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.      

Energy requirement of magnetic reconnection during magnetospheric substorms

Methods are discussed to estimate energy transfer from the solar wind to the magnetosphere during substorm growth phases. Observational and modeling constraints are then used to assess quantitatively the total amount of energy stored in the magnetotail. The major avenues of energy dissipation are examined and the energy that is released in the form of plasma sheet heating, ionospheric Joule heating, plasmoids, and energetic particle production during substorms is assessed. Energy sources are evaluated to drive substorm evolution in a particularly well-observed case: stored tail-lobe energy is sufficient to drive observed substorm dissipation processes (by large margins). On the other hand, energy in the closed field lines of the plasma sheet is insufficient to supply the substorm energy. Hence, magnetic reconnection is required on energy grounds during well-observed substorm cases.     

Outstanding issues in understanding the dynamics of the inner plasma sheet and ring current during storms and substorms

The paper deals with five selected issues of the dynamical coupling of the near-Earth plasma sheet and magnetosphere, (1) substorm initiation, (2) dipolarization, (3) pressure release of the outer magnetosphere via the auroral energy conversion process, (4) magnetization of the very high beta plasma assembling at the inner edge of the tail, and (5) penetration of energetic particles into the ring current below L 4. One outstanding and strongly debated subject is not discussed here, the origin of the substorm current wedge. The main conclusions (or personal preferences) are: (1) the substorm is initiated by formation of a near-Earth neutral line; (2) dipolarization occurs through magnetic flux transport by the earthward reconnection flow and not by current diffusion; (3) the auroral energy conversion process, the “auroral pressure valve”, contributes substantially to the pressure release during the substorms; (4) high beta ( 10) plasma breaks up into smaller scale blobs under slow magnetization; and (5) deep and prolonged penetration of hot plasma sheet plasma into the middle magnetosphere produces currents and electric fields which lead to the growth of the storm-time ring current.     

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.     

Saw-tooth substorms: Inconsistency of repetitive bay-like magnetic disturbances with behavior of aurora

The relationships between the magnetic disturbance onsets, aurora dynamics and particles injections at the geostationary orbit have been analyzed in detail for 25 sawtooth substorms. It is shown that inconsistency between the above signatures of the substorms onset is typical of the powerful sawtooth substorms, unlike the isolated (“classical”) magnetospheric substorms. The distinguishing feature of the aurora in case of saw-tooth substorms is permanently high level of auroral activity irrespective of the magnetic disturbance onsets and the double oval structure of the aurora display. The close relationship between the aurora behavior and the particle injections at geostationary orbit is also broken. The conclusion is made, that the classical concept of the substorm development, put forward by Akasofu (1964) for isolated substorms, is not workable in cases of the sawtooth disturbances, when the powerful solar wind energy pumping into the magnetosphere provides a permanent powerful aurora particle precipitation into the auroral zone.     

Plasma flow in the near and distant geomagnetic tail

Measurements of the bulk flow of plasma in the outer magnetosphere were first made a little over a decade ago with Los Alamos instruments on the Vela satellites. During the intervening years, as flow measurements have been made with improved instruments and by other satellites they have come to play a crucial role in the development of our understanding of the structure and dynamics of the magnetosphere. For example, they were the means of discovery of the magnetosphere's boundary layer and of plasma vortices within the plasma sheet. They were the essential ingredient in the identification of signatures of magnetic reconnection at the magnetopause. And they were indispensible in clarifying the complex phenomena in the magnetotail accompanying substorms and in showing that these phenomena are consistent with a substorm model involving magnetic reconnection at a near-earth neutral line. Most recently, magnetotail plasma flow measurements by the ISEE-3 satellite at distances as great as 230 R_E have been instrumental in fixing the average location of the “distant” neutral line at ～ 60 to 120 R_E and in identifying plasmoids (i.e., severed sections of the plasma sheet), released during substorms and escaping down-tail. This paper reviews the features of magnetotail plasma flow, describes the most recent observations, and discusses their implications for magnetospheric physics.     

Old tail lobes effect on the solar-wind – Magnetosphere energy transport for the 27 August 2001 substorm

The magnetic flux of tail lobes Ψ is divided in two parts of comparable values Ψ₁ and Ψ₀₂, with the first that appears during substorm and the second, observed before substorm start. The first was named “new magnetic flux”, the second – “old magnetic flux”. The first, Ψ₁, is known to play a definitive role in the energy transport from the solar wind into the magnetosphere-ionosphere-atmosphere system, but the role of Ψ₀₂ in this transport is not well known. From the 27 August 2001 substorm data we study the involvement in the above transport process of the old flux Ψ₀₂. This involvement is observed in the polar cap (PC) area, which existed prior to the substorm and is called respectively “the old PC”. In this study, as distinct from earlier works, we use the balance equation of the energy stored in magnetosphere and energy consumed. Activation of the old PC magnetic flux Ψ₀₂ was found to increase the energy input by ∼85% in the event under consideration.     

磁层亚暴恢复期间等离子片边界层区的Kelvin—Helmholtz不稳定性

本文根据两个理想的磁流体力学模型研究了磁层亚暴恢复期间等离子体片边界层区的Kelvin-Helmholtz不稳定性。一个模型由三个均匀的等离子体区组成--北尾瓣、等离子体片和南尾瓣。在此模型里,不稳定模式在北(或南)尾瓣和等离子体片之间的边界面上激发。另一个模型由五个均匀的等离子体区组成--北尾瓣、等离子体片北边界层、等离子体片中间层、等离子体片南边界层和南尾瓣,在等离子体片边界层区存在着两种不稳定模式,一种在等离子体片的北(或南)边界层和中间层之间的边界表面上激发(模式Ⅰ);另一种在等离子体片北(或南)边界层和北(或南)尾瓣之间的边界面上激发(模式Ⅱ).模式Ⅰ多数时间是不稳定的。这些不稳定模式可能引起卫星观测到的等离子体边界层的一些特征。      

晨昏电场对磁亚暴触发的影响

本文讨论了晨昏电场存在时磁尾等离子体片内撕裂模不稳定性的激发和增长问题。得到的结果可以用来解释晨昏电场对磁亚暴的触发以及加快等离子体片贮存能量的耗散等现象。我们推导了存在电场漂移时的撕裂模方程,并在均匀电流片模型下求解了该方程的解析解。结果表明触发不稳定性所允许的临界电流片宽度与电场大小近似成正比,这表明考虑晨昏电场效应后磁亚暴更易被激发。此外对于厚度相同的电流片来说,长波撕裂模的增长率也随晨昏电场的加强而增大。      

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.     

磁尾等离子体片对流对亚暴的影响

采用2(1/2)维全粒子电磁模拟方法研究了等离子体片中稳态对流及局地爆发高速流对磁层亚暴触发过程的影响.研究发现,地向瞬时局地高速流可触发磁场重联,导致储存于磁尾磁场能量的快速释放.但是,等离子体片稳态对流可抑制磁尾磁场重联过程.此项研究结果表明,局地爆发高速流能够触发磁层亚暴;而行星际磁场(IMF)持续南向时的稳态磁层对流期间,不易发生亚暴.      

Magnetic cloud and magnetosphere — Ionosphere response to the 6 November 1997 CME

In the present work we analyse the magnetic cloud (MC) at 1 AU on 9 November 1997. The appearance of a hotter and dense part (dense filament), with radial extent 10⁶ km, immediately behind the frontal part of the MC, is a distinctive feature of the event. The INTERBALL - Aurora] Probe had a chance to observe field-aligned currents in the mid - altitude magnetosphere during the substorm expansion phase intensification related to the dense filament. We emphasise the appearance of unusual “N”- shape magnetic structure, duration 3 min, amplitude 50 nT between field-aligned current region 1 and the magnetosphere lobe in the late evening hours. To explain some of the MC features we refer to a model scenario of the 6 November 1997 coronal mass ejection.     

A comparative analysis of the role of interplanetary magnetic field (IMF) and sudden impulse (SI) in triggering a substorm

During a typical Akasofu-type of substorm, the southward component of IMF Bz is necessary prior to the onset. However, a sudden compression of solar wind, if intense enough, can also sometimes trigger a substorm, and is independent of the IMF orientation. The Akasofu-type substorm and the Impulse-induced substorm may differ in their occurrence mechanism and ground-based observations. This is shown using the initial four substorm events discussed in this paper having distinctly different IMF and sudden impulse conditions. A question then arises is how will these signatures vary when both sudden impulse and a southward component of IMF Bz are present prior to the onset. To account for the same, we analyze two substorm events of 05th April 2010 and 22nd June 2015. The substorm onsets on these days not just coincided with the sudden impulse but also a southward component of IMF Bz was present prior to the onsets. The present study accounts for the similarities and differences among isolated IMF induced substorms, isolated impulse-induced substorms and when both sudden impulse and a southward component of IMF Bz are present. We examined the relative dominance between the two factors in triggering a substorm using ground-based and satellite-based observations. If IMF Bz is near zero, a strong pressure pulse and/or large IMF By can lead to particle precipitation away from the usual midnight. To further ensure whether a pressure pulse or IMF By predominantly influences the substorm onset location, a statistical analysis of isolated substorms will be needed.     

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.     

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.     

Comparison of global MHD simulation results with actual storm and substorm events

Global MHD models hold the promise of providing a physics-based understanding of magnetospheric structure and dynamics. As such models have become more sophisticated, and computing power has increased, it is now possible to model actual events using solar wind data as a boundary condition. In this paper we present some results MHD simulations of actual storm and substorm events. We will demonstrate that not only can the simulations reproduce details of events, they also are reproducing fundamental aspects of energy coupling between the solar wind and the magnetosphere in such a manner that we can distinguish storms and substorms.     

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.     

亚暴起始位置和膨胀相持续时间对行星际磁场Bz分量的响应

磁层亚暴是太阳风–磁层–电离层耦合过程中的重要爆发性事件,其特性受太阳风参数的影响很大。本文利用对IMAGE卫星在2000 － 2005年观测到的4193个亚暴起始事件,统计研究了在不同的行星际磁场（IMF）B_z 条件下亚暴起始位置和膨胀相持续时间。结果表明,南向IMF发生的亚暴比北向IMF下发生的亚暴要多。南向IMF条件下亚暴AE指数最大值的平均值基本上>600 nT,并有随南向IMF持续时间增大而增大的趋势。北向IMF条件下亚暴AE指数最大值的平均值基本上<500 nT,并有随北向IMF持续时间增大而减小的趋势。亚暴的起始磁纬度基本上位于65° － 70°之间。当南向IMF或北向IMF的持续时间增大,超过80 min时,北半球的亚暴起始磁纬度会降低。亚暴起始磁地方时大部分位于22:15 － 23:15 MLT之间。但整体分布比较分散,显示不出特别清晰的随IMF B_z持续时间变化的趋势。相比于南向的IMF,北向IMF期间发生亚暴的平均膨胀相持续时间增大了将近10 min,表明南向IMF期间,亚暴强度虽然较大,但其膨胀相持续时间较短,亚暴能量释放和耗散的速度更快。      

Statistical survey on sawtooth events,SMCs and isolated substorms

Solar wind driving can cause a variety of different responses in the magnetosphere. Strong and steady driving during geomagnetic storms may result in sawtooth events. Strong to moderate driving may be followed by either sawtooth events or steady magnetospheric convection (SMC) events. Lower solar wind energy input typically leads to the formation of isolated non-storm substorms. This study uses superposed epoch analysis to reveal the typical properties of these three event groups as well as their similarities and differences. We use IMF and solar wind parameters, as well as ground-based indices (AL, SYM-H, ASY-H, PCN) to examine the level of solar wind driving and its response in the magnetosphere. Our results show that sawtooth events are associated with the strongest ionospheric activity. The subgroups of events during constant solar wind _EY$E_Y$ show that the key difference between the events is the average solar wind speed. Particularly, the high activity during sawtooth events is driven by high solar wind speed, while the lowest average speed during the SMCs may explain the lack of substorm activity during the steady convection periods.