Phase Of Substorm Research Articles

Comment on “Where is the magnetic energy for the expansion phase of auroral substorms accumulated?” by S.‐I. Akasofu

Comment on “Where is the magnetic energy for the expansion phase of auroral substorms accumulated?” by S.‐I. Akasofu

Simulation of substorm‐time acceleration of oxygen ions on azimuthally directed magnetic field lines in the near‐Earth plasma sheet

AbstractAcceleration of energetic ions is one of the drastic and common phenomena in the near‐Earth plasma sheet during the expansion phase of substorms. When a magnetic field line is stretched, the curvature radius of the magnetic field line becomes small, and many of energetic ions are accelerated nonadiabatically under the presence of electric fields. In a global magnetohydrodynamics (MHD) simulation, when the magnetic field line is further stretched, a flux rope structure appears in the near‐Earth plasma sheet (at ~8.5 RE for this particular simulation) for about 10 min before a substorm onset under the presence of the Y component of the interplanetary magnetic field. The magnetic field lines are azimuthally (in the east‐west direction) elongated near the equatorial plane, and the structure is different from that directly associated with magnetic reconnection. Based on a test particle simulation, we show that the oxygen ions departing in the flux rope structure a few minutes before the onset go around in the near‐Earth plasma sheet twice, experience strong dawn‐dusk electric field, and the ions gain kinetic energy as high as ~200 keV in ~10 min. The large acceleration results from nonadiabation motion together with geometry of magnetic field lines having a kink. The acceleration process (passing through or near the kink and energization by dawn‐dusk electric field) is not common. However, most of the particles that are accelerated more than 150 keV passed through or near the kink. The azimuthally elongated magnetic field line seems to have a large influence of substorm‐time acceleration of the oxygen ions that preexist before the onset.

The quiet evening auroral arc and the structure of the growth phase near‐Earth plasma sheet

AbstractThe plasma pressure and current configuration of the near‐Earth plasma sheet that creates and sustains the quiet evening auroral arc during the growth phase of magnetospheric substorms is investigated. We propose that the quiet evening arc (QEA) connects to the thin near‐Earth current sheet, which forms during the development of the growth phase enhancement of convection. The current sheet's large polarization electric fields are shielded from the ionosphere by an Inverted‐V parallel potential drop, thereby producing the electron precipitation responsible for the arc's luminosity. The QEA is located in the plasma sheet region of maximal radial pressure gradient and, in the east‐west direction, follows the vanishing of the approximately dawn‐dusk‐directed gradient or fold in the plasma pressure. In the evening sector, the boundary between the Region1 and Region 2 current systems occurs where the pressure maximizes (approximately radial gradient of the pressure vanishes) and where the approximately radial gradient of the magnetic flux tube volume also vanishes in an inflection region. The proposed intricate balance of plasma sheet pressure and currents may well be very sensitive to disruption by the arrival of equatorward traveling auroral streamers and their associated earthward traveling dipolarization fronts.

Substorm onset process: Ignition of auroral acceleration and related substorm phases

AbstractThe substorm onset process was studied on the basis of the vertical evolution of auroral acceleration regions derived from auroral kilometric radiation (AKR) spectra and Pi pulsations on the ground. The field‐aligned auroral acceleration at substorm onset demonstrated two distinct phases. Low‐altitude acceleration (h~3000–5000 km), which accompanied auroral initial brightening, prebreakup Pi2, and direct current of ultralow frequency (DC‐ULF) pulsation were first activated and played an important role (precondition) in the subsequent substorm expansion phase onset. Prebreakup Pi2 is suggestive of the ballooning mode wave generation, and negative decrease in DC‐ULF suggests increasing field‐aligned current (FAC). We called this stage the substorm initial phase. A few minutes after this initial phase onset, high‐altitude acceleration, which accompanied auroral breakup and poleward expansion with breakup Pi1 and Pi2 pulsations, suddenly broke out in an altitude range from 8000 to 16,000 km. Thus, substorm expansion onset originated in the magnetosphere‐ionosphere (M‐I) coupling region, i.e., substorm ignition in the M‐I coupling region. Statistical investigations revealed that about 65% of earthward flow bursts observed in the plasma sheet were accompanied by enhanced low‐altitude AKR, suggesting that flow braking of bursts causes FAC and resulting low‐altitude field‐aligned acceleration in the M‐I coupling region. On the basis of these observations, we propose a substorm onset scenario in which FAC that originated from the braking of plasma flow bursts first enhances low‐altitude acceleration (substorm initial phase onset) and then the increasing FAC induces current‐driven instability in the M‐I coupling region, which leads to high‐altitude acceleration and resulting substorm expansion phase onset.

Observations of GPS scintillation during an isolated auroral substorm

This paper reports simultaneous observations of ionospheric scintillation during an auroral substorm that were made using an all-sky full-color digital single-lens reflex (DSLR) camera (ASC) and a Global Positioning System (GPS) ionospheric scintillation and total electron content monitor (GISTM) in Tromsø (69.60 N, 19.20 E), Norway. On the night of November 19, 2009, a small substorm occurred in northern Scandinavia. The ASC captured its temporal evolution from the beginning of the growth phase to the end of the recovery phase. The amplitude scintillation, as monitored by the S4 index from the GISTM, did not increase in any substorm phase. By contrast, phase scintillation, as measured by the σ φ index, occurred when discrete auroral arcs appeared on the GPS signal path. In particular, the phase scintillation was significantly enhanced for a few minutes immediately after the onset of the expansion phase. During this period, bright and discrete auroral forms covered the entire sky, which implies that structured precipitation on the scale of a few kilometers to a few tens of kilometers dominated the electron density distribution in the E region. Such inhomogeneous ionization structures probably produced significant changes in the refractive index and eventually resulted in the enhancement of the phase scintillation.

Where is the magnetic energy for the expansion phase of auroral substorms accumulated?

AbstractCharacteristics of the ionospheric current system (the directly driven (DD) / unloading (UL) currents) and the power ε during auroral substorms are examined semiquantitatively in terms of input‐output relationship. The study confirms (a) The magnetic energy for the expansion phase is accumulated with the rate of a little more than 1018 erg/s during the growth phase which lasts for about 1 h (because the ionosphere cannot dissipate the input power). (b) The global energy dissipation rate in the ionosphere (including the Joule heat production and the particle precipitation) is more than 3.3 × 1018 erg/s. (c) Thus, the total energy consumption by the ionosphere during the impulsive expansion phase (lasting for about 1 h) is 1.2 × 1022 erg or up to 1023 erg for medium substorms. (d) However, it is shown that even the total magnetic energy in the magnetotail at a distance between 10 RE and 20 RE (6.5 × 1021 ergs = 6.5 × 1014 J) cannot supply the energy for the expansion phase. This estimate is consistent with the most recent satellite observations (4.3 × 1013 J (= 4.3 × 1020 ergs)). Thus, the following conclusions may be applicable for medium intensity substorms: (i) The major part of the magnetic energy for the expansion phase must be accumulated within a distance of 10 RE and be released almost totally during the expansion phase (because the duration of the growth phase and the expansion phase is about the same). (ii) This suggests that there is an upper limit of the energy which the magnetosphere can accumulate even at distances less than 10 RE. Further, it is confirmed that: (a) The expansion phase is characterized by a single cell current system UL (not a twin cell current system DD which is driven by the earthward plasma convection from the magnetotail). (b) The UL current is independent of the DD current, requiring a different origin. (c) During the recovery phase, the UL current is absent and the DD current approximately follows ε (no accumulation of the energy), because the ionosphere is capable of dissipating the power even if the power is moderate). (d) Based on these results, as an example, one possible chain of processes within 10 RE is proposed as the release process of the accumulated magnetic energy by taking into account the fact that the frozen‐in field condition is observed to be broken down.

Analysis of EMIC‐wave‐moderated flux limitation of measured energetic ion spectra in multispecies magnetospheric plasmas

A differential Kennel‐Petschek (KP) flux limit for magnetospheric energetic ions is devised taking into account multiple ion species effects on electromagnetic ion cyclotron (EMIC) waves that scatter the ions. The idea is that EMIC waves may limit the highest ion intensities during acceleration phases of storms and substorms (~ hour) while other mechanisms (e.g., charge exchange) may account for losses below those limits and over longer periods of time. This approach is applied to published Earth magnetosphere energetic ion spectra (~ keV to ~1 MeV) for radial positions (L) 3 to 6.7 RE. The flatness of the most intense spectral shapes for <100 keV indicate sculpting by just such a mechanism, but modifications of traditional KP parameters are needed to account for maximum fluxes up to 5.4 times greater than expected. Future work using the new capabilities of the Van Allen Probes mission will likely resolve outstanding uncertainties.

Statistical comparison of isolated and non‐isolated auroral substorms

AbstractThe present study compares isolated and non‐isolated substorms in terms of their global morphology and energy deposition. The analysis is based on a list of geomagnetic substorm onsets identified with magnetometer data from SuperMAG and published previously by Newell and Gjerlove (2011a). Isolated substorms are defined as those with separation of two consecutive onsets no less than 3 h. The auroral data are obtained from the global ultraviolet imager (GUVI) on board the TIMED satellite and are rebinned into typical magnetic latitude‐magnetic local time maps. The auroral maps are then averaged in 1 min intervals to show the dynamic change of the aurora. The three phases of the substorm are clearly demonstrated in both isolated and non‐isolated substorms. However, there are noticeable differences between the two types of substorms: (1) While the nighttime auroral power for both types of substorms slightly increases in the growth phase, isolated (non‐isolated) substorms are associated with smaller (greater) nighttime auroral power. (2) In the expansion phase, isolated substorms are associated with greater and more explosive energy release than non‐isolated substorms. (3) The time for the recovery phase is ~2 times longer for isolated than for non‐isolated substorms. (4) The winter‐to‐summer auroral power ratio is approximately constant throughout the three substorm phases and the ratio is larger for isolated (~30%) than that for non‐isolated (~10%) substorms. It is also found that the polar cap area increases during the growth phase until ~10 min prior to the magnetic substorm onset and decreases rapidly thereafter. The decrease is found to result from the closure of the nightside polar cap associated with substorm expansion. It is found that the observed differences between the two types of substorms simply reflect the differences in the solar wind and EUV drivers. Thus, we conclude that there is no intrinsic difference between isolated and non‐isolated substorms in terms of auroral energy release and subsequent auroral power decay.

The relationship between the magnetosphere and magnetospheric/auroral substorms

Abstract. On the basis of auroral and polar magnetic substorm studies, the relationship between the solar wind-magnetosphere dynamo (the DD dynamo) current and the substorm dynamo (the UL dynamo) current is studied. The characteristics of both the DD and UL currents reveal why auroral substorms consist of the three distinct phases after the input power ε is increased above 1018 erg s−1. (a) The growth phase; the magnetosphere can accumulate magnetic energy for auroral substorms, when the ionosphere cannot dissipate the power before the expansion phase. (b) The expansion phase; the magnetosphere releases the accumulated magnetic energy during the growth phase in a pulse-like manner in a few hours, because it tries to stabilize itself when the accumulated energy reaches to about 1023 erg s−1. (c) The recovery phase; the magnetosphere becomes an ordinary dissipative system after the expansion phase, because the ionosphere becomes capable of dissipating the power with the rate of 1018 ~ 1019 erg s−1. On the basis of the above conclusion, it is suggested that the magnetosphere accomplishes the pulse-like release process (resulting in spectacular auroral activities) by producing plasma instabilities in the current sheet, thus reducing the current. The resulting contraction of the magnetic field lines (expending the accumulated magnetic energy), together with break down of the "frozen-in" field condition at distances of less than 10 RE, establishes the substorm dynamo that generates an earthward electric field (Lui and Kamide, 2003; Akasofu, 2011). It is this electric field which manifests as the expansion phase. A recent satellite observation at a distance of as close as 8.1 RE by Lui (2011) seems to support strongly the occurrence of the chain of processes suggested in the above. It is hoped that although the concept presented here is very crude, it will serve in providing one way of studying the three phases of auroral substorms. In turn, a better understanding of auroral substorms will also be useful in studying the magnetosphere, because various auroral activities can be the visible guide for this endeavor.

The substorm current wedge and midnight sector partial ring current near substorm onset: A synthesis based on a magnetotail magnetic field geometry model

The Substorm Current Wedge (SCW) occurrence in the late growth and onset phases of substorms was proposed as thecurrent system which disrupts cross-tail current by diverting it to the ionosphere. The closure current for the SCW originally wassuggested to be the strong westward auroral electrojet (WEJ). However, the SCW-WEJ system has no viable generator current.Similarly, the asymmetric or Partial Ring Current (PRC) increases in strength during the growth phase, and is sometimes associatedwith an enhanced Region 2 field-aligned current (FAC) closing to the ionosphere, but specifics of that closure have been lacking.Here we present a unifying picture which includes the SCW post- and pre-midnight (AM and PM, respectively) currents and agenerator current in the midnight portion of the PRC system, with these currents based upon a model of the nightside magnetotailmagnetic geometry. That geometry consists of open north and south lobe regions surrounding a plasmasheet with two types ofclosed field line regions-stretched lines in the central part of the plasmasheet (SPS) and dipolar lines (DPS) between the low latitudeboundary layer (LLBL) regions and the SPS. There is also an important plasmasheet transition region (TPS) in which thedipolar field near the plasmapause gradually transforms to stretched lines near the earthward edge of the SPS, and in which themidnight part of the PRC flows. We propose that our proposed near-onset current system consists of a central current which becomespart of the midnight sector PRC and which is the generator, to which are linked two three-part current systems, one on thedawnside and one on the duskside. The three-part systems consist of up and down FACs closing as Pedersen currents in the ionosphere.These 3-part systems are not activated until near-onset is reached, because of a lack of ionospheric conductivity in the appropriatelocations where the Pedersen current closure occurs. The initial downward FAC of the 3-part dawnside system and thefinal upward FAC of the 3-part duskside system correspond to the AM and PM current segments, respectively, of the originallyproposed SCW.

Statistical properties of substorms during different storm and solar cycle phases

Abstract. Substorm properties during different storm phases have been studied using an automated recognition of substorm and storm phases in the auroral electrojet (AL) and ring current (Dst) index data from 1995–2009. The large number of events (about 500 storms and 15 000 substorms) provides statistically reliable distributions, average behaviour and long time series of simple parameters, such as durations and intensities. The phases of storms and substorms have been examined independently. Substorm phases have been further combined to single and multi-cycle events. The former consist of one growth, one expansion and one recovery phase, while the latter include multiple expansion and recovery phases after one growth phase. Our findings show that most substorms take place during non-storm times, and substorms during storm initial phases resemble isolated non-storm time substorms. Both during storm initial phases and non-storm times, the substorm growth phases may last longer than the other substorm phases. Substorm recovery phase is typically the longest phase but its duration also varies most. The longest substorm recovery phase duration was observed during multi-cycle substorms. The longest substorm expansion and storm main phases were found during the years close to the solar maximum. The shortest substorm events (the shortest phase durations) are the single-cycle substorms. The period of expansion onsets during multi-cycle substorms varied hugely for events with a small number of expansion phases. For events with a larger number of expansions, a clearer periodicity of about one hour (median value) was suggested.

Effect of prompt penetration on the low latitude ASY indices

Onset of a geomagnetic substorm often intensifies the westward auroral electrojet, as well as produces asymmetric magnetic field at low-/mid-latitudes. Auroral electrojet and low latitude asymmetric indices are known to correlate well during substorms. These indices have been widely used to monitor the duration and strength of substorm activities. However, several processes, other than substorms, introduce local time asymmetry in magnetic field at low latitudes, which can substantially influence the ASY indices. Large number of substorms are observed in association with changes in the interplanetary magnetic field (IMF). It is known that sharp IMF Bz orientation changes result in penetration of interplanetary electric field (IEF) to lower latitudes, which affects the geomagnetic fields to different degree in different local times. In the present study, we demonstrate that sharp IMF Bz fluctuations during the expansion phases of substorms introduce additional asymmetry at low latitudes. The effect is clearly seen in ASYH, whereas ASYD remains almost unaltered.

An empirical model of magnetospheric chorus amplitude using solar wind and geomagnetic indices

Magnetospheric chorus is an Extremely Low Frequency/Very Low Frequency (ELF/VLF, 0.3–30 kHz) electromagnetic wave phenomenon which plays an important role in the acceleration and loss of energetic electrons in the Earth's radiation belts. One must therefore possess accurate estimates of chorus amplitudes in order to model radiation belt dynamics. The goal of this study is to design an empirical model of chorus amplitude, the output of which can be used as input to models of radiation belt dynamics. In pursuit of this goal, we compare two related empirical models of chorus amplitude that we have developed based on THEMIS data from June 2008 through December 2011 which use multiple regression to predict equatorial chorus amplitudes as a function of L and MLT. One model uses only AE* and Kp as model inputs, and the other model utilizes solar wind measurements and geomagnetic indices. The models perform similarly, with each one achieving a median RMS prediction error of 0.39 log10 pT (a factor of 2.5 in amplitude). The coefficients of determination of chorus amplitude for the full model and the AE*/Kp model are 0.034 and 0.026, respectively, meaning that these models explain 3.4 and 2.6 percent of the variance of chorus amplitude. We present a parametric analysis, showing the expected effects on chorus amplitude from a modeled substorm and solar wind pressure pulse, as well as modeled chorus amplitude over the course of the month of September 2008. The model outputs give important insight into the global evolution of equatorial chorus amplitude as a function of geomagnetic storm and substorm phase.

Spectral characteristics of the plasma dispersionless injection during the storm recovery phase on 11 March 1998

A substorm dispersionless injection event observed during the storm recovery phase on 11 March 1998 at geosynchronous orbit is carefully studied. The event shows the notable characteristics that for energetic ions the flux enhancement ratio before and after injection increases and remains elevated with increasing energy, while for energetic electrons it tends to decrease with increasing energy. In order to explain the unique injection feature, the authors propose a possible mechanism that velocity space diffusion in common to electric acceleration adjusts the particle injection state. Spectral characteristics of four different phases (pregrowth phase, the growth phase, the substorm expansion phase, and the recovery phase) have been investigated. The differential fluxes of electrons from 50 keV to 1.5 Mev and ions from 50 keV to 1.2 MeV measured by Synchronous Orbit Plasma Analyzer (SOPA) instrument onboard LANL satellite 1991–080 are found to be best fitted with the three‐parameter kappa distribution function (f ∼ A0 · E[1 + E / (κE0)]−κ−1) by Levenberg‐Marquardt and Universal Global Optimization methods. The evolutions of the three parameters in the above kappa distribution in different substorm phases have been depicted for both electrons and ions. In each phase, E0 and κ show an approximately linear relationship κ(E0) = κ0 + ηE0. This linear relationship can be obtained by solving the velocity space diffusion equation with an initial superthermal kappa distribution. Ion and electron are found to have opposite trend of parameters κ0 and η in each phase, which indicates that the different species of particles exert different velocity space diffusion processes so that their flux enhancement ratios before and after injection are rather different. This implies that not only electric field acceleration, but also velocity space diffusion plays a very important role in the particle injection.

Fine scale structures of pulsating auroras in the early recovery phase of substorm using ground‐based EMCCD camera

We have carried out ground‐based observations, optimized to temporal and spatial characteristics of pulsating auroras (PAs) in the micro/meso scale, using an electron multiplying charge coupled device (EMCCD) camera with a wide field of view corresponding to 100 × 100 km at an altitude of 110 km and a high sampling rate up to 100 frames per second. We focus on transient PAs propagating southward around 1100 UT, in the early recovery phase of the substorm, on 4th March 2011. Three independent patches (PA1–3) each with different periods between 4 and 7 s were observed, which means that the periodicity was not explained by the electron bounce motion and strongly depended on local plasma conditions in the magnetosphere or in the ionosphere. One more insight is that only PA1 had also a sharp peak of modulations around 1.5 Hz, with a narrow frequency width of 0.30 Hz, and the strong modulations existed as a small spot in the center of PA1. We have also conducted cross spectrum analysis and have obtained coherence and phase distributions for auroral variations between 0.1 and 3.0 Hz. The results indicated that low frequency variations from 0.2 to 0.5 Hz inside PA1–3 propagated as a collective motion in well‐defined directions. The estimated horizontal propagation velocities ranged from 50 to 120 km/s at the auroral altitude. The velocities are almost consistent with the Alfven speed at the magnetic equator, which suggests that compressional waves have an effect on PA via modulations of the ambient plasma environment.

Emergence of the active magnetotail plasma sheet boundary from transient, localized ion acceleration

Observations of the Earth's magnetotail plasma sheet boundary layer (PSBL) have been typically accompanied by field‐aligned crescent‐shaped ion beams, thought to emanate at distant or mid‐tail semi‐permanent or impulsive acceleration sites. Typically such observations, and the theoretical and modeling efforts to explain them, have been disjoint from the adjacent plasma sheet properties near the equatorial projection of the observation. Thus the plasma sheet boundary layer has been thought of as a harbinger of remote, rather than local plasma sheet activity, exception of plasma sheet expansions during the recovery phase of substorms. Using case and statistical studies from THEMIS, obtained simultaneously at the near‐Earth PSBL and at its adjacent central plasma sheet (CPS), we study the transient and impulsive nature of PSBL beams and their inherent connection with CPS bursty bulk flows and associated dipolarization fronts. We show that PSBL beams typically commence a few minutes before CPS flow bursts, which in turn are seen tens of seconds ahead of the arrival of dipolarization fronts. These timing correlations, the crescent shapes of PSBL ion beams, the CPS ion flux enhancements in the earthward and dawnward directions, and other particle distribution characteristics can all be well reproduced by a simple model of ion reflection and acceleration at earthward‐propagating dipolarization fronts associated with CPS flow bursts. The emerging paradigm, therefore, unifies impulsive transport phenomena across latitudes in the near‐Earth magnetotail plasma sheet.

The double auroral oval in the dusk‐midnight sector: Formation, mapping and dynamics

The present study observationally examines the double auroral oval structure in the evening‐premidnight sector. The results are summarized as follows; (1) The poleward auroral branch can be attributed to accelerated auroral precipitation, occasionally accompanied by Alfvénic precipitation, and it is collocated with the upward R1 current; (2) The equatorward auroral branch, which is embedded in the downward R2 current, very often if not always consists of the diffuse precipitation of energetic electrons and extends mostly poleward of the b2i boundary collocated with energetic ion precipitation; (3) The associated ionospheric convection is consistent with the two‐cell pattern, and the poleward branch is located at the zonal flow shear. It is inferred that the equatorward branch is mapped to the geosynchronous region and outside, whereas the poleward branch is mapped farther down the tail to the source region of the R1 system. It is suggested that the intense ring current and injection of energetic electrons are necessary for the equatorward branch to be distinct, but the substorm expansion phase is not favorable for the formation of the emission gap. This explains why the double oval often takes place during magnetospheric storms and during the recovery phase of intense substorms. The equatorial magnetic field is expected to be strong in the mid‐tail region, which might be related to the auroral gap region. It is also found that the poleward branch occasionally intensifies in an extended MLT sector in a few minutes, which may be explained in terms of the sudden enhancement of the return (sunward) convection flow.

Diversion of plasma due to high pressure in the inner magnetosphere during steady magnetospheric convection

Steady magnetospheric convection (SMC) events in the Earth's magnetosphere are thought to result from balancing the rate of opening flux through solar wind‐magnetosphere reconnection at the dayside magnetopause to the rate of closing flux through reconnection in the magnetotail. For this to occur, reconnected flux in the tail must return to the dayside to balance the dayside reconnection rate. Using Geotail and THEMIS data over a span of 14 years, we examine the average plasma conditions and fast Earthward flows during SMC intervals and compare them to other types of geomagnetic activity, such as quiet intervals, isolated substorm phases, and the two hours before an SMC (Pre‐SMC intervals). We show that the average total pressure in the inner magnetosphere is higher during SMC events than for other types of activity. This higher pressure region extends to larger radial distances, and causes fast Earthward flows to divert toward the dawn or dusk flanks and continue to the dayside. This pattern is contrasted to substorms, during which flows are directed toward the inner magnetosphere and flux remains there in the “pile‐up region.” We suggest that the SMC pattern of flow deflection carries enough flux from the tail to the dayside to allow for balanced reconnection. Finally, the Pre‐SMC intervals have plasma conditions that are similar to, but slightly weaker than, SMC events. Since most SMCs begin with a substorm, this indicates that preconditioning of the magnetosphere by prior geomagnetic activity is important in setting up the magnetotail for an SMC state.

Multi-spacecraft observations of earthward flow bursts

On the basis of the plasma, electric and magnetic fields jointly observed by Cluster and the Double Star TC-1 spacecraft in the Earth’s magnetotail, we have investigated the earthward flow bursts by introducing the momentum equation in the X-direction in the ideal conditions of magneto hydrodynamics (MHD). One earthward flow burst with a peak in excess of 500 km/s was selected, when the four spacecraft of Cluster were located around −16 RE and TC-1 was located around −10 RE in the X-direction. The inter-spacecraft distances in Y and Z directions were smaller than the statistical spatial scales of the bursty bulk flows. When the Y components of E and −V×B were compared, there was no clear breakdown of the frozen-in condition during the earthward flow burst. With the measured plasma and magnetic parameters from two spacecraft at different positions in the magnetotail, the X component of the pressure gradient was calculated. Magnetic tension was calculated using the magnetic field measured at four points, which could be compared with the assumed constant in the past research with single satellite. When the pressure gradient and the magnetic tension were put into the MHD momentum equation, some samples of the earthward flow bursts were accelerated and some were decelerated. The braking process of the earthward flow burst was more complicated than what the past results had shown. The accelerated samples accounted for about one third of the whole earthward flow bursts and discontinuously located among the decelerated elements. The original single earthward flow burst event might be split into several short flow bursts when it was moving to the Earth. Our results may partly illustrate that the duration of fast flows during three phases of substorm becomes short near the Earth. The results are consistent with the past results that fast flows intrude to places earthward the typical braking region.

Earthward plasma sheet flows during substorm phases

[1] Magnetic reconnection in the Earth's magnetotail converts open magnetic flux to closed, producing fast flows in the plasma sheet. Earthward fast flows can thus be used as a proxy for energy and momentum transfer into the closed magnetosphere during the course of a substorm. Characteristics of flows observed before substorm onset can provide information on the role of the flows in substorm onset triggering as well as the sequence of substorm-related processes. We use 15 years of data obtained by Geotail, Cluster, and THEMIS to study statistically the distribution of earthward plasma sheet flows during substorms. Substorm phases are determined from time series of the AL index and the z-component of the interplanetary magnetic field. We find that the occurrence frequency of medium-speed (100 km/s 500 km/s) flows, on the other hand, does not clearly increase during the substorm growth phase. The increase at substorm onset is more gradual than for the medium-speed flows. The occurrence frequency of high-speed flows peaks at the beginning of substorm recovery. These results indicate that any effect of the flows on substorm onset triggering would be caused by medium rather than very high speed flows.