亚暴期间场向电流的特征

分析了由ISEE-1和ISEE-2飞船在亚暴期间观测到的场向电流的特性,包括场向电流所伴随的行星际磁场、地方时与L值分布以及场向电流强度和密度随亚暴位相的变化,并着重分析了亚暴电流楔的特征。结果表明,亚暴膨胀相的场向电流强度和密度都达到极大值;亚暴的膨胀相是由行星际磁场转向或南向分量变小触发的,其起始位置很可能位于近地等离子体片内边缘。     

Field-aligned current distribution and response to interplanetary conditions during a superstorm―CHAMP observation

With geomagnetic measurements on board of CHAMP satellite, the characteristics of global large-scale field-aligned currents （FACs） in the topside ionosphere are investigated along with their responses to interplanetary conditions for the superstorm of November, 2003. It is found that （1） The storm-time FAC densities enhanced greatly in comparison with quiet period and the enhancements show hemispheric asymmetry of both summer-winter and sunlit-dark. （2） For the first time, it is revealed that the lati- tude-integrated FAC density is controlled mainly by solar wind dynamic pressure rather than iMF. （3） FACs expanded equatorward dramatically, with the lowest latitude being 45° MLat or more; on the dayside this expansion was controlled directly by IMF Bz, showing an interaction time scale of about 25 min in the solar wind-magnetosphere-ionosphere coupling system, and a nonlinear saturation of the equatorward expansion when IMF Bz 〈 -30 nT; while on the nightside, the expansion and recovery lagged about 3 h behind the IMF changes but nearly in phase with changes of SYM-H index. （4） During the storm main phase, the nightside FAC latitude coverage extended to 25° or wider, appearing multi-sheet current structure with more than 10 sheets.     

Pressure gradient evolution in the near-Earth magnetotail at the arrival of BBFs

Using in situ observations from THEMIS A, D and E during the 2008-2011 tail season, we present a statistical study of the evolution of pressure gradients in the near-Earth tail during bursty bulk flow （BBF） convection. We identified 138 substorm BBFs and 2,197 non-substorm BBFs for this study. We found that both the pressure and the Bz component of the magnetic field were enhanced at the arrival of BBFs at the spacecraft locations. We suggest that the increase of Bz during non-substorm BBFs is associated with flux pile-up. However, the much stronger enhancement of Bz during substorm BBFs implies the occurrence of magnetic field dipolarization which is caused by both the flux pile-up process and near-Earth current disruption. Furthermore, a bow-wave-like high pressure appears to be formed at the arrival of substorm BBFs, which is responsible for the formation of region-1-sense FACs. The azimuthal pressure gradient associated with the arrival of substorm BBFs lasts for about 5 min. The enhanced pressure gradient associated with the bow wave is caused by the braking and diversion of the Earthward flow in the inner plasma sheet. The results from this statistical study suggest that the braking and azimuthal diversion of BBFs may commonly create azimuthal pressure gradients, which are related to the forrnation of the FAC of the substorm current wedge.     

Continuous lobe reconnection in the mid-tail and its relationship to substorms： Cluster observations of continuous lobe reconnection in the mid-magneto tail

When the IMF turns southward, a great amount of magnetic energy is stored in the magnetotail, and the electric field across the magnetotail substantially enhances. As long as magnetic reconnection （MR） in the magnetotail initiates and continues, the magnetic field and plasma in the central plasma sheet are carried away to the near-Earth and down to the tail, the magnetic field and plasma in the lobe region enter the CPS and are involved in MR. We call this process “Continuous Lobe Reconnection （CLR）”. In this paper a detailed analysis of Cluster observation of MR through 2001--2003 is made. Plenty of CLR events are found that led to considerable changes of tail configuration, appearance of BBF, as well as large-scale bubbles in which both plasma temperature and number density substantially decrease. It is shown that in general CLR events last for dozens of minutes and have good correspondence to substorm initiation under the condition of continuous southward IMF.     

场向电流与地磁和亚暴活动的相关性

检验了场向电流与地磁和亚暴活动的相关性。按极光电集流指数AL的大小,地磁活动被分为4个水平,即|AL|<=50,50<|AL|<=150,150<|AL|<=300和|AL|>300分别相应于非常平静、平静、扰动和强烈扰动。统计结果表明,场向电流的发生率、强度和密度都随地磁活动的增加而增大。在最低活动水平,发生率只有8.9%,而73.2%的事件发生在扰动水平以上。对于夜间的场向电流,65.5%的事件伴随着磁层亚暴。     

Continuous tailward flow in the near-Earth magnetotail observed by TC-1 satellite

On July 11, 2004, a substorm process in the period of continuous tailward flow was observed by the joint exploration of the TC-1, IMAGE and ACE satellites. The substorm observed by the TC-1 in the near-Earth has three stages: the growth phase (from 11:43 to 12:19), the pre-expansion process (from 12:19 to 12:28) and the dipolarization process. The auroral brightening was at 12:26 recorded by the FUV instrument on IMAGE, and the dipolarization occurred two minutes later. During the 45 min period of the tailward flow, the magnetotail experienced the growth phase and the pre-expansion process. When the dipolarization process began, the TC-1 entered the plasma sheet and observed a high speed earthward flow. The field-aligned tailward flow is characterized by the low temperature and high density, which is consistent with the properties of the flow from the ionosphere detected in the near-Earth magnetotail by other satellites. The tailward flow is closely related with the southward interplanetary magnetic field (IMF), and may have an important effect on the substorm.     

Auroral substorm response to solar wind pressure shock

Two cases of auroral substorms have been studied with the Polar UVI data, which were associated with solar wind pressure shock arriving at the Earth. The global aurora activities started about 1–2 min after pressure shocks arrived at dayside magnetopause, then nightside auroras intensified rapidly 3–4 min later, with auroral substorm onset. The observations in synchronous orbit indicated that the compressing effects on magnetosphere were observed in their corresponding sites about 2 min after the pressure shocks impulse magnetopause. We propose that the auroral intensification and substorm onset possibly result from hydromagnetic wave produced by the pressure shock. The fast-mode wave propagates across the magnetotail lobes with higher local Alfven velocity, magnetotail was compressed rapidly and strong lobe field and cross-tail current were built in about 1–2 min, and furthermore the substorm was triggered due to an instability in current sheet.     

磁尾等离子体片边界层场向电流与行星际磁场时钟角的关系

根据ClusterII卫星的探测数据,对1 839个磁尾等离子体片边界层场向电流事件与行星际磁场(IMF)时钟角Φ的关系进行了统计分析。结果显示等离子体片边界层场向电流的发生率明显受到IMF时钟角的调控:当0°<|Ф|<90°,场向电流发生率随IMF时钟角的变化曲线为一"V"型结构。当90°<Ф<180°时场向电流发生率保持在很高的水平上;当-180°<Ф<-90°时,场向电流的发生率先减小后增大,其拐点在Ф=-130°附近。这说明场向电流不仅受行星际Bz的影响,也受行星际磁场By的影响。     

Dependence of magnetic field just inside the magnetopause on subsolar standoff distance: Global MHD results

The subsolar magnetopause is the boundary between the solar wind and the Earth’s magnetosphere,where reduced solar wind dynamic pressure is equal to the magnetic pressure of the Earth’s outer magnetosphere.We use a global magnetohydrodynamic (MHD)model to estimate the ratio f of the compressed magnetic field just inside the subsolar magnetopause to the purely dipolar magnetic field.We also compare our numerical results to a similar work by Shue,which used Time History of Events and Macroscale Interactions during Substorms(THEMIS)data.Our results show that the ratio f is linearly proportional to the subsolar magnetopause standoff distance(r0)for both the northward and southward interplanetary magnetic field,properties consistent with Shue but with a smaller proportionality constant.However,previous theoretical studies show that f is nearly independent of the subsolar standoff distance.The global model results also show that f is smaller for the southward Interplanetary Magnetic Field(IMF)under the same r0,and that the proportionality constant for the southward IMF is larger than that for the northward IMF.Both conclusions agree with statistical results from observations by Shue.     

Enhanced anti-sunward flow near local noon during a period of horizontal IMF and high solar wind velocity <Emphasis Type="Italic">V</Emphasis><Subscript><Emphasis Type="Italic">Y</Emphasis></Subscript>

It is believed that a southward interplanetary magnetic field (IMF) is mainly responsible for the energy input from solar wind into the magnetosphere. This paper presents an unusual case of strong anti-sunward plasma flow (up to 2 km/s) in the polar cap ionosphere and large cross-polar cap potential (CPCP) during a period of horizontal IMF (|B _Z | < 2 nT) observed by both ACE (at the L1 point) and Geotail (on the dusk flank of the magnetosheath). The CPCP is even higher than that under preceding B _Z ≈ −23 nT. Furthermore, GOES8 observed that the magnetosheath field turns northward as the anti-sunward plasma flow and CPCP start to increase, which implies that the magnetosheath field interacting with the Earth’s magnetopause has significantly rotated and differs from the IMF observed by ACE and Geotail. In accordance with previous theoretical work, we suggest that the magnetic field line draping produces a southward magnetosheath field and enhances anti-sunward plasma flow and the CPCP.     

磁层亚暴过程中高速流分布

采用实际的磁尾位型,利用数值模拟方法研究磁层亚暴过程中高速流分布。结果表明垂直磁场的高速流主要分布在电流片内,范围在|Z|<0.25RE;平行磁场高速流分布区域在两个区域较大,一个是距离地球较近的磁场较强的区域,另一个是距离等离子体片较近磁场较强的区域。     

Magnetic Storm Effects in the Auroral Ionosphere Observed with EISCAT Radar -Two Case Studies

Storm-time changes of main plasma parameters in the auroral ionosphere are analyzed for two intense storms occurring on May 15, 1997 and Sept. 25, 1998, with emphasis on their relationship to the solar wind dynamic pressure and the IMF Bz component. Strong hard particle precipitation occurred in the initial phase for both storma,associated with high solar wind dynamical pressure. During the recovery phase of the storms, some strong particle precipitation was neither concerned with high solar wind pressure nor southward IMF Bz. Severe negative storm effects depicted by electron density depletion appeared in theF-region during the main and recovery phase of both storms, caused by intensive electric field-related strong Joule/frictional heating when IMF was largely southward. The ion temperature behaved similarly in E- and F-region, but the electron temperature did quite different, with a strong increase in the lower E-region relating to plasma instability excited by strong electric field and a slight decrease in the F-region probably concerning with a cooling process. The field-aligned ion velocity was high and apparently anticorrelated with the northward component of the ion convection velocity.     

Responses of properties in the plasma sheet and at the geosynchronous orbit to interplanetary shock

On July 22, 2004, the WIND spacecraft detected a typical interplanetary shock. There was sustaining weak southward magnetic field in the preshock region and the southward field was suddenly enhanced across the shock front （i.e., southward turning）. When the shock impinged on the magnetosphere, the magnetospheric plasma convection was abruptly enhanced in the central plasma sheet, which was directly observed by both the TC-1 and Cluster spacecraft located in different regions. Simultaneously, the Cluster spacecraft observed that the dawn-to-dusk electric field was abruptly enhanced. The variations of the magnetic field observed by TC-1, Cluster, GOES-10 and GOES-12 that were distributed in different regions in the plasma sheet and at the geosynchronous orbit are obviously distinct. TC-1 observations showed that the magnetic intensity kept almost unchanged and the elevation angle decreased, but the Cluster spacecraft, which was also in the plasma sheet and was further from the equator, observed that the magnetic field was obviously enhanced. Simultaneously, GOES-12 located near the midnight observed that the magnetic intensity sharply increased and the elevation angle decreased, but GOES-10 located in the dawn side observed that the magnetic field was merely compressed with its three components all sharply increasing. Furthermore, the energetic proton and electron fluxes at nearly all channels observed by five LANL satellites located at different magnetic local times （MLTs） all showed impulsive enhancements due to the compression of the shock. The responses of the energetic particles were much evident on the dayside than those on the nightside. Especially the responses near the midnight were rather weak. In this paper, the possible reasonable physical explanation to above observations is also discussed. All the shock-induced responses are the joint effects of the solar wind dynamic pressure pulse and the magnetic field southward turning.     

Correlation between continuous lobe reconnection in the mid magnetotail and substorm expansion onset

Data on plasma sheet crossing measured by Cluster/HIA and Cluster/FGM during the period from July to October in 2001 -2003 are analyzed. Based on previous work on the characteristic features of continuous lobe reconnection (CLR) described in reference, two case studies and a statistical analysis were carried out on correlation between CLR in the mid magnetotail and substorm expansion onset for the events occurring during this period. It is found that almost all CLR events are in close connection with substorms. The beginning of CLR is almost always a few minutes ahead of substorm activities seen in the near Earth magnetotail and on the ground-based stations. This provides a clear indication that CLR is the virtual cause of substorm expansion onset during the period of continuous southward interplanetary magnetic field.     

Responses of the magnetotail plasma sheet to two interplanetary shocks: TC-1 observations

Two interplanetary shocks are examined to determine the responses of the magnetic field and plasma in the plasma sheet upon the shock impacts by using TC-1 observational data.The two shocks are observed by WIND on November 7,2004.Prior to and after the shock,the IMF is either weakly southward or northward.The responses of the plasma sheet to the two shocks are intense and much similar.When the shock interacts with the magnetosphere,the magnetic field impulsively increases 1-2 min after the geomagnetic field...     

Downward current electron beams observed at the dipolarization front

The strong field-aligned pitch angle distribution of electrons is observed right at the dipolarization front (DF) before the arriving of a high speed flow when the four Cluster satellites are traveling in the magnetotail around 15 R E on July 22, 2001. The increased electron fluxes only last for a short time period at the DF, corresponding to just a few bouncing periods for 1 keV electrons. The field-aligned current contributed by these electrons agrees well with that calculated by the magnetic field observations by four satellites at the front. These electron streams are found in the energy range of 0.2-2 keV, peak around 1 keV. It is suggested that these downward current electrons may be originated near the aurora region by some kinds of potential structure. The occurrence of these electrons implies that the formation of the dipolarization front and the associated field-aligned current play an important role in the magnetosphere-ionosphere coupling.     

Observations of kinetic Alfvén waves by THEMIS near a substorm onset

Low frequency electromagnetic fluctuations in the vicinity of a magnetospheric substorm onset were investigated using simultaneous observations by THEMIS multiple probes in the near-Earth plasma sheet in the magnetotail.The observations indicate that in the vicinity of a substorm onset,kinetic Alfvén waves can be excited in the high-βplasma sheet(β=2μ0nT/B 2 ,the ratio of plasma thermal pressure to magnetic pressure)within the near-Earth magnetotail.The kinetic Alfvén wave has a small spatial scale in the high-βplasma.The parallel electric field accompanying kinetic Alfvén waves accelerates the charged particles along the magnetic field.The kinetic Alfvén waves play an important role in the substorm trigger process,and possibly in the formation of a substorm current wedge.     

Convective high-speed flow and field-aligned high-speed flows explored by TC-1

From June 1, 2004 to October 31, 2006, a total 465 high-speed flow events are observed by the TC-1 satellite in the near-Earth region （-13 RE〈X〈-9 RE, ｜Y｜〈10 RE, ｜Z＋〈5 RE）, Based on the angle between the flow and the magnetic field, the high-speed flow events are further divided into two types, that is, field-aligned high-speed flow （FAHF） in the plasma sheet boundary and convective bursty bulk flow （BBF） in the center plasma sheet, Among the total 465 high-speed flow events, there are 371 FAHFs, and 94 BBFs, The CHF are mainly concentrated in the plasma sheet, the intersection angle between the flow and the magnetic field is larger, the magnetic field intensity is relatively weak, The FHF are mainly distributed near the boundary layer of the plasma sheet, the intersection angle between the flow and magnetic field is smaller, and the magnetic field intensity is relatively strong, The convective BBFs have an important effect on the substorm,     

外场中超导体/半导体/超导体结的直流Josephson电流

当外加磁场时,超导体/半导体/超导体结的临界电流受外加磁场的影响.超导结的临界电流与磁场的曲线非常类似于单色光在单狭缝衍射的夫琅和费图样.在未考虑外加磁场的超导体/半导体/超导体结的临界电流的基础上,进一步对外加磁场的情形进行了研究.     

Statistical survey on the magnetic field in magnetotail current sheets: Cluster observations

The distribution properties of the magnetic field in magnetotail current sheets have been explored statistically with the magnetic measurement data of the Cluster mission from June to November of the years 2001–2005. It is found that, on average, the strength of the magnetic field and its B_z component in the current sheet are weaker in the region close to midnight but stronger near the dawnside and duskside flanks, which implies that, in general, a thinner current sheet occurs near midnight and thicker ones near both flanks. The occurrence of tail current sheet flapping is higher on both flanks than in the midnight region, although it is most frequent in the dawn flank. Current sheets with a negative B_z component or a strong B_y component have a higher probability of occurring at magnetic local times of 21:00–01:00, indicating that magnetic activity, e.g. magnetic reconnection and current disruption occur more frequently there. Statistically, the probability distributions of the B_y component and the tilt angle of magnetic field lines in the current sheet are approximately normal distributions, and the occurrence probability of the flattened current sheet is about one third that of the normal current sheet. The magnetic field and B_z component in the current sheet mainly vary from 1 nT to 10 nT. The B_y component in the tail central current sheet is on average twice the IMF B_y at 1 AU.