摘要:

It is proposed here that the expansion phase of substorms results from a reduction in the large‐scale electric field imparted to the magnetosphere from the solar wind, following a ≳ 30‐min growth phase due to an enhancement in this electric field. The reduction in the electric field is assumed to propagate antisunward within the magnetosphere. Triggering by a reduction in the electric field is suggested by the observation that substorms are often triggered by northward turnings of the interplanetary magnetic field (IMF). However, under the theory presented here, substorms may be triggered by anything that causes an electric field reduction such as a reduction in the magnitude of the y component of the IMF. A reduction in the large‐scale electric field disrupts both the inward motion and energization of plasma sheet particles that occurs during the growth phase. It is suggested here that this can lead to formation of the expansion phase current wedge and active aurora. The current wedge results from the magnetic drift of ions, which has a speed proportional to particle energy, and a large azimuthal gradient in mean particle energy that is expected to develop in the vicinity of magnetic midnight during the growth phase. Current wedge formation will most likely be initiated near the radial distance (∼6‐10RE) of the peak in the growth phase plasma pressure distribution, and then propagate tailward from that region. Order‐of‐magnitude calculations show that the above proposal can account for the rapid development of the expansion phase relative to the growth phase, the magnitude of the reduction in the cross‐tail current within the current wedge, the speeds of tailward and westward expansion of the current reduction region, the speeds of poleward and westward motion of active aurora in the ionosphere, and the magnitude of wedge field‐aligned currents that connect the ionospheric region of active auroral to the divergent cross‐tail current wit

ISSN:0148-0227    

DOI:10.1029/95JA01344

年代:1995

数据来源: WILEY

摘要:

We report the first global magnetohydrodynamic (MHD) simulation of an actual magnetospheric substorm, which was recorded by the Viking spacecraft on October 19, 1986. The simulation is driven by IMP 8 solar wind parameters measured upstream of the Earth's bow shock. The substorm, which had expansion onset at 1132 UT, was caused by a brief period of southward interplanetary magnetic field (IMF) and two weak solar wind shocks. The simulation model includes a self‐consistent auroral ionospheric conductance depending directly on the MHD magnetospheric plasma parameters and magnetic field. Synthetic auroral emissions, derived from simulation results, are compared to the Viking images, which show considerable dayside activity preceding the substorm. We also compare model‐derived syntheticAUandALindices to geomagnetic measurements. The simulation results are seen to be in reasonable agreement with the observations throughout the growth phase and expansion onset. Moreover, the results allow us to form conclusions concerning which essential processes were responsible for the substorm occurence. These results are a highly encouraging first step leading toward development of a space weather forecasting methodology based on the directly measured solar in

ISSN:0148-0227    

DOI:10.1029/95JA01524

年代:1995

数据来源: WILEY

摘要:

A substorm on July 24, 1986, exhibiting a rather unusual auroral morphology is analyzed with data from 10 spacecraft (Viking; DMSP F6 and F7; GOES 5 and 6; three LANL geosynchronous satellites; CCE; and IMP 8). This substorm occurred during high solar wind dynamic pressure (>5 nPa). Several notable features for this substorm are (1) the substorm onset activity was preceded by prominent auroral activations in the morning sector with spatial separations between adjacent bright regions ranging from ∼160 to 640 km, and their intensity was modulated at ∼3.2‐min intervals; (2) the initial substorm activity was concentrated in the morning sector, followed by a sudden activation in the dusk sector, leaving the midnight sector relatively undisturbed, in sharp contrast to the traditional substorm development in which the major activity occurs in the midnight sector; (3) while a substorm injection was observed at a geocentric distance of ∼8.4REby CCE in association with the substorm onset, particle injections (detectable with three LANL geosynchronous satellites) and dipolarization signatures (detectable by the two GOES satellites) were not observed until subsequent intensifications; (4) timing subsequent substorm intensifications from injections at the geosynchronous altitude differed from timing intensifications based on Viking auroral images by as much as ∼3 min even when the auroral activity occurred in the same magnetic local time sector as the geosynchronous satellite; (5) the polar cap boundary was at a significantly higher latitude than the poleward boundary delineated by detectable auroral luminosity in the auroral oval, supporting the previous result that auroral activity at substorm onset occurs substantially equatorward of the boundary between open and closed magnetic field lines. Detailed timing analysis suggests the substorm onset to be associated with southward interplanetary magnetic field (IMF), possibly with the crossing of an IMF sector boundary (interplanetary current sheet). The dimming of auroral luminosity in the midnight region was associated with a sudden northward turning of the IMF during high solar wind dynamic pressure

ISSN:0148-0227    

DOI:10.1029/95JA01181

年代:1995

数据来源: WILEY

摘要:

Field and particle data recorded on the geostationary satellite GEOS 2 are used to investigate the electric and magnetic signatures of a substorm characterized by a dispersionless injection of energetic electrons and ions. Three types of field variations are observed: (1) Long‐period oscillations with period of ∼ 300 s, interpreted as oscillations of entire field lines. These oscillations develop as second harmonic standing waves and correspond to coupled shear Alfvén‐slow magnetosonic modes. They grow after the most active period of the breakup. (2) Short‐period transient oscillations with periods of ∼ 45–65 s, interpreted as wave modes trapped in a current layer which develops prior to the substorm breakup and is disrupted at breakup. These oscillations also correspond to a coupled shear Alfvén‐slow magnetosonic mode (coupled via magnetic field curvature effects in a high‐β plasma). The short‐period transient oscillations are only observed during the most active period of the breakup. (3) A nonoscillatory sharp increase observed on both the parallel magnetic component and the energetic ion flux, averaged over one satellite rotation, interpreted as evidence for the fast magnetosonic mode which in view of the simultaneous large impulsive increase in the azimuthal electric field, appears to propagate radially outwards, transporting the subst

ISSN:0148-0227    

DOI:10.1029/95JA00990

年代:1995

数据来源: WILEY

摘要:

On April 15, 1979 at 0645 UT, several minutes prior to the expansion phase onset of a weak substorm, the ISEE 1 and 2 spacecraft were located 16REdowntail and close to local midnight. The two spacecraft were on opposite sides of the current sheet, allowing us to better constrain the possible interpretations of the available plasma, magnetic field, and energetic particle data. Plasma sheet acceleration occurring earthward of the spacecraft was evident in the energetic particle data 2–3 min before the first ground‐based signatures of substorm onset. The ensuing magnetic field and particle changes are consistent with anX‐type neutral line moving tailward of ISEE 1 and 2 with an apparent speed of ∼30 km/s, while the plasma sheet thickness decreased to a minimum of less than 0.2RE. Although the substorm effects at geosynchronous altitude and on the ground were weak, the reconnection rate was locally quite intense (>1 mV/m) and produced earthward bursty bulk flows of peak velocity>600 km/s. Ion velocity‐space distributions confirm that the flows are indeed convective flows of a single‐ion population and not counterstreaming beams, as is often the case at the plasma sheet boundary. The fast tailward flows observed at the initial stage of reconnection were localized to a thin layer surrounding the neutral sheet. The magnitude of the current disruption and the region 1 sense field‐aligned currents associated with reconnection were sufficiently large to explain the intensity of the substorm current wedge, as inferred from ground

ISSN:0148-0227    

DOI:10.1029/95JA01471

年代:1995

数据来源: WILEY

摘要:

The objective of the present study is to assess the mechanism of substorm‐associated tail current disruption on the basis of magnetic field observations in the near‐Earth tail. We examined 15 events observed by the Charge Composition Explorer (CCE) of the Active Magnetospheric Particle Tracer Explorers (AMPTE), with an emphasis on the August 28, 1986, event. In these events the satellite observed magnetic fluctuations to start almost simultaneously with ground substorm onsets, strongly suggesting that these fluctuations are related to the trigger of substorms. In this study we applied the new method, fractal analysis, to these fluctuations. This method enables us to examine fluctuations quantitatively and to pick up characteristic timescale(s) of fluctuations, even if fluctuations are far from sinusoidal. The results are summarized as follows: (1) Whereas before the onset of tail current disruption, magnetic fluctuations are suppressed in each of the magnetic components, after the onset, the magnitude of theH(north‐south) component fluctuations is about 30% larger than the magnitudes of the fluctuations of the other components. (2) The magnetic fluctuations have a characteristic timescale, which is several times the proton gyroperiod. The first result suggests that observed magnetic fluctuations are actually related to changes in the tail current intensity, that is, tail current disruption. This result also indicates that the microprocess of tail current disruption should be described in terms of turbulent perturbation electric currents, although away from the onset region the effects of tail current disruption may be approximated by those of an orderly decrease in the tail current intensity. The second result strongly suggests that tail current disruption is driven by a certain instability, which grows most rapidly around that characteristic time scale, and in which ions should play an important

ISSN:0148-0227    

DOI:10.1029/95JA00903

年代:1995

数据来源: WILEY

摘要:

The modified‐two‐stream instability (MTSI) is one of the modes of the cross‐field current instability which has recently been considered as one plausible mechanism for substorm expansion onset and intensification. This paper extends the previous analysis of this instability to nonlocal treatment under the simplifying assumptions of electrostatic perturbations and neglect of thermal and kinetic effects. The result enables us to gain new insights on the onset threshold of the instability. We find that a current sheet with a constant velocity associated with the current (exemplified by the Harris equilibrium current sheet) is stable with respect to MTSI, whereas one with a prominent velocity peak at its center is unstable when it is sufficiently thin. This result therefore indicates that the onset threshold of the instability is very sensitive to the global characteristics of the current sheet, and its value can be determined only through nonlocal analysis. The higher onset threshold than that predicted by the local theory allows one to better understand why a substantial cross‐tail current enhancement can develop and a thin current sheet can remain stable for an extended period before the onset of substorm expansion and current dis

ISSN:0148-0227    

DOI:10.1029/95JA01404

年代:1995

数据来源: WILEY

摘要:

Time series of theALindex and rectified solar wind electric field are used to study solar wind coupling to auroral ionospheric currents during individual substorms. Calculation of the individual filters is accomplished by representing them as the product of two simple filters: a low‐pass filter that accounts for the response of the ionospheric electric field to the reconnection electric field, and a filter composed of delta functions. The delta functions each have the effect of delaying and scaling the response ofALrelative toVBswithout distorting the waveform ofVBs. Nearly all significant correlation betweenALandVBscan be accounted for when the second filter contains only two components. This result suggests that the westward electrojet indexed byALis controlled by two distinct processes, both proportional toVBs. For an ensemble of 117 isolated substorms the average prediction efficiency of a filter with two delays is 71%. If we were able to determine the exact values of the two delays and the two gain factors prior to an event, then we would be able to estimate the instantaneous magnetospheric response function. We examined season, universal time, and prior activity as possible factors controlling the magnitude of the parameters but found no apparent relationships. Until such relations are found the average parameters provide the best estimate but account for only 47% of the variance in theALinde

ISSN:0148-0227    

DOI:10.1029/95JA01341

年代:1995

数据来源: WILEY

摘要:

Particle acceleration near anXline in the magnetotail and the resulting ion distribution functions in the central plasma sheet are studied by using 2½‐dimensional test particle simulations. The electric and magnetic fields are taken from a three‐dimensional global magnetohydrodynamic (MHD) simulation model describing the solar wind‐magnetosphere interaction. A southward interplanetary magnetic field is chosen as the input solar wind condition. Test particles are generated from a model ion source distribution consistent with the macroscopic MHD parameters in the lobe region. The particles are calculated earthward and tailward of theXline. It is found that ion distributions consist of two main components: a low‐energy population similar to the input distribution and a high‐energy population. The former corresponds to particles that do not cross the region in which theBχfield reverses direction, while the latter consists of particles which cross the midplane, gaining energies on the order of keV. The spatial dependence of the relative population of low‐energy versus high‐energy components is discussed. The details of the acceleration process are determined from the motion of test particles in

ISSN:0148-0227    

DOI:10.1029/95JA00963

年代:1995

数据来源: WILEY

摘要:

We are reporting the first direct comparison of in situ observations of plasmas and magnetic fields in Earth's distant magnetotail with the results of a time‐dependent, global magnetohydrodynamic (MHD) simulation of the interaction of the solar wind with the magnetosphere. The magnetotail observations were taken with the Geotail spacecraft during the period 0300–0630 UT on October 27, 1992 at a position near the dawnside magnetopause at a downstream distance of about 81RE. During this period a dense, cold ion stream similar in density and speed to that expected for the magnetosheath plasmas was intermittently observed. When the cold ion stream was not present, the spacecraft was located in the northern magnetotail lobe. The dense, cold ion stream differed from that expected for the magnetosheath in theYandZcomponents of ion bulk flow and in theYcomponent of the magnetic field. These cold ion streams are associated with a magnetopause accommodation region positioned just outside the classical magnetopause, as identified by a well‐defined transition from magnetic fields typical of those found in the lobe to the lesser and more fluctuating fields in the magnetosheath. This accommodation region exhibits perturbations in plasma flows and magnetic fields that appear to be related to the complex topology of the magnetopause at these large downstream positions. Simultaneous observations of the solar wind ions and the interplanetary magnetic field (IMF) with the IMP 8 spacecraft upstream from Earth provided the driving input for a global MHD model. The solar wind ion flow was steady during this period, and the IMF exhibited a series of rotations from northward to duskward. The dynamics of the magnetotail were controlled by theYandZcomponents of the IMF. When thisBywas strongly positive, the magnetotail lobe appeared at the downstream Geotail position. Examination of the modeled plasma parameters in theY‐Zplane through the spacecraft position shows that thisByprovides a torque on the magnetotail about its central axis. The MHD model also accurately positions the spacecraft alternately in the magnetopause accommodation region and the magnetotail lobe as the IMF clock angle varied from northward to duskward, respectively. The temporal variations of modeled parameters, i.e., ion densities, temperatures, and bulk flow velocities and the magnetic field components, are directly compared with the Geotail measurements. This first comparison of the Geotail observations with the modeled plasma parameters and magnetic fields provides substantial encouragement that a global MHD model can provide a valid description of important aspects of the large‐scale topology and dynamics of the ma

ISSN:0148-0227    

DOI:10.1029/95JA00571

年代:1995

数据来源: WILEY