Showing papers on "Substorm published in 1997"

Braking of high‐speed flows in the near‐Earth tail

Abstract: We have studied possible braking mechanisms of high-speed ion flows in the near-Earth central plasma sheet for radial distances between 9 and 19 Earth Radii (RE) on the basis of observations made by the AMPTE/IRM satellite. Flows with velocities in excess of 400 km/s are almost always Earthward for this range, indicating that the source of the flows is beyond 19 RE. Though the occurrence rate of the high-speed flows substantially decreases when the satellite comes closer to the Earth, high-speed flows with velocities higher than 600 km/s are still observed. We suggest that the high-speed flows are stopped at a clear boundary between the regions of dipolar field and tail-like field in the plasma sheet. The boundary corresponds to the inner edge of the neutral sheet. The average jump of the magnetic field at the boundary, which is estimated from the observations by assuming a pressure balance, is 6.7 nT. The inertia current caused by the braking of the flow and the current caused by pileup of the magnetic flux at the stopping point are quantitatively estimated and discussed in relation to the formation of the substorm current wedge.

Characteristic plasma properties during dispersionless substorm injections at geosynchronous orbit

Abstract: The substorm-associated behavior of the thermal plasma (30eV < E < 40keV) in the plasma sheet is examined by means of a superposed epoch analysis, using a full year of data from a spacecraft in geosynchronous orbit. The zero epoch time is taken to be substorm onset as indicated by a dispersionless energetic particle injection observed on the same spacecraft. Five classes of injection events are found to be well ordered by their average local times. These range from pure ion injections ∼3 hours prior to local midnight, to ion injections followed a few minutes later by an electron injection ∼2 hours before midnight, to simultaneous ion and electron injections close to midnight, to electron injections that are followed by an ion injection ∼1 hour postmidnight, and finally to pure electron injections ∼2 hours postmidnight. The thermal electrons show a significant increase in temperature (from ∼1 to ∼2keV) and pressure at substorm onset, while the density and the thermal ion signatures (below ∼30keV) are typically weak and may even vary with local time. However, energetic ions (above ∼30keV), which contribute significantly to the total ion pressure, show clear flux enhancements, leading to a rise in the total temperature from ∼10 to ∼16keV. Preexisting perpendicular anisotropies in the thermal electrons are reduced during the substorm growth phase but become enhanced again after onset, sometimes after a brief period of parallel anisotropy. Similar anisotropy signatures are found for the thermal ions, although somewhat less pronounced.

Coordinated observations demonstrating external substorm triggering

Abstract: To understand the magnetospheric substorm, it is necessary to determine whether its onset is externally triggered by the interplanetary magnetic field (IMF). We analyze the relationship between the IMF and the onset of classical substorms with well-defined onset times. A classical substorm is one that has auroral brightening and electrojet formation at onset, followed by poleward expansion of the region of bright aurora. Substorms meeting these criteria are identified using Canadian Auroral Network for the OPEN Program United Study ground photometer data. We find that a clear IMF trigger (a northward turning or a reduction in the magnitude of the y component) can be identified for 14 of the 20 substorms used in our study. All but one of the identified triggers are northward turnings. We develop a rigorous set of criteria that represents these triggers. By applying the criteria to a large set of IMF data, we find that it is essentially impossible for the observed association between triggers and substorms to happen by chance. This demonstrates that substorm triggering is a real phenomenon and not the result of the requirement that the IMF be southward before but not after a substorm. We also find that spatial structure in the plane perpendicular to the Earth-Sun line critically affects whether or not a trigger is observed from a particular IMF monitor; the probability of seeing a trigger for the substorms in our study is 89% for monitors that are < 30 RE from the Earth-Sun line but only 50% for monitors 30 RE to 56.7 RE from the Earth-Sun line. Thus a well-defined IMF trigger is associated with most of substorms considered here, and the probability of trigger identification is a strong function of IMF monitor distance from the Earth-Sun line. Given this limitation of trigger identification due to spatial structure, our observations imply that a large majority of classical substorms are triggered by the IMF. We also obtain estimates of ∼9 min for the mean time delay between magnetopause contact of an IMF trigger and substorm onset and ∼64–72 min for the median growth-phase period of southward IMF that precedes triggered classical substorms.

Magnetotail flow bursts: Association to global magnetospheric circulation, relationship to ionospheric activity and direct evidence for localization

Abstract: A series of bursty bulk flow events (BBFs) were observed by GEOTAIL and WIND in the geomagnetotail. IMP8 at the solar wind showed significant energy coupling into the magnetosphere, while the UVI instrument of POALR evidenced significant energy transfer to the ionosphere during two substorms. There was good correlation between BBFs and ionospheric activity observed by UVI even when ground magnetic signatures were absent, suggesting that low ionospheric conductivity at the active sector may be responsible for this observation. During the second substorm no significant flux transport was evidenced past WIND in stark contrast to GEOTAIL and despite the small intersatellite separation ((3.54, 2.88, -0.06) Re). Throughout the intervals studied there were significant differences in the individual flow bursts at the two satellites, even during longitudinally extended ionospheric activations. We conclude that the half-scale-size of transport bearing flow bursts is less than 3 Re.

Substorm ion injections: Geosynchronous observations and test particle orbits in three-dimensional dynamic MHD fields

Abstract: We investigate particle acceleration and the flux increases associated with substorm particle injections using geosynchronous observations and test proton orbits in the dynamic fields of a three-dimensional MHD simulation of neutral line formation and dipolarization in the magnetotail. The energetic particle flux changes obtained from the test particle orbits agree well with observations that demonstrate rapid ion flux increases at energies above {approximately}20keV but little change at lower energies. The {open_quotes}injection region{close_quotes} inferred from the test particles not only has a sharp earthward boundary (the usual injection boundary) but also a sharp but ragged tailward boundary. The earthward portion of enhanced ion flux can be traced to the enhanced cross-tail electric field associated with the collapse and dipolarization of the inner tail, whereas the tailward edge is closely associated with the near-Earth x-type neutral line. Because of the rapid earthward motion of accelerated ions away from the neutral line, this boundary is displaced earthward to where the energetic ions become more adiabatic in the stronger dipolar field. Lower-energy ions are not affected by cross-tail acceleration in the strong E{sub y} fields because their earthward {bold E}{times}{bold B} drift dominates the cross-tail drift, except very close to the neutral line.{copyright} 1997more » American Geophysical Union« less

Remote determination of auroral energy characteristics during substorm activity

Abstract: Ultraviolet auroral images from the Ultraviolet Imager (UVI) onboard the POLAR satellite can be used as quantitative remote diagnostics of the auroral regions, yielding estimates of incident energy characteristics, compositional changes, and other higher order data products. Here incident energy estimates derived from UVI are compared with in situ measurements of the same parameters from an overflight by the DMSP F12 satellite coincident with the UVI image times during substorm activity occurring on May 19, 1996. This event was simultaneously observed by WIND, GEOTAIL, INTERBALL, DMSP and NOAA spacecraft as well as by POLAR.

Formation of density troughs embedded in the outer plasmasphere by subauroral ion drift events

Abstract: A dynamic global core plasma model (DGCPM) is used to investigate the effects of subauroral ion drift (SAID) events on the formation of trough density profiles in the outer plasmasphere during periods of high magnetic activity. The DGCPM includes the influences of convection on the changing flux tube volumes, as well as daytime refilling and nighttime draining of plasma, to calculate the plasma tube contents and equatorial plasma density distribution versus time throughout the magnetosphere. SAIDs are regions of latitudinally narrow westward flow of plasma equatorward of the auroral zone. We present DGCPM results for various presumed SAID locations and durations relative to enhanced substorm convection onset and decay, to parametrically elicit the formation of plasmaspheric density trough structures resulting from SAID effects. It is found that imposing a SAID event in the dusk-evening sector for 30 min leads to the formation of a narrow (less than 1 RE near the equatorial plane) embedded plasma density trough in the dusk bulge region. The modeled plasmasphere density profiles with troughs generally resemble plasmasphere density profiles observed from DE 1 measurements.

A quantitative assessment of energy storage and release in the Earth's magnetotail

Abstract: Magnetospheric substorms represent a global interaction between the solar wind, the magnetosphere, and the ionosphere. Energy extracted from the solar wind is episodically stored in the magnetosphere, with a large fraction of this energy being in the form of excess magnetic flux in the magnetotail lobes. The stored energy is periodically dissipated in an explosive instability that occurs in the near-Earth plasma sheet at substorm expansive phase onset. Methods are discussed to estimate the amount of energy transferred from the solar wind to the magnetosphere during substorm growth phases. Observational and modeling constraints are then used to assess quantitatively the total amount of energy stored in the plasma sheet and magnetotail lobes. Possible energy sources to drive substorm evolution are evaluated in one particularly well-observed case (May 3, 1986). By examining the major avenues of energy dissipation, the energy that is released in the form of plasma sheet heating, ionospheric Joule heating, etc. is assessed during the substorm. It is found that stored tail-lobe energy is sufficient (by a factor of 3 or more) to drive observed substorm dissipation processes. On the other hand, energy in the closed field lines of the plasma sheet is insufficient (by a substantial margin) to supply the substorm energy.

Effects of intense storms and substorms on the equatorial ionosphere/thermosphere system in the American sector from ground‐based and satellite data

Abstract: Equatorial ionospheric responses to magnetospheric storm/substorm-associated electric fields are investigated for a few intense events of the equinoctial months of solar maximum years 1978–1979. All the magnetic storms considered here are the result of the transit at Earth of interplanetary magnetic clouds. The interplanetary magnetic field data Bz from the ISEE 3 satellite, the auroral electrojet activity index AE, and the ring current index Dst are used as indicators of the disturbed magnetospheric conditions, and the ionospheric response features are analyzed using the F layer critical parameters h′F, h′F3, hpF2, and ƒ0F2. Focus is given to identify, when a large number of sequential substorms occurs, (1) the responses to prompt penetration electric field (from individual substorm events) as different from the delayed effect from the disturbance dynamo electric field and (2) the verification of local time dependences of the disturbance electric field polarity as predicted from the existing theoretical models. We have found evidence of near-midnight polarity reversal of prompt penetration disturbance electric field during the course of a developing substorm. Evidence is provided also on the near-midnight polarity reversal for the disturbance dynamo electric field. The prereversal enhancement electric field at sunset, produced by the F layer dynamo, is found to undergo drastic day-to-day variations in the course of a disturbed interval. However, the competing influences of the prompt versus delayed electric fields after a series of substorms could result at times in partial, or even complete, cancellation of the effects, so that the prereversal enhancement in the vertical drift could appear unaffected by the disturbances. There are indications that the disturbance dynamo electric field effects on the equatorial ionosphere last for one more day past the end of the substorm recovery.

Ring current modeling in a realistic magnetic field configuration

Abstract: A 3-dimensional kinetic model has been developed to study the dynamics of the storm time ring current in a dipole magnetic field. In this paper, the ring current model is extended to include a realistic, time-varying magnetic field model. The magnetic field is expressed as the cross product of the gradients of two Euler potentials and the bounce-averaged particle drifts are calculated in the Euler potential coordinates. A dipolarization event is modeled by collapsing a tail-like magnetosphere to a dipole-like configuration. Our model is able to simulate the sudden enhancements in the ring current ion fluxes and the corresponding ionospheric precipitation during the substorm expansion.

Geotail observations of energetic ion species and magnetic field in plasmoid‐like structures in the course of an isolated substorm event

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

MHD drift ballooning instability near the inner edge of the near-Earth plasma sheet and its application to substorm onset

Abstract: The MHD drift ballooning mode (DBM) instability near the inner edge of the near-Earth plasma sheet is studied by using both the one-fluid generalized progressing wave expansion method and the two-fluid approach. It is found that in the frame of reference at rest relative to the bulk plasma the DBM may become a purely growing mode in two distinct circumstances, which, for convenience, are called the DBM1 and DBM2, respectively. The β threshold for the DBM1 is identical with that derived by Ohtani and Tamao [1993] and Southwood and Kivelson [1987], while the criterion of the DBM2 covers that of Miura et al. [1989]. Comparisons of the theory with GEOS 2 data show that the DBM2 is more easily excited in the late substorm growth phase. There is considerable evidence that the DBM is generated at expansion onsets. The characteristic features of magnetic field dipolarization can be interpreted in terms of the development of the DBM. The extremely thin current sheet cases should be studied with approaches other than those used in this work.

Magnetospheric source region of discrete auroras inferred from their relationship with isotropy boundaries of energetic particles

Abstract: According to observations, the discrete auro- ral arcs can sometimes be found, either deep inside the auroral oval or at the poleward border of the wide (so- called double) auroral oval, which map to very diAerent regions of the magnetotail. To find common physical conditions for the auroral-arc generation in these magnetotail regions, we study the spatial relationship between the diAuse and discrete auroras and the isotropic boundaries (IBs) of the precipitating energetic particles which can be used to characterise locally the equatorial magnetic field in the tail. From comparison of ground observation of auroral forms with meridional profiles of particle flux measured simultaneously by the low-altitude NOAA satellites above the ground obser- vation region, we found that (1) discrete auroral arcs are always situated polewards from (or very close to) the IB of >30-keV electrons, whereas (2) the IB of the >30- keV protons is often seen inside the diAuse aurora. These relationships hold true for both quiet and active (substorm) conditions in the premidnight-nightside (18- 01-h) MLT sector considered. In some events the auroral arcs occupy a wide latitudinal range. The most equatorial of these arcs was found at the poleward edge of the diAuse auroras (but anyway in the vicinity of the electron IB), the most poleward arcs were simultaneous- ly observed on the closed field lines near the polar-cap boundary. These observations disagree with the notion that the discrete aurora originate exclusively in the near- Earth portion of plasma sheet or exclusively on the PSBL field lines. Result (1) may imply a fundamental feature of auroral-arc formation: they originate in the current-sheet regions having very curved and tailward- stretched magnetic field lines.

HF-radar observations of the dayside magnetic merging rate: A Geospace Environment Modeling boundary layer campaign study

Abstract: Goose Bay HF-radar data have been used to determine the dayside reconnection electric field which transports energy from the solar wind into the Earth's magnetosphere and ionosphere. The speed of the ionospheric plasma flow perpendicular to the open/closed boundary is determined in the rest frame of the boundary along each of the 16 beam directions of the HF radar. The observations were made during one of the Geospace Environment Modeling program's boundary layer campaigns. The period from 1200 to 1600 UT on March 29, 1992, was one of generally southward interplanetary magnetic field (IMF). The y component of the IMF was negative for most of the time. Despite the generally steady IMF conditions, the merging rate observed by the radar shows a great deal of temporal structure. The radar observations have been compared with the results from the assimilative mapping of ionospheric electrodynamics (AMIE) procedure. Initially, the merging inferred from the radar observations accounts for a significant portion of the total polar cap potential drop, suggesting that a majority of the potential drop was generated within the radar field of view and must therefore be due to magnetic merging at the magnetopause. At the end of the period, however, the potential drop derived from the radar measurements is distinctly less than that derived from the AMIE procedure. At that time, however, satellite and ground magnetometer data show that a substorm was in progress, and there is substantial evidence for a strong nightside contribution to the polar cap potential drop. An additional feature that appears in this data set is that the orientation of the open/closed magnetic field separatrix with respect to magnetic latitude is well correlated to the y component of the IMF.

Asymmetric multiple auroral arcs and inertial Alfvén waves

Abstract: High-resolution optical observations by the University of Calgary Portable Auroral Imager show a frequent occurrence of asymmetric multiple small-scale auroral arc structures during auroral substorms. Whereas the classical multiple arc array tends to exhibit a fairly symmetrical configuration, with parallel motions within individual discrete arcs being opposite in direction across the center of the arc array, the multiple arcs to be discussed herein are distinguished by the presence of discrete arcs strictly equatorward of the two bright counter-streaming arcs that would ordinarily define the center of the arc array. The intensity of these parallel equatorward-lying arcs were in most cases found to decrease rapidly in the equatorward direction. By considering the topology of the structures and the spacing between arcs, observations are found to be consistent with recent theories suggesting inertial Alfven waves as a possible cause of fine-scale auroral arcs.

WIND, GEOTAIL, and GOES 9 observations of magnetic field dipolarization and bursty bulk flows in the near‐tail

Abstract: The WIND, GEOTAIL, and GOES 9 spacecraft were all in the nightside magnetosphere and Interball was in the solar wind when a substorm began at ∼07:25 UT on April 18, 1996. WIND and GEOTAIL were located at downstream distances of X=−12 to −14 Re. The separation between these spacecraft was ∼10 Re in the Y direction and less than ∼1 Re in Z. While the northward field component at GOES 9 began to increase immediately, little or no dipolarization was observed at WIND and GEOTAIL until a series of very rapid Bz increases were observed ∼25–30 min later. About ∼1–2 min prior to each dipolarization event at WIND and GEOTAIL an earthward flow burst with peak speeds of 100–500 km/s was initiated. The duration of these bursty bulk flows were 1–7 min. These unique observations are interpreted as strong evidence for the existence of spatially localized, but sometimes temporally overlapping flow bursts in the near-tail during substorm expansion phase which result, cumulatively, in sufficient earthward magnetic flux transport to produce the well known dipolarization at geosynchronous orbit.

Quasi-neutral sheet tearing instability induced by electron preferential acceleration from stochasticity

Abstract: The role of tearing instability in substorm expansion onset is examined. From a consideration of observational constraints on the onset mechanism we conclude that this instability must have a considerable initial stage when the equilibrium magnetic field topology (without neutral lines) is still conserved. Moreover, this instability is shown to have no linear stage. Instead, either explicitly nonlinear or pseudolinear instability of negative energy eigenmodes can develop. The latter possibility may arise from electron preferential acceleration. This acceleration occurs for the electrons near the equilibrium chaos/adiabatic boundary that interact with the perturbation electric field and provide a means of dissipation to restore positive feedback for the instability development. We compute a set of single-particle orbits with realistic pitch angle distributions in a given tearing field model and evaluate statistical averages over the ensemble of particles and wave phases to verify the preferential acceleration of electrons. The resulting instability is analyzed by using a quasi-hydrodynamical multifluid model. Its amplitude can be less than that required for neutral line formation. It has a fast growth rate well within the timescales for substorm expansion onset. Its excitation leads to earthward acceleration of electrons and ions as well as local current amplification in the near-Earth region for excitation of cross-field current instability. In the theme of substorm as a low-dimension catastrophe for the current sheet, this nonlinear tearing instability approximates a fold catastrophe near the upper energy state of the system, while the unstable whistler waves from the cross-field current instability correspond to positive energy oscillations near a new lower-energy state of the current sheet achieved in the catastrophe. The nonlinear saturation of these whistler waves provides the irreversible relaxation of this state.

CUTLASS/IMAGE observations of high-latitude convection features during substorms

Abstract: . The CUTLASS Finland HF radar has been operational since February 1995. The radar frequently observes backscatter during the midnight sector from a latitude range 70–75° geographic, latitudes often associated with the polar cap. These intervals of backscatter occur during intervals of substorm activity, predominantly in periods of relatively quiet magnetospheric activity, with Kp during the interval under study being 2- and ΣKp for the day being only 8-. During August 1995 the radar ran in a high time resolution mode, allowing measurements of line-of-sight convection velocities along a single beam with a temporal resolution of 14 s, and measurement of a full spatial scan of line-of-sight convection velocities every four minutes. Data from such scans reveal the radar to be measuring return flow convection during the interval of substorm activity. For three intervals during the period under study, a reduction in the spatial extent of radar backscatter occurred. This is a consequence of D region HF absorption and its limited extent in the present study is probably a consequence of the high latitude of the substorm activity, with the electrojet centre lying between 67° and 71° geomagnetic latitude. The high time resolution beam of the radar additionally demonstrates that the convection is highly time dependent. Pulses of equatorward flow exceeding ~600 m s–1 are observed with a duration of ~5 min and a repetition period of ~8 min. Their spatial extent in the CUTLASS field of view was 400–500 km in longitude, and 300–400 km in latitude. Each pulse of enhanced equatorward flow was preceded by an interval of suppressed flow and enhanced ionospheric Hall conductance. The transient features are interpreted as being due to ionospheric current vortices associated with field aligned current pairs. The relationship between these observations and substorm phenomena in the magnetotail is discussed.

On storm weakening during substorm expansion phase

Abstract: Iyemori and Rao recently presented evidence that the strength of a magnetic storm, as measured by -Dst, weakens, or its rate of growth slows, during the substorm expansion phase. Yet the expansion phase is known to inject energetic particles into the ring current, which should strengthen the storm. We propose to reconcile these apparently contradictory results by combining the virial theorem and a principle of energy partitioning between energy storage elements in a system with dissipation. As applied to the unloading description of the substorm expansion phase, the virial theorem states that -Dst is proportional to the sum of the total magnetic energy and twice the total kinetic energy in the magnetosphere including the tail. Thus if expansion phase involves converting magnetic energy stored in the tail into kinetic energy stored in the ring current, a drop in -Dst during expansion phase requires that less than half the drop in magnetic energy goes into the ring current, the rest going into the ionosphere. Indeed Weiss et al., have estimated that the energy dissipated in the ionosphere during expansion phase is twice that injected into the ring current. This conclusion is also consistent with the mentioned energy partitioning principle, which requires that more energy be dissipated than transferred between storage elements. While Iyemori and Rao's observations seem to contradict the hypothesis that storms consist at least in part of a sum of substorms, this mode of description might none- theless be preserved by including the substorm's growth- phase contribution. Then the change in storm strength measured from before the growth phase to after the expansion phase is positive, even though the expansion phase alone makes a negative contribution.

The electric field response to the growth phase and expansion phase onset of a small isolated substorm

Abstract: . We capitalise on the very large field of view of the Halley HF radar to provide a comprehensive description of the electric field response to the substorm growth phase and expansion phase onset of a relatively simple isolated substorm ( |AL| 12 h) of magnetic quiescence, such that prior to the start of the growth phase, the apparent latitudinal motion of the radar backscatter returns is consistent with the variation in latitude of the quiet-time auroral oval with magnetic local time. The growth phase is characterised by an increasing, superimposed equatorward motion of the equatorward edge of the radar backscatter as the auroral oval expands. Within this backscatter region, there is a poleward gradient in the Doppler spectral width, which we believe to correspond to latitudinal structure in auroral emissions and magnetospheric precipitation. During the growth phase the ionospheric convection is dominated by a relatively smooth large-scale flow pattern consistent with the expanding DP2 (convection) auroral electrojets. Immediately prior to substorm onset the ionospheric convection observed by the radar in the midnight sector has a predominantly equatorward flow component. At substorm onset a dramatic change occurs and a poleward flow component prevails. The timing and location are quite remarkable. The timing of the flow change is within one minute of the dispersionless injection observed at geostationary orbit and the Pi2 magnetic signature on the ground. The location shows that this sudden change in flow is due to the effect of the upward field aligned current of the substorm current wedge imposed directly within the Halley radar field of view.

Convection‐driven reconnection and the stability of the near‐Earth plasma sheet

Abstract: Driven convection in the near-Earth plasma sheet is investigated using 3-D, full particle simulations. The inductive response to an externally imposed, uniform convection electric field leads to the effective tailward propagation of magnetic flux, the erosion of the equatorial magnetic field, the development of an embedded thin current sheet, and the eventual formation of a neutral line at the inward edge of the plasma sheet. In addition to creating a magnetic island, the neutral line permits the growth of a kink-like mode with a characteristic scale of 1-2 R E in the east-west (y) direction. The combined plasma flows from reconnection and the kink mode are in the range of 200-400 km/s and exhibit a reversal between earthward and tailward flow as a function of y. These modifications of the plasma sheet resemble the substorm breakup changes which are observed in the current disruption region.

Substorm precipitation in the polar cap and associated Pc 5 modulation

Abstract: This paper reports substorm-related cosmic radio noise absorption events observed deep into the polar cap, well poleward of the typical nightside auroral oval. We find that such events usually occur in the late expansion/recovery phase of a substorm during moderate-to-strong planetary disturbances (Kp = 3-8) and at times of high solar wind speed conditions. We present riometer data from U.S. manned and automatic geophysical observatories in Antarctica to track the morphology and progression of auroral absorption features over 5 hours of local time at ≥ 80° magnetic latitude. The azimuthal motions observed, westward premidnight, eastward post-midnight, are similar to the dynamics expected for lower latitude substorm-related auroral absorption. ULF modulation of the absorption (and hence of the causative fluxes of precipitating energetic electrons) are often observed. These modulations are usually dominated by spectral peaks between 1-4 mHz and, in the one case examined in detail, were confined to a narrow, ∼ 1° latitude band embedded within more uniform precipitation. Pc 5 (150 to 600 s periods) magnetic variations with similar spectral peaks, and auroral arcs are often observed concurrently. Several mechanisms for producing the particle modulations are examined within the context of these observations.

Phase bunching during substorm dipolarization

Abstract: We investigate the dynamics of near-Earth plasma sheet ions during storm time dipolarization of the magnetospheric field lines. We more specifically examine the behavior of O+ ions that are trapped in the equatorial vicinity. We show that during field line dipolarization, these particles may be transported in a nonadiabatic manner and experience large magnetic moment enhancement together with prominent bunching in gyration phase. An analytical estimate is obtained that is in good agreement with numerical trajectory calculations and allows detailed analysis of the adiabatic-nonadiabatic transition. We focus on the gyrophase bunching effect and demonstrate that it occurs below some threshold energy at the dipolarization onset, this threshold energy being controlled by the amplitude of the magnetic transition and by the injection depth in the magnetotail. In the near-Earth tail, the phase-bunched particles possibly experience intense (up to the hundred of keV range) nonadiabatic energization and are radially distributed in a well-structured manner. As a result of this, systematic trajectory computations reveal that phase bunching during storm time dipolarization can lead to significant though localized variations of the cross-tail current and may thus be responsible for large fluctuations of the magnetic field.

Magnetotail reconnection rate during magnetospheric substorms

Abstract: The magnetotail reconnection rate during substorms is studied using data taken by the Sondrestrom incoherent scatter radar during an ensemble of 24 substorms A superposed epoch analysis indicates that magnetotail reconnection increases marginally significantly above its average level near the time of substorm expansion phase onset Our data set shows this increase with 87% confidence within ±5 min of onset and with greater than 99% confidence at 10± 5 min after onset However, the median reconnection rate measurement does not begin to increase until 20 min after onset This effect is shown to be due to a spatial variation in the development of magnetotail reconnection during substorms; magnetotail reconnection increases very soon after onset in a small magnetic local time sector near midnight, while over most of the separatrix, the increase in the reconnection rate is delayed The average delay in the local time sectors greater that 15 hours from midnight is approximately 20 min We assign functional forms to the average dependence of the magnetotail reconnection rate on magnetic local time, on the interplanetary magnetic field, and on substorm activity, and analyze the generalized linear combination of these functions The resulting expression consists of the sum of the average interplanetary magnetic field and substorm dependence of the reconnection rate gated by the average spatial variation and accounts for 36% of the variance in the reconnection rate (055 linear correlation), which is most of the low-frequency (f < 05 h -1 ) variation The residual variance in the reconnection rate is mostly noise at a level consistent with the reconnection rate measurement uncertainty

Drift-shell splitting of energetic ions injected at pseudo-substorm onsets

Abstract: One feature of a magnetospheric substorm is the injection of energetic particles into closed drift orbits. Injections are routinely observed by geosynchronous satellites and have been used to identify the occurrence of substorms and the local time of particle energization. In this study we examine pitch angle distributions of ion injections in the 50-to 300-keV energy range observed by the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE/CCE) satellite, hereinafter CCE. In a dipole field, all pitch angles follow the same drift shell, but the day{endash}night asymmetry of the magnetospheric magnetic field introduces a pitch angle dependence in particle drift orbits, so that particles with different pitch angles disperse radially as they drift. The effect is known as drift-shell splitting. For satellite observations near noon at a fixed geocentric distance, the guiding center orbits of ions detected at small pitch angles intersect the midnight meridian at larger geocentric distances than do ions with near-90{degree} pitch angles. The ion pitch angle distributions detected on the dayside therefore provide information about the radial distance of the nightside acceleration region. We apply this principle to study ion injection events observed on September 17{endash}18, 1984, in association with pseudo-substorm onsets. CCE was atmore » 13 hours local time near its apogee (8.8R{sub E}) and observed a series of ion flux enhancements. Energy dispersion of the timing of the flux increases assures that they are due to injections on the nightside. The flux increases were observed only at pitch angles from 0{degree} to 60{degree}. We calculate drift orbits of protons using the Tsyganenko 89c magnetic field model and find that the drift orbits for 60{degree} pitch angle protons observed at the satellite pass through midnight at 9R{sub E}, well outside of geostationary orbit, indicating that the ion injections occurred tailward of 9R{sub E}. Energetic ion data from geostationary satellites for the same time interval show no evidence of injections at 6.6R{sub E}, consistent with the calculated inner boundary of the injections. Model calculations are presented demonstrating that dispersive injections observed near noon outside geosynchronous orbit provide the greatest sensitivity to drift-shell splitting effects and are therefore most suitable for remote sensing the radial boundaries of substorm injections. {copyright} 1997 American Geophysical Union« less

Changes in the distant tail configuration during geomagnetic storms

Abstract: Changes in the structure of the distant tail associated with geomagnetic storms are studied by using plasma and magnetic field data obtained from Geotail. Thirteen storm intervals between October 1993 and October 1994 are examined when the satellite was located in the distant tail between X = −83 RE and X = −210 RE. Geotail observed the magnetosheath during all storms including those when the satellite was located near the nominal tail axis. Assuming the flow direction in the magnetosheath to be parallel to the magnetotail axis, we estimated the dimension and the flux of the tail for the magnetopause crossing events. Systematic changes of the distant tail structure are found in association with the development of the storms. Before the main phase onset of the storms, when the Dst shows positive excursion, the enhanced solar wind pressure reduces the average radius of the distant tail to ∼23 RE as compared with the quiet time value, ∼31 RE. During the storm main phase the dimension of the tail is comparable to the quiet time value in spite of the high solar wind pressure. This is attributed to the enhanced magnetic flux in the tail in association with the southward interplanetary magnetic field (IMF) Bz. An average energy of ∼5 × 1015 J is stored also in the distant tail during the storm main phase, which is a comparable value to that stored in the midtail during an intense substorm growth phase reported in the previous studies. During storm time, when the Bz component of the magnetosheath field was larger compared with the By component, the average tail cross section has a north-south elongated elliptical shape. The average tail for quiet time, however, was elongated toward the dawn-dusk direction. By component was larger than Bz component in our quiet time data set. These observations imply that the anisotropy in the magnetosheath magnetic field pressure for different IMF orientation changes the shape of the tail, which was also shown in global MHD models. Changes in the average magnetosheath parameters (density, speed, direction) observed during the storms are found to be consistent with solar wind fluctuations expected from corotating interaction regions.

A study of the CDAW 9C substorm of May 3, 1986, using magnetogram inversion technique 2, and a substorm scenario with two active phases

Abstract: One of the ('DAW 9(' substornts is investigated in this pa. per using the database reported by Hones et al. and supplelnented with magnetogram inversion technique (MIT) 2 data.. These latter have provided information about the dynamics of the open ta. il magnetic flux, current syst. ems in the ionosphere, and the size a. nd dyna, lnics of the current wedge. We have identified the growth, expansion, and recovery phases of this substorm, with characteristics expected from a generally accepted scenario. However, specific signatures were observed in the interval (0919- 0935) (?T, i.e., between the growth and expansion phases, indicating the concurrent development of the substorln onset and corresponding instabilities in the innermost current sheet,, and slna,ll-scale cross-tail current disruptions without the open tail reconnection. In addition to signatures of small-scale dipolariza, tion, an increase of the open tail magnetic flux, and a current system of the type close to D P 2 were observed a,t. (0919-0935) tIT, which is more likely to suggest predominance of the tail-stretching process than magnetic colla.pse. This fact was interpreted in letres of a releva, nt simple model a.s a signa.ture of the growth of the energy input h'om the solar wind which ensures the observable disturbance power. Hence the disturba,nce a,t (0919-0935) UT was more likely a driven one than a,n mfioading one. The aforementioned signa,tures make it possible t.o identify the interva.l (0919-0935) UT a,s the "pha._e of multiple onsets" or (equivalently) the "first a, ctive phase," which was previously defined by Mishin (1991, and references therein) a,s one of the four standard phases of a typical substorln (in addition to the expansion phase). Thus the ca. se st.tdy supports the substorm scenario with two active phases and, a. ccordingl.x..', with two different kinds of physics. This case study illustrates also the inforlnativity of MIT 2 data and their ability to effectively complement the database tra, ditionally used in substorm studies.