Do we understand the inner magnetotail dynamics during substorms?

Substorm evolution of the near-Earth (|X|<15 R_E) plasma sheet has been emphasized recently because the inner tail is thought to link closely to the substorm auroral activity in the ionosphere during the early stage of substorms. In this paper, we discuss how the inner tail substorm phenomena during the late substorm growth phase and early expansion phase are accounted for by the two prevailing substorm models, namely, the near-Earth neutral line model and the current disruption model. We find that the late growth phase features are more satisfactorily accounted for by the current disruption model than by the near-Earth neutral line model. In addition, detailed observations on current disruption show evidence inconsistent with the proposed idea of dipolarization being due to plasma flow braking from reconnection in the mid-tail region, which poses a difficulty to the near-Earth neutral line model as well.     

TC-1和Geotail对亚暴过程中近地等离子体片磁扰动的联合观测

以2004年9月28日02:53:20 UT的亚暴为例, 通过TC-1在磁尾约12.5 R_e 和Geotail卫星在近地磁尾等离子体片约8～9 R_e的联合观测, 研究亚暴触发过程中近地磁尾等离子体片中等离子体波动特征. 结果表明, 亚暴触发区是近地磁尾中心等离子体片中较小的一个区域, 在亚暴触发区中低混杂不稳定性在近地磁尾等离子体片中存在, 准垂直传播的低混杂波发生在亚暴触发过程中, 而亚暴触发过程中近地磁尾等离子体片外边界区内的磁场偶极化信号和扰动都非常微弱. 在亚暴触发和亚暴膨胀相过程中出现了多次具有不同特征的磁场偶极化现象.      

Plasma sheet oscillations and their relation to substorm development: Cluster and double star TC1 case study

We examined two consecutive plasma sheet oscillation and dipolarization events observed by Cluster in the magnetotail, which are associated with a pseudo-breakup and a small substorm monitored by the IMAGE spacecraft. Energy input from the solar wind and an associated enhancement of the cross-tail current lead to current sheet thinning and plasma sheet oscillations of 3–5 min periods, while the pseudo-breakups occur during the loading phase within a spatially limited area, accompanied by a localized dipolarization observed by DSP TC1 or GOES 12. That is, the so-called “growth phase” is a preferable condition for both pseudo-breakup and plasma sheet oscillations in the near-Earth magnetotail. One of the plasma sheet oscillation events occurs before the pseudo-breakup, whereas the other takes place after pseudo-breakup. Thus there is no causal relationship between the plasma sheet oscillation events and pseudo-breakup. As for the contribution to the subsequent small substorm, the onset of the small substorm took place where the preceding plasma sheet oscillations can reach the region.     

The energy release of substorm and magnetic storm: Its present state and future improvements by KuaFu mission

Considering the KuaFu mission, state of the energy release of substorm and storm is simply presented and it’s improvements by KuaFu mission are investigated. The KuaFu mission will provide us an opportunity to improve our understanding of the energy release during the storm and the substorms. The two KuaFu-B satellites flying in 180° phase-lagged formation in a polar orbit will allow synoptic observations of the auroral oval, central plasma sheet, ring current and other regions. It can monitor the polar region 24/7 continuously. The advantage of the KuaFu mission is to provide the data during all phases of storm and substorm time that can be used to study the global energy release during all phases continuously. The data from auroral imager and other in-situ instruments on board KuaFu-B can be used to study the auroral dynamics and Joule heating during a storm and substorm. The data from the neutral atom imager instrument can be used to study the dynamics and the energy release in the ring current region from sudden commencement to complete storm recovery. Furthermore the data from KuaFu-A, which is around L1 point, can be used to study the interplanetary conditions along with the data from the plasma sheet to study the triggering process and energy release during a substorm. So, KuaFu mission with its continuous time monitoring facilities would enable us to make much progress towards solving the underlying problems.     

Study of three dimensional structure of the fast convection flow in the plasma sheet by MHD simulations on the basis of spontaneous fast reconnection model

Three dimensional structure of the fast convection flow in the plasma sheet is examined using magnetohydrodynamic (MHD) simulations on the basis of spontaneous fast reconnection model. The fast flow observed in the near-Earth magnetotail is one of the key phenomena in order to understand the causal relationship between magnetic substorm and magnetic reconnection. In this paper, we focus on this earthward fast flow in the near-Earth magnetotail. Our previous studies have shown that the fast reconnection produces the Alfvénic fast reconnection outflow and drastic magnetic field dipolarization in the finite extent. In this paper, the results of our simulations are compared with those of the in-situ observations in the geomagnetotail. They have consistent temporal profiles of the plasma quantities. It is suggested that the fast convection flows are caused by spontaneous fast reconnection.     

利用THEMIS卫星观测结果分析亚暴期间等离子体片尾向膨胀

利用THEMIS卫星观测结果,分析2008年3月13日10:40UT-12:10UT的一次中等亚暴事件在磁尾的全球演化过程.在该过程中,THEMIS的5颗卫星在午夜区附近沿x轴依次排列,离地心距离约8.7~13.2R_e.亚暴触发开始后,磁场偶极化和等离子体片的膨胀依次被在磁尾不同位置的卫星观测到.等离子体尾向膨胀的平均速度约为140km·s-1.在此次亚暴事件中可观测到两种类型的偶极化.一种为偶极化锋面,其与爆发性整体流（BBF）密切相关;另一种为全球偶极化,其与等离子体片的膨胀密切相关.亚暴触发开始约7min后,THEMIS卫星在低中高纬都可以观测到Pi2脉动的发生,且Pi2脉动的振幅随着纬度的升高逐渐变大.此次亚暴事件中的离子整体流速度主要是由离子电漂移速度引起的,测得的电场为局地磁通量变化导致的感应电场.      

Particle simulation study of substrom triggering with a southward IMF

This paper reports the spatial and temporal development of bursty bulk flows (BBFs) created by reconnection as well as current disruptions (CDs) in the near-Earth tail using our 3-D global electromagnetic (EM) particle simulation with a southward turning interplanetary magnetic field (IMF) in the context of the substorm onset. Recently, observations show that BBFs are often accompanied by current disruptions for triggering substorms. We have examined the dynamics of BBFs and CDs in order to understand the timing and triggering mechanism of substorms. As the solar wind with the southward IMF advances over the Earth, the near-Earth tail thins and the sheet current intensifies. Before the peak of the current density becomes maximum, reconnection takes place, which ejects particles from the reconnection region. Because of earthward flows the peak of the current density moves toward Earth. The characteristics of the earthward flows depend on the ions and electrons. Electrons flow back into the inflow region (the center of reconnection region), which provides current closure. Therefore the structure of electron flows near the reconnection region is rather complicated. In contrast, the ion earthward flows are generated far from the reconnection region. These earthward flows pile up near the Earth. The ions mainly drift toward the duskside. The electrons are diverted toward the dawnside. Due to the pile-up, dawnward current is generated near Earth. This dawnward current dissipates rapidly with the sheet current because of the opposite current direction, which coincides with the dipolarization in the near-Earth tail. At this time the wedge current may be created in our simulation model. This simulation study shows the sequence of the substorm dynamics in the near-Earth tail, which is similar to the features obtained by multisatellite observations. Identification of the timing and mechanism of triggering substorm onset requires further studies in conjunction with observations.     

Application of an MHD simulation to the study of substorms

The substorm mechanism is studied by the numerical solutions obtained from a resistive magnetohydrodynamic (MHD) simulation. After a southward turning of the interplanetary magnetic field (IMF), the simulation results reproduce observed features of the growth phase. The numerical solutions show that the plasma sheet thinning during the growth phase is formed under the dynamic balance between the flux pileup from the midtail and the flux removal toward the dayside controlled by the convection in the magnetosphere-ionosphere (M-I) coupling system. After the growth phase, dipolarization is generated in the near-earth tail accompanied by a plasma injection into the inner magnetosphere, the formation of plasmoid in the midtail, and the enhancement of the nightside field-aligned currents (FACs). The direct cause of this onset is the state (phase space) transition of the convection system from a thinned state to a dipolarized state associated with a self-organization in the nonlinear system.     

Sawtooth substorms generated under conditions of the steadily high solar wind energy input into the magnetosphere: Relationship between PC, AL and ASYM indices

Periodicity in occurrence of magnetic disturbances in polar cap and auroral zone under conditions of steady and powerful solar wind influence on the magnetosphere is analyzed on the example of 9 storm events with distinctly expressed sawtooth substorms (N = 48). Relationships between the polar cap magnetic activity (PC-index), magnetic disturbances in the auroral zone (AL-index) and value of the ring current asymmetry (ASYM index) were examined within the intervals of the PC growth phase and the PC decline phase inherent to each substorm. It is shown that the substorm sudden onsets are always preceded by the PC growth and that the substorm development does not affect the PC growth rate. On achieving the disturbance maximum, the PC and AL indices are simultaneously fall down to the level preceding the substorm, so that the higher the substorm intensity, the larger is the AL and PC drop in the decline phase. The ASYM index increases and decreases in conformity with the PC and AL behavior, the correlation between ASYM and PC being better than between ASYM and AL. Level of the solar wind energy input into the magnetosphere determines periodicity and intensity of disturbances: the higher the coupling function E_KL, the higher is substorm intensity and shorter is substorm length. Taking into account the permanently high level of auroral activity and inconsistency of aurora behavior and magnetic onsets during sawtooth substorms, the conclusion is made that auroral ionosphere conductivity is typically high and ensures an extremely high intensity of field-aligned currents in R1 FAC system. The periodicity of sawtooth substorms is determined by recurrent depletions and restorations of R1 currents, which are responsible for coordinated variations of magnetic activity in the polar cap and auroral zone.     

A statistical analysis of substorm associated tail activity

Substorm onset timing is a critical issue in magnetotail dynamics research. Solar wind energy is accumulated in the magnetosphere and the configuration of the magnetosphere evolves toward an unstable state during the growth phase. At some point, the expansion phase begins and the stored energy is released through a variety of processes that return the magnetosphere to a lower energy state. In recovery the various processes die away. Unfortunately, the ground and magnetospheric signatures of onset, i.e. energy release, can be seen both in the growth phase prior to onset and in the expansion phase after onset. Some investigators refer to each of these events as a substorm. Tail observations suggest that most substorms have one event that differentiates the behavior of the tail field and plasma. We refer to this time as the “main substorm onset”. Each substorm associated phenomenon is timed independently and then compared with main substorm onsets. ISEE-2 tail observations are used to examine the tail lobe magnetic conditions associated with substorms because ISEE-2 orbit has a high inclination and frequently observes lobe field. Approximately 70 ∼ 75% of tail lobe B_t and B_z change are associated with the main substorm onset. If the satellite is more than 3 Re above (below) the neutral sheet, 86% (57%) of plasma pressure dropouts are associated with substorms. We interpret our results as evidence that the effect of the growth phase is to drive the magnetosphere towards instability. As it approaches global instability local regions become temporarily unstable but are rapidly quenched. Eventually one of these events develops into the global instability that releases most of the stored energy and returns the magnetosphere to a more stable configuration.     

太阳风扰动对极光电急流和环电流指数的影响

利用WIND卫星的太阳风观测数据和地磁活动指数, 研究了太阳风扰动对环电流SYM-H指数, 西向极光电急流AL指数和东向极光电急流AU指数的影响. 结果表明, 太阳风动压增长和减少能够同步或延迟引起地磁活动指数的强烈扰动, 其包括环电流指数的上升, 西向极光电急流指数的下降和东向极光电急流指数的上升. 太阳风动压的突然剧烈增加还能够触发超级亚暴和大磁暴. 太阳风动压脉冲引起的地磁效应具有复杂的表现形式, 这说明太阳风动压脉冲的地磁效应不仅与太阳风动压脉冲大小和持续时间有关, 还与磁层本身所处的状态有关. 时间尺度较长, 消耗能量较大的磁暴只有大的持续时间较长的太阳风动压脉冲才能激发.      

Circulation of energetic ions of terrestrial origin in the magnetosphere

Energetic ion composition measurements have now been performed from earth orbiting satellites for more than a decade. As early as 1972 we knew that energetic (keV) ions of terrestrial origin represented a non-negligible component of the storm time ring current. We have now assembled a significant body of knowledge concerning energetic ion composition throughout much of the earth's magnetosphere. We know that terrestrial ions are a common component of the hot equatorial magnetospheric plasma in the ring current and the plasma sheet out to ? 23 R_E. During periods of enhanced geomagnetic activity this component may become dominant. There is also clear evidence that the terrestrial component (specifically O⁺) is strongly dependent on solar cycle. Terrestrial ion source, transport, and acceleration regions have been identified in the polar auroral region, over the polar caps, in the magnetospheric boundary layers, and within the magnetotail lobes and plasma sheet boundary layer. Combining our present knowledge of these various magnetospheric ion populations, it is concluded that the primary terrestrial ion circulation pattern associated with enhanced geomagnetic activity involves direct injection from the auroral ion acceleration region into the plasma sheet boundary layer and central plasma sheet. The observed terrestrial component of the magnetospheric boundary layer and magnetotail lobes are inadequate to provide the required influx. They may, however, contribute significantly to the maintenence of the plasma sheet terrestrial ion population, particularly during periods of reduced geomagnetic activity. It is further concluded, on the basis of the relative energy distributions of H⁺ and O⁺ in the plasma sheet, that O⁺ probably contributes significantly to the ring current population at energies inaccessible to present ion composition instrumentation (? 30 keV).     

ISEE-1 and 2 observations of field-aligned currents in the distant midnight magnetosphere

Magnetic field measurements obtained in the nightside magnetosphere by the co-orbiting ISEE-1 and 2 spacecraft have been examined for signatures of field-aligned currents (FAC). Such currents are found on the boundary of the plasma sheet both when the plasma sheet is expanding and when it is thinning. Plasma sheet boundary layer current structure and substorm associated dynamics can be determined using the two spacecraft, although for slow traversals of the FAC sheet the spatial/temporal ambiguity is still an issue. We often find evidence for the existence of waves on the plasma sheet boundary, leading to multiple crossings of the FAC sheet. At times the boundary layer FAC sheet orientation is nearly parallel to the X-Z GSM plane, suggesting ‘protrusions’ of plasma sheet into the lobes. The boundary layer current polarity is, as expected, into the ionosphere in the midnight to dawn local time sector, and outward near dusk. Current sheet thicknesses and velocities are essentially independent of plasma sheet expansion or thinning, having typical values of 1500 km and 20–40 km/s respectively. Characteristic boundary layer current densities are about 10 nanoamps per square meter.     

Remote sensing of the ring current using energetic neutral atoms

Energetic neutral atom (ENA) images of the storm-time ring current obtained from the ISEE-1 spacecraft provide information for a “zero-order” global model of the energetic ion distribution. With the assumption of isotropic pressure and magnetostatic, non-convective pressure balance, the global system of electrical currents driven by the ion pressure can be calculated using Euler potentials for the divergenceless current density. Radial pressure gradients drive azimuthal currents, and azimuthal pressure gradients drive radial currents. The radial currents cause current lines in the inner magnetosphere to close in the ionosphere, forming a “partial” ring current. The intensities and locations of these field-aligned currents driven into and out of the ionosphere resemble those of the observed Region 2 current system, but not all observed properties of the Region 2 system are reproduced by the “zero-order” model.     

地球磁层亚暴统计分析

利用AL和AE指数对第24个太阳活动周发生的亚暴事件进行统计分析.主要统计了关于磁层亚暴的强度，亚暴初值与恢复值的关系，亚暴持续时间，亚暴恢复相与增长相（包括膨胀相）持续时间的关系等.统计结果表明:在第24个太阳活动周中2008-2016年发生的亚暴事件大部分比较剧烈，其峰值大都在200～1200nT；初值和恢复值大都在30～100nT，并且事件占比符合正态分布；大部分亚暴都能恢复到亚暴初值60nT以内，并且差值越小，事件的占比越大.大部分亚暴的持续时间较长，在100～400min之间，其中增长相（包括膨胀相）持续时间均在120min以内，并且持续时间越长，其事件占比越小；大部分亚暴事件的恢复相持续时间在60～300min之间，并且呈现出正态分布特征.绝大多数亚暴事件的恢复相持续时间为增长相持续时间的10倍以下，其中约一半亚暴事件的恢复相持续时间为增长相持续时间的1～4倍.这说明亚暴的能量聚集速度约为能量释放速度的1～4倍.      

Relationship of the diffuse aurora and SAR arc dynamics to substorms and storms

Investigation results of a diffuse aurora (DA) and stable auroral red (SAR) arc dynamics based on spectrophotometric observations at the Yakutsk meridian (199°E geomagnetic longitude) are presented. The relationship of an equatorward extension of DA in the 557.7 nm emission to a substorm growth phase during the magnetospheric convection intensification after the turn of IMF B_Z to the south is shown. The formation of SAR arc during the substorm expansion phase is investigated. The association of SAR arc dynamics with the development of asymmetric ring current (substorm injection) during the main phase of a storm is analyzed. It is shown how the pulsating precipitations of energetic ring current particles develop in the outer plasmasphere based on photometric observations.     

近地尾向流和磁场偶极化——TC-1卫星观测结果

2004年10月12日, 在01:30---04:30 UT期间, 位于向阳侧磁层顶附近的Geotail卫星探测到行星际磁场为持续南向. 此太阳风条件驱动了一个小磁暴, Sym-H指数在04:12 UT达到最小值-33nT. 在磁暴主相期间, AE指数维持在较高的水平, 其最大值达400nT. 02:00---03:00 UT期间, TC-1卫星在近地磁尾(-10.6, 3.2, -0.1)Re处观测到明显的亚暴膨胀相特征和磁场偶极化过程. 在偶极化前1min, 有较强的(v_x<-100 km/s)持续时间超过3min的尾向流发生. 分析发现该尾向流具有低温、高密度和沿磁场流动的特点, 这说明尾向流具有来源于电离层风的特征. 尾向流期间, TC-1观测的磁场分量Bx和总的磁场强度增加, 磁倾角减小, 磁场结构变成非偶极型, 说明尾向流对磁场结构有一定的影响, 文中尝试给出了相应的物理解释. 观测表明, 该事例中的近地磁尾尾向流可能对磁场偶极化过程的发生有重要意义.      

Substorms in the inner plasma sheet

Thin Current Sheets (TCS) are regularly formed prior to substorm breakup, even in the near-Earth plasma sheet, as close as the geostationary orbit. A self-consistent kinetic theory describing the response of the plasma sheet to an electromagnetic perturbation is given. This perturbation corresponds to an external forcing, for instance caused by the solar wind (not an internal instability). The equilibrium of the configuration of this TCS in the presence of a time varying perturbation is shown to produce a strong parallel thermal anisotropy (T T) of energetic electrons and ions (E>50keV) as well as an enhanced diamagnetic current carried by low energy ions (E<50keV). Both currents tend to enhance the confinement of this current sheet near the magnetic equator. These results are compared with data gathered by GEOS-2 at the geostationary orbit, where the magnetic signatures of TCS, and parallel anisotropics are regularly observed prior to breakup. By ensuring quasi-neutrality everywhere we find, when low frequency electromagnetic perturbations are applied, that although the magnetic field line remains an equipotential to the lowest order in T_e/T_i, a field-aligned potential drop exists to the next order in (T_e/T_i). Thus the development of a TCS implies the formation of a field-aligned potential drop ( few hundred volts) to ensure the quasi-neutrality everywhere. For an earthward directed pressure gradient, a field-aligned electric field, directed towards the ionosphere, is obtained, on the western edge of the perturbation (i.e. western edge of the current sheet). Thus field aligned beams of electrons are expected to flow towards the equatorial region on the western edge of the current sheet. We study the stability of these electron beams and show that they are unstable to “High Frequency” (HF) waves. These “HF” waves are regularly observed at frequencies of the order of the proton gyrofrequency (f_H+) just before, or at breakup. The amplitude of these HF waves is so large that they can produce a strong pitch-angle diffusion of energetic ions and a spatial diffusion that leads to a reduction of the diamagnetic current. The signature of a fast ion diffusion is indeed regularly observed during the early breakup; it coincides with the sudden development of large amplitude transient fluctuations, ballooning modes, observed at much lower frequencies (fH+). These results suggest that the HF waves, generated by field-aligned electron beams, provide the dissipation which is necessary to destabilize low frequency (ballooning) modes.     

Images of the Earth's aurora and geocorona from the dynamics explorer mission

Several results from analyses of auroral and geocoronal images from the Dynamics Explorer Mission are summarized. (1) The motion of the transpolar arc of a theta aurora is found to be correlated with the y-component of the interplanetary magnetic field. The arc motion is in the general direction of the y-component. (2) A sequence of global images of a small auroral substorm shows the initial development of intense luminosities in a relatively small spatial region, or ‘bright spot’, in the pre-midnight sector of the auroral oval and a subsequent appearance of an expanding area of lesser intensities at lower latitudes and contiguous to the midnight boundary of the bright spot. This evolution of auroral luminosities is interpreted in terms of acceleration of electrons in the boundary layer of the magnetotail plasma sheet to produce the bright spot and subsequent injection into, and eastward drift within, the plasma sheet to form the diffuse area of lesser intensities. (3) A series of images of the Earth's geocorona in scattered solar Ly α emissions is used to obtain a best-fit spherical model of atomic hydrogen densities in the Earth's exosphere. A Chamberlain model provides an adequate fit to radial distances of 4.5 R_E, beyond which an exponential fit is used. The geocoronal tail is detected as an asymmetric increase in scattered Ly α intensities in the anti-solar direction.     

Controlling factors of Region 2 field-aligned current and its relationship to the ring current: Model results

One essential component of magnetosphere and ionosphere coupling is the closure of the ring current through Region 2 field-aligned current (FAC). Using the Comprehensive Ring Current Model (CRCM), which includes magnetosphere and ionosphere coupling by solving the kinetic equation of ring current particles and the closure of the electric currents between the two regions, we have investigated the effects of high latitude potential, ionospheric conductivity, plasma sheet density and different magnetic field models on the development of Region 2 field-aligned currents, and the relationship between R2 FACs and the ring current. It is shown that an increase in high latitude potential, ionospheric conductivity or plasma sheet density generally results in an increase in Region 2 FACs’ intensity, but R2 FACs display different local time and latitudinal distributions for changes in each parameter due to the different mechanisms involved. Our simulation results show that the magnetic field configuration of the inner magnetosphere is also an important factor in the development of Region 2 field-aligned current. More numerical experiments and observational results are needed in further our understanding of the complex relationship of the two current systems.