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。      

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.     

Implication of multiple dipolarization event near 9 RE for the physics of substorms

In 2001, 2002 and 2003, the Polar spacecraft probed the near equatorial plasma sheet at 9 R_E near local midnight. Using the magnetic field observations, the signatures at substorm onsets are studied. Close to the flux pile-up region, the Polar spacecraft readily detects the dipolarization front, especially for pseudo onsets. An event with two distinct onsets has been examined. The signatures are found to be consistent with the multiple-onset model suggested by Russell [Russell, C.T. How northward turnings of the IMF can lead to substorm expansion onsets. Geophys. Res. Lett. 27, 3257–3259, 2000] which is a modified Near-Earth Neutral Line (NENL) model. Another similar event is also examined showing the effects of different Interplanetary Magnetic Field (IMF) conditions upon substorms. Moreover, ground effects can be very weak compared to in situ observations, especially for pseudo onsets, because these signatures appear to be localized and not global.     

On the association of quiet-time Pi2 pulsations with IMF variations

The association of quiet-time Pi2 pulsations with the variations of the interplanetary magnetic field (IMF) has been examined by using three reported events, occurring during extremely quiet intervals, of which the first was on 10 March 1997, the second 27 December 1997, and the third 11 May 1999. For the first event, the onset time of ground Pi2s maps to the IMF structure bearing a variation cycle of north-to-south and north again as seen by Wind in the upstream region and Geotail in the magnetosheath. Likewise, the second and the third events have respectively, four and three recurrent turnings propagating to the Earth sensed by multiple satellites. The comparison of geomagnetic perturbations, auroral brightenings, and energetic particle data in the magnetotail with the IMF observations shows successive substorm-like activations accompanied by ground Pi2 onsets. For a clear variation cycle, the first Pi2 burst appears 36 ± 8 min after southward turning of the IMF and the second one follows14 ± 4 min after a northward turning. Moreover, ground Pi2 onsets recur under low IMF clock angle conditions. These observational results can be interpreted with the prevailing models of externally triggered substorm. But the solar wind coupling to the magnetosphere under quiet conditions proceeds in a less efficient way than under substorm time conditions. Consequently, we suggest that recurrent quiet-time Pi2s can be associated with IMF variations and their cause can be the same as those for substorm times.     

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.     

Relationship between substorm activity and the interplanetary medium parameters during the main phase of strong magnetic storms

In the present paper dependences of substorm activity on the solar wind velocity and southward component (Bz) of interplanetary magnetic field (IMF) during the main phase of magnetic storms, induced by the CIR and ICME events, is studied. Strong magnetic storms with close values of Dst_min?≈??100?±?10?nT are considered. For the period of 1979–2017 there are selected 26 magnetic storms induced by the CIR and ICME (MC?+?Ejecta) events. It is shown that for the CIR and ICME events the average value of the AE index (AE_aver) at the main phase of magnetic storm correlates with the solar wind electric field. The highest correlation coefficient (r?=?0.73) is observed for the magnetic storms induced by the CIR events. It is found that the AE_aver for magnetic storms induced by ICME events, unlike CIR events, increases with the growth of average value of the southward IMF Bz module. The analysis of dependence between the AE_aver and average value of the solar wind velocity (Vsw_aver) during the main phase of magnetic storm shows that in the CIR events, unlike ICME, the AE_aver correlates on the Vsw_aver.     

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.     

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.      

2000年7月空间天气大事件对地磁场的影响

2000年7月空间大事件对地磁场产生了巨大影响，7月15日至18日发生大磁暴(K=9)．磁暴为急始型，在我国地区初相期变幅有200—300nT，主相最大幅度有500—600nT，为多年来所罕见．在行星际磁场Bz由北向转向南向时，磁暴主相开始；南向分量达到最大值后大约2h，地磁H分量达到最小值，恢复相开始．并且，这次磁暴与太阳风也存在一定的对应关系。     

2004年12月13日IMF北向期间极区亚暴电离层电动力学特征

利用KRM地磁反演方法, 结合北半球中高纬度地磁台站数据, 研究了2004年12月13日行星际磁场北向期间发生的亚暴事件, 极区电离层电动力学参量(电流矢量、等效电流函数以及电势)的分布特征. 结果表明, 在该亚暴膨胀相起始后, 午夜之前西向电集流急剧增强, 且等效电流体系表现为夜侧双涡, 同时伴随夜侧增强的南向电场. 由于极弱的直接驱动过程, 卸载过程引起的电离层效应得到清楚显示. 卸载过程在膨胀相期间起绝对主导性作用. 同时, 夜侧电导率的增强是电集流区域电流急剧增强的主要原因.      

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.     

1998年5月空间天气大事件的地磁场响应

地磁场与1998年5月空间大事件相对应的是5月1日至16日发生的大磁暴（k=8)。磁暴主相开始的几个小时伴随有丰富的Pc型地磁脉动，包括P c2,Pc3,Pc4等。在增暴的恢复相，甚至还有Pc5巨型脉动，在行星标磁场Bz由北向转向南向时，磁暴主相开始；南向分量达到最大值后大约2小时，地磁H分量达到最小值，恢复相开始，并且，这次磁暴与太阳风电场也存在一定的对应关系。     

TC-1和Cluster对向阳侧磁层顶通量传输事件的联合观测研究

2004年2至4月期间,探测一号(TC-1)卫星和Cluster卫星有25次同时处在向阳侧磁层顶附近的磁鞘内,TC-1卫星在低纬区,Cluster卫星在中高纬区.利用这一期间两卫星探测到的27个通量传输事件(FTE),分析行星际磁场(IMF)横向分量BT={By,Bz}对磁层顶重联发生位置的影响,以及分量重联的观测事实,得到如下主要结果.(1)当IMF南向分量Bz占优势(|Bz|＞|By|)时,FTE大多(约占87.5%)能在低纬观测到,而当IMF By分量占优势(|Bz|＜|By|)时,则FTE大部分能在中高纬观测到(占84.2%);(2)很少观测到相关联的事件(关联事件指在低纬生成的FTE,向高纬运动中先后被TC-1卫星和Cluster卫星探测到的事件),表明在低纬形成的FTE可能大多沿磁层顶两侧滑向磁尾,只有少数可能运动到高纬地区;(3)中纬地区探测到的FTE大多是以分量重联方式产生于该区,而非来自磁赤道附近成对形成的FTE.      

On the source region of flux transfer events

We have examined the region of occurrence of flux transfer events for three distinct orientations of the interplanetary magnetic field: nearly horizontal in the solar magnetospheric equator, diagonally southward at 45° to the magnetospheric equator and nearly due south. For horizontal IMF conditions the FTE's occur in a horizontal band about ± 6 R_E wide. For diagonally southward IMF conditions, the FTE's occur in a diagonal swath about ± 6 R_E wide passing through the subsolar point. For duskward but nearly due southward IMF conditions, our observations reveal FTE's throughout the northern morning quadrant. These observations are consistent with a near equatorial source for flux transfer events and hence with component merging and not anti-parallel merging. These observations also help understand the energetic ion anisotropies seen in these events.     

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.     

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.      

亚暴起始地磁指数判断的统计研究

利用2004年地磁西向电急流 AL指数, 亚暴电急流AE指数和场向电流AF指数来确定亚暴起始, 并与2004年亚暴极光起始进行对比. 研究发现, 如果以极光亚暴起始为时间零点, 亚暴的西向电急流AL起始和电急流AE起始主要分布于-5~+6 min的时间范围内, 但在-9~+9 min的时间范围内也有个别事例. 场向电流 AF 起始分布较宽, 可以分布于-8~+7 min的时间范围内. 平均西向电急流AL起始, 电急流AE起始和场向电流AF起始分别为0.5, 0.5, -0.1min. 通常西向电急流AL起始与极光起始同时的概率最高, 而多数情况下电急流AE起始和场向电流AF起始提前极光起始1min. 这些地面磁场指数确定的亚暴起始分布, 随着亚暴强度的增大(即最小AL指数减少, 最大AE指数增大, 最大AF指数增大)而向极光亚暴起始靠近. 对于5个超级亚暴来说, 其西向电急流AL起始和电急流AE起始都发生在极光起始之前. 这些结果说明对于大亚暴, 电急流的增加要早于极光爆发.      

Two sources of magnetosheath ions observed by Cluster in the mid-altitude polar cusp

Double cusps have been observed on a few occasions by polar orbiting spacecraft and ground-based observatories. The four Cluster spacecraft observed two distinct regions, showing characteristics of a double cusp, during a mid-altitude cusp pass on 7 August 2004. The Wind spacecraft detected a southward turning of the Interplanetary Magnetic Field (IMF) at the beginning of the cusp crossings and IMF–Bz stayed negative throughout. Cluster 4 observed a high energy step in the ion precipitation around 1 keV on the equatorward side of the cusp and a dense ion population in the cusp centre. Cluster 1, entering the cusp around 1 min later, observed only a partial ion dispersion with a low energy cutoff reaching 100 eV, together with the dense ion population in the cusp centre. About 9 min later, Cluster 3 entered the cusp and observed full ion dispersion from a few keV down to around 50 eV, together with the dense ion population in the centre of the cusp. The ion flow was directed poleward and eastward in the step/dispersion, whereas in the centre of the cusp the flow was directed poleward and westward. In addition the altitude of the source region of ion injection in the step/dispersion was found 50% larger than in the cusp centre. This event could be explained by the onset of dayside reconnection when the IMF turned southward. The step would be the first signature of component reconnection near the subsolar point, and the injection in the centre of the cusp a result of anti-parallel reconnection in the northern dusk side of the cusp. A three-dimensional magnetohydrodynamic (MHD) simulation is used to display the topology of the magnetic field and locate the sources of the ions during the event.     

Interplanetary magnetic field control of plasma distribution in the polar ionosphere

We used the TEC (Total electron content) data of 5 min resolution obtained from the Madrigal database during solar-maximum winter (Nov. 6, 2000–Feb. 4, 2001) to study statistically the polar ionospheric plasma distribution response to different intensity and orientation of IMF By/Bz components. The sunlit high-density plasma extension from dayside to nightside is favored in negative IMF By and Bz conditions. With the magnitude of the negative Bz increasing, the time range corresponding to the distinct high-density extension feature expands, and the plasma density along the extension path enhances, which can be attributed to the interaction between dayside solar-produced ionization whose poleward limit is decided by terminator and convection extent mainly modulated by IMF Bz component. As for IMF By component influence on the sunlit plasma extension, the combination effect of convection and corotation electric fields is necessary to be considered.     

Multisatellite investigations of substorm onsets

Substorm onsets, identified by Pi2 pulsations observed on the AFGL Magnetometer Network, have been studied using ISEE 1 electric and magnetic field data and GOES 2 and GOES 3 magnetic field data. The relative positions of the spacecraft with respect to the substorm current system were determined from the Pi2 polarizations. One onset occurred when ISEE 1 and GOES 2 were on the same field line but in opposite hemispheres. During this onset ISEE 1 and GOES 2 observed magnetic signatures which appear to be due to conjugate field-aligned-currents flowing out of the western edge of the westward electrojets. A broadband burst of wave noise was seen in the ISEE 1 electric field at the same time as field-aligned-currents were observed. These may be current driven ion cyclotron waves. A three minute perturbation in the electric field data prior to the initial substorm onset indicates that there was an azimuthal westward flow of plasma starting ～ 1 min before the substorm onset.