Recent studies by the author and his colleagues on solar flares based on the magnetic reconnection model are reviewed. The studies have been done mainly by using the data sets of space-craft observations and supercomputer simulations. The covered topics axe an EUV observation of reconnection inflows, MHD simulations of the reconnection model, and a microwave observation of freely- streaming high-energy electrons. The universal scaling law to explain the relation of the temperature and the emission measure of the solar/stellar flares is also shown.
We carried out particle-in-cell simulation to investigate the mechanism for steady and unsteady shock front formation depending on Mach number (M-A) and plasma beta. When even under the same MA, depending on beta, the electric and magnetic field structures in the shock transition region (STR) are much different. At lower beta or/and higher MA condition, the shock front tends to be unsteady and electrons play more important role in the shock dissipation process through two-stream instability.
We statistically and systematically studied the dawn-dusk magnetosheath asymmetry by using both thermal/middle energy electrons (<= 40 keV: LEP) and energetic electrons (>= 38 keV: EPIC) experiments onboard the Geotail spacecraft. We found the dawn-dusk magnetosheath asymmetry of energetic electrons distribution (> 38 keV), while the clear asymmetry could not be observed, for the lower energy electrons. It was also found that energetic electrons flux did not strongly change across the magnetopause as compared with that of the thermal/middle energy electrons in the dawn side magnetopause, and that the intensity of the energetic electrons increases with going towards the dawn side magnetopause from the bow shock. Those results suggest that energetic electrons leak out of magnetosphere into magnetosheath.
The magnetospheric community has conducted active wave measurements, which are made by radiating known signals to space and measuring the dispersion of the signals when propagating through magnetospheric plasma, for more than three decades. Because of the advances in space electronics and signal processing technology, active wave experiments can now cover a large dynamic range in frequency and be made in a large spatial range for versatile applications. They are becoming a promising new technique to probe the space plasma conditions. In this review we demonstrate the capability of magnetospheric sounding technique employed by the radio plasma imager (RPI) on board the IMAGE satellite. This new technique, combined with the mathematical density inversion algorithm, measures the plasma density in situat the satellite location and remotely and instantaneously along the magnetic field line from one hemisphere to the other down to as low as half an Earth radius (Re) in altitude. The technique has been formally validated. The database from the RPI active measurements covers all local times from 1.5 Re to 5 Re under different geomagnetic activities. Empirical models that are being developed specify the density as functions of radial distance, latitude, local time, distance along the field line from the earth's surface, solar wind conditions, geomagnetic indices, and other possible variables that affect the density distribution. The models can describe the statistical behaviors of the plasma distribution and can also provide snapshots of the plasma conditions on occasions. Dynamical processes that cause variations from the average models, such as depletion/refilling processes and plasma convection tail formation, can be studied. When applied to multiple satellites, or a constellation of satellites, the active wave measurements also make it possible for magnetospheric tomography, in which transmissions and reception of various waves within the constellation are used to derive the plasma density and magnetic field component in the constellation plane.
A thin current sheet with thickness comparable to the ion inertial length gives rise to decoupling of ions from electrons. This situation is favorable for a number of dynamic processes leading to particle acceleration and plasma turbulence. In this article, we investigate the cross-field current instability that can be excited in a thin current sheet with a non-zero magnetic field component normal to the current sheet surface. There is strong observational evidence for this instability and substantial theoretical work on it. We first discuss briefly the observational basis, followed by a brief overview of theoretical predictions. We then present particle simulation of this instability for a thin current sheet using a two-dimensional fully electromagnetic particle-in- cell code. The instability is found to develop rapidly, resulting in particle acceleration, current filamentation, current density reduction at the sheet center, current sheet broadening, and undulations of the current sheet profile. These features revealed from simulation are quite similar to those observed for current disruption.
We have studied several topics related to the 2f(p) radiation generated in the terrestrial electron foreshock. Our investigation started from the macroscopic geometry cf the radio source, and is expanding to the microscopic processes. In this paper, we present a summary of latter studies, especially about the generation mechanism of electrostatic and electromagnetic 2f(p) waves and the electron acceleration at the quasi-perpendicular shock.
Whistler waves axe considered to play an important role in the electron dynamics for the collisionless shock formation process at interplanetary shocks (IPSs). In this report, we analyze IPS events observed by GEOTAIL on 21 February 1994 and 15 July 2000, focusing on whistler wave properties in their upstream region extending the previous work of Shimada et al. (1999). In both events, we have identified the existence of whistler mode waves in the upstream region of the IPS as well as the tendency of the intensity increase toward the shock front. At the same time we have found the detailed features differing between these events: While the intermittent but clear wave bursts were found on. 21 February 1994 event, the waves were more or less continuous on 15 July 2000 event.
In the present study we aim to derive empirical equations relating source plasma of the ring current to the solar wind. We used the energy spectra at energies of 9-135 keV obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft. The plasma parameters (i.e., number density and temperature) of H+, O+, and He were estimated by fitting the K-distribution function to the energy spectra in the region of a geocentric distance of 8.5-10.5 RE and magnetic local time of 2200-0200 hour. The results showed that the H+ number density in the plasma sheet correlated with the solar wind density, while the O+ and He+ number density had no correlation with the solar wind parameters. Thus the origin of H+ ions in the plasma sheet is thought to be the solar wind. O+ and He+ ions in the plasma sheet are expected to have different origin from the solar wind. It was also found that the temperature of H+, O+ and He+ has a good correlation with the solar wind velocity and that gradients of the derived empirical equations can be ordered by ion mass. This implies that ions are accelerated in a mass-dependent way.
Lobe trapped continuum radiation (LTCR) has been observed by the plasma wave instrument (PWI) on-board the GEOTAIL spacecraft at frequencies as low as 1 kHz in the distant geomagnetic tail region. From the direction finding analysis with the wave form capture (WFC) data, the arrival directions of LTCR are almost parallel to the dawn-dusk direction. A 3-D ray tracing analysis shows that the initially radiated Earth-tail ray directions of LTCR are transformed into the dawn-dusk directions by the reflection at the cylindrical tail magnetopause. The propagation characteristics of LTCR can give us very important information on the macroscopic structure such as magnetotail flapping. By comparing the results of the direction finding with the 3-D ray tracing analysis, we find that the possible source regions for LTCR are located at the plasma sheet boundary layer away from the nominal tail axis and the low latitude boundary layer.
Since its launch in 1992, GEOTAIL has extensively surveyed the magnetotail with a full set of plasma and field instruments over a wide range of distances from 9 Re to 220 Re away from the Earth by means of a sophisticated orbit strategy. Ill the first two years, the orbit was optimized to explore the distant tail, and thereafter was changed to study substorm processes in the near-Earth tail region. The near-tail orbit has also facilitated exploration of the dayside outer magnetosphere, the magnetopause, the magnetosheath, and the bow shock, as well as the upstrearn solar wind. GEOTAIL observations have revealed a number of new phenomena in these regions, and as of the end of 2003, about 600 papers have been published in refereed journals. Recent GEOTAIL studies have significantly advanced our understanding of the structure and formation of thin current sheets in the mid-tail plasma sheet during substorms, and have elucidated new kinetic aspects of magnetic reconnection. GEOTAIL has operated far beyond the designed mission life of three and half years. Most of the onboard instruments are still functioning well, and it is expected that GEOTAIL will continue to generate scientifically useful data.
We have studied by MHD simulations the effects of the guide field in three-dimensional magnetic reconnection. The guide field is introduced by adding a constant B-y component B-y0 to the anti-parallel reconnecting component B-x = tanh(z). An ad-hoc anomalous resistive region, which facilitates the study of reconnection in MHD, is assumed to have a finite extent in the gamma direction and this gives rise to a three-dimensional situation. It is shown that the guide field makes the U-shaped reconnected field lines as well as the reconnection jet to be inclined from the z axis. The guide field also acts as an obstacle to the jet and this effect produces a pair of helical streamlines in the jet leading part. Time series data from a virtual spacecraft that encounters or skims the jet leading part are found to show good agreement with the observed FTE signatures on either side of the dayside magnetopause.
Measurements of the magnetic field and low-energy plasma by Geotail have been used to study mean and instantaneous characteristics of plasma and magnetic field in the distant tail X = -(79-200) RE under extremely quiet and weakly disturbed conditions. The analysis has been carried out separately for the tail lobes and the plasma sheet. A good consistency between variations of the plasma and magnetic parameters is observed only in the tail lobes, the correspondence being considered for appropriate components (Vx and Bx, Vy and By, Vz and Bz). The distant plasma sheet seems to be constantly in regime of turbulence, i.e. such a regime when the magnetic field and plasma parameters are steadily subjected to fluctuations of different periods, the variations in magnetic field and plasma velocity being inconsistent in time. The mean values of V and B in the plasma sheet are not compatible with their instantaneous characteristics at all. The coefficient of diffusion across the plasma sheet, estimated from experimental data, is in harmony with predictions of theory for the plasma sheet with turbulence. The applicability of the frozen-in approximation to description of processes in the plasma sheet is discussed.
On the basis of our recent studies (Sakurai, et al., 1999a, 1999b and 2001) the present paper is intended to review oscillation and propagation characteristics of Pc 3 and Pc 5 ULF waves observed near the dayside magnetopause by the Geotail satellite. These characteristics are studied on the basis of the simultaneous observations of the magnetic and electric fields, and low energy plasmas measured with the excellent instruments on board the satellite. The study reveals that the dominant ULF waves observed near the dayside magnetopause are Pc 3 and Pc 5 oscillations. The Pc 3 oscillations appear with a peak power around noon at the frequency of 25 mHz in the azimuthal component of the magnetic field. This frequency component shows clear resonant oscillations. In addition fast mode earthward propagation is recognized. The Poynting flux of Pc 3 waves is about 1-10 nW/m(2) on average, the strongest being along the magnetic field-line. Pc 5 oscillation,; are also dominant, and are observed mainly in the dawn and dusk-side flanks, They appear as clear oscillations in the radial component of the electric field, suggesting that resonant oscillations along the magnetic field-line are well established. The Poynting flux of Pc 5 can be estimated as 10 - 100 nW/m2 in both directions across and along the magnetic field-line. This result suggests that Pc 5 wave energy is carried into both the ionosphere and the inner magnetosphere during an hour with the energy of 10(10) - 10(13) J. which is one or two orders of magnitude less than the substorm energy. By taking into account the continuous activation of ULF waves in the outer magnetosphere, these waves should play an important role in the energetics of the magnetosphere.
The double probe technique is one of the standard techniques for in situ DC electric field measurements in tenuous plasmas, but it is known for its sensitiveness to the plasma environment around a spacecraft. In the present study we compare the double-probe electric fields obtained by Geotail/EFD-P with the convection electric fields -V x B in various regions of the distant tail. We have found that the sensitivity (effective length of the double probes) varies as a function of the spacecraft potential, and the offset weakly depends on the electron temperature of ambient plasma. Using this result, an empirical calibration formula for the double-probe electric fields is obtained.
yWe study the electron inertial effects on an MHD-scale Kelvin-Helmholtz (K-H) vortex. An LLBL like situation, that is, an MHD/ion scale velocity shear layer collocated with a current layer, is set up and the evolution of an MHD/ion-scale K-H mode is followed. The magnetic field is assumed to be perpendicular to the flow and the simulation plane. For a duskside-like situation, it is shown that smaller vortices appear within the MHD-scale parent K-H vortex. For the reasonable initial condition we adopt here, these appear only when the electron inertia effects are turned on. The smaller vortices grow as they are entrained into the center of the larger vortex and eventually destroy the parent's vortex pattern. For a dawnside situation, in contrast, such a decay process is not observed. We will present detailed analyses revealing the nature of these more or less surprising results.
Between 1000 and 1800 UT on October 10, 2001, when Cluster was approaching the plasma sheet from the northern lobe, Geotail traversed the southern lobe also approaching the plasma sheet during two substorm intensifications identified in the Kakioka magnetogram. In this paper we examine the temporal change of the tail configuration and evolution of the plasma sheet between 11 and 13 UT, when multiple intensifications of the first substorm took place. Changes in the tail configuration toward a dipole-like field were identified during all intensifications, suggesting plasma sheet expansion which were embedded in a gradual northward motion of the tail. We emphasize the importance of simultaneous measurements in both hemispheres in order to identify plasma sheet expansion or dipolarization, because flapping motions or local enhancements of the tail current density may mask and mimic the signatures. During a later intensification, Geotail and Cluster encountered the plasma sheet boundary layer, which is the most direct signature of expansion of the plasma sheet. Using the time difference among the Cluster four spacecraft and Geotail, the possible propagation speed of the dipolarization and spatial scale of the disturbance is discussed. In addition to the usual dawn-to-dusk electric field, significant contribution from a north-south electric field were observed both in the lobe and at the boundary of the plasma sheet associated with the dipolarization. This suggests the importance of the effects from a localized source region for dipolarization and/or reconnection process.
It is now known that there are several different types of auroral zone disturbances, each reflecting fundamentally different processes within the magnetosphere-ionosphere system. Four types known to be dynamically important are described here. Substorms follow a greater than or similar to 0.5 hr growth period of enhanced convection. Onset occurs within the near-Earth plasma sheet and is often followed by new magnetic X-line in the mid-tail. Evidence now indicates that substorm onset results from a transition from a stable to unstable state of the nightside magnetosphere that initiates a few minutes prior to onset as the result of a reduction in the strength of convection caused by an appropriate change in the interplanetary magnetic field impacting the magnetosphere. Convection driven auroral enhancements occur at midnight-to-dawn MLTs and result from plasma sheet electrons, which are strongly energized during periods of enhanced convection and magnetic drift around the dawn side. Enhancements in solar wind dynamic pressure cause global disturbances that can be quite dramatic. These disturbances consists of increases in magnetospheric and ionospheric currents and electric fields, enhancements in auroral emissions and broadening of the auroral oval, and significant shrinkage of the polar cap size. Poleward boundary intcnsifications (PBIs) are auroral enhancements that initiate along the poleward boundary of the auroral oval. They occur during all overall levels of geomagnetic activity and are associated with few minute flow bursts in the tail plasma sheet. They are the most common disturbance. They are often repetitive and may be a manifestation of a large-scale ULF oscillation.
The interplanetary magnetic field (IMF) Bz was continuously southward for 16 hours on April 18, 2002. During this period, the spacecraft Geotail was in the northern tail lobe at radial distances of 24-28 R-E, and it observed sequences of increasing and decreasing in the magnetic field intensity in association with substorms. The increases in the magnetic field took approximately 40 minutes, and the decreases took approximately 50 minutes. The time scale of the increase is consistent with the typical time scale of the substorm growth phase, which usually begins with a clear southward turn of the IMF Bz. The time scale of the decrease agrees well with the typical time scale of the substorm expansion phase, although the substorm expansion phase frequently starts with a northward turn of the IMF Bz. It is suggested that the magnetotail has an intrinsic time scale for the loading-unloading process for substorms irrespective of the IMF Bz conditions.
We investigated upstream events observed by the ion composition system (ICS) sensor of the energetic particles and ion composition (EPIC) instrument on board the Geotail spacecraft. We examined how occurrence probability and spatial distribution of upstream events depend on the geomagnetic activity. The results showed that the upstream events were observed more frequently in the dawn side during intense geomagnetic activity in particular. We also analyzed carbon-nitrogen-oxygen ions during the upstream events. From the above results we discuss origin of the upstream energetic ions.
The structure of the plasma sheet under northward IMF is studied by data from the Geotail spacecraft. The plasma sheet is known to become cold and dense (T < 2 keV, n > 1 cm(-3)) during extended northward IMF periods. We show that such cold-dense ions (CDIs) appear at vertical bar Y-gsm vertical bar > 10 R-e that is, 10 R-e off the tail axis to both flanks. CDIs on the dawnside have higher temperatures and some reach the upper-limit of 2 keV that we use to select CDIs. These CDIs having the highest temperature are distributed at the dawnside plasma sheet inner-edge (R < 15 R-e) and is connected to the hot-dense ions (HDIs: T > 2 keV, n > 1 cm(-3)) in the further inner region. A survey shows that HDIs under nominal solar wind dynamic pressure appear mostly in the dawnside inner-magnetosphere during extended northward IMF intervals. Both results point to the idea that HDIs are the inner-magnetosphere extension of dawnside CDIs, while such a partner to the duskside CDIs cannot be identified. This structure of the plasma sheet suggests that there is significant dawn-dusk asymmetry in heating and transport in the magnetotail under northward IMF.
