RE beyond its nominal position and engulf Interball-1. A timing study employing nearby Interball-1 and Magion-4 observations demonstrates that this motion corresponded to an antisunward and northward moving wave on the magnetopause. The same wave then engulfed Geotail, which was nominally located downstream in the outer dawn magnetosheath. Despite its large amplitude, the wave produced only minor effects in GOES-8 geosynchronous observations near local dawn. Polar Ultraviolet Imager (UVI) observed a sudden brightening of the afternoon aurora, followed by an even more intense transient brightening of the morning aurora. Consistent with this asymmetry, the discontinuity produced only weak near-simultaneous perturbations in high-latitude postnoon ground magnetometers but a transient convection vortex in the prenoon Greenland ground magnetograms. The results of this study indicate that the solar wind interaction with the bow shock is far more dynamic than previously imagined and far more significant to the solar wind-magnetosphere interaction.

Poleward boundary intensifications are nightside geomagnetic disturbances that have an auroral signature that moves equatorward from the poleward boundary of the auroral zone. They occur repetitively, so that many individual disturbances can occur during time intervals of ∼1 hour, and they appear to be the most intense auroral disturbance at times other than the expansion phase of substorms. We have used data from three nightside conjunctions of the Geotail spacecraft in the magnetotail with the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) ground-based array in central Canada to investigate the relation between the poleward boundary intensifications and bursty plasma sheet flows and to characterize the bursty flows associated with the disturbances. We have found a distinct difference in plasma sheet dynamics between periods with, and periods without, poleward boundary intensifications. During periods with identifiable poleward boundary intensifications, the plasma sheet has considerable structure and bursty flow activity. During periods without such poleward boundary intensifications, the plasma sheet was found to be far more stable with fewer and weaker bursty flows. This is consistent with the intensifications being the result of the mapping to the ionosphere of the electric fields that give rise to bursty flows within the plasma sheet. Two different types of plasma sheet disturbance have been found to be associated with the poleward boundary intensifications. The first consists of plasma sheet flows that appear to be the result of Speiser motion of particles in a localized region of thin current sheet. The second, seen primarily in our nearest-to-the-Earth example, consists of energy-dispersed ion structures that culminate in bursts of low-energy ions and isotropic low-energy electrons and are associated with minima in magnetic field and temperature and maxima in ion density and pressure. Both types of plasma sheet disturbance are associated with localized regions of enhanced dawn-to-dusk electric fields and appear to be associated with localized enhanced reconnection. Our analysis has shown that poleward boundary intensifications are an important aspect of geomagnetic activity that is distinct from substorms. In addition to their very distinct auroral signature, we have found them to be associated with a prolonged series of ground magnetic Pi 2 pulsations and ground X component perturbations, which peak at latitudes near the ionospheric mapping of the magnetic separatrix, and with a series of magnetic Bz oscillations near synchronous orbit. Like substorms, the tail dynamics associated with the poleward boundary intensifications can apparently extend throughout the entire radial extent of the plasma sheet. Color versions of figures are available at http://www.atmos.ucla.edu/∼larry/geotail.html.

High-velocity magnetotail flow bursts measured by the Geotail Low Energy Plasma experiment in the premidnight equatorial region between 10 and 15 RE have been compared with other magnetospheric phenomena. These bursts, typically characterized by earthward velocities approaching 1000 km/s and lasting for times of the order of l min, are associated with magnetotail dipolarizations and large magnetic field fluctuations. Using supporting measurements of the International Solar Terrestrial Physics program it is found that the flow bursts are closely associated with auroral brightenings, AKR onsets, geosynchronous particle injections, and ground magnetic activity. Flow bursts for which Polar UVI images are available showed auroral brightenings that developed near the footpoint Geotail field line. AKR intensifications usually accompanied the flow bursts in close time coincidence, whereas dispersionless geosynchronous particle injections tended to be delayed by 1–3 min. Since flow bursts often exhibit the earliest onsets of these various phenomena, it seems likely that this chain of events is initiated in the tail beyond 15 RE, presumably by magnetic reconnection. It is concluded that flow bursts are a fundamental magnetotail process of limited spacial extent that are important in energy and magnetic flux transport in the magnetosphere. Magnetotail flow bursts are intimately connected to auroral acceleration processes and AKR generation at several thousand kilometer altitude and a full explanation of substorms will have to explain this relationship.

Over a wide range of the tailward distances in the lobe/mantle regions (|X GSM' | ≤210 R E ), the Geotail spacecraft sometimes observed cold O + beams (COBs) streaming tailward at a velocity nearly equal to that of the major proton component. The discovery of the ionospheric ions in the distant lobe/mantle leads us to a reexamination of the conventional view which expects these O + ions to descend into the plasma sheet because of the ExB drift. In order to clarify the quantitative property of the COBs, we have projected COB parameters onto the polar ionosphere with a simple model which takes into account the velocity filter effect alone. The statistical data set from October 1993 to March 1995 in a solar-minimum period is used in the calculation. The calculated average COB flux in the lobe/mantle corresponds to at least ∼10 8 cm -2 s -1 O + flux at the 1000-km altitude. From the comparison with polar outflows, it is suggested that the cusp/cleft originating O + ions sometimes need significant additional energization(s) so as to supply the COBs in the distant tail. When we assume the typical flux and temperature of polar O + outflows, the required bulk energy of upward flowing ions (UFIs) turns out to be of the order of 3 keV, which is much higher than the typical energy of cusp/clef outflows (≤ 1 keV). The energy difference between the tail lobe/mantle and the cusp/cleft O + provides clues to the supply and/or energization mechanisms of the ionospheric ions. Calculations with individual COB events suggest that the rate of COB events in which the ionospheric O + outflow flux exceeds 10 9 cm -2 s -1 increases with the tailward distance, and it corresponds to 27% of all events in the distant tail of |X GSM' | ≥150 R E .

We present ground-based, plasma sheet, and magnetosheath observations of two subsequent short-duration (10–20 min) increases of the postmidnight westward electrojet on November 28, 1995. Appearing as though small (150–200 nT) substorms, they were not accompanied by any substorm expansion onset signatures. Auroral breakup, worldwide Pi2 pulsations, and the corresponding plasma sheet activity, such as fast flows, current disruption, and plasmoid generation, were all observed only at the recovery of the second electrojet increase. These convection bays were associated with the equatorward expansion of the auroras and simultaneous magnetic variations in the polar cap and middle latitudes. Growth phase signatures of the lobe field increase and tailward stretching of magnetic field were also observed in the plasma sheet. Bursty bulk flows in the plasma sheet seem to be quenched at the onset of first convection bay and did not resume until the auroral breakup which concluded the second convection bay. A point of interest of this event was the “incomplete” convection/current system with a well-developed dawn vortex in the absence of well-defined dusk vortex; instead, a complicated transient activity dominated over the afternoon-dusk local time sector. We interpret this asymmetry either in terms of the magnetopause encounter with the edge of the solar wind driver, i.e., strong southward IMF, which hits only the dawn part of the magnetosphere, or with an extremely slant interplanetary discontinuity. This unique configuration was inferred from observations of uncorrelated strong southward Bz events by the Wind and IMP 8 spacecraft in the dusk and dawn magnetosheath, respectively, as well as from the directional analysis of the interplanetary discontinuities which form the edges of these structures. We suggest that interaction of the magnetosphere with very slant solar wind discontinuities may bring various specific features to magnetospheric and ionospheric dynamics that have not been reported.

On the basis of Geotail high energy particle - low energy particle detector (HEP-LD) observations, this paper reports on three energetic (144 – 4000 keV) oxygen burst sequences detected in the distant tail (Xgse = −40 to −66 RE) and their relation to substorm signatures. Those energetic oxygen ion bursts lasted only 20 to 30 min and exhibited strong beam-like structures. Two of the events (at about 1000 and 1900 UT on February 13, 1994) occurred in tailward flowing plasma after the flow direction changed from earthward to tailward; geostationary and ground based observations detected intense substorm activity during these periods, and the local magnetic field component Bz assumed predominantly negative values. The occurrence of a magnetic field with southward polarity and oxygen bursts embedded in tailward flowing plasma is consistent with the basic signatures of reconnection (formation of a neutral line) in the magnetotail. The third energetic oxygen burst with earthward flow was observed relatively close to Earth on August 27, 1993, X= −40 RE. No reversal in the plasma flow direction was seen, and the magnetic field polarity was essentially positive throughout the event. This is consistent with a “near-Earth” neutral line that had formed beyond X=−40 RE. We conclude that (1) a large amount of heavy ions from the ionosphere can be transferred to the distant tail and accelerated to high energies during substorm activity and that (2) these oxygen O+ ions from the polar ionosphere can be considered as “tracer ions” in the substorm dynamical process.

On January 11, 1994 an interplanetary corotating interaction region (CIR) passed the Earth's magnetosphere (final phase of solar cycle 22). Ground-based magnetometers, geosynchronous satellites, and Geotail (GSE −91, 15.5 and −3.5 RE), in or near the plasma sheet at the dusk flank of the magnetotail, detected a series of disturbances throughout this day, which culminated in the development of an isolated substorm between 1400 and 1710 UT (event 2). Small substorm activity early in the day (event 1) produced an energetic particle population with rather normal composition (relative helium abundance 10%). The CIR-related high-speed stream started to interact with the magnetosphere at about midday, which appears to have contributed to the initiation of the major substorm at 1400 UT (event 2). A geosynchronous Los Alamos National Laboratory (LANL) satellite at 2100 LT detected a rapid flux dropout for electrons at 1400 UT followed by a transient recovery around 1500 UT. The main injection phase, which started at 1551 UT, resulted in a massive electron flux increase. Throughout the 120-min long growth phase of this substorm the energetic particle spectrometer (HEP-LD) on board Geotail measured a rather slow flux buildup for protons and helium ions in the distant plasma sheet (PS). The azimuthal angular distributions showed only very small anisotropies in this phase. An unusual feature is the high helium abundance of 26% compared to about 10% normally found in the magnetosphere. After the onset of the electron injection at 6.6 RE, HEP-LD observed the appearance of tailward beams of energetic protons and helium ions (relative helium abundance 26%) in the PS/plasma sheet boundary layer (PSBL) and oxygen ions in the central plasma sheet (CPS). The oxygen ions were delayed by 27 min relative to the P/He beam. Plasmoid-like structures in the magnetic field accompanied the streaming ions. The synoptic observations in the geostationary orbit and in the distant tail suggest a source location at X=−50 RE (near-Earth neutral line (NENL)) for this substorm. Substorm-related extraction of oxygen ions from the polar ionosphere and subsequent drift to the acceleration region (NENL) can explain the observed delay for these ions, event 3, the last disturbance of the day, started at 2000 UT and lasted until 2300 UT. The proton/helium population was again broad in angular distribution and helium-rich in composition (20.7%) but the geostationary/distant tail association is less dynamic. The appearance of accelerated helium-rich energetic plasmas in event 2 and 3, confined to the plasma sheet and absent in the lobes, is most likely a result of the arrival of solar wind plasma with an enhanced helium content, which gained access to the magnetosphere shortly before or during the growth phase of the event 2 substorm. Helium-rich solar wind compositions are frequently observed in CIR events.

An extended interval of strong northward interplanetary magnetic field (IMF) was observed by the Wind spacecraft located at an upstream distance of ∼193 RE from February 8–10, 1995, with a brief break of southward IMF from 0200 to 0400 UT on February 9. This brief interval of southward IMF led to an isolated substorm of moderate intensity (∼500 nT) with expansion phase starting at ∼0431 UT. This substorm may be triggered by the northward turning of the IMF since its onset time matched well with the time expected for the arrival of the northward turning of the IMF at Earth. The substorm activities were monitored by 11 spacecraft in space (Wind, IMP 8, Geotail, six geosynchronous satellites, one DMSP satellite, and Freja) and two networks of ground stations (Canopus and SuperDARN) covering both the northern and southern hemispheres. The extensive coverage of this event provides us with results (1) showing some unusual characteristics possibly related to the isolated nature of the substorm and (2) revealing some surprising features difficult to reconcile with the traditional substorm model. In the first category is unusually long duration of the growth phase and the long time delay between substorm expansion onset and particle injection onset at the geosynchronous orbit. In the second category is new evidence for multiple particle acceleration sites during substorm expansion and for sunward flow during the late expansion phase of a substorm being unrelated to a single acceleration site (X line) moving from the near-Earth tail to the more distant tail. We also present observations which show the possible optical signature on the ground of bursty bulk flows in the magnetotail.

Accepted: 1997-07-07, 資料番号: SA1002534000

At 1130 UT on November 28, 1995, two spacecraft, Interball-Tail and Geotail, were in a favorable position to study the plasma sheet activity and an auroral breakup observed on the ground near the spacecraft ionospheric footpoints. Both spacecraft were near the neutral sheet, and they were nearly aligned along the magnetic meridian. During the auroral breakup observed at the equatorward half of the auroral oval (also registered as an AKR burst at Interball) both spacecraft simultaneously detected signatures of a reconnection pulse: The earthward plasma streaming and magnetic field dipolarization were observed at 12 R E at Interball, while the tailward energetic ion beam, then the tailward flow and the passage of a plasmoid were observed at 28 R E at Geotail. This pulse seem to proceed inside of the plasma sheet closed field lines, in the region of small (∼ 1nT) background magnetic field at the neutral sheet. At Interball position the onset of fast earthward ion flow, likely initiated by the reconnection pulse, was followed by other manifestations (dipolarization, enhancements of the magnetic turbulence and the energetic particle flux, the intensification of field-aligned currents). Auroral observations showed initial brightening delayed an approximately 1 min after the commencement of the reconnection pulse. The auroral intensification was not accompanied by a significant magnetic disturbance on the ground, and therefore the event can be classified as the pseudobreakup. We estimate magnetic flux transport characteristics and possible location of the onset region in the plasma sheet. We conclude that observations during this event are consistent with the initiation of an auroral breakup by some disturbance (e.g., Alfven wave) generated by the reconnection pulse that commenced in the neutral sheet at ∼15 R E distance.

We present observations of two sequential substorm onsets on May 15, 1996. The first event occurred during persistently negative IMF B-Z, whereas the second expansion followed a northward turning o ...

We have examined several Geotail passes through the magnetosheath region downstream of the Earth, when the solar wind conditions as observed by the Wind spacecraft were steady. We have found that near the magnetopause the bulk speed of the magnetosheath plasma is strongly dependent upon the local direction of the magnetic field with respect to the local velocity vector. In one set of observations, the observed magnetosheath speed is slowest when the magnetosheath magnetic field and velocity vectors are aligned (either parallel or antiparallel), and the speed is larger when the magnetic field is oriented perpendicular to the magnetosheath velocity. However, the speed of the magnetosheath plasma is not observed to exceed that of the solar wind plasma. The enhanced speed for perpendicular flows is due to acceleration of the plasma by means of magnetic field line tension, as the magnetosheath plasma is dragged around the magnetosphere. The maximum enhancement in magnetosheath plasma speed can be quantitatively estimated by a consideration of the intermediate mode of the magnetohydrodynamic (MHD) relations. A simple model for the velocity enhancement is derived, and we find extremely good agreement between the Geotail magnetosheath observations and this model. The second set of observations are similar to the first, except that when the magnetosheath velocity and magnetic field vectors are parallel to one another, the magnetosheath flow is higher than when the vectors are antiparallel or perpendicular to each other, and is even larger than the solar wind speed. This behavior cannot be explained by nonresistive ideal MHD theory, and other explanations are investigated.

On January 15, 1994, the ion spectrometer high energy particle - low energy particle detector (HEP-LD) on the Japanese spacecraft Geotail observed five quasi-periodic energetic ion bursts in the deep tail (X=−96 RE). These bursts were associated with plasmoid-like structures in the magnetic field components. In. addition, three multiple TCR groups were identified in the interval. The observations in the distant tail occurred during a time interval of substorm activity which also produced multiple injections in the geosynchronous orbit region. The HEP-LD observations show that Bz bipolar plasmoid-like structures are associated with tailward flowing particle bursts. However, earthward flowing particle bursts are predominantly associated with bipolar signatures in By. In addition, an oxygen burst was seen in the back of a plasmoid (postplasmoid) which showed both By and Bz bipolar magnetic field signatures. The oxygen burst lasted for 23 min, and the density ratio (O/H) reached 15% for the HEP-LD energy range (in the same plasmoid, this ratio was approximately 1% before the oxygen burst). The oxygen burst exhibited a strong beam-like structure which occupied only 6 ∼ 7% of the full solid angle (4π). We suggest that energized oxygen ions of ionospheric origin travel downtail in the narrow postplasmoid-plasma sheet which trails the plasmoid. Furthermore, we suggest that the magnetosphere dissipated larger quantities of energy during this very intense substorm event by ejecting multiple relatively small plasmoids rather than through the formation and ejection of a single large plasmoid.