Showing papers on "Substorm published in 1999"

Flow braking and the substorm current wedge

Abstract: Recent models of magnetotail activity have associated the braking of earthward flow with dipolarization and the reduction and diversion of cross-tail current, that is, the signatures of the substorm current wedge. Estimates of the magnitude of the diverted current by Haerendel [1992] and Shiokawa et al. [1997, 1998] tend to be lower than results from computer simulations of magnetotail reconnection and tail collapse [Birn and Hesse, 1996], despite similar underlying models. An analysis of the differences between these estimates on the basis of the simulations gives a more refined picture of the diversion of perpendicular into parallel currents. The inertial currents considered by Haerendel [1992] and Shiokawa et al. [1997] contribute to the initial current reduction and diversion, but the dominant and more permanent contribution stems from the pressure gradient terms, which change in connection with the field collapse and distortion. The major effect results from pressure gradients in the z direction, rather than from the azimuthal gradients [Shiokawa et al., 1998], combined with changes in B y and B x . The reduction of the current density near the equatorial plane is associated with a reduction of the curvature drift which overcompensates changes of the magnetization current and of the gradient B drift current. In contrast to the inertial current effects, the pressure gradient effects persist even after the burst of earthward flow ends.

Substorm dipolarization and recovery

Abstract: On the basis of ∼2 years of Geotail data, we use a superposed epoch approach to study the average behavior of plasma and magnetic fields at different radial distances, between 11 and 31 RE, during 66 substorms in the premidnight sector. Magnetic field dipolarization is first seen in the innermost region (11–16 RE) around substorm onset and subsequently moves tailward at a rate of 35 km/s. Fast earthward and tailward ion bulk flows in the central plasma sheet indicate that during substorm expansion the near-Earth neutral line is located between 21 and 26 RE, with a tendency to be closer to 21 RE near substorm onset. About 45 min after onset, the tailward moving dipolarization front reaches the distance range where the near-Earth neutral line is located. Thereafter the near-Earth neutral line disappears beyond 31 RE. This is the classical signature of the start of the recovery phase. We conclude that substorm recovery sets in when the tailward moving dipolarization front reaches the near-Earth neutral line, because the near-Earth neutral line cannot operate in a dipolar field geometry.

Generation of Pi2 pulsations by bursty bulk flows

Abstract: We present the first observations directly relating bursty bulk flows (BBFs) to Pi2 pulsations. At ISEE 2, a large earthward flow was observed. The Institute of Geological Sciences (IGS) magnetometer chain located at the same local time as ISEE 2 recorded Pi2 pulsations, slightly delayed relative to flow onset, consisting of a long-period (7–8 mHz) component associated with oscillations of the substorm current wedge. We demonstrate that the ionospheric current began to increase 90 s before the arrival of these Pi2 pulsations. We argue that this precursor was generated by earthward convection of plasma sheet plasma. Phase skips at midlatitudes on the nightside are associated with new flow bursts at ISEE 2. The Air Force Geophysics Laboratory (AFGL) magnetometer chain, which was located on the dusk flank, measured Pi2 pulsations that started ∼90 s after the flow bursts. We demonstrate that there is a one-to-one correlation between impulsive flow onset in the tail and Pi2 pulsations. We also show that oscillations at AFGL are directly related to temporal variations of the flow. Thus we suggest that the characteristic frequencies of low-latitude Pi2 pulsations are established by the temporal structure of processes in the near-Earth magnetotail.

Pi2 pulsations and substorm onsets: A review

Abstract: Pi2 pulsations have been the subject of continuous study since they were recognized to be an integral part of the magnetospheric substorm. With the advent of arrays of ground instruments the nature of the Pi2 has begun to be understood. As adopted by the 13th General Assembly of the International Union of Geodesy and Geophysics in 1963, Pi2 is a designation that includes impulsive pulsations in the period range from 40 to 150 s. The Pi2 signal encompasses a class of pulsations that represents two fundamental processes. The first process is the sudden generation of field-aligned currents in association with the disruption of cross-tail currents in the plasma sheet and their subsequent effects on the ionosphere. The ionosphere appears to be something more than a passive load for this electrodynamic impulse. It responds, sending currents back into a magnetosphere whose topology is changing and, perhaps producing the feedback necessary to cause the explosive growth of the substorm current system. Oscillations of these currents are detected across the nightside of the Earth at onset as the midlatitude and high-latitude portions of Pi2. The second process is the impulse response of the inner magnetosphere to the compressional waves that are generated at substorm onset. Traveling inward, they stimulate field line resonances and surface waves at the plasmapause and excite global oscillations in the inner magnetosphere. The two processes produce wave modes that couple and cross-couple threading energy into the inner magnetosphere and ultimately to the ground. The purpose of this review is to construct a phenomenological overview of the Pi2.

Earthward flow bursts in the inner magnetotail and their relation to auroral brightenings, AKR intensifications, geosynchronous particle injections and magnetic activity

Abstract: High-velocity magnetotail flow bursts measured by the Geotail Low Energy Plasma experiment in the premidnight equatorial region between 10 and 15 RE have been compared with other magnetospheric phenomena. These bursts, typically characterized by earthward velocities approaching 1000 km/s and lasting for times of the order of l min, are associated with magnetotail dipolarizations and large magnetic field fluctuations. Using supporting measurements of the International Solar Terrestrial Physics program it is found that the flow bursts are closely associated with auroral brightenings, AKR onsets, geosynchronous particle injections, and ground magnetic activity. Flow bursts for which Polar UVI images are available showed auroral brightenings that developed near the footpoint Geotail field line. AKR intensifications usually accompanied the flow bursts in close time coincidence, whereas dispersionless geosynchronous particle injections tended to be delayed by 1–3 min. Since flow bursts often exhibit the earliest onsets of these various phenomena, it seems likely that this chain of events is initiated in the tail beyond 15 RE, presumably by magnetic reconnection. It is concluded that flow bursts are a fundamental magnetotail process of limited spacial extent that are important in energy and magnetic flux transport in the magnetosphere. Magnetotail flow bursts are intimately connected to auroral acceleration processes and AKR generation at several thousand kilometer altitude and a full explanation of substorms will have to explain this relationship.

Storm‐like dynamics of Jupiter's inner and middle magnetosphere

Abstract: The discovery of energy-time dispersed, charged particle signatures of dynamic, longitudinally confined charged particle injections within Jupiter's inner magnetosphere has been reported previously as measured in >20-keV particle intensities by the Galileo energetic particles detector (EPD). While these events have similarities to so-called substorm injections observed within the Earth's magnetosphere, it is unknown whether the driving mechanisms are similar. Over 100 Jovian injection events have now been documented between radial distances of ∼9 RJ (the inner boundary of most of the observations) and 27 RJ Injections occur at all System III longitudes and local time positions. Similar to Earth magnetospheric injections, the Jovian injections occur throughout the broad radial region of transition between the quasi-dipolar and neutral sheet magnetic field configurations, and where the charged particle energy densities are competitive with the magnetic energy densities. The Jovian injections can be clustered in time, analogous to what often happens during well-known magnetic storms that occur in the Earth's magnetosphere. During one particular periapsis of Galileo's orbital trajectory, the magnetosphere was observed by EPD to become suddenly very disturbed with multiple injections following a prolonged period (>24 Earth hours) of relative quiescence. Because of the prestorm coincidence of a signature of an apparent Earth-like global magnetospheric disturbance, we hypothesize that this Jovian storm occurred when the inner and middle magnetosphere were triggered out of marginal stability by the passage of a magnetohydrodynamic fast mode wave launched at the magnetopause by a pressure variation in the interplanetary (solar wind) environment.

Modeling of inner plasma sheet and ring current during substorms

Abstract: The evolution of the inner plasma sheet and the ring current during substorm dipolarizations is simulated. A substorm cycle is treated by stretching and dipolarizing the magnetosphere according to the Tsyganenko 89 model. In order to clarify the relative influences of steady convection and induction electric field on ring current development, the inductive electric field is superposed on two baseline convective states: a nonstorm state using a weak electric field, and a storm-time state using a stronger electric field. Ion distributions on the nightside at 12 Earth radii (RE) during these two substorms are obtained using our single-particle code to trace particle trajectories backward in time to source regions assumed to have steady characteristics. The subsequent acceleration and transport of these boundary ions into the inner magnetosphere is modeled by our kinetic model of the ring current. The simulation generates many frequently observed features of substorm injections, including the sudden appearance of hot plasma tailward of a sharply defined "injection boundary," the earthward motion of an "injection front," the azimuthal and tailward expansion of this enhanced region, and the creation of characteristic ion dispersion patterns near geosynchronous orbit. Comparison of the nonstorm and storm cases suggests that substorms occurring without a convection enhancement produce mainly an enhancement of the cross-tail current but little change in the ring current. With strong convection, the role of substorms is to enable the convection enhancement to create robust ring current in the inner magnetosphere.

Monitoring spatial and temporal variations in the dayside plasmasphere using geomagnetic field line resonances

Abstract: It is well known that the resonant frequency of geomagnetic field lines is determined by the magnetic field and plasma density. We used cross-phase and related methods to determine the field line resonance frequency across 2.4≤

On relative timing in substorm onset signatures

Abstract: We investigate the timing of popular substorm onset signatures to understand their temporal relationship. Proxies for substorm onsets include auroral breakups, high-latitude magnetic bays, low-latitude Pi2 bursts, dispersionless injections at geostationary orbits, and auroral kilometric radiation. We use the auroral breakup, identified with Polar UVI images, as a common reference time frame to calibrate the others. Results, illustrated by two well-defined auroral substorms, unambiguously indicate that none of the four frequently used substorm onset proxies can provide a consistent timing of substorm onset. This inconsistency in substorm onset timing is attributed as a consequence of temporal and spatial limitations on each observational technique. A delay between the proxy identifiers and the auroral breakup is found to be typical. It is therefore strongly suggested from this study result that a common reference time frame for substorm onset is necessary, and we propose it should be auroral breakups. We argue that there is a need for an intercalibration of magnetospheric substorm phenomenology by using a unified definition of the substorm onset.

Polar cap area and boundary motion during substorms

Abstract: The area of the polar cap as a function of local time and substorm phase was measured using images from the Polar Ultraviolet Imager (UVI) for different interplanetary magnetic field (IMF) orientations during three substorms in January 1997. We measured changes in the polar cap area and motion of the poleward and equatorward boundary of the auroral oval. It was found that the polar cap boundary is strongly influenced by thinning of the oval, decrease in polar cap structures, the poleward expansion of the substorm at midnight, and the fading of luminosity below the instrument sensitivity threshold. Generally, these effects dominate over the latitudinal motion of the auroral oval at its equatorward edge. A new feature is that the polar cap region clears of precipitation during the substorm growth phase, which expands the size of the polar cap but is not necessarily related to an expansion of the open flux region. Another finding is that the increase in polar cap area prior to onset can be independent of the strength of the southward IMF component. For one case the polar cap area increased while the southward component of the IMF was 0 ± 0.5 nT. These observations have strong implications for models that use the polar cap area to estimate the magnitude of energy storage in the lobe magnetic field and loss during substorms.

GEOTAIL observations of flow velocity and north‐south magnetic field variations in the near and mid‐distant tail associated with substorm onsets

Abstract: Temporal changes in the plasma flow and magnetic fields of the Earth's magnetotail in the region of −7.5 > X(RE) > −52.5 and −15 < Y(RE) < 15 in the GSM coordinates were studied by applying a superposed epoch analysis method to 263 substorm events observed by GEOTAIL spacecraft. The onset times of the substorms were determined with Pi-2 pulsations at Kakioka station. It was found that the tailward flow associated with plasmoids started about 2 or 3 min before the Pi-2 onsets, and the earthward flow became somewhat coherent 1 min before the onsets. The variation of the northward magnetic field component was investigated by obtaining the deviation of this component from that in the interval from 10.5 to 7.5 min before the Pi-2 onsets. We found that the northward magnetic field increased at X ∼ −10RE and decreased at X ∼ −28RE simultaneously in the premidnight tail immediately after the onsets. These variations definetely correspond to the dipolarization and plasmoid formation, respectively.

MHD waves in a three‐dimensional dipolar magnetic field: A search for Pi2 pulsations

Abstract: ULF pulsations have been numerically studied in a new three-dimensional dipole model, which allows a realistic Alfven speed profile for the plasmasphere and outer magnetosphere in the tailward region. This model includes more realistic boundary conditions at the outer boundary, allowing for partial reflection at the magnetopause and escape of wave energy down the tail. We investigate how Pi2 modes develop in time when an impulse associated with the substorm onset is assumed. It is shown that discrete compressional modes, which are initially excited by the plasmasphere, persistently arise in the nightside region. Our numerical results suggest that Pi2 pulsations are strongly associated with these virtual resonances in the plasmasphere. Wave spectra are examined both at the equator and the ionosphere. We discuss and compare them with current theoretical and observational characteristics.

Sausage mode instability of thin current sheets as a cause of magnetospheric substorms

Abstract: . Observations have shown that, prior to substorm explosions, thin current sheets are formed in the plasma sheet of the Earth's magnetotail. This provokes the question, to what extent current-sheet thinning and substorm onsets are physically, maybe even causally, related. To answer this question, one has to understand the plasma stability of thin current sheets. Kinetic effects must be taken into account since particle scales are reached in the course of tail current-sheet thinning. We present the results of theoretical investigations of the stability of thin current sheets and about the most unstable mode of their decay. Our conclusions are based upon a non-local linear dispersion analysis of a cross-magnetic field instability of Harris-type current sheets. We found that a sausage-mode bulk current instability starts after a sheet has thinned down to the ion inertial length. We also present the results of three-dimensional electromagnetic PIC-code simulations carried out for mass ratios up to Mi / me=64. They verify the linearly predicted properties of the sausage mode decay of thin current sheets in the parameter range of interest. Key words. Magnetospheric physics (plasma waves and instabilities; storms and substorms) · Space plasma physics (magnetic reconnection)

Substorm onset timing: The December 31, 1995, event

Abstract: The objective of the present study is to examine the timing of various onset-associated signatures and address the cause-and-effect relationship between the formation of a near-Earth neutral line (NENL) and the trigger of tail current disruption. An event selected for this study took place on December 31, 1995. In this event the Geotail satellite was located at X = −30.3 RE in the midnight sector at a local time between the GOES 8 and 9 geosynchronous satellites. The timing of the Geotail observation of a fast (950-km/s) tailward convection flow accompanied with southward Bz (< −10 nT) indicates that the near-Earth reconnection process started at least 4 min before the ground substorm onset, which was identified by various signatures such as an auroral expansion, a Pi2 onset, a positive bay onset, and a negative bay onset. Both GOES satellites observed dipolarization. GOES 9 was located closer to the onset meridian and observed a sudden recovery (dipolarization) of the local magnetic field but with a noticeable (≈1 min) delay from the ground onset. This delay can be interpreted in terms of the earthward expansion of tail current disruption initiated outside of geosynchronous orbit. The timing of all these features is consistent with the idea that dipolarization is a pileup of magnetic flux conveyed from the NENL. However, a sharp decrease in the H component at GOES 9 prior to the local dipolarization onset and the sudden start of a substorm are difficult to explain in terms of this idea. It is asserted that tail current disruption is a unique process rather than a direct consequence of the NENL formation, although it is possible that the reconnection process sets up a favorable condition for triggering tail current disruption. The fast plasma flow in the plasma sheet ceased soon after the substorm onset, suggesting that during the expansion phase, the tail current disruption took over the near-Earth reconnection process as a major role in the substorm dynamics.

Hybrid Input Algorithm: An event‐oriented magnetospheric model

Abstract: We introduce and test a new approach suitable to model the magnetotail configuration during individual events. This Hybrid Input Algorithm (HIA) uses, in addition to spacecraft magnetic measurements in the tail, other complementary information (in this version, the isotropic boundaries of energetic particles observed at low altitudes), therefore increasing the amount of input data for the modeling and making models more accurate for mapping purposes as well as for evaluation of the current sheet thickness and current density. We test the HIA on two previously well-studied and modeled events, compare different models as well as the model- based and observation-based estimates for the plasma pressure and current density, and discuss the uncertainty in the resulting mapping. We apply the HIA-based models to evaluate the location of substorm onset in the tail during Coordinated Data Analysis Workshop 6A (CDAW-6A) substorm event and the characteristics of the current sheet at 7-15 Rjust before the substorm onset obtained for several other events. We found moderate values for the maximal current densities in the thin current sheet region (10-35 nA/m 2) and the minimal current sheet half thickness between 0.1 and 0.7 R. The X scale of the thinning region was sire 5 R. By assembling the modeling results for many events we found that the maximum current densities in the tail current sheet could be effectively predicted by taking into account the observed positions of isotropic boundaries and dipole tilt angle. We also briefly discuss possible future extension of the Hybrid Input Algorithm.

Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities

Abstract: . Giant pulsations are nearly monochromatic ULF-pulsations of the Earth's magnetic field with periods of about 100 s and amplitudes of up to 40 nT. For one such event ground-magnetic observations as well as simultaneous GEOS-2 magnetic and electric field data and proton flux measurements made in the geostationary orbit have been analysed. The observations of the electromagnetic field indicate the excitation of an odd-mode type fundamental field line oscillation. A clear correlation between variations of the proton flux in the energy range 30-90 keV with the giant pulsation event observed at the ground is found. Furthermore, the proton phase space density exhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce resonance instability as a possible generation mechanism, the azimuthal wave number of the pulsation wave field may be determined using a generalized resonance condition. The value determined in this way, m = - 21 ± 4, is in accord with the value m = - 27 ± 6 determined from ground-magnetic measurements. A more detailed examination of the observed ring current plasma distribution function f shows that odd-mode type eigenoscillations are expected for the case ∂f / ∂W > 0, much as observed. This result is different from previous theoretical studies as we not only consider local gradients of the distribution function in real space, but also in velocity space. It is therefore concluded that the observed giant pulsation is the result of a drift-bounce resonance instability of the ring current plasma coupling to an odd-mode fundamental standing wave. The generation of the bump-on-the-tail distribution causing ∂f / ∂W > 0 can be explained due to velocity dispersion of protons injected into the ring current. Both this velocity dispersion and the necessary substorm activity causing the injection of protons into the nightside magnetosphere are observed. Key words. Magnetospheric physics (energetic particles , trapped; MHD waves and instabilities) · Space plasma physics (wave-particle interactions).

Substorms: A global instability of the magnetosphere-ionosphere system

Abstract: Observational and numerical modeling evidence demonstrates that substorms are a global, coherent set of processes within the magnetosphere and ionosphere This supports the view that substorms are a configurational instability of the coupled system since the entire magnetosphere changes during the expansion phase onset It is shown that the magnetosphere progresses through a specific sequence of energy-loading and stress-developing states until the entire system collapses This energy loading-unloading sequence is the essential basis of the Faraday Loop non-linear dynamics model which has been quite successful in describing the fundamental behavior of substorms without invoking detailed treatments of the internal substorm instability mechanism Present-day MHD models also are seen to produce substorm-like global instabilities despite the fact that they do not treat realistically the extremely thin current sheets that play such an essential role in the near-tail dynamics prior to substorm onset This paper discusses three-dimensional kinetic simulations that have recently shown a variety of initial plasma kinetic instability modes which all evolve quickly to a similar, globally unstable reconnection configuration Continuing research concerning the substorm onset location and mechanisms addresses important questions of when and exactly how the substorm expansion develops However, the loaded magnetosphere almost always progresses rapidly to the same basic reconnection configuration irrespective of the detailed localized initiation mechanism This is likened to the catastrophic collapse of a sand dune that has reached a highly unstable configuration: Any small local perturbation can cause a complete and large-scale collapse irrespective of the perturbation details It is concluded that the global magnetospheric substorm problem has now largely been solved and that future work should be directed toward understanding the detailed plasma physical processes that occur during substorms

Substorm influence on the ionospheric electric potentials and currents

Abstract: Characteristic patterns of the high-latitude, ionospheric electric potential have been derived for two groups, with and without the occurrence of magnetospheric substorms, and for all orientations of the interplanetary magnetic field (IMF). These maps were derived from all available, simultaneous Dynamics Explorer 2 and solar wind data, using a least error fit technique. There are distinctive patterns for each orientation of the IMF, both with and without substorms in progress. In comparison to the nonsubstorm electric potential patterns, the substorm group tends to have a more pronounced Harang discontinuity near midnight. The averages of the AE indices that were measured simultaneously with the electric potentials have also been analyzed. The AU index is found to be linearly proportional to the electric potential in the dusk cell, while the |AL| index compared with the dawn cell's potential has a nonlinear trend. The |AL| index is distinctively greater for the substorm group, as expected, while the electric potential and AU index remain relatively unchanged. The measured electric potential patterns have been combined with a conductivity model in order to derive ionospheric and field-aligned currents. The Harang discontinuity is prominent in the horizontal ionospheric currents, particularly for the substorm group. Model calculations of |AL| indicate that some of the substorm increase can be accounted for by an enhanced auroral conductivity, while the changes in the electric field's magnitude and orientation near midnight, in distortions that do not significantly increase the overall potential drop, also contribute to the substorm electrojet. The field-aligned currents show a systematic evolution of the region 1 and region 2 current belts as the IMF changes from a +Y to −Y orientation. For negative BY the upward currents form a continuous oval, linking the region 2 current on the dawn side with the region 1 current on the dusk side, through both noon and midnight. For positive BY the upward currents are no longer continuous through noon and are divided by downward current linking the region 2 current on the duskside with the region 1 current on the dawnside. The “region 0” or cusp currents, which move from prenoon to postnoon as the IMF BY changes from positive to negative, appear to be continuations of the post/prenoon region 1 current belts.

Influence of the substorm current wedge on the Dst index

Abstract: One of the major questions confronting researchers studying the nature of the solar-terrestrial interaction centers around whether or not the substorm expansive phase has any causal effect on the growth of the storm time ring current. This question is often addressed by using the Dst index as a proxy for the storm time ring current and inspecting the main phase growth of Dst in the context of the substorm expansive phases which occur in the same time frame as the ring current growth. In the past it has been assumed that the magnetic effects of the substorm current wedge have little influence on the Dst index because the current wedge is an asymmetric current system, while Dst is supposed to reflect changes in the symmetric component of the ring current. In this paper we shall show that the substorm current wedge can have a significant effect on the present Dst index, primarily as a consequence of the fact that only four stations are presently used to formulate the index. Calculations are made assuming the instantaneous magnitude of the wedge current is constant at 1 MA. Hourly values of Dst may be as much as 50° smaller than those presented here because of variation of the wedge current over the hour. We shall show how the effect of the current wedge depends on the UT of the expansive phase onset, the angular extent of the current wedge, and the locale of the closure current in the magnetosphere. The fact that the substorm current wedge is a conjugate phenomenon has an important influence on the magnitude of the expansive phase effect in the Dst index.

Electron pitch angle distributions following the dipolarization phase of a substorm: Interball‐Tail observations and modeling

Abstract: We investigate Interball-Tail observations of electron pitch angle distributions after the dipolarization phase of a substorm. For 10 keV electrons we observe beamlike, coniclike, and perpendicularly peaked distributions at L ∼ 11, L ∼ 9, and L ∼ 7, respectively. We examine the efficiency of betatron heating and Fermi acceleration associated with adiabatic transport of the electrons during the substorm dipolarization phase. This dipolarization phase was modeled using transition between different K p levels within a realistic magnetic field model. The calculations reproduce well the evolution of the high-energy electron flux in the parallel and perpendicular directions. They also reproduce well the pitch angle distribution observed by Interball-Tail at 10 keV, after the dipolarization phase. It is shown that Fermi acceleration is the leading process, compared to betatron heating. The production of the coniclike distributions is narrowly linked to the existence of a transition region between dipolelike and taillike magnetic fields, at about L ∼ 9.

Sporadic plasma sheet ion injections into the high‐altitude auroral bulge: Satellite observations

Abstract: We report on a new feature of auroral substorms, namely, the sporadic though recurrent injections of magnetospheric ions throughout the auroral bulge. These injections are interpreted as time of flight dispersed ion structures (TDIS). Our analysis builds on a combination of measurements from Interball-Auroral, from UV imagery onboard Polar, from ground magnetometers, and also from observations on Geotail and from geostationary spacecraft. Backward tracing of ion trajectories from Interball-Auroral orbit using realistic three-dimensional magnetic and electric field models indicates that the injection region can extend over a wide range of radial distances, from ∼7–40 RE in the nearly equatorial magnetosphere. Both hydrogen and oxygen ions are shown to be injected toward the Earth's upper ionosphere. At Interball altitudes we find that ion injections are associated with two types of low-frequency torsional oscillations of the magnetic field: (1) shear Alfven waves with a period of a few minutes with the highest amplitude near the bulge front and decreasing amplitude at lower latitudes and (2) higher-frequency shear Alfven waves of the PlB type, strictly restricted to the poleward boundary of the surge, with a characteristic period of ∼40 s. The systematic observation of sporadic TDIS during the auroral bulge expansion leads us to conclude that the same physical process is at work throughout the midtail. We also show that ion injections are detected well inside the bulge, which suggests that the injection fronts propagate from the outer to the inner magnetosphere over large distances. This topic is more extensively studied by Sergeev et al. [1999]. We also show that the poleward boundary of the surge is associated with a prominent outflow of ionospheric H+ and O+. These ions in the hundred of eV to the keV range are heated perpendicularly to the local magnetic field and subsequently transported into the magnetotail. The expanding auroral bulge thus forms a significant source of ionospheric ions for the midtail magnetosphere.

Ballooning instability in the presence of a plasma flow: A synthesis of tail reconnection and current disruption models for the initiation of substorms

Abstract: The drift ballooning mode (DBM) instability near the inner edge of the plasma sheet (IEPS) is studied further by including a nonstationary earthward flow and flow shear in the analysis. Both equatorial and off-equatorial regions are considered. It is found that the presence of a decelerated earthward flow destabilizes both the M− and M+ branches of the DBM in a large portion of the current sheet near the IEPS and substantially increases the growth rate of the instability. The flow shear in the premidnight sector causes the conventional ballooning mode to weakly subside, while it slightly enhances the growth rate for the Alfvenic ballooning mode. The combination of the earthward flow and flow shear makes both the Alfvenic ballooning mode and conventional ballooning mode grow much faster than they would without the flow, giving rise to coupled Alfvenic slow magnetosonic waves, field-aligned currents, and the formation of a current wedge. A synthesis of tail reconnection and cross-tail current disruption scenarios is proposed for the substorm global initiation process: When the fast flow produced by magnetic reconnection in the midtail abruptly decelerates at the IEPS, it compresses the plasma populations earthward of the front, transports momentum to them, and pushes them farther earthward. This creates the configuration instability in a large portion of the inner tail magnetic field lines on both the tailward side and earthward side of the braking point. As soon as the ionospheric conductance increases over a threshold level, the auroral electrojet is greatly intensified, which leads to the formation of the substorm current wedge and dipolarization of the magnetic field. This substorm paradigm combines the near-Earth neutral line and near-Earth current disruption scenarios for the initiation of substorms and may also synthesize dynamical processes in the rnagnetosphere-ionosphere coupling and field line resonance during the substorm onset. We intend to use this global model to explain substorm expansion onsets occurring under the southward interplanetary magnetic field condition.

Plasma sheet dynamics in the Jovian magnetotail: Signatures For substorm-like processes ?

Abstract: During Galileo's orbit G2 in 1996 the Energetic Particles Detector (EPD) onboard the spacecraft detected a number of particle bursts with large radial/antisunward anisotropies in the distant Jovian magnetotail [Krupp et al., 1998]. In this letter we focus on a detailed analysis of one of the bursts. Prior to the onset of the burst, particle intensities at low energies increase over several hours. This phase can be interpreted as a plasma loading phase. It ends after the onset of strong distortions in the magnetic field with a bipolar excursion of the north-south component being the most prominent feature. The subsequent plasma sheet encounters show that the plasma sheet has thinned considerably. Accelerated/heated ion beams first from the Jovian direction and then later from the tail direction are seen at the plasma sheet and lobe interfaces and intense radio and plasma wave emissions are detected. The event is tentatively interpreted as a dynamical process, where the Jovian magnetotail is internally driven unstable by mass loading of magnetic flux tubes.

Coherent HF radar backscatter characteristics associated with auroral forms identified by incoherent radar techniques: A comparison of CUTLASS and EISCAT observations

Abstract: Backscatter from decameter-wavelength field-aligned F region irregularities, as measured by the Cooperative UK Twin Located Auroral Sounding System (CUTLASS) Finland HF coherent radar, is compared with common volume plasma parameters and the electric field deduced by the European Incoherent Scatter (EISCAT) UHF incoherent radar system, for a 12 hour period from June 18 to June 19, 1996. During this interval we find an excellent agreement between irregularity Doppler velocity and bulk ion drift resolved along the CUTLASS beam. Backscatter is found to exist only in regions of nonzero electric field, as the E×B instability growth rate is dependent on E. Following a substorm expansion phase onset, backscatter largely disappears for a period of several hours, thought to be a consequence of nondeviative absorption of the HF radio wave in the D region or a quenching of the F region instability mechanism by enhanced E region Pedersen conductivity. Finally, the presence of auroral arcs within the scatter volume increases the intensity of backscatter returns and introduces a subsidiary peak, displaced from the preexisting peak, in the backscatter spectra; this subsidiary peak results in an increase in the apparent spectral width of the backscatter. We show how this allows the location of precipitation features within the field of view to be determined.

Characteristics of pseudobreakups and substorms observed in the ionosphere, at the geosynchronous orbit, and in the midtail

Abstract: We present a comprehensive study of a sequence of two substorms and multiple pseudobreakups using optical, magnetic and incoherent scatter radar measurements, energetic particles from two geosynchronous satellites and particle and field data from the Geotail spacecraft located at Xgsm ∼ −86 RE. Following conventional nomenclature, we classified as pseudobreakups those auroral breakups which did not exhibit significant poleward expansion (< 2° magnetic latitude). Auroral intensifications following substorm breakups were also observed, and were classified separately. Pseudobreakups were found not to differ from substorm breakups in longitudinal extent (from 1.3 to 6.1 hours of magnetic local time), or in duration (from 5 to 16 minutes). In general, the ionospheric currents producing ground magnetic disturbances were more intense during substorms than pseudobreakups. We found that pseudobreakups are associated with the same magnetospheric processes as substorm breakups which involve current wedge formation, midlatitude magnetic Pi2 pulsations and energetic particle injections at the geosynchronous altitude. Moreover, pseudobreakups are associated with magnetic reconnection in the near-Earth region, evidenced by the typical subsequent detection of a plasmoid at Geotail. This implies that the magnetotail volume influenced by a pseudobreakup is quite large in radial distance. We conclude that there is no definitive qualitative distinction between pseudobreakups and substorms but there is a continuum of states between the small pseudobreakups and large substorms.

Magnetostatically equilibrated plasma sheet with developed medium-scale turbulence : Structure and implications for substorm dynamics

Abstract: A simple theory of the magnetostatic equilibrium of the magnetospheric plasma sheet is proposed It takes into consideration the effects of the large-scale dawn-to-dusk electric field and developed medium-scale turbulence This makes it possible to describe the main features of the plasma sheet dynamics The suggested model assumes that the magnetic field lines are not equipotential and that the regular velocity of a plasma parcel is much lower than the chaotic velocity Hence the plasma is intensely mixed One of the main consequences of such mixing is the experimentally observed temperature equalization across the plasma sheet The obtained dependence of plasma pressure on the magnetic vector potential permits one to solve the Grad-Shafranov equation and reconstruct, in the tail approximation, the magnetic field line configuration for the case of inhomogeneous distribution of the dawn-to-dusk electric field along the tail The thickness of the plasma sheet in the model is determined by the magnitudes of the regular and stochastic electric fields, their scales, and plasma temperature A quasi three-dimensional model of the plasma sheet is developed The substorm dynamics of the plasma sheet is analyzed The model can explain the thinning of the plasma sheet without variations in the plasma pressure at the tail axis (thinning without compression) during the substorm growth phase The plasma sheet expansion during substorm expansion phase is related to the growth of turbulent fluctuations

A statistical study of Pc3–Pc5 magnetic pulsations observed by the AMPTE/Ion Release Module satellite

Abstract: Magnetic field data from the Active Magnetospheric Particle Tracer Explorers/Ion Release Module satellite are used to complete a statistical study yielding occurrence rates of a number of different types of pulsations. Two hour panels of dynamic spectra and detrended line plots were inspected to determine occurrence rates over all local times from L = 6 to L = 20. Event types include fundamental field line resonances, harmonic resonances, storm time pulsations, and signatures of bursty bulk flows and fast flows. However, we also include observations of Pc3 compressional pulsations and note their association with harmonic events. Likewise, we include high-frequency events (40–70 mHz) and show a relation to storm time pulsations. On the basis of the occurrence distributions, we are able to make a number of conclusions. We determine that the excitation source of fundamental resonances is likely band limited from 3 to 10 mHz and that harmonic resonances are at least sometimes associated with compressional Pc3 pulsations. Storm time pulsations, compressional in nature, are sometimes associated with relatively high frequency transverse events and often occur in regions very close to the magnetopause. On the basis of other works that associate these pulsations with instabilities in the partial ring current, we suggest that particles that form the partial ring current may extend to the magnetopause during storms and substorms. Finally, we note that bursty bulk flows and fast flows in general have a magnetic signature that is predominantly compressional, and we discuss the relevance this may have regarding substorm dipolarization.

Substorm-associated pressure variations in the magnetotail plasma sheet and lobe

Abstract: Simultaneous pressure measurements by Interball-Tail in the high-latitude lobe and by Geotail in the equatorial plasma sheet were analyzed for 30 substorms which exhibited significant pressure changes. At the onset of a few substorms we observed equatorial pressure peaks with magnitudes up to 50% higher than those in the lobe. These pileups are probably rather localized, and their properties are consistent with plasma sheet thickening between two active regions in the tail. During expansion and recovery phases of more than half of substorms, we observed equatorial pressure depletions relative to the high-latitude lobe pressure. These depletions can last more than 2 hours and are likely formed during the substorm expansion phase near the equatorial plane behind (tailward of) the strongly dipolar near-Earth magnetotail region. The observed pressure gradient is probably a nonstationary feature and can be compensated partially by magnetic tension on the curved field lines. Magnitude and history of the solar wind dynamic pressure appear to significantly influence substorm scenarios in the magnetotail. Possible existence of the pressure difference should be taken into account in single-spacecraft substorm studies.

Formation and evolution of subauroral ion drifts in the course of a substorm

Abstract: We propose a physical mechanism that explains how “polarization jets” (PJ) or “subauroral ion drifts” (SAID) are formed in the course of a substorm and how they evolve. A PJ/SAID is considered to be the ionospheric signature of an inward moving injected plasma front. The flow shear that exists across this interface when it arrives in the vicinity of the plasmapause is responsible for the generation of intense electric fields in the premidnight sector, where PJ/SAID are observed. Quantitative simulation of this mechanism accounts for PJ/SAID width and peak drift velocity. The mechanism explains why PJ/SAID are observed poleward of or in the vicinity of the plasmapause. The inward traveling time of the injected plasma agrees with the delay between substorm onset and the apparition of PJ/SAID; the evolution of the ionospheric signature is consistent with observations as well.

A sandpile model with dual scaling regimes for laboratory, space and astrophysical plasmas

Abstract: There is increasing evidence that the Earth’s magnetosphere, like other macroscopic confined plasma systems (magnetic fusion plasmas, astrophysical accretion discs), displays sandpile-type phenomenology so that energy dissipation is by means of avalanches which do not have an intrinsic scale. This may in turn imply that these systems evolve via self-organized criticality (SOC). For example, the power law dependence of the power spectrum of auroral indices, and in situ magnetic field observations in the Earth’s geotail, indicate that the coupled solar wind-magnetospheric system can to some extent be described by an avalanche model. However, substorm statistics exhibit probability distributions with characteristic scales. In this paper a simple sandpile model is discussed which yields for energy discharges due to internal reorganization a probability distribution that is a power law, implying SOC, whereas systemwide discharges (flow of “sand” out of the system) form a distinct group whose probability distri...