Magnetopause magnetic barrier parameters in dependence on the solar wind magnetic field orientation

Some theories predict the magnetosheath magnetic field strength will decrease and the density increase just outside the dayside magnetopause as the interplanetary magnetic field turns southward. Two studies have recently reported results which confirm these expectations. In contrast, we briefly review our own theoretical predictions which indicate that precisely the opposite effect is expected. We survey new and previously reported magnetosheath observations and demonstrate that they are consistent with the predictions of our model. The conflicting results indicate a need for further theoretical and observational work.     

Cluster Observes the High-Altitude CUSP Region

This paper gives an overview of Cluster observations in the high-altitude cusp region of the magnetosphere. The low and mid-altitude cusps have been extensively studied previously with a number of low-altitude satellites, but only little is known about the distant part of the magnetospheric cusps. During the spring-time, the trajectory of the Cluster fleet is well placed for dayside, high-altitude magnetosphere investigations due to its highly eccentric polar orbit. Wide coverage of the region has resulted and, depending on the magnetic dipole tilt and the solar wind conditions, the spacecraft are susceptible to encounter: the plasma mantle, the high-altitude cusp, the dayside magnetosphere (i.e. dayside plasma sheet) and the distant exterior cusp diamagnetic cavity. The spacecraft either exit into the magnetosheath through the dayside magnetopause or through the exterior cusp–magnetosheath interface. This paper is based on Cluster observations made during three high-altitude passes. These were chosen because they occurred during different solar wind conditions and different inter-spacecraft separations. In addition, the dynamic nature of the cusp allowed all the aforementioned regions to be sampled with different order, duration and characteristics. The analysis deals with observations of: (1) both spatial and temporal structures at high-altitudes in the cusp and plasma mantle, (2) signatures of possible steady reconnection, flux transfer events (FTE) and plasma transfer events (PTE), (3) intermittent cold (<100 eV) plasma acceleration associated with both plasma penetration and boundary motions, (4) energetic ions (5–40 keV) in the exterior cusp diamagnetic cavity and (5) the global structure of the exterior cusp and its direct interface with the magnetosheath. The analysis is primarily focused on ion and magnetic field measurements. By use of these recent multi-spacecraft Cluster observations we illustrate the current topics under debate pertaining to the solar wind–magnetosphere interaction, for which this region is known to be of major importance.     

Determining the thickness of the low-latitude boundary layer in the Earth’s magnetosphere

We describe a method for determining the thickness of the low-latitude boundary layer (LLBL) of the Earth’s magnetosphere at the dayside near the equatorial plane based on the data gathered by a single satellite that traverses the layer and measures the plasma velocity. The method may be applied when the position of the magnetopause and the magnetosheath parameters fluctuate. The necessity of taking the presence of outer and inner LLBL regions into account is analyzed. The developed method is tested using the analysis results of two almost simultaneous close traverses of the magnetopause completed by the THEMIS mission satellites that provided relatively precise data on the LLBL thickness. It is shown that the developed method makes it possible to determine the LLBL thickness with an accuracy of ～10%.     

Broadband Plasma Waves In The MagnetopauseAnd Polar Cap Boundary Layers

Boundary layers occurring in the magnetosphere can support a wide spectrumof plasma waves spanning a frequency range of a few mHz to tens of kHz andbeyond. This review describes the main characteristics of the broadband plasma waves observed in the Earth's low-latitude magnetopause boundary layer (LLBL), in the polar cap boundary layer (PCBL), and the possible generation mechanisms. The broadband waves at the low-latitude boundary layer are sufficiently intense to cause the diffusion of the magnetosheath plasma across the closed magnetospheric field lines at a rate rapid enough to populate and maintain the boundary layer itself. The rapid pitch angle scattering of energetic particles via cyclotron resonant interactionswith the waves can provide sufficient precipitation energy flux to the ionosphere to create the dayside aurora. In general, the broadband plasma waves may play an important part in the processes of local heating/acceleration of the boundary layer plasma.     

2001年1月26日高纬磁层顶通量管事件的观测研究

2001年1月26日11:10～11:40UT, ClusterⅡ卫星簇位于午后高纬磁鞘边界层和磁鞘区,此 时行星际磁场Bz为南向. 本文对在此期间观测到的多次磁通量管事件作了详细的研究 ,获得一系列的新发现：（1）高纬磁鞘边界层磁通量管的出现具有准周期性,周期约为78s ,比目前已知的磁层顶向阳面FTE的平均周期（8～11min）小得多. （2）这些通量管都具有 强的核心磁场;其主轴多数在磁场最小变化方向,少数在中间变化方向,有些无法用PAA判 定其方向（需要用电流管PAA确定）,这与卫星穿越通量管的相对路径有关. （3）每个事件 都存在很好的HT参考系,在HT参考系中这些通量管是准定常态结构;所有通量管都沿磁层顶 表面运动,速度方向大体相同,都来自晨侧下方. 通量管的径向尺度为1～2RE, 与通 常的FTE通量管相当. （4）起源于磁层的强能离子大体上沿着管轴方向由磁层向磁鞘运动; 起源于太阳风的热等离子体沿管轴向磁层传输. 通量管为太阳风等离子体向磁层输运和磁层 粒子向行星际空间逃逸提供了通道. （5）每个通量管事件都伴随有晨昏电场的反转,该电 场为对流电场.     

Cluster Observations of the CUSP: Magnetic Structure and Dynamics

This paper reviews Cluster observations of the high altitude and exterior (outer) cusp, and adjacent regions in terms of new multi-spacecraft analysis and the geometry of the surrounding boundary layers. Several crossings are described in terms of the regions sampled, the boundary dynamics and the electric current signatures observed. A companion paper in this issue focuses on the detailed plasma distributions of the boundary layers. The polar Cluster orbits take the four spacecraft in a changing formation out of the magnetosphere, on the northern leg, and into the magnetosphere, on the southern leg, of the orbits. During February to April the orbits are centred on a few hours of local noon and, on the northern leg, generally pass consecutively through the northern lobe and the cusp at mid- to high-altitudes. Depending upon conditions, the spacecraft often sample the outer cusp region, near the magnetopause, and the dayside and tail boundary layer regions adjacent to the central cusp. On the southern, inbound leg the sequence is reversed. Cluster has therefore sampled the boundaries around the high altitude cusp and nearby magnetopause under a variety of conditions. The instruments onboard provide unprecedented resolution of the plasma and field properties of the region, and the simultaneous, four-spacecraft coverage achieved by Cluster is unique. The spacecraft array forms a nearly regular tetrahedral configuration in the cusp and already the mission has covered this region on multiple spatial scales (100–2000 km). This multi-spacecraft coverage allows spatial and temporal features to be distinguished to a large degree and, in particular, enables the macroscopic properties of the boundary layers to be identified: the orientation, motion and thickness, and the associated current layers. We review the results of this analysis for a number of selected crossings from both the North and South cusp regions. Several key results have been found or have confirmed earlier work: (1) evidence for magnetically defined boundaries at both the outer cusp/magnetosheath interface and the␣inner cusp/lobe or cusp/dayside magnetosphere interface, as would support the existence of a distinct exterior cusp region; (2) evidence for an associated indentation region on the magnetopause across the outer cusp; (3) well defined plasma boundaries at the edges of the mid- to high-altitude cusp “throat”, and well defined magnetic boundaries in the high-altitude “throat”, consistent with a funnel geometry; (4) direct control of the cusp position, and its extent, by the IMF, both in the dawn/dusk and North/South directions. The exterior cusp, in particular, is highly dependent on the external conditions prevailing. The magnetic field geometry is sometimes complex, but often the current layer has a well defined thickness ranging from a few hundred (for the inner cusp boundaries) to 1000 km. Motion of the inner cusp boundaries can occur at speeds up to 60 km/s, but typically 10–20 km/s. These speeds appear to represent global motion of the cusp in some cases, but also could arise from expansion or narrowing in others. The mid- to high-altitude cusp usually contains enhanced ULF wave activity, and the exterior cusp usually is associated with a substantial reduction in field magnitude.     

MHD model of magnetosheath flow: comparison with AMPTE/IRM observations on 24 October, 1985

We compare numerical results obtained from a steady-state MHD model of solar wind flow past the terrestrial magnetosphere with documented observations made by the AMPTE/IRM spacecraft on 24 October, 1985, during an inbound crossing of the magnetosheath. Observations indicate that steady conditions prevailed during this about 4 hour-long crossing. The magnetic shear at spacecraft entry into the magnetosphere was 15°. A steady density decrease and a concomitant magnetic field pile-up were observed during the 40 min interval just preceding the magnetopause crossing. In this plasma depletion layer (1) the plasma beta dropped to values below unity; (2) the flow speed tangential to the magnetopause was enhanced; and (3) the local magnetic field and velocity vectors became increasingly more orthogonal to each other as the magnetopause was approached (Phan et al., 1994). We model parameter variations along a spacecraft orbit approximating that of AMPTE/IRM, which was at slightly southern GSE latitudes and about 1.5 h postnoon Local Time. We model the magnetopause as a tangential discontinuity, as suggested by the observations, and take as input solar wind parameters those measured by AMPTE/IRM just prior to its bow shock crossing. We find that computed field and plasma profiles across the magnetosheath and plasma depletion layer match all observations closely. Theoretical predictions on stagnation line flow near this low-shear magnetopause are confirmed by the experimental findings. Our theory does not give, and the data on this pass do not show, any localized density enhancements in the inner magnetosheath region just outside the plasma depletion layer.     

Predicted signatures of pulsed reconnection in ESR data

Early in 1996, the latest of the European inco-herent-scatter (EISCAT) radars came into operation on the Svalbard islands. The EISCAT Svalbard Radar (ESR) has been built in order to study the ionosphere in the northern polar cap and in particular, the dayside cusp. Conditions in the upper atmosphere in the cusp region are complex, with magnetosheath plasma cascading freely into the atmosphere along open magnetic field lines as a result of magnetic reconnection at the dayside magnetopause. A model has been developed to predict the effects of pulsed reconnection and the subsequent cusp precipitation in the ionosphere. Using this model we have successfully recreated some of the major features seen in photometer and satellite data within the cusp. In this paper, the work is extended to predict the signatures of pulsed reconnection in ESR data when the radar is pointed along the magnetic field. It is expected that enhancements in both electron concentration and electron temperature will be observed. Whether these enhancements are continuous in time or occur as a series of separate events is shown to depend critically on where the open/closed field-line boundary is with respect to the radar. This is shown to be particularly true when reconnection pulses are superposed on a steady background rate.     

Structure of the low-latitude magnetopause: MAGION-4 observations

The aims of this paper are (1) briefly to describe the plasma devices onboard the MAGION-4 satellite, launched on 3 August 1995, of the INTER-BALL project, and (2) to discuss first observations made near the magnetopause region. During the presented boundary crossings the MAGION–4 observed quasi-periodic pulses of magnetosheath-like plasma in a region of low plasma density. This region is located just earthwards of the magnetopause and is populated by a plasma which, except for the density, has the same parameters as in the magnetosheath. Deeper in the magnetosphere, the encounter of a layer of hot electrons and high-energy ions was interpreted as low-latitude boundary layer.     

Density fluctuations measured by ISEE 1–2 in the Earth’s magnetosheath and the resultant scattering of radio waves

Radio waves undergo angular scattering when they propagate through a plasma with fluctuating density. We show how the angular scattering coefficient can be calculated as a function of the frequency spectrum of the local density fluctuations. In the Earths magnetosheath, the ISEE 1–2 propagation experiment measured the spectral power of the density fluctuations for periods in the range 300 to 1 s, which produce most of the scattering. The resultant local angular scattering coefficient can then be calculated for the first time with realistic density fluctuation spectra, which are neither Gaussian nor power laws. We present results on the variation of the local angular scattering coefficient during two crossings of the dayside magnetosheath, from the quasi-perpendicular bow shock to the magnetopause. For a radio wave at twice the local electron plasma frequency, the scattering coefficient in the major part of the magnetosheath is b(2f_p) 0.5–4 × 10^–9 rad²/m. The scattering coefficient is about ten times stronger in a thin sheet (0.1 to IR_E) just downstream of the shock ramp, and close to the magnetopause.     

Low-latitude variations in the geomagnetic field caused by solar wind disturbances

In view of the actual question regarding the effect of a solar-wind pressure jump on disturbances in the Earth’s magnetosphere, events with high velocity and density gradients are of special interest. In this work, we consider the response of the current at the dayside magnetopause to these events and the corresponding strengthening of the geomagnetic field in the low-latitude magnetosphere. A transient process is studied that accompanies reconfiguration of the magnetosphere under the effect of disturbances of solar wind parameters. An analytical equation is received for estimation of an increase in the northern component of low-latitude magnetic field of the magnetosphere in a transient current system (transient ring current) versus initial values of the solar-wind velocity and density and their disturbances.     

非均匀动压冲击和停止后产生的瞬时重联过程

在非均匀动压冲击期间和冲击突然停止,可引起等离子体边界层的局部瞬时重联过程．本文用二维可压缩ＭＨＤ数值模拟方法研究了这两个过程．结果表明：当大尺度的均匀横向流从一侧边冲击边界层时,磁力线不弯曲,也不发生磁场重联,只是边界层被推着向下游运动;当局部的非均匀动压（特别是横向剪切流）冲击边界层时,被冲击的同向磁场区磁力线逐渐弯曲,在弯曲的反磁场区,出现磁岛,然后在电流片区发生磁场重联,且逐渐形成准稳态的“反Ｋ型”重联结构;当横向剪切流冲击停止后,边界层区变为非常不稳定的系统,产生多种流体涡旋和流型,并相应地产生多种类型的磁场重联结构,直到涡旋消失变为湍动状态时,磁场拓扑才逐渐恢复到未扰动状态。我们提出,外力作用的突然停止,可能是驱动重联的一种新机制,并对这种重联过程在磁层物理中可能的应用进行了讨论．     

Energy and pitch-angle dispersions of LLBL/cusp ions seen at middle altitudes: predictions by the open magnetosphere model

Numerical simulations are presented of the ion distribution functions seen by middle-altitude spacecraft in the low-latitude boundary layer (LLBL) and cusp regions when reconnection is, or has recently been, taking place at the equatorial magnetopause. From the evolution of the distribution function with time elapsed since the field line was opened, both the observed energy/observation-time and pitch-angle/energy dispersions are well reproduced. Distribution functions showing a mixture of magnetosheath and magnetospheric ions, often thought to be a signature of the LLBL, are found on newly opened field lines as a natural consequence of the magnetopause effects on the ions and their flight times. In addition, it is shown that the extent of the source region of the magnetosheath ions that are detected by a satellite is a function of the sensitivity of the ion instrument . If the instrument one-count level is high (and/or solar-wind densities are low), the cusp ion precipitation detected comes from a localised region of the mid-latitude magnetopause (around the magnetic cusp), even though the reconnection takes place at the equatorial magnetopause. However, if the instrument sensitivity is high enough, then ions injected from a large segment of the dayside magnetosphere (in the relevant hemisphere) will be detected in the cusp. Ion precipitation classed as LLBL is shown to arise from the low-latitude magnetopause, irrespective of the instrument sensitivity. Adoption of threshold flux definitions has the same effect as instrument sensitivity in artificially restricting the apparent source region.     

2004年3月18日高纬磁层顶多重通量管事件分析

2004-03-18 23:10～23:50 UT期间,“双星（Double Star）”探测一号卫星（TC 1）在向阳面磁层顶高纬晨侧由内向外穿越磁层顶,其时TC_1的GSM坐标为 (75RE, -55RE, -54RE), RE为地球半径.穿越过程中TC_1观测到了8个通量管和1个磁通量传输事件（FTEs）.在此期间Cluster星簇位于向阳面太阳风内,其GSM坐标为(180RE, -31RE, -62RE),其4颗卫星监测到行星际磁场（IMF）的BZ分量持续南向,BＹ有较大的负值.本文的研究表明：TC_1观测到的前7个通量管具有准周期重现性,周期大约是1～4 min,明显小于以前所观测到的FTEs的平均周期(8～11 min);所有的通量管都具有较强的核心场.本文分别使用最小方差分析法（MVA）和Grad_Shafranov反演方法（GSR）对通量管的轴向进行了分析和对比,发现所有的通量管主轴基本沿晨昏向,结果显示GSR方法在轴向分析上比MVA优越.本文使用GSR方法对通量管的磁场结构进行了分析,恢复出了通量管的磁场在卫星穿越面的结构图;此外,本文还对这次多重通量管事件进行了deHoffmann Teller(HT)分析,结果表明,所有通量管大致朝南极方向运动,均来源于向日面低纬区域.这说明它们可能起源于向日面低纬区,由该区的磁场分量重联产生.     

2001年3月2日磁通量传输事件特性的研究

2001年3月2日11:00 至11:15 UT 期间,Cluster Ⅱ在南半球极尖区晨侧附近磁鞘内探测到3个通量传输事件（简称FTEs）. 本文利用Cluster Ⅱ星簇4颗卫星观测到的磁场和等离子体资料研究了这些通量传输事件的磁场形态和粒子特征. 并利用它们探测到的空间磁场梯度资料由安培定律直接求出星簇所在区域的电流分布. 结果指出：（1）BY占优势的行星际磁场结构在磁层顶的重联可以在极尖区附近发生;（2）FTEs通量管形成初期内外总压差和磁箍缩应力不一定平衡,达到平衡有一发展过程;（3）FTEs通量管截面在L M平面内的线度约为1.89RE;（4）FTEs通量管中等离子体主要沿轴向场方向流动,整个通量管以慢于背景等离子体的速度沿磁层顶向南向尾运动;（5）FTEs通量管中不仅有轴向电流,也存在环向电流. 轴向电流基本沿轴向磁场方向流动. 轴向和环向电流在管内均呈体分布,因而轴向电流产生的环向磁场接近管心时不断减小到零,而环向电流生成的轴向场则不断增大到极值;（6）在通量管的磁鞘部分观测到磁层能量粒子流量的增强,这表明通量管通过磁层顶将磁鞘和磁层内部连通起来了.     

Magnetopause stand-off distance in dependence on the magnetosheath and solar wind parameters

A model of the magnetosheath structure proposed in a recent paper from the authors is extended to estimate the magnetopause stand-off distance from solar wind data. For this purpose, the relationship of the magnetopause location to the magnetosheath and solar wind parameters is studied. It is shown that magnetopause erosion may be explained in terms of the magnetosheath magnetic field penetration into the magnetosphere. The coefficient of penetration (the ratio of the magnetospheric magnetic field depression to the intensity of the magnetosheath magnetic field B_mz = -B_m sin²/2, is estimated and found approximately to equal 1. It is shown that having combined a magnetosheath model presented in an earlier paper and the magnetosheath field penetration model presented in this paper, it is possible to predict the magnetopause stand-off distance from solar wind parameters.     

The Influence of Parameters of the Interplanetary Medium and Magnetosheath Boundaries on the Correlation Coefficient between the Ion Flux Measured in the Solar Wind and the Magnetosheath

In this paper, the correlation coefficient between the ion fluxes in the solar wind and the magnetosheath is analyzed with the use of data of two satellites of the THEMIS mission and the THEMIS/Spektr-R satellites obtained in 2008 and 2011?2014, respectively. We have distinguished the conditions in which a high level of correlation between the measurements in the solar wind and the magnetosheath is observed, i.e., the correlation coefficient exceeds 0.7. As key factors, we consider both direct parameters of the solar wind, such as the density, the magnetic field magnitude, the magnetosonic Mach number, and the ratio β of the thermal pressure to the magnetic, and a more general factor—the type of large-scale structure of the solar wind. In addition, the effect of the satellite location in the magnetosheath relative to its boundaries—the bow shock and the magnetopause—on the correlation level is considered. It has been shown that, in roughly one third of cases, the plasma structures of the solar wind undergo a strong modification at the bow shock and in the magnetosheath, which results in a low correlation level corresponding to a correlation coefficient of less than 0.5; a high correlation level is observed in half of cases, i.e., the plasma structures are weakly disturbed. It has been determined that (1) the low correlation level in the magnetosheath behind quasi-perpendicular bow shock is more often observed near the magnetopause than in region just behind the bow shock, (2) the probability of observations of a high correlation level is independent of the profile shape of the quasi-perpendicular bow shock, and (3) the high correlation is more probable for the events corresponding to the solar wind of the Corotating Interaction Region (CIR) type than for those with the other solar wind types observed in the considered period.     

IMP 8 magnetosheath field comparisons with models

This study presents cross-sectional vector maps of the magnetic field derived from IMP 8 magnetometer in the magnetosheath at 30 R_e behind the Earth. In addition the vector patterns of the magnetosheath field for northward, southward, and east-west interplanetary magnetic field (IMF) directions are qualitatively compared with those obtained from the Spreiter-Stahara gas dynamic (GD) and Fedder-Lyon magnetohydrodynamic models (MHD). The main purpose is to display the cross-sectional differences in relation to the dayside merging with different IMF directions, allowing the reader to make direct visual comparisons of the vector patterns. It is seen that for east-west IMF directions, the data-based and MHD-based patterns differ noticeably in a similar way from the GD model, presumably reflecting the influence of dayside magnetic merging of the Earths magnetic field with the y-component of the interplanetary magnetic field. All three northward IMF cross sections show comparable field draping patterns as expected for a closed magnetosphere. For southward IMF case, on the other hand, differences between the three cross-sectional patterns are greater as seen in the field vector sizes and directions, especially closer to the magnetopause where more disturbed magneto-spheric conditions are known to be exist. The data comparisons with the MHD and GD models show that the differences result from the magnetic field-flow coupling and that the effects of dayside reconnection are present in IMP 8 magnetic field measurements.     

Characteristics of ≥ 290 keV magnetosheath ions

We performed a statistical analysis of 290–500 keV ion data obtained by IMP-8 during the years 1982–1988 within the earth’s magnetosheath and analysed in detail some time periods withdistinct ion bursts. These studies reveal the following characteristics for magnetosheath 290–500 keV energetic ions: (a) the occurrence frequency and the flux of ions increase with increasing geomagnetic activity as indicated by the Kp index; the occurrence frequency was found to be as high as P ≥ 42% for Kp ≥ 2, (b) the occurrence frequency in the dusk magnetosheath was found to be slightly dependent on the local time and ranged between ≈30% and ≈46% for all Kp values; the highest occurrence frequency was detected near the dusk magnetopause (21 LT), (c) the high energy ion bursts display a dawn-dusk asymmetry in their maximum fluxes, with higher fluxes appearing in the dusk magnetosheath, and (d) the observations in the dusk magnetosheath suggest that there exist intensity gradients of energetic ions from the bow shock toward the magnetopause. The statistical results are consistent with the concept that leakage of magnetospheric ions from the dusk magnetopause is a semi-permanent physical process often providing the magnetosheath with high energy (290–500 keV) ions.