Showing papers on "Substorm published in 1985"
TL;DR: In this paper, the average and substorm conditions in the distant magnetotail were investigated using ISEE 3 magnetic field and plasma observations, and the results were in qualitative agreement with leaky magnetopause model of Pilipp and Morfill (1978).
Abstract: Both average and substorm conditions in the distant magnetotail were investigated using ISEE 3 magnetic field and plasma observations. The diameter of the tail, the strength of the lobe magnetic fields, and their dependence on downstream distance were all found to agree well with the flaring tail models of magnetopause geometry and pressure balance. The gradual filling of the lobes by mantle plasma reported in previous ISEE 3 studies were further investigated, and the results were found to be in qualitative agreement with leaky magnetopause model of Pilipp and Morfill (1978). The variations of plasma parameters with X + or -Y, and AE in the plasma sheet were examined. At all distances, the greatest tailward flow speeds were found to be directly proportional to the embedded southward B(z). THe magnitudes of tailward V(x) and southward B(z) are directly proportional to the level of substorm activity near the earth as measured by the AE index.
365 citations
TL;DR: In this paper, a model of purely collisionless, time-dependent, ion-dominated reconnection was proposed to describe the reconnection collapse of the initially thick plasma sheet in weakly magnetized, thin current sheets.
Abstract: In the current 'conceptual' model of magnetospheric substorms, the growth phase terminates and the expansion phase commences with the onset of rapid reconnection at a new, near-earth X-type neutral line. Physical concepts developed in the analysis of the collisionless tearing mode and the flow of collisionless plasma in weakly magnetized, thin current sheets are combined to construct a model of purely collisionless, time-dependent, ion-dominated reconnection. Formulated in the context of time-dependent magnetospheric convection, the model describes the reconnection collapse of the initially thick plasma sheet. In the nonlinear phase the reconnection rate grows explosively in time and saturates into a steady collisionless reconnection flow when the initial magnetic flux in the current sheet has reconnected; at saturation the reconnection rate is comparable to the maximum Petschek rate. The time scale and dynamics of the explosive reconnection model are broadly consistent with observations of substorm growth phase and expansion phase onset. For typical plasma sheet parameters the explosive reconnection electromotive force across the tail approaches 1 MV at saturation.
137 citations
TL;DR: In this paper, the March 22, 1979 substorm interval selected for analysis by the CDAW 6 has been investigated using a ground magnetic data inversion scheme which computes equivalent current patterns as well as real ionospheric and field aligned current distributions.
Abstract: The March 22, 1979 substorm interval selected for analysis by the CDAW 6 has been investigated using a ground magnetic data inversion scheme which computes equivalent current patterns as well as real ionospheric and field aligned current distributions. These have been computed at 5-minute increments during the two substorm intervals on this day. The computed results have been analyzed using a differential technique which permits us to isolate the current development which occurs during selected time intervals. We find that both DP 1 and DP 2 currents develop during the course of the substorm activity. The first substorm is characterized by the gradual enhancement of a DP 2 equivalent current system following the southward turning of the interplanetary magnetic field. The major magnetic disturbance associated with the expansion phase of the substorm is dominated by the development of a DP 1 current system. During the second interval which consists of several hours of magnetic disturbance, the DP 2 system grows to be much stronger and is the dominant contribution to much of the auroral magnetic activity. The peak of the activity during the second substorm interval appears to be composed of both strong DP 2 and DP 1 currents coexisting. Following the decay of the DP 1 system a strong DP 2 system continues to exist during the enhanced convection phase of the substorm.
134 citations
TL;DR: In this article, the authors focused on the 1054 UT March 22, 1979, substorm event, which had been selected for detailed examination in connection with the studies performed by the Coordinated Data Analysis Workshop (CDAW 6).
Abstract: The Coordinated Data Analysis Workshop (CDAW 6) has the primary objective to trace the flow of energy from the solar wind through the magnetosphere to its ultimate dissipation in the ionosphere. An essential role in this energy transfer is played by magnetospheric substorms, however, details are not yet completely understood. The International Magnetospheric Study (IMS) has provided an ideal data base for the study conducted by CDAW 6. The present investigation is concerned with the 1054 UT March 22, 1979, substorm event, which had been selected for detailed examination in connection with the studies performed by the CDAW 6. The observations of this substorm are discussed, taking into account solar wind conditions, ground magnetic activity on March 22, 1979, observations at synchronous orbit, observations in the near geomagnetic tail, and the onset of the 1054 UT expansion phase. Substorm development and magnetospheric dynamics are discussed on the basis of a synthesis of the observations.
117 citations
TL;DR: A survey of the relative frequency of encounters with slow-mode shocks observed by ISEE 3 in the distant geomagnetic tail is made in this paper, and the association of these shocks with the phase of substorm activity as evidenced by enhanced currents in the auroral ionosphere and enhanced energetic particle populations at geostationary orbit is documented.
Abstract: A survey is made of the relative frequency of encounters with slow-mode shocks observed by ISEE 3 in the distant geomagnetic tail. The association of these shocks with the phase of substorm activity as evidenced by enhanced currents in the auroral ionosphere and enhanced energetic-particle populations at geostationary orbit is also documented. During the 25 days between January 18 and February 11, 1983, 26 unambiguous examples of slow shocks were observed. Although a very strong association with the level of geomagnetic activity is found, shocks were observed during all phases of substorm activity including one during quiet conditions. Slow-mode shocks must therefore be a semipermanent feature of the bounding surfaces which separate lobe and plasma sheet particle populations in the distant geomagnetic tail.
115 citations
TL;DR: In this paper, the authors examined the energetic (>35 keV) ion bursts in the deep geomagnetic tail associated with the passage of 37 plasmoids using data from the energetic particle anisotropy spectrometer (EPAS) instrument on ISEE 3.
Abstract: Energetic (>35 keV) ion bursts in the deep geomagnetic tail associated with the passage of 37 plasmoids are examined using data from the energetic particle anisotropy spectrometer (EPAS) instrument on ISEE 3. These bursts can usually be divided into four distinct phases: (1) strongly tailward streaming ions observed in the lobe for a few minutes prior to plasmoid entry, commencing ∼25 min after geomagnetic substorm onset; (2) the plasmoid interval, when the energetic ions have a broader tailward angular distribution arising from convection with the plasmoid; (3) the “post-plasmoid” plasma sheet, where more strongly tailward streaming ions are observed in the plasma sheet on field lines disconnected from the earth at the substorm neutral line; and (4) a strongly tailward streaming ion population extending into the lobe for a few minutes after exit from the plasma sheet. We concentrate here on the streaming ion “boundary layers” observed in the lobe at the leading and trailing edges of these bursts. In a majority of these layers, a clear dawn-dusk gradient anisotropy and energy dispersion are evident at the leading edge, and a similar gradient anisotropy with “reverse” dispersion is evident at the trailing edge. It is shown however that the dispersion at onset is not consistent with simple time of flight from a near-earth neutral line or from a neutral line retreating tailward during substorm recovery. Instead, observations of 90° pitch angle ions with a time resolution of 16 s are used to infer that the ion onset is due to a layer of energetic ions expanding outward from the tail center plane and engulfing the spacecraft. At the trailing edge of the burst, this layer contracts back across the spacecraft toward the center plane. Mean expansion and contraction speeds are 94±74 km s−1 and 99±100 km s−1 respectively, with boundary layer thicknesses of ∼3 RE. From these observations, it is concluded that the expansion of the ion layer is caused predominantly by the ion layer being swept across the spacecraft by the arrival of the plasmoid in the deep tail, contributing ∼60 km s−1 to the expansion speed, rather than by a thickening of the region of lobe field lines disconnected at the substorm neutral line which expands at ∼35 km s−1. The energy dispersion at the leading edge can be reconciled with a near-earth neutral line in this case. Using this dispersion and the measured expansion speed of the layer, the electric field along the near-earth substorm neutral line is deduced. Values derived from layer expansions and contractions are both ∼0.4 mV m−1, equivalent to ∼110 kV across a ∼40-RE tail width.
99 citations
TL;DR: In this article, the authors used a Monte Carlo diffusion code to compute the transport of ions from the polar ionosphere to the inner magnetosphere during storm time conditions, where the effect of electrostatic turbulence assumed to be present during the substorm expansion phase was simulated by a process that accelerated the ions stochastically perpendicular to the magnetic field with a diffusion coefficient proportional to the rate of energization of the ions by the induced electric field.
Abstract: The transport of ions from the polar ionosphere to the inner magnetosphere during storm time conditions has been computed using a Monte Carlo diffusion code. The effect of the electrostatic turbulence assumed to be present during the substorm expansion phase was simulated by a process that accelerated the ions stochastically perpendicular to the magnetic field with a diffusion coefficient proportional to the rate of energization of the ions by the induced electric field. This diffusion process was continued as the ions were convected from the plasma sheet boundary layer to the double-spiral injection boundary. Inward of the injection boundary the ions were convected adiabatically. By using as input an O(+) flux of 2.8 x 10 to the 8th per sq cm per s (w greater than 10 eV) and an H(+) flux of 5.5 x 10 to the 8th per sq cm per s (w greater than .63 eV) the computed distribution functions of the ions in the ring current were found to be in good agreement, over a wide range in L (4-8), with measurements made with the ISEE 1 satellite during a storm. This O(+) flux and a large part of the H(+) flux appear to be consistent with the DE-1 and DE-2 satellite measurements of the polar ionospheric outflow during disturbed times.
80 citations
TL;DR: In this paper, the authors examined the evidence for the storage of solar wind energy in the magnetotail prior to substorm expansion phase onsets and determined the time delays between loading of energy into the magnetospheric system and subsequent unloading of this energy.
Abstract: The concept of the Coordinated Data Analysis Workshop (CDAW) grew out of the International Magnetospheric Study (IMS) program. According to this concept, data are to be pooled from a wide variety of spacecraft and ground-based sources for limited time intervals. These data are to provide the basis for the performance of very detailed correlative analyses, usually with fairly limited physical problems in mind. However, in the case of the CDAW 6 truly global goals are involved. The primary goal is to trace the flow of energy from the solar wind through the magnetosphere to its ultimate dissipation by substorm processes. The present investigation has the specific goal to examine the evidence for the storage of solar wind energy in the magnetotail prior to substorm expansion phase onsets. Of particular interest is the determination, in individual substorm cases, of the time delays between the loading of energy into the magnetospheric system and the subsequent unloading of this energy.
78 citations
TL;DR: In this paper, a color movie film of the substorm changes of the three-dimensional current system as well as the associated potential pattern is presented, on the basis of the space-time distribution of the key parameters, which can be referenced to in terms of the sub-storm phases when discussing other magnetospheric and ionospheric records.
Abstract: Using a recently developed numerical scheme combined with International Magnetospheric Study magnetometer data and the Rice University Ionospheric conductivity model as input, the global distribution of the key ionospheric parameters is estimated for the Coordinated Data Analysis Workshop (CDAW) 6 intervals. These outputs include ionospheric electric fields and currents, field-aligned currents and Joule heat production rate at high latitudes, and are compiled in the form of a color movie film, which demonstrates dynamics of substorm changes of the three-dimensional current system as well as of the associated potential pattern. The present paper gives, on the basis of the space-time distribution of the key parameters, the substorm time frame that can be referenced to in terms of the substorm phases when discussing some other magnetospheric and ionospheric records. The distinction between ''substorm expansion'' and ''enhanced convection'' current systems is presented on the basis of the conventional equivalent current and potential patterns and ''true'' ionospheric currents. Although the auroral electrojets flow rather contiguously throughout the dark sector, there are several separate source regions of Joule heating from the electrojet currents. This indicates that the relative importance of the ionospheric conductivity and the electric field in the ionospheric currents varies considerably depending uponmore » latitude and local time. A possible difference in the generation mechanisms of isolated and continuous substorm activity is also discussed to some extent in the light of the two CDAW 6 intervals.« less
75 citations
TL;DR: In this article, the authors studied the behavior of polar cusp auroras and the local magnetic field related to changes in the interplanetary magnetic field (IMF) and irregularities in the solar wind plasma.
Abstract: Photometric observations from two stations on Svalbard, Norway, have been used to map the location and dynamics of polar cusp auroras. Cases showing the behavior of cusp auroras and the local magnetic field related to changes in the interplanetary magnetic field (IMF) and irregularities in the solar wind plasma are presented. Dynamical phenomena with different time scales are studied. Southward and northward expansions of the midday sector of the auroral oval are discussed in relation to southward and northward turnings of the magnetosheath magnetic field and geomagnetic substorm activity. Examples showing no direct relationship to substorm activity are presented. Intensifications and rapid motions of discrete auroral structures in the cusp region are shown to be associated with local Pi-type magnetic pulsations, each event lasting a few minutes. These small-scale dynamical phenomena are discussed in relation to the concept of impulsive penetration of plasma irregularities across the dayside magnetopause, from the magnetosheath to the polar cusp region of the magnetosphere.
75 citations
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TL;DR: In this paper, the position of the dayside cusp is largely controlled by substorm processes internal to the magnetosphere rather than by direct merging and erosion processes with the interplanetary field.
Abstract: The position of dayside aurora (as measured from South Pole station in 1981) is compared to the interplanetary magnetic field Bz component and to the AE index. The results support our earlier work showing a close relationship with AE and little correlation with Bz. Two recent papers have presented data that purport to contradict this interpretation, claiming a dominant Bz correlation. A reanalysis of the data sets used in those papers does not support Bz dependence, and in fact reaffirms a close dependence on AE. We conclude that the position of the dayside cusp is largely controlled by substorm processes internal to the magnetosphere rather than by direct merging and erosion processes with the interplanetary field.
TL;DR: A time-dependent, three-dimensional, multi-ion model of the ionospheric F region at 120-800 km altitude is presented in this paper, where account is taken of field-aligned diffusion, cross-field electrodynamic drifts in equatorial and high latitude regions, interhemispheric flow, thermospheric winds, polar wind escape, energy dependent chemical reactions and neutral composition changes.
Abstract: A time-dependent, three-dimensional, multi-ion model of the ionospheric F region at 120-800 km altitude is presented. Account is taken of field-aligned diffusion, cross-field electrodynamic drifts in equatorial and high latitude regions, interhemispheric flow, thermospheric winds, polar wind escape, energy-dependent chemical reactions and neutral composition changes. Attention is also given to the effects of ion production by solar EUV radiation and auroral precipitation, thermal conduction, diffusion-thermal heat flow, local heating and cooling processes, offsets between the geomagnetic and geographic poles, and bending of field lines near the magnetic equator. The model incorporates all phenomena described by previous models and can be applied to tracing magnetic storm and substorm disturbances from high to low latitudes on a global scale. Sample results are provided for ionospheric features during a June solstice, the solar maximum and in a period of low geomagnetic activity. The model will eventually be used to study coupled ionosphere-thermosphere activity.
TL;DR: In this article, the UCL 3-dimensional time-dependent thermospheric model with atomic and molecular components is used to study composition changes in the neutral gas at F-layer heights produced by a severe magnetic storm.
TL;DR: In this article, the authors show that the growth and expansion phase of this substorm occurred while the outer magnetosphere was composed dominantly of solar wind (H+ and He++) plasmas.
Abstract: Extensive ground-based arrays of magnetometers and numerous satellite platforms in the outer magnetosphere have established that two separate substorm expansion onsets occurred on March 22, 1979. The first of these occurred at 1055 UT and is demonstrated to be localized in the 0200–0300 LT sector. Concurrent plasma sheet ion composition measurements are used to show that the growth and expansion phase of this substorm occurred while the outer magnetosphere was composed dominantly of solar wind (H+ and He++) plasmas. The 1055 UT substorm greatly perturbed and altered the ion composition of the plasma in the outer magnetosphere such that the second substorm expansion onset (1436 UT) occurred while the outer magnetospheric plasmas were dominantly of ionospheric (O+) origin. The 1436 UT substorm is shown to have a component of the westward electrojet localized further westward in local time relative to the first substorm. These results are a consistent, well-documented example of the possible important role of heavy ions in the localization and initiation of plasma sheet instabilities during substorms.
TL;DR: In this paper, a series of images of the geocorona in scattered solar Ly-alpha emissions is used to obtain a best-fit spherical model of atomic hydrogen densities in the earth's exosphere; a Chamberlain model provides an adequate fit to radial distances of 4.5 earth-radii, beyond which an exponential fit is used.
TL;DR: In this article, the occurrence probability of Pc5 waves with azimuthal polarization (A-class) is small at GOES 3 (4.7°N in magnetic latitude), as compared with that at GoES 2 (9.3-11.4°N).
Abstract: Statistical analyses were made to clarify the latitude dependence of occurrence and other characteristics of Pc5 waves near L=6.6, using magnetic field data obtained with GOES 2 and 3 during 1978-1984. It is found that the occurrence probability of Pc5 waves with azimuthal polarization (A-class) is small at GOES 3 (4.7°N in magnetic latitude), as compared with that at GOES 2 (9.3-11.4°N). Most of A-class waves observed at GOES 2 are simultaneously detected near the subsatellite point of GOES 2. A typical amplitude ratio of ground Pc5 pulsations to A-class Pc5 waves at synchronous orbit is about 5 (50nT/10nT). In contrast to the A-class waves, compressional waves with radial polarization, observed mostly in the afternoon around 15h LT during magnetically disturbed conditions, have a larger occurrence probability at GOES 3 than at GOES 2. Amplitudes of the compressional Pc5 are larger at GOES 3 than at GOES 2, when the wave simultaneously occurs at both the satellites. The difference of Pc5 characteristics at GOES 2 and 3 suggests that the compressional Pc5 wave is confined to a latitudinally narrow region near the magnetic equator. The compressional Pc5 can be further divided into two types. One is of long duration and mostly occurs around noon in the recovery phase of magnetic storm. The other is of short duration and is observed on the afternoon-dusk side. The occurrence of short-duration events is well correlated with the individual substorm onset. A systematic delay of Pc5 occurrence behind the substorm onset is statistically obtained as a function of the local time of the satellite position. This delay suggests that the source region of the short-duration compressional Pc5 drifts westward with a speed of the gradient drift of 50-60keV protons.
TL;DR: A small auroral substorm is investigated with auroral imaging photometers carried on the spacecraft Dynamics Explorer 1 as mentioned in this paper, which is interpreted as the signature of electron acceleration along magnetic field lines threading the boundary layer of the plasma sheet in the magnetotail.
Abstract: A small auroral substorm is investigated with auroral imaging photometers carried on the spacecraft Dynamics Explorer 1. Initial brightening along the auroral oval and the subsequent westward and poleward motions of intense, localized emission regions are associated with auroral surges. Following substorm onset, another region of less intense emissions is observed to develop at lower latitudes and adjacent to the bright region near local midnight. This second region expands towards the east. The bright zone of auroral emissions associated with the surges is interpreted as the signature of electron acceleration along magnetic field lines threading the boundary layer of the plasma sheet in the magnetotail. The more diffuse, less intense region is identified with eastward-drifting electrons injected into the plasma sheet and ring current following substorm onset. No rapid poleward motion of the discrete aurora is detected during substorm recovery.
TL;DR: In this article, the authors used the basic methodology of examining substorms which occurred during intervals when the interplanetary magnetic field was steady to remove the obvious ambiguity in the interpretation of dayside auroral dynamics.
Abstract: In a study of the dynamics of dayside aurora, Horwitz and Akasofu (1977) adopted the basic methodology of examining substorms which occurred during intervals when the interplanetary magnetic field was steady. By using this approach, it was possible to remove the obvious ambiguity in the interpretation of dayside auroral dynamics which arises if interplanetary medium variations are not excluded. It is believed that for an understanding of the natural, internal instability behavior of the magnetosphere it will be necessary to employ the same methodology in many 'substorm' studies. The present investigation has the objective to present some examples of substorms occurring during intervals of steady interplanetary magnetic field. Subsequently, the approximate time scales of expansion and recovery for such substorms are determined. The obtained results are compared with a substorm model proposed by Hill and Reiff (1980).
TL;DR: In this article, the ISEE 1 and 2 observations in the plasma sheet near 15 R/sub E/ radial distance for the 1054 substorm on March 22, 1979 were reported.
Abstract: The paper reports ISEE 1 and 2 plasma observations in the plasma sheet near 15 R/sub E/ radial distance for the 1054 substorm on March 22, 1979. Noteworthy features are plasma sheet thinning and the brief appearance of rapid tailward flows after substorm onset, strong earthward flows (superimposed on dawnward convection) at the beginning of plasma sheet expansion and a quieting of all plasma properties at substorm recovery. Much of the time, however, plasma flows are small and switch directions. In particular, no strong flows are observed during neutral sheet crossings. Two-dimensional velocity distribution functions show a complex behavior, with several plasma components often coexisting and at times streaming in opposite directions. The interpretation of the observations in terms of the formation of a neutral line earthward of and close to ISEE at substorm onset is tempting, but three-dimensional spatial effects as well as rapid temporal changes have to be invoked to obtain consistency.
TL;DR: In this paper, the authors examined temporal and spatial variations of the distance between the electron precipitation boundaries across the polar cap: the Defense Meteorological Satellite Program satellite data, within a few hours of the dawn-dusk and noon-midnight meridians, are used.
Abstract: We examine temporal and spatial variations of the distance between the electron precipitation boundaries across the polar cap: the Defense Meteorological Satellite Program satellite data, within a few hours of the dawn-dusk and noon-midnight meridians, are used. It is found that changes in the distance between the precipitation boundaries across the polar region in both the dawn-dusk and noon-midnight sectors tend to follow grossly changes of the AE index. On the basis of the simultaneous ISEE 3 data, it is found that the increase of the distance begins at about the time when the southward turning of the interplanetary magnetic field reaches the magnetosphere, but prior to the corresponding increase of the AE index. The period of the maximum distance approximately coincides with the period of the maximum AE value. The decrease of the distance begins at about the same time as the substorm activity begins to subside, but it continues well after the AE index has reduced to very small values. These findings are discussed in terms of changes of the total open magnetic flux by assuming that the precipitation boundaries coincide approximately with the boundaries of the open field line region.
TL;DR: In this article, the behavior of energetic O(+) ions in the earth's plasma sheet, taking into account observations by the ISEE 1 spacecraft during a magnetically active time interval encompassing two major substorms on March 22, 1979, was investigated.
Abstract: The present investigation has the objective to report on the behavior of energetic (approximately 130 keV) O(+) ions in the earth's plasma sheet, taking into account observations by the ISEE 1 spacecraft during a magnetically active time interval encompassing two major substorms on March 22, 1979. Attention is also given to suprathermal H(+) and He(++) ions. ISEE 1 plasma sheet observations of the proton and alpha particle phase space densities as a function of energy per charge during the time interval 0933-1000 UT on March 22, 1979 are considered along with the proton phase space density versus energy in the energy interval approximately 10 to 70 keV for the selected time periods 0933-1000 UT (presubstorm) and 1230-1243 UT (recovery phase) during the 1055 substorm on March 22, 1979. A table listing the proton energy density for presubstorm and recovery periods is also provided.
TL;DR: In this paper, a detailed case study of the relation between geomagnetic substorm activity and the ion composition in the plasma sheet is presented, showing that a decrease in solar wind ion flow at ISEE 1 and the subsequent increase in the flow of ionospheric ions is interpreted as evidence of a fundamental change in the particle flow pattern following the start of substorm activities.
Abstract: One of the potentially most useful aspects of mass spectroscopy in magnetospheric research is related to the ability to distinguish between ions of solar wind and terrestrial origins Knowing the source of the plasmas in various regions and under various geomagnetic conditions is crucial in improving the understanding of particle flows in the magnetosphere Sharp et al (1982) found that the plasma sheet ions in the 01- to 16-keV/e range, were predominantly of solar wind origin during low geomagnetic activity, while there was a large terrestrial component during active times This paper presents a set of data from March 22, 1979 The reported investigation represents the first detailed case study of the relation between geomagnetic substorm activity and the ion composition in the plasma sheet A decrease in solar wind ion flow at ISEE 1 and the subsequent increase in the flow of ionospheric ions is interpreted as evidence of a fundamental change in the particle flow pattern following the start of substorm activity
TL;DR: In this article, magnetic observations made by the midlatitude Air Force Geophysics Laboratory (AFGL) Magnetometer Network, the GOES 2 and 3 synchronous satellites, and the near-geosynchronous P78-2 Spacecraft Charging at High Altitudes (SCATHA) satellite are used to detect a clear substorm onset.
Abstract: It is pointed out that magnetospheric substorms have been referred to as the 'fundamental instability' of the magnetosphere which results from coupling between the solar wind and the earth's magnetic and plasma environment. The present paper is mainly concerned with magnetic observations made by the midlatitude Air Force Geophysics Laboratory (AFGL) Magnetometer Network, the GOES 2 and 3 synchronous satellites, and the near-geosynchronous P78-2 Spacecraft Charging at High Altitudes (SCATHA) satellite. Ground-based Pi 2 and magnetic bay observations are used to detect a clear substorm onset. An idealized model of the substorm current system is utilized to relate the observations to the spatial location and temporal development of the substorm disturbance near synchronous orbit.
TL;DR: In this paper, a series of 12 successive images of the northern auroral oval obtained by the University of Iowa's auroral imager on DE 1 during a substorm on November 8, 1981 were discussed.
Abstract: This paper discusses a recently published series of 12 successive images of the northern auroral oval obtained by the University of Iowa's auroral imager on DE 1 during a substorm on November 8, 1981. The images showed a sudden (within <12 min) thickening and poleward expansion of the midnight sector of the oval that occurred at the “maximum epoch” of the substorm, i.e., as auroral zone bays reached their peaks and began to subside about 90 min after the expansive phase onset. We propose that this was the auroral manifestation of the poleward leap of the auroral electrojet that has been reported on many occasions to coincide with thickening of the plasma sheet in the outer magnetotail (beyond ∼15 RE) late in a substorm. The plasma sheet thickening is thought to be associated with a tailward retreat of the substorm magnetic neutral line. We note evidence that the neutral line's tailward retreat influences auroral currents and particle precipitation even at low auroral latitudes. Modifications of the present auroral substorm model are suggested.
TL;DR: In this article, the authors studied the dynamic evolution of the geomagnetic tail in response to a dawn-to-dusk electric field E/sub y/ applied at the high-latitude boundary by means of a two-dimensional nonlinear MHD code.
Abstract: We studied the dynamic evolution of the geomagnetic tail in response to a dawn-to-dusk electric field E/sub y/ applied at the high-latitude boundary by means of a two-dimensional nonlinear MHD code. We found that the electric field propagates inward through the lobes toward the plasma sheet. Only a small fraction, however, penetrates into the interior of the plasma sheet, and eventually an almost linear decrease from the boundary to the equatorial plane is assumed. There was no tendency to evolve toward a steady or nearly steady state which would require E/sub y/ roughly-equal const in a larger area. The resulting time-dependent changes are consistent with quasi-static analytic models of Schindler and Birn (1982) and Birn and Schindler (1983) and with observed substorm growth phase effects in the near tail. If the external electric field E/sub y/ is constant or varies only slightly along the tail, the convection speed within the plasma sheet remains small. A strongly nonuniform external electric field E/sub y/ leads to similar spatial and temporal variation of E/sub y/. It does, however, produce a locally much stronger thinning which favors the onset of a tail instability such as the tearing mode. It can also produce stronger flowsmore » primarily parallel to the magnetic field.« less
01 Jul 1985
TL;DR: The relationship between the Pi 2 polarization pattern observed at a mid-latitude longitudinal chain of stations and synchronous orbit magnetic activity is explored in this article, where the pattern center is also observed to separate regions of dipolarization and more taillike magnetic signatures.
Abstract: Pi 2 pulsations are often used to indicate the start of the substorm expansion phase. In this report the relationship between the Pi 2 polarization pattern observed at a mid-latitude longitudinal chain of stations and synchronous orbit magnetic activity is explored. Eighteen single- and multiple-substorm-onset intervals, consisting of 55 Pi 2's, are examined using data from the Air Force Geophysics Laboratory Magnetometer Network and the synchronous orbit satellites GOES 2 and 3. Magnetic fluctuations at synchronous orbit are found to be enhanced when the satellite is near the Pi 2 pattern center, defined as the longitude where the major axis of the polarization ellipse is along the H axis. The pattern center is also observed to separate regions of dipolarization and more taillike magnetic signatures. East of the pattern center the field inclination becomes larger, i.e., more dipolelike, while west of the center the field inclination becomes smaller, i.e., more taillike. Although regions of substorm activation, as determined by the Pi 2 pattern, move between onsets during multiple-onset events, no consistent azimuthal motion was found.
TL;DR: In this paper, the substorm associated Pi 2's indicate that these pulsations may be caused by the transient reflection of an Alfven wave from the auroral ionosphere, which is not connected with auroral forms such as the westward traveling surge but are more likely connected with the initial brightening of the arc associated with the breakup.
Abstract: Ground-based measurements of the phase velocities, group velocities, and rise times of the magnetic fields of high-latitude (60°–68°N corrected geomagnetic latitude), substorm associated Pi 2's indicate that these pulsations may be caused by the transient reflection of an Alfven wave from the auroral ionosphere. Individual pulsation wave packets can be very short, typically one cycle, with a duration of 100–150 s. The phase velocities are eastward to the east of the region of the onset of the field-aligned currents, and westward to the west of the region of the onset. The group velocities follow a pattern similar to that of the phase velocities but tend to have values somewhat lower than the phase velocities. The longitudinal phase velocities are very high, typically 20 km/s or greater, suggesting that the Pi 2's are not connected with auroral forms such as the westward traveling surge but are more likely connected with the initial brightening of the arc associated with the breakup. Consequently, the longitudinal expansion of the field-aligned currents and Alfven waves causing the Pi 2's may be a direct result of the mechanism leading to the onset of the substorm expansive phase in the magnetotail.
TL;DR: In this article, simultaneous observations of energetic particle measurements from the geosynchronous satellite 1982-019 and magnetic field, electron plasma, and energetic proton and electron measurements obtained with ISEE 3 in the deep tail are presented.
Abstract: Simultaneous observations of energetic particle measurements from the geosynchronous satellite 1982-019 and magnetic field, electron plasma, and energetic proton and electron measurements obtained with ISEE 3 in the deep tail are presented. The data are supplemented by ground magnetograms. A substorm occurred on March 22, 1983, close to 0300 UT as identified in the ground magnetograms and by a particle injection at geosynchronous orbit. About 10 min later, ISEE 3 observed (at a distance of approximately 130 RE in the deep tail) magnetic field, plasma, and energetic particle signatures consistent with the passage of a plasmoid. After the passage of the plasmoid the satellite enters shortly into a lobelike environment, in which an energetic proton beam is observed. High-resolution magnetic field data are indicative of small-scale structures in the postplasmoid plasma sheet. From the plasma sheet flow speed during the plasmoid's passage it is concluded that the 0300 UT substorm is responsible for its origin. This allows an approximate timing of the plasmoid release at a near-earth neutral line and of the plasma sheet recovery after substorm onset, and it indicates a close relationship between processes in the near-earth plasma sheet and the deep tail during substorms.
TL;DR: In this article, the mid-latitude Pi 2 pulsation signatures observed using the Air Force Geophysics Laboratory Magnetometer Network during the March 22, 1979, Sixth Coordinated Data Analysis Workshop interval were compared to the magnetic variations observed at geosynchronous orbit and to ionospheric current distributions derived from high latitude magnetometer data and an ionosphere conductivity model.
Abstract: We relate the mid-latitude Pi 2 pulsation signatures observed using the Air Force Geophysics Laboratory Magnetometer Network during the March 22, 1979, Sixth Coordinated Data Analysis Workshop interval to the magnetic variations observed at geosynchronous orbit and to ionospheric current distributions derived from high latitude magnetometer data and an ionospheric conductivity model. We are able to show that at least three separate substorm onsets or intensifications occurred around 1100 UT, specifically at 1053:50, 1104:10 and 1110:30 UT. We contrast an earlier onset which occurred at 0614 UT during a geomagnetically quiet interval to these more disturbed events. In both cases the Pi 2 polarization pattern locates the region of strong westward electrojet. Changes in the Pi 2 polarization during a single event appear to accompany small shifts in the electrojet position. These events do not obey the Pi 2 period-electrojet latitude relationship obtained by Kuwashima and Saito (1981).