Showing papers on "Substorm published in 1998"
TL;DR: In this paper, the structure and dynamics of magnetic reconnection were studied in the premidnight sector of the magnetotail at 20-30 RE for substorm onsets in Geotail observations.
Abstract: Fast tailward ion flows with strongly southward magnetic fields are frequently observed near the neutral sheet in the premidnight sector of the magnetotail at 20–30 RE for substorm onsets in Geotail observations. These fast tailward flows are occasionally accompanied by a few keV electrons. With these events, we study the structure and dynamics of magnetic reconnection. The plasma sheet near the magnetic reconnection site can be divided into three regions: the neutral sheet region (near the neutral sheet with the absolute magnitude of Bx of 10 nT), and the off-equatorial plasma sheet (the rest). In the neutral sheet region, plasmas are transported with strong convection, and accelerated electrons show nearly isotropic distributions. In the off-equatorial plasma sheet, two ion components coexist: ions being accelerated and heated during convection toward the neutral sheet and ions flowing at a high speed almost along the magnetic field. In this region, highly accelerated electrons are observed. Although electron distributions are basically isotropic, high-energy (higher than 10 keV) electrons show streaming away from the reconnection site along the magnetic field line. In the boundary region, ions also show two components: ions with convection toward the neutral sheet and field-aligned ions flowing out of the reconnection region, although acceleration and heating during convection are weak. In the boundary region, high-energy (10 keV) electrons stream away, while medium-energy (3 keV) electrons stream into the reconnection site. Magnetic reconnection usually starts in the premidnight sector of the magnetotail between XGSM = −20 RE and XGSM = −30 RE prior to an onset signature identified with Pi 2 pulsation on the ground. Magnetic reconnection proceeds on a timescale of 10 min. After magnetic reconnection ends, adjacent plasmas are transported into the postreconnection site, and plasmas can become stationary even in the expansion phase.
554 citations
TL;DR: In this paper, the authors summarize the current understanding of the storm/substorm relationship by clearing up a considerable amount of controversy and by addressing the question of how solar wind energy is deposited into and is dissipated in the constituent elements that are critical to magnetospheric and ionospheric processes during magnetic storms.
Abstract: This paper attempts to summarize the current understanding of the storm/substorm relationship by clearing up a considerable amount of controversy and by addressing the question of how solar wind energy is deposited into and is dissipated in the constituent elements that are critical to magnetospheric and ionospheric processes during magnetic storms. (1) Four mechanisms are identified and discussed as the primary causes of enhanced electric fields in the interplanetary medium responsible for geomagnetic storms. It is pointed out that in reality, these four mechanisms, which are not mutually exclusive, but interdependent, interact differently from event to event. Interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) are found to be the primary phenomena responsible for the main phase of geomagnetic storms. The other two mechanisms, i.e., HILDCAA (high-intensity, long-duration, continuous auroral electrojet activity) and the so-called Russell-McPherron effect, work to make the ICME and CIR phenomena more geoeffective. The solar cycle dependence of the various sources in creating magnetic storms has yet to be quantitatively understood. (2) A serious controversy exists as to whether the successive occurrence of intense substorms plays a direct role in the energization of ring current particles or whether the enhanced electric field associated withmore » southward IMF enhances the effect of substorm expansions. While most of the {ital Dst} variance during magnetic storms can be solely reproduced by changes in the large-scale electric field in the solar wind and the residuals are uncorrelated with substorms, recent satellite observations of the ring current constituents during the main phase of magnetic storms show the importance of ionospheric ions. This implies that ionospheric ions, which are associated with the frequent occurrence of intense substorms, are accelerated upward along magnetic field lines, contributing to the energy density of the storm-time ring current. An apparently new controversy regarding the relative importance of the two processes is thus created. It is important to identify the role of substorm occurrence in the large-scale enhancement of magnetospheric convection driven by solar wind electric fields. (3) Numerical schemes for predicting geomagnetic activity indices on the basis of solar/solar wind/interplanetary magnetic field parameters continue to be upgraded, ensuring reliable techniques for forecasting magnetic storms under real-time conditions. There is a need to evaluate the prediction capability of geomagnetic indices on the basis of physical processes that occur during storm time substorms. (4) It is crucial to differentiate between storms and nonstorm time substorms in terms of energy transfer/conversion processes, i.e., mechanical energy from the solar wind, electromagnetic energy in the magnetotail, and again, mechanical energy of particles in the plasma sheet, ring current, and aurora. To help answer the question of the role of substorms in energizing ring current particles, it is crucial to find efficient magnetospheric processes that heat ions up to some minimal energies so that they can have an effect on the strength of the storm time ring current. (5) The question of whether the {ital Dst} index is an accurate and effective measure of the storm time ring-current is also controversial. In particular, it is demonstrated that the dipolarization effect associated with substorm expansion acts to reduce the {ital Dst} magnitude, even though the ring current may still be growing. {copyright} 1998 American Geophysical Union« less
368 citations
TL;DR: In this article, the onset timing of earthward high-speed ion flow observed by the AMPTE/IRM satellite at 12.3 Earth radii (RE) and 0100 MLT in the central plasma sheet during an isolated substorm event on March 1, 1985 was studied.
Abstract: We have studied the onset timing of earthward high-speed ion flow observed by the AMPTE/IRM satellite at 12.3 Earth radii (RE) and 0100 MLT in the central plasma sheet during an isolated substorm event on March 1, 1985. The timing of this onset is compared with that of the substorm current wedge and Pi 2 magnetic pulsations observed by a large number of ground-based stations and the AMPTE/CCE, GOES 5, and ISEE 1 satellites and with that of high-energy particle injection observed at Los Aimos geosynchronous satellite 1982-019. The onset of earthward high-speed flow is observed 3 min before the onset of the global current wedge formation and 6 min before the onset of high-energy particle injection. The three bursts of the high-speed flow observed at AMPTE/IRM are likely to correspond to three compressional pulses observed at AMPTE/CCE at 6 RE and three Pi 2 wave packets observed at midlatitude ground stations. On the basis of these observations we conclude that the substorm current wedge is caused by inertia current and the current due to flow shear at the braking point of the earthward high-speed flow during the initial stage of the substorm expansion phase. The braking point is well separated from the near-Earth neutral line. It is also suggested that the compressional pulses and fluctuations of field-aligned currents generated at the flow braking point can be the initial cause of the Pi 2 magnetic pulsations in the inner magnetosphere.
298 citations
TL;DR: In this article, the authors used low energy particle (LEP) and magnetic field (MGF) data from the Geotail spacecraft to study the evolution of plasmoids in the Earth's magnetotail.
Abstract: Plasmoids in the Earth's magnetotail were studied statistically, using low energy particle (LEP) and magnetic field (MGF) data from the Geotail spacecraft. Their evolution along the tail axis from XGSM′ = −16 to −210 RE was investigated with 824 plasmoid events. Their dependence on YGSM′ was studied as well to derive the three-dimensional structure of the plasmoids. (The coordinates are aberrated to remove the average effects of the orbital velocity of the Earth about the Sun.) We defined a plasmoid as a structure with rotating magnetic fields and enhanced total pressure. In the near tail (XGSM′ ≥ −50 RE), there was a tendency for the plasmoids to be observed in the premidnight sector around the tail axis (|YGSM′ − 3| ≤ 10 RE), while they were observed widely (|YGSM′ ≤ 20 RE) in the middle tail (−50 > XGSM′ ≥ −100 RE) and in the distant tail (−100 RE > XGSM′). The plasmoids expanded in the ±YGSM′ direction with typical velocities of ±130 ± 100 km/s in the near tail. This strongly supports the view that plasmoids are initially formed at the near-Earth neutral line which has a limited extent in the YGSM′ direction. The plasmoids accelerated in the downtail direction from 400 ± 200 km/s to 700 ± 300 km/s from the near to the middle tail. Then, it is suggested that they decelerated to 600 ± 200 km/s as they traveled to the distant tail. The ion temperature inside plasmoids was 4.5 ± 2 keV in the near and middle tail, and then rapidly decreased to 2 ± 1 keV from the middle to the distant tail region. The ion temperature in the distant tail was 2 times higher than the values deduced previously. Typical plasmoid dimensions were estimated to be 10 RE (length) × 40 RE (width) × 10 RE (height) in the middle and distant tail. The energy carried by each plasmoid was ∼2 × 1014 J in the middle tail, half of which was lost from the middle to the distant tail. Inside plasmoids, the thermal energy flux exceeded the bulk energy flux and Poynting flux. The energy released tailward in the course of a substorm was estimated to be roughly 1015 J.
250 citations
TL;DR: In this article, the authors show that dispersionless injections can be understood as a consequence of changes in the electric and magnetic fields by modeling an electron injection event observed early on January 10, 1997 by means of a test-particle simulation.
Abstract: The term “dispersionless injection” refers to a class of events which show simultaneous enhancement (injection) of electrons and ions with different energies usually seen at or near geosynchronous orbit. We show that dispersionless injections can be understood as a consequence of changes in the electric and magnetic fields by modeling an electron injection event observed early on January 10, 1997 by means of a test-particle simulation. The model background magnetic field is a basic dipole field made asymmetrical by a compressed dayside and a weakened nightside. The transient fields are modeled with only one component of the electric field which is westward and a consistent magnetic field. These fields are used to model the major features of a dipolarization process during a substorm onset. We follow the electrons using a relativistic guiding center code. Our simulation results, with an initial kappa electron energy flux spectrum, reproduce the observed electron injection and subsequent drift echoes and show that the energization of injected electrons is mainly due to betatron acceleration of the preexisting electron population at larger radial distances in the magnetotail by transient fields.
216 citations
TL;DR: In this article, the acceleration and the flux increases associated with energetic electron injections on the basis of geosynchronous observations and test-electron orbits in the dynamic fields of a three-dimensional MHD simulation of neutral line formation and dipolarization in the magnetotail were investigated.
Abstract: We investigate electron acceleration and the flux increases associated with energetic electron injections on the basis of geosynchronous observations and test-electron orbits in the dynamic fields of a three-dimensional MHD simulation of neutral line formation and dipolarization in the magnetotail. This complements an earlier investigation of test protons [Birn et al., 1997b]. In the present paper we consider equatorial orbits only, using the gyrocenter drift approximation. It turns out that this approximation is valid for electrons prior to and during the flux rises observed in the near tail region of the model at all energies considered (∼ 100 eV to 1 MeV). The test particle model reproduces major observed characteristics: a fast flux rise, comparable to that of the ions, and the existence of five categories of dispersionless events, typical for observations at different local times. They consist of dispersionless injections of ions or electrons without accompanying injections of the other species, delayed electron injections and delayed ion injections, and simultaneous two-species injections. As postulated from observations [Birn et al., 1997a], these categories can be attributed to a dawn-dusk displacement of the ion and electron injection boundaries in combination with an earthward motion or expansion. The simulated electron injection region extends farther toward dusk at lower energies (say, below 40 keV) than at higher energies. This explains the existence of observed energetic ion injections that are accompanied by electron flux increases at the lower energies but not by an energetic electron injection at energies above 50 keV. The simulated distributions show that flux increases are limited in energy, as observed. The reason for this limitation and for the differences between the injection regions at different energies is the localization in the dawn-dusk direction of the tail collapse and the associated cross-tail electric field, in combination with a difference in the relative importance of E × B drift and gradient drifts at different energies. The results demonstrate that the collapsing field region earthward of the neutral line appears to be more significant than the neutral line itself for the acceleration of electrons, particularly for the initial rise of the fluxes and the injection boundary. This is similar to the result obtained for test ions [Birn et al., 1997b].
187 citations
TL;DR: In this paper, a new theory of kinetic ballooning instability (KBI) is proposed to explain the high critical beta threshold (the high critical β threshold is greater than or equal to 50) of the low-frequency global instability observed by the AMPTE/CCE.
Abstract: A new scenario of AMPTE/CCE observation of substorm onset and current disruption and the corresponding physical processes is presented. Toward the end of late growth phase plasma beta increases to greater than or equal to 50 and a low-frequency instability with a wave period of 50-75 seconds is excited and grows exponentially to a large amplitude at the onset of current disruption. At the current disruption onset, higher-frequency instabilities are excited so that the plasma and electromagnetic magnetic field form a turbulent state. Plasma transport takes place to modify the ambient plasma pressure and velocity profiles so that the ambient magnetic field recovers from a tail-like geometry to a more dipole-like geometry. To understand the excitation of the low-frequency global instability, a new theory of kinetic ballooning instability (KBI) is proposed to explain the high critical beta threshold (the high critical beta threshold is greater than or equal to 50) of the low-frequency global instability observed by the AMPTE/CCE. The stabilization is mainly due to kinetic effects of trapped electrons and finite ion Larmor radii which give rise to a large parallel electric field and hence a parallel current that greatly enhances the stabilizing effect of field line tension tomore » the ballooning mode. As a result, the high critical beta threshold for excitation of KBI is greatly increased over the ideal-MHD ballooning instability threshold by greater than or equal to O(10 exp 2). The wave-ion magnetic drift resonance effect produces a perturbed resonant ion velocity distribution with a duskward velocity roughly equal to the average ion magnetic (gradient B and curvature) drift velocity. Higher-frequency instabilities such as cross-field current instability (CCI) can be excited by the additional velocity space free energy associated with the positive slope in the perturbed resonant ion velocity distribution in the current disruption phase.« less
179 citations
TL;DR: In this article, the effect of self-organized criticality (SOC) is proposed as an internal mechanism to generate 1/f b fluctuations in the magnetosphere, where localized spatially current instabilities, developing in the magnetic tail at the initial substorm phase can be considered as SOC avalanches or dynamic clusters.
Abstract: . Low frequency stochastic variations of the geomagnetic AE-index characterized by 1/f b-like power spectrum (where f is a frequency) are studied. Based on the analysis of experimental data we show that the Bz-component of IMF, velocity of solar wind plasma, and the coupling function of Akasofu are insufficient factors to explain these behaviors of the AE-index together with the 1/f b fluctuations of geomagnetic intensity. The effect of self-organized criticality (SOC) is proposed as an internal mechanism to generate 1/f b fluctuations in the magnetosphere. It is suggested that localized spatially current instabilities, developing in the magnetospheric tail at the initial substorm phase can be considered as SOC avalanches or dynamic clusters, superposition of which leads to the 1/f b fluctuations of macroscopic characteristics in the system. Using the sandpile model of SOC, we undertake numerical modeling of space-localized and global disturbances of magnetospheric current layer. Qualitative conformity between the disturbed dynamics of self-organized critical state of the model and the main phases of real magnetospheric substorm development is demonstrated. It is also shown that power spectrum of sandpile model fluctuations controlled by real solar wind parameters reproduces all distinctive spectral features of the AE fluctuations. Key words. Magnetospheric physics (MHD waves and instabilities; solar wind · magnetosphere interactions; storms and substroms).
130 citations
TL;DR: In this article, a geoeffective magnetic cloud impacted the Earth early on 15 May 1997, which exhibited strong initial southward interplanetary magnetic field (BZ∼−25 nT), which caused intense substorm activity and an intense geomagnetic storm (Dst ∼−170 nT).
Abstract: A geoeffective magnetic cloud impacted the Earth early on 15 May 1997. The cloud exhibited strong initial southward interplanetary magnetic field (BZ∼−25 nT), which caused intense substorm activity and an intense geomagnetic storm (Dst ∼−170 nT). SAMPEX data showed that relativistic electrons (E ≳ 1.0 MeV) appeared suddenly deep in the magnetosphere at L=3 to 4. These electrons were not directly “injected” from higher altitudes (i.e., from the magnetotail), nor did they come from an interplanetary source. The electron increase was preceded (for ∼2 hrs) by remarkably strong low-frequency wave activity as seen by CANOPUS ground stations and by the GOES-8 spacecraft at geostationary orbit. POLAR/CEPPAD measurements support the result that high-energy electrons suddenly appeared deep in the magnetosphere. Thus, these new multi-point data suggest that strong magnetospheric waves can quickly and efficiently accelerate electrons to multi-MeV energies deep in the radiation belts on timescales of tens of minutes.
125 citations
TL;DR: The first geomagnetic storm of 1997 began on January 10 and lasted for approximately 1 day, and is associated with the strong compression of the magnetosphere as discussed by the authors, while the second enhancement showed a more characteristic time delay.
Abstract: The first geomagnetic storm of 1997 began on January 10. It is of particular interest because it was exceptionally well observed by the full complement of International Solar Terrestrial Physics (ISTP) satellites and because of its possible association with the catastrophic failure of the Telstar 401 telecommunications satellite. Here we report on the energetic electron environment observed by five geosynchronous satellites. In part one of this paper we examine the magnetospheric response to the magnetic cloud. The interval of southward IMF drove strong substorm activity while the interval of northward IMF and high solar wind density strongly compressed the magnetosphere. At energies above a few hundred keV, two distinct electron enhancements were observed at geosynchronous orbit. The first enhancement began and ended suddenly, lasted for approximately 1 day, and is associated with the strong compression of the magnetosphere. The second enhancement showed a more characteristic time delay, peaking on January 15. Both enhancements may be due to transport of electrons from the same initial acceleration event at a location inside geosynchronous orbit but the first enhancement was due to a temporary, quasi-adiabatic transport associated with the compression of the magnetosphere while the second enhancement was due to slower diffusive processes. In the second part of the paper we compare the relativistic electron fluxes measured simultaneously at different local times. We find that the >2-MeV electron fluxes increased first at noon followed by dusk and then dawn and that there can be difference of two orders of magnitude in the fluxes observed at different local times. Finally, we discuss the development of data-driven models of the relativistic electron belts for space weather applications. By interpolating fluxes between satellites we produced a model that gives the >2-MeV electron fluxes at all local times as a function of universal time. In a first application of this model we show that, at least in this case, magnetopause shadowing does not contribute noticeably to relativistic electron dropouts.
121 citations
TL;DR: In this article, the instability mode of low-frequency (1-10 Hz) electromagnetic turbulence observed at the neutral sheet during substorms has been examined on the basis of wave and plasma observations of the Geotail satellite.
Abstract: On the basis of wave and plasma observations of the Geotail satellite, the instability mode of low-frequency (1-10 Hz) electromagnetic turbulence observed at the neutral sheet during substorms has been examined. Quantitative estimation has also been made for the anomalous heating and resistivity resulting from the electromagnetic turbulence. Four possible candidates of substorm onset sites, characterized by the near-Earth neutral line, are found in the data sets obtained at substorm onset times. In these events, wave spectra obtained by the search-coil magnetometer and the spherical double-probe instrument clearly show the existence of electromagnetic wave activity in the lower hybrid frequency range at and near the neutral sheet. The linear and quasi-linear calculations of the lower hybrid drift instability well explain the observed electromagnetic turbulence quantitatively. The calculated characteristic electron heating time is comparable to the timescale of the expansion onset, while that of ion heating time is much longer. The estimated anomalous resistivity fails to supply enough dissipation for the resistive tearing mode instability.
TL;DR: In this paper, the ground signatures of dynamic substorm features with particular emphasis on the event interpretation capabilities provided by the IMAGE magnetometer network were presented, and the authors stressed the importance of multi-instrument observation for a reliable interpretation of dynamic auroral processes.
Abstract: We present the ground signatures of dynamic substorm features with particular emphasis on the event interpretation capabilities provided by the IMAGE magnetometer network. This array covers the high latitudes from the sub-auroral to the cusp/cleft region. An isolated substorm on 11 Oct. 1993 during the late evening hours exhibited many of well-known features such as the Harang discontinuity, westward travelling surge and poleward leap, but also discrete auroral forms, known as auroral streamers, appeared propagating westward along the centre of the electrojet. Besides the magnetic field measurements, there were auroral observations and plasma flow and conductivity measurements obtained by EISCAT. The data of all three sets of instruments are consistent with the notion of upward field-aligned currents associated with the moving auroral patches. A detailed analysis of the electro-dynamic parameters in the ionosphere, however, reveals that they do not agree with the expectations resulting from commonly used simplifying approximations. For example, the westward moving auroral streamers which are associated with field-aligned current filaments, are not collocated with the centres of equivalent current vortices. Furthermore, there is a clear discrepancy between the measured plasma flow direction and the obtained equivalent current direction. All this suggests that steep conductivity gradients are associated with the transient auroral forms. Also self-induction effects in the ionosphere may play a role for the orientation of the plasma flows. This study stresses the importance of multi-instrument observation for a reliable interpretation of dynamic auroral processes.
TL;DR: In this paper, the authors used an appropriately modified IMF-dependent terrestrial magnetosphere model scaled for Mercury, together with an assumed, reasonable IMF time series, to reproduce the magnetic field signature during the disturbed outbound pass segment.
Abstract: In addition to providing the first in situ evidence of a magnetosphere at Mercury, the first flyby by Mariner 10 inspired reports of Earth-like substorms. While the small scales at Mercury should make the substorm timescale there much shorter than it is at the Earth, these early interpretations may have too readily assumed that the substorm requirement of an energy storage and release phase occurs. Instead, its size should make Mercury's magnetosphere especially prone to disturbances that are purely “driven” by the changing external boundary conditions on the magnetosphere imposed by the solar wind. These result simply from the magnetosphere's relatively unhindered reconfiguration in response to the varying interplanetary parameters, including the IMF southward component. In this paper we demonstrate that the “disturbed” structure observed outbound from closest approach during the first Mariner 10 flyby can alternately be explained as a consequence of a typical period of rotating IMF. We use an appropriately modified IMF-dependent terrestrial magnetosphere model scaled for Mercury, together with an assumed, reasonable IMF time series, to reproduce the magnetic field signature during the disturbed outbound pass segment. This result suggests that rapid restructuring of the small magnetosphere in response to changing local interplanetary conditions may dominate the magnetospheric dynamics at Mercury. Future Mercury magnetosphere missions should be instrumented to distinguish between this driven magnetospheric dynamism and any internal Earth-like substorm processes. These results also remind us that driven reconfigurations must always be considered in studies of transients in the terrestrial magnetosphere.
TL;DR: In this article, the authors examined substorm-associated magnetic field fluctuations observed by the AMPTE/CCE and SCATHA satellites in the near-Earth tail and found that the magnetic fluctuations do have a characteristic timescale, which is several times the proton gyroperiod.
Abstract: This study examines substorm-associated magnetic field fluctuations observed by the AMPTE/CCE and SCATHA satellites in the near-Earth tail. Three tail reconfiguration events are selected, one event on August 28, 1986, and two consecutive events on August 30, 1986. The fractal analysis was applied to magnetic field measurements of each satellite. The result indicates that (1) the amplitude of the fluctuation of the north-south magnetic component is larger, though not overwhelmingly, than the amplitudes of the other two components and (2) the magnetic fluctuations do have a characteristic timescale, which is several times the proton gyroperiod. In the examined events the satellite separation was less than 10 times the proton gyroradius. Nevertheless, the comparison between the AMPTE/CCE and SCATHA observations indicates that (3) there was a noticeable time delay between the onsets of the magnetic fluctuations at the two satellite positions, which is too long to ascribe to the propagation of a fast magnetosonic wave, and (4) the coherence of the magnetic fluctuations was low in the August 28, 1986, event and the fluctuations had different characteristic timescales in the first event of August 30, 1986, whereas some similarities can be found for the second event of August 30, 1986. Result 1 indicates that perturbation electric currents associated with the magnetic fluctuations tend to flow parallel to the tail current sheet and are presumably related to the reduction of the tail current intensity. Results 2 and 3 suggest that the excitation of the magnetic fluctuations and therefore the trigger of the tail current disruption is a kinetic process in which ions play an important role. It is inferred from results 3 and 4 that the characteristic spatial scale of the associated instability is of the order of the proton gyroradius or even shorter, and therefore the tail current disruption is described as a system of chaotic filamentary electric currents. However, result 4 suggests that the nature of the tail current disruption can vary from event to event.
TL;DR: In this paper, the authors present observations of two sequential substorm onsets on May 15, 1996 during persistently negative IMF B-Z, and the second expansion followed a northward turning o...
Abstract: 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 ...
TL;DR: In this article, a strong correlation between high-speed solar wind speeds and the occurrence of high energy electrons in the Earth's outer radiation belts has been found, and the processes by which electrons are accelerated to high energies (E>1 MeV) are found to consist of two temporally distinct steps.
TL;DR: A substorm onset was documented by ground magnetograms, auroral kilometric radiation, and magnetic field and particle data from four spacecraft at and near geosynchronous orbit.
Abstract: On April 26, 1995, while Geotail was in the near-equatorial magnetotail at 13 RE and 2300 LT, a substorm onset occurred that was documented by ground magnetograms, auroral kilometric radiation, and magnetic field and particle data from four spacecraft at and near geosynchronous orbit. Although Geotail was initially outside a greatly thinned current sheet, plasma sheet thickening associated with the substorm dipolarization quickly caused Geotail to move into the plasma sheet where it observed field-aligned earthward moving ions with velocities of 400 km/s. During the subsequent few minutes as the magnetic field became more northward, the velocities increased with particles moving increasingly into the energy range of the energetic particle experiment. These flows culminated with 1-min worth of earthward flow of 2000 km/s that was perpendicular to the northward B field. Such flow, probably the largest ever detected at 13 RE, was confirmed by the observation of an intense dc electric field of 50 mV/m (0.3 megavolts/RE). This large field is probably inductive, caused by reconnection that occurred tailward of the spacecraft, and related to the acceleration processes associated with particle injection at geosynchronous orbit. Energy and magnetic flux conservation arguments suggest that this rapid flow has a small cross-tail dimension of the order of 1 RE. The data appear to support a simulation of Birn and Hesse [1996] which showed rapid earthward flows from a reconnection line at 23 RE that caused a tailward expansion of a region of dipolarized flux. Subsequent to the onset, Geotail observed plasma vortices with typical velocities of 50–100 km/s that occurred in a high-beta plasma sheet with a 15-nT northward magnetic field. The vortices were punctuated by occasional flow bursts with velocities up to 400 km/s, one of which was accompanied by a violently varying magnetic field where north/south field components were as large as 30 nT and as small as −8 nT.
TL;DR: In this article, 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.
Abstract: 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.
TL;DR: In this paper, the origin of multiple energetic particle injections into the inner magnetosphere is addressed using a rare opportunity of measuring the energetic particle fluxes at different radial distances under known electric and magnetic fields.
Abstract: The origin of multiple energetic particle injections into the inner magnetosphere is addressed using a rare opportunity of measuring the energetic particle fluxes at different radial distances under known electric and magnetic fields. During a strong substorm on February 10, 1991, the CRRES spacecraft measured {ital E} and {ital B} fields and high-energy particle fluxes near the magnetic equator at r{approximately}5R{sub e}, whereas particle injections, their azimuthal locations, and some other parameters were simultaneously monitored by three geostationary spacecraft and ground networks. We show a multitude of impulsive short-duration injection events which correlate with 1{endash}2 min long pulses of dawn-dusk electric field. The observations suggest that some {ital E} field pulses recorded deep in the inner magnetosphere were fast magnetosonic waves radiated by the current disruption region. This supports the concept of impulsive dissipation event as an elementary building block of substorm expansion. Furthermore, our modeling results indicate that most of the flux variations of energetic particles can be explained by the global convective transport and corresponding particle acceleration. However, we emphasize that, depending on particle spectra and radial flux gradient, one can observe either flux increase, or decrease, or no variation (often seen in different energy ranges simultaneously andmore » at the same point) as a response to the electric field pulse. Both the cloud of injected particles and magnetic field dipolarization region had a sharp inner boundary (injection front) which propagated inward at the convection speed. We document the complicated structure of this front, consisting of a diamagnetic hot proton layer followed by the dipolarization front which contains enhanced energetic electron fluxes. Further study is required to understand how common this structure is and, if common, how it may be formed. {copyright} 1998 American Geophysical Union« less
TL;DR: In this article, a time series of the ionospheric equivalent current function with time resolution of 5 min during March 17-19, 1978 is calculated on the basis of six meridian chains magnetometer data during the International Magnetospheric Study in order to obtain the fundamental orthogonal basis set.
Abstract: This paper attempts to separate objectively the directly driven and unloading components in substorm processes by applying the method of natural orthogonal components (MNOC). A time series of the ionospheric equivalent current function with time resolution of 5 min during March 17–19, 1978 is calculated on the basis of six meridian chains magnetometer data during the International Magnetospheric Study in order to obtain the fundamental orthogonal basis set. The first and second natural components of the set thus obtained dominate over the rest of the natural components. The first natural component is found to have a two-cell pattern, which is well known to be associated with global plasma convection in the magnetosphere. It is enhanced during the growth phase and expansion phase of substorms and decays during the recovery phase of substorms. Further, it is in fair correlation to the ϵ parameter with time lag of 20–25 min. This can be identified as the directly driven component. The second natural component reveals itself as an impulsive enhancement of the westward electrojet around midnight between 65° and 70° latitude during the expansion phase only. It is much less correlated with the ϵ parameter than the first one. Thus, as a first approximation, we identify it as the unloading component. It is shown that the directly driven component tends to dominate over the unloading component except for a brief period soon after substorm onset. This is the first clear determination of the time profile of the unloading component.
TL;DR: In this article, a global MHD simulation of an isolated substorm that occurred on March 9, 1995 is presented, driven with solar wind data provided by the Wind satellite, which reproduced to a surprising degree the evolution of substorm activity.
Abstract: A global MHD simulation of an isolated substorm that occurred on March 9, 1995 is presented. The simulation, driven with solar wind data provided by the Wind satellite, reproduced to a surprising degree the evolution of substorm activity. The onset of the expansion phase was coincident with the penetration of an electric field spike into the near- Earth region. This impulse launched a tailward propagating signal (rarefaction wave) that enhanced reconnection in the mid tail. Substorm intensification was correlated with th'e enhancement of the reconnection rate at the preexisting reconnection region located at 30 Re. The importance of the electric field spike in correlating ionospheric and magnetospheric aspects of the substorm is emphasized.
TL;DR: In this article, a nonlinear numerical simulation model using different background ionospheric conditions was used to understand observations of modulated irregularities in a localized region on the bottomside of the equatorial ionosphere without the presence of vertically rising large-scale plume events.
Abstract: In order to understand observations of modulated irregularities in a localized region on the bottomside of the equatorial ionosphere without the presence of vertically rising large-scale plume events, an investigation is carried out by means of a nonlinear numerical simulation model using different background ionospheric conditions. The investigation revealed that the development of long wavelength (200 km) perturbations can only be confined to the bottomside of the ionosphere in a localized region with the combined effects of shear in the zonal plasma drift and a nighttime westward electric field. The variabilities in the occurrence characteristic are shown to depend on the variabilities in the prereversal enhancement in the zonal electric field. Further, the resurgence of a plume event at a later time, as seen by the Jicamarca radar, is explained on the basis of confined (preseeded) structures subject to a fresh instability source. First evidence is presented that a source of fresh instability may actually be a wind and/or a temperature disturbance launched hours earlier by a small substorm.
TL;DR: In this article, a wavelet analysis is used to detect Pi 2 pulsations by using real-time geomagnetic field data, with a sampling rate of 1 second, obtained at mid-and low-latitude stations.
Abstract: Wavelet analysis is suitable for investigating waves, such as Pi 2 pulsations, which are limited in both time and frequency. We have developed an algorithm to detect Pi 2 pulsations by wavelet analysis. We tested the algorithm and found that the results of Pi 2 detection are consistent with those obtained by visual inspection. The algorithm is applied in a project which aims at the nowcasting of substorm onsets. In this project we use real-time geomagnetic field data, with a sampling rate of 1 second, obtained at mid- and low-latitude stations (Mineyama in Japan, the York SAMNET station in the U.K., and Boulder in the U.S.). These stations are each separated by about 120° in longitude, so at least one station is on the nightside at all times. We plan to analyze the real-time data at each station using the Pi 2 detection algorithm, and to exchange the detection results among these stations via the Internet. Therefore we can obtain information about substorm onsets in real-time, even if we are on the dayside. We have constructed a system to detect Pi 2 pulsations automatically at Mineyama observatory. The detection results for the period of February to August 1996 showed that the rate of successful detection of Pi 2 pulsations was 83.4% for the nightside (18-06MLT) and 26.5% for the dayside (06-18MLT). The detection results near local midnight (20-02MLT) give the rate of successful detection of 93.2%.
TL;DR: In this article, the authors examined the spatial development of tail current disruption in the near-Earth tail by making use of magnetic field measurements made by two satellites located close to each other.
Abstract: The present paper addresses the spatial development of tail current disruption in the near-Earth tail by making use of magnetic field measurements made by two satellites located close to each other. The paper consists of two parts: a statistical study and a case study of a substorm event that took place on August 30, 1986. In the first part the relative timing of the onsets of tail magnetic field reconfiguration (dipolarization) at the AMPTE/CCE (r < 8.8 RE) and GOES geosynchronous satellites is statistically examined. The result shows that the tail current disruption region expands azimuthally outside geosynchronous orbit. Another, and most unexpected, finding is that the tail current disruption region often expands earthward. The substorm event of August 30, 1986, observed by the AMPTE/CCE and SCATHA satellites provides a good opportunity to examine the earthward expansion of the tail current disruption region. In this event, AMPTE/CCE was located on the earthward side of SCATHA by 0.4 RE and observed the onset of magnetic fluctuations 15 s after the SCATHA onset. The earthward expansion velocity is estimated at 180 km/s, much slower than the propagation velocity of the fast magnetosonic mode, indicating that the observed earthward expansion is not an effect of the wave propagation. The very initial signature observed by AMPTE/CCE in this event is a decrease in the north-south magnetic component accompanied by the enhancement of the flux of energetic ions having their guiding centers tailward. This result suggests that the tail current is intensified by injected particles at the front of the earthward expanding region of the tail current disruption. This enhanced current may trigger the local disruption of the tail current, making the disruption region expand further earthward.
TL;DR: In this paper, the global conditions during a moderate geomagnetic disturbance event on May 15, 1996, are examined by comparing data from several ground-based instruments and inner tail satellites with global MHD simulations of the same event.
Abstract: The global conditions during a moderate geomagnetic disturbance event on May 15, 1996, are examined by comparing data from several ground-based instruments and inner tail satellites with global MHD simulations of the same event. The ground-based data show two substorm intensifications about 40 min apart, the first one being small and localized (a pseudobreakup) and the second leading to a major rearrangement of both the ionospheric auroral distribution and the magnetotail configuration. The simulation shows that during the pseudobreakup, open field lines were reconnecting in the midtail, but the flows were mainly tailward and very few effects were observable in the inner magnetosphere. The result that pseudobreakups can be associated with activity producing topological changes in the tail is an important new aspect that has not been discussed in earlier studies. Both the observations and the simulation show two distinct regions of activity: a thin current sheet in the inner tail magnetically connected with the auroral bulge and a reconnection region in the midtail associated with the most intense electrojet currents.
TL;DR: In this paper, the authors identified a list of auroral breakups based on global imaging data from the Ultraviolet Imager of the POLAR spacecraft for two favorable viewing periods and examined the simultaneous plasma measurements in the tail from GEOTAIL.
Abstract: Auroral breakups are the first visible sign of a substorm expansion onset. Keying the plasma sheet behavior to onset times of auroral breakups may help to identify the substorm onset process. With this goal in mind, we have identified a list of auroral breakups based on global imaging data from the Ultraviolet Imager of the POLAR spacecraft for two favorable viewing periods and have examined the simultaneous plasma measurements in the tail from GEOTAIL. Synoptic patterns of plasma transport and magnetic field changes in the tail surrounding the times of auroral breakups are constructed. The results indicate that the plasma sheet activities associated with auroral breakups are transient and spatially localized. These findings are consistent with the scenario in which expansion phase activities are dominated by localized, transient disturbances as portrayed by the substorm synthesis model.
TL;DR: In this paper, it was shown that a pseudo-breakup is, in fact, a disturbance which represents the same physical process as the onset of an expansive phase or any of the subsequent intensifications.
Abstract: It is common to view the expansive phase of a magnetospheric substorm as a large amplitude disturbance. As a result, any magnetic disturbance that has the normal characteristics of an expansive phase onset but has an amplitude below some subjective limit is not classed as an expansive phase onset but is labelled, instead, as a pseudo-breakup. In this paper it will be shown that a pseudo-breakup is, in fact, a disturbance which represents the same physical process as the onset of an expansive phase or any of the subsequent intensifications. Examination of simultaneous interplanetary magnetic field data suggests the possibility that a pseudo-breakup is distinguished from the onset or intensifications of an expansive phase by its occurrence during a period of increasing energy input into the magnetosphere. Thus, the pseudo-breakup is not accompanied by a large scale poleward motion of the high latitude edge of the auroral oval in the substorm disturbed region characteristic of an expansive phase.
TL;DR: In this paper, a theoretical study on how Pi 2 pulsations are excited in the magnetosphere is presented, where impulsive disturbances associated with the substorm onset are assumed at the tailward region, their propagation toward the sunward direction is investigated with a wave equation.
Abstract: We present a theoretical study on how Pi 2 pulsations are excited in the magnetosphere. When impulsive disturbances associated with the substorm onset are assumed at the tailward region, their propagation toward the sunward direction is investigated with a wave equation. In order to examine the effect of the plasmapause on the initial disturbances, we analytically solve the wave equation based on the model of a reasonable Alfven speed profile. The exact solution shows that virtual resonant states exist in the plasmasphere and plasmapause. We obtain the result that these unique modes strongly persist for arbitrary incoming impulses from the source in the magnetotail, which corresponds to the signature of Pi 2 pulsations.
TL;DR: In this paper, the azimuthal pressure gradient in the central plasma sheet during substorms using plasma and magnetic field data obtained by the AMPTE/IRM satellite at nightside in radial distances of 9-15 RE.
Abstract: We have studied the azimuthal pressure gradient in the central plasma sheet during substorms using plasma and magnetic field data obtained by the AMPTE/IRM satellite at nightside in radial distances of 9–15 RE. The pressure gradient is statistically estimated for the interval when the magnetic field shows a dipolar configuration (elevation angle >45°). It is found that by this criterion, most data are obtained during and after the passage of high-speed ion flow in the vicinity of the neutral sheet during magnetically active times. We show that there is an azimuthal gradient of plasma pressure in the dipolar field region. The pressure gradient can drive a substantial amount of field-aligned current (4.1×105 A per 2-hour local time sector). We suggest that this current is a source of the substorm current system after the high-speed ion flow stops.