共查询到20条相似文献，搜索用时 39 毫秒
1.
Shear flow instability is studied in the Earth’s magnetotail by treating plasma as compressible. A dispersion relation is derived from the linearized MHD equations using the oscillating boundary conditions at the inner central plasma sheet/outer central plasma sheet (OCPS) interface and OCPS/plasma-sheet boundary layer (PSBL) interface, whereas the surface-mode boundary condition is used at the PSBL/lobe interface. The growth rates and the real frequencies are obtained numerically for near-Earth (\midX\mid\sim10-15 RE) and far-Earth (\midX\mid\sim100 RE) magnetotail parameters. The periods and wavelengths of excited modes depend sensitively on the value of plasma-sheet half thickness, L, which is taken as L=5 RE for quiet time and L=1 RE for disturbed time. The plasma-sheet region is found to be stable for constant plasma flows unless MA3>1.25, where MA3 is the Alfvén Mach number in PSBL. For near-Earth magnetotail, the excited oscillations have periods of 2–20 min (quiet time) and 0.5-4 min (disturbed time) with typical transverse wavelengths of 2–30 RE and 0.5-6.5 RE, respectively; whereas for distant magnetotail, the analysis predicts the oscillation periods of \sim8-80 min for quiet periods and 2–16 min for disturbed periods. 相似文献
2.
《Journal of Atmospheric and Solar》2008,70(17):2118-2131
The magnetotail lobes are two vast regions between the plasma sheet (PS) and the magnetotail boundary layers at the magnetopause, where the plasma has very low temperature and densities. The open magnetic field lines of the lobes directly couple the ionospheric polar caps with the solar wind (SW) through the magnetosheath. The survey of 576 h INTERBALL-1 measurements in the near (XGSM>−27RE) lobes in October–November 1997 shows that they are populated with plasmas of various origin and properties. Presented and discussed in details are four cases of lobe measurements under different geomagnetic conditions. Discrete plasma structures encountered in the lobes could originate from the PS, from the magnetosheath or the mantle. A ubiquitous picture in the lobes is the registration of ‘clouds’ of anisotropic electrons with energies up to 300–500 eV, with no accompanying ions. The electron distributions are highly variable and complex, with different degree of anisotropy. The earthward flowing electrons originate in the SW, the anisotropy of the electron fluxes reflects the anisotropy of the SW electrons. In some cases the tailward electrons are not only mirrored earthward fluxes but an additional source earthward of the observations is present. The positive spacecraft potential plays a substantial role in modifying the observed electron distributions. 相似文献
3.
Simultaneous energetic particle and magnetic field observations from the GEOTAIL spacecraft in the distant tail (XGSM -150 Re) have been analysed to study the response of the Earths magnetotail during a strong substorm (AE 680 nT). At geosynchronous altitude, LANL spacecraft recorded three electron injections between 0030 UT and 0130 UT, which correspond to onsets observed on the ground at Kiruna Ground Observatory. The Earths magnetotail responded to this substorm with the ejection of five plasmoids, whose size decreases from one plasmoid to the next. Since the type of magnetic structure detected by a spacecraft residing the lobes, depends on the Z extent of the structure passing underneath the spacecraft, GEOTAIL is first engulfed by a plasmoid structure; six minutes later it detects a boundary layer plasmoid (BLP) and finally at the recovery phase of the substorm GEOTAIL observes three travelling compression regions (TCRs). The time-of-flight (TOF) speed of these magnetic structures was estimated to range between 510 km/s and 620 km/s. The length of these individual plasmoids was calculated to be between 28 Re and 56 Re. The principal axis analysis performed on the magnetic field during the TCR encountered, has confirmed that GEOTAIL observed a 2-D perturbation in the X-Z plane due to the passage of a plasmoid underneath. The first large plasmoid that engulfed GEOTAIL was much more complicated in nature probably due to the external, variable draped field lines associated with high beta plasma sheet and the PSBL flux tubes surrounding the plasmoid. From the analysis of the energetic particle angular distribution, evidence was found that ions were accelerated from the distant X-line at the onset of the burst associated with the first magnetic structure. 相似文献
4.
D. V. Sarafopoulos M. A. Athanasiu E. T. Sarris T. Yamamoto S. Kokubun 《Annales Geophysicae》1999,17(9):1121-1133
We study an interval of 56 h on January 16 to 18, 1995, during which the GEOTAIL spacecraft traversed the duskside magnetosheath from X ≅ −15 to −40 RE and the EPIC/ICS and EPIC/STICS sensors sporadically detected tens of energetic particle bursts. This interval coincides with the expansion and growth of a great geomagnetic storm. The flux bursts are strongly dependent on the magnetic field orientation. They switch on whenever the Bz component approaches zero (Bz ≅ 0 nT). We strongly suggest a magnetospheric origin for the energetic ions and electrons streaming along these “exodus channels”. The time profiles for energetic protons and “tracer” O+ ions are nearly identical, which suggests a common source. We suggest that the particles leak out of the magnetosphere all the time and that when the magnetosheath magnetic field connects the spacecraft to the magnetotail, they stream away to be observed by the GEOTAIL sensors. The energetic electron fluxes are not observed as commonly as the ions, indicating that their source is more limited in extent. In one case study the magnetosheath magnetic field lines are draped around the magnetopause within the YZ plane and a dispersed structure for peak fluxes of different species is detected and interpreted as evidence for energetic electrons leaking out from the dawn LLBL and then being channelled along the draped magnetic field lines over the magnetopause. Protons leak from the equatorial dusk LLBL and this spatial differentiation between electron and proton sources results in the observed dispersion. A gradient of energetic proton intensities toward the ZGSM= 0 plane is inferred. There is a permanent layer of energetic particles adjacent to the magnetosheath during this interval in which the dominant component of the magnetic field was Bz. 相似文献
5.
A. Belehaki E. T. Sarris G. Tsiropoula R. W. McEntire S. Kokubun T. Yamamoto 《Annales Geophysicae》1997,15(12):1515-1531
Geotail energetic particle, magnetic field data and plasma observations (EPIC, MGF and CPI experiments) have been examined for a number of energetic particle bursts in the distant tail (120Re < |XGSM| < 130 Re), associated with moving magnetic field structures, following substorm onsets. The features obtained from this data analysis are consistent with the distant magnetotail dynamics determined first by ISEE3 observations and explained in terms of the neutral line model. At the onset of the bursts, before plasma sheet entrance, energetic electrons appear as a field-aligned beam flowing in the tailward direction, followed by anisotro-pic ions. Within the flux rope region, suprathermal ions exhibit a convective anisotropy, which allows determination of the plasma flow velocity, assuming that the anisotropy arises from the Compton-Getting effect. The velocities thus determined in the plasma sheet are estimated to be 200–650 km/s, and compare favourably with the velocities derived from the CPI electron and proton experiment. The estimated length of magnetic field structures varies between 28 and 56 Re and depends on the strength of the westward electrojet intensification. Finally, the three structures reported here show clear magnetic field signatures of flux rope topology. The existence of a strong magnetic field aligned approximately along the Y-axis and centred on the north-to-south excursion of the field, and the bipolar signature in both By and/or Bz components, is consistent with the existence of closed field lines extending from Earth and wrapping around the core of the flux rope structure. 相似文献
6.
E. E. Grigorenko R. Koleva L. M. Zelenyi J. -A. Sauvaud 《Geomagnetism and Aeronomy》2010,50(6):720-732
We present an analysis of two crossings of the plasma sheet boundary layer (PSBL) in Earth’s magnetotail by a quartet of Cluster
satellites, accomplished both in the absolutely quiet geomagnetic interval and in the restoration period between two substorms.
It is found that in the quiet period, field-aligned electric currents were not observed in the PSBL, in spite of presence
in this region of high-velocity ions moving toward Earth along the magnetic field lines at velocities of ∼1400 km/s. This
means that in the given event, ions and electrons moved together; i.e., high-velocity ions were a part of the accelerated
plasma flow. In the disturbed period, the Cluster satellites detected a system of two oppositely directed field-aligned currents,
which can be associated with the presence of the X line rather close to the Earth. Owing to multipoint Cluster observations, we managed to estimate the spatial size of a current
structure (along the normal to the PSBL surface), which was equal, for a current flowing toward Earth along the high-latitude
PSBL boundary, to about 1600 km, which is comparable with the value of the inertial radius of ions. This agrees with the scenario
of spatial separation of charges and formation of the Hall system of currents in the magnetic reconnection region. The duration
of observation of the given current structure was ∼12 min., which points to the quasi-steady-state character of reconnection. 相似文献
7.
"嫦娥"一号、二号绕月飞行经历地球磁尾边界层区域时,分别在2007年11月26日—2008年2月5日和2010年10月3日—2011年2月28日,发现了15次月球轨道0.1~2 MeV电子急剧增加(Bursts of 0.1~2 MeV Energetic Electrons,BEE),卫星周围等离子体离子加速的现象.统计研究表明,这类现象发生在稳定太阳风和弱行星际磁场条件下,且无显著空间环境扰动事件发生时,离子的加速滞后于高能电子爆发,离子能量的变化与高能电子通量的时间演化正相关,地球磁鞘内侧或边界层过渡区域是该类现象的高发区,离子能量增加时卫星表面电位大幅下降可达负几千伏.为了研究高能电子爆发与绕月卫星表面电位变化的关系及其对月球表面电位的影响,本文用电流平衡法建立绕月卫星和月球表面充电模型,并假设能量电子(2eV~2 MeV)满足幂律谱的分布,模拟急剧增加的能量电子对卫星和月球表面电位的影响.模拟结果表明,能量电子急剧增加使得绕月卫星和月球表面电位大幅下降;能量电子总流量1011 cm-2时,绕月卫星和月球表面充电电位可达负上千伏;月球充电到大的负电位的时间仅为卫星充电时间的1/10.鉴于高能电子急剧增加事件的高发生率(~125次/年),能量电子急剧增加使得绕月卫星表面电位大幅下降的发生率应大于实测等离子体离子加速现象的发生率(~25次/年). 相似文献
8.
Measuring the low-energy ions in the Earth's magnetotail lobes is difficult, because a spacecraft becomes positively charged in a sunlit and tenuous plasma environment. Recent studies have introduced a new method, making use of the positive electric potential on the Cluster spacecraft, to measure the low-energy ions(less than a few tens of electronvolts) in the polar caps/magnetotail lobes in the years 2001–2010. With the measured velocities, we are able to study the trajectories of these low-energy ions. Particle tracing has been used in previous studies, confirming that ions of ionospheric origin are the dominant contributor to the ion population in the Earth's magnetotail lobes. In this work, we continue to study the source of low-energy ions measured in the lobes. We found that not all of the low-energy ions in the lobes come directly from the ionosphere. Particle tracing infers that some of the low-energy ions start to move tailward from the cusp/near-cusp region with a zero parallel velocity. In the following, we refer to these low-energy ions as stagnant low-energy ions. On the other hand, the in situ measurements by Cluster show a population of low-energy ions in the cusp/near-cusp region with pitch angles near 90°(i.e., no significant parallel velocity).The locations of stagnant low-energy ions are determined by particle tracing and in situ measurements. Similar ion energies and spatial distributions determined by these two methods confirm the presence of the stagnant low-energy ion population. 相似文献
9.
Quasi-periodic Pc 5 pulsations have been reported inside and just outside the Earth’s magnetotail during intervals of low geomagnetic activity. In order to further define their characteristics and spatial extent, we present three case studies of simultaneous magnetic field and plasma observations by IMP-8, ISEE-1 (and ISEE-2 in one case) in the Earth’s magnetotail and ISEE-3 far upstream of the bow shock, during intervals in which the spacecraft were widely separated. In the first case study, similar pulsations are observed by IMP-8 at the dawn flank of the plasma sheet and by ISEE-1 near the plasma sheet boundary layer (PSBL) near midnight local time. In the second case study, simultaneous pulsations are observed by IMP-8 in the dusk magnetosheath and by ISEE-1 and 2 in the dawn plasma sheet. In the third case study, simultaneous pulsations are observed in the north plasma sheet boundary layer and the south plasma sheet. We conclude that the pulsations occur simultaneously throughout much of the nightside magnetosphere and the surrounding magnetosheath, i.e. that they have a global character. Some additional findings are the following: (a) the observed pulsations are mixed mode compressional and transverse, where the compressional character is more apparent in the close vicinity of the plane ZGSM=0; (b) the compressional pulsations of the magnetic field in the dusk magnetosheath show peaks that coincide (almost one-to-one) with similar peaks observed inside the dawn plasma sheet; (c) in the second case study the polarization sense of the magnetic field and the recurrent left-hand plasma vortices observed in the dawn plasma sheet are consistent with antisunward moving waves on the magneto-pause; (d) pulsation amplitudes are weaker in the PSBL(or lobe) as compared with those in the magneto-tail’s flanks, suggesting a decay with distance from the magnetopause; (e) the thickness of the plasma sheet (under extremely quiet conditions) is estimated to be \sim22 RE at an average location of (X, Y)GSM=(16, 17) RE, whereas at midnight local time the thickness is \sim14 RE. The detected pulsations are probably due to the pressure variations (recorded by ISEE-3) in the solar wind, and/or the Kelvin Helmholtz instability in the low-latitude boundary layer or the magnetopause due to a strongly northward IMF. 相似文献
10.
太阳风的动量涨落将通过磁层边界在磁尾激发磁流体力学波。快磁声波携带扰动能量传到等离子体片中,发展为激波,或者通过激波的相互作用而耗散能量,使等离子体加热。等离子体片中的随机费米加速机制,使麦克斯韦分布尾巴部分的高能量粒子被加速到更高能。在宁静态时,加热、加速与耗散过程平衡。当太阳风的动量或者其涨落较大时,整个加热和加速过程加剧,更多的高能粒子产生,并从等离子体片中逃逸,形成高速的等离子体流注入近地轨道和极区,表现为磁层亚暴过程。利用这种机制,可以解释地球磁层亚暴的定性特征。 相似文献
11.
S. Y. Fu Q. G. Zong Z. Y. Pu C. J. Xiao A. Korth P. Daly H. Reme 《Surveys in Geophysics》2005,26(1-3):241-254
In this paper we report energetic ion behavior and its composition variations observed by the Cluster/RAPID instrument when the spacecraft was travelling in the high latitude magnetospheric boundary region on the day of the 31 March, 2001, strongest magnetic storm in the past 50 years. The Dst index reached −360 nT at about 09:00 UT. During its early recovery phase, large amounts of oxygen and helium ions were observed; the ratio of oxygen to hydrogen in the RAPID energy range reached as high as 250%, which suggests that the observed energetic particles might be of magnetospheric origin. The observations further show that enhanced energetic electron fluxes are confined in a very narrow region, while protons have occupied a larger region, and heavy ions have been observed in an even larger region. The flux of energetic electrons show a slight enhancement in a region where the magnetic field magnitude is around zero. These observed energetic ions could be quasi-trapped by the current sheet in the stagnation region of the cusp. 相似文献
12.
According to observations, the discrete auroral arcs can sometimes be found, either deep inside the auroral oval or at the poleward border of the wide (so-called double) auroral oval, which map to very different regions of the magnetotail. To find common physical conditions for the auroral-arc generation in these magnetotail regions, we study the spatial relationship between the diffuse and discrete auroras and the isotropic boundaries (IBs) of the precipitating energetic particles which can be used to characterise locally the equatorial magnetic field in the tail. From comparison of ground observation of auroral forms with meridional profiles of particle flux measured simultaneously by the low-altitude NOAA satellites above the ground observation region, we found that (1) discrete auroral arcs are always situated polewards from (or very close to) the IB of > 30-keV electrons, whereas (2) the IB of the > 30-keV protons is often seen inside the diffuse aurora. These relationships hold true for both quiet and active (substorm) conditions in the premidnight-nightside (18– 01-h) MLT sector considered. In some events the auroral arcs occupy a wide latitudinal range. The most equatorial of these arcs was found at the poleward edge of the diffuse auroras (but anyway in the vicinity of the electron IB), the most poleward arcs were simultaneously observed on the closed field lines near the polar-cap boundary. These observations disagree with the notion that the discrete aurora originate exclusively in the near-Earth portion of plasma sheet or exclusively on the PSBL field lines. Result (1) may imply a fundamental feature of auroral-arc formation: they originate in the current-sheet regions having very curved and tailward-stretched magnetic field lines. 相似文献
13.
《Journal of Atmospheric and Solar》2000,62(6):421-436
More than 30 years after the prediction of the polar wind outflow from the high latitude ionosphere, the exact magnitude and ultimate fate of the ionospheric plasma supply remains unknown. Estimates made more than a decade ago suggested that the polar ion outflow might well be of sufficient strength to populate the different regions of the Earth’s magnetosphere. Direct measurements in the high altitude magnetosphere became possible only with the launch of the Polar spacecraft. The combination of the Thermal Ion Dynamics Experiment and the Plasma Source Instrument has revealed the presence of low energy (<10 eV) ions moving through the polar regions and into the lobes of the magnetotail. These ions would have been invisible to previous un-neutralized satellites because of the high positive spacecraft potentials. Through the use of a recently developed single particle trajectory and energization code, the movement and energy transformation of these measured particles can be estimated. They are found to move into the plasma sheet region and to be energized to typical plasma sheet energies. The magnitude of the flux of the highly variable out-flowing ions mapped to 1000 km altitude is 1 − 3 × 108 ions/cm2 s in agreement with the original estimates. Future observations by the TIDE/PSI instruments will be required to determine the extent of the total ionospheric contribution. 相似文献
14.
磁尾电流片在磁尾动力学过程中起着重要作用.卫星观测表明磁尾电流片经常处于拍动状态.但磁尾电流片拍动的特性和产生机制至今仍然没有被完全弄清楚.本文主要利用欧洲空间局Cluster卫星数据,研究一个伴随高速离子流的电流片拍动事件.该电流片拍动事件具有很强的周期性.拍动的周期约是2min,磁场振荡幅度约为20nT.能量电子和离子的通量具有周期性增强和减弱的特征.电流密度X和Y分量也具有周期性的振荡,并且振荡周期与磁场振荡周期一致.通过对粒子流速矢量与电流矢量的分析,发现粒子运动具有涡旋的特征.因此可以推断,该磁尾电流片的拍动不是由磁尾等离子体片高速流产生的,而是与局地等离子体不稳定性有关. 相似文献
15.
Q. G. Zong T. A. Fritz A. Korth P. W. Daly M. Dunlop A. Balogh J. F. Fennell J. D. Sullivan R. W. H. Friedel H. Reme 《Surveys in Geophysics》2005,26(1-3):215-240
Energetic electrons (e.g., 50 keV) travel along field lines with a high speed of around 20 REs−1. These swift electrons trace out field lines in the magnetosphere in a rather short time, and therefore can provide nearly instantaneous information about the changes in the field configuration in regions of geospace. The energetic electrons in the high latitude boundary regions (including the cusp) have been examined in detail by using Cluster/RAPID data for four consecutive high latitude/cusp crossings between 16 March and 19 March 2001. Energetic electrons with high and stable fluxes were observed in the time interval when the IMF had a predominately positive Bz component. These electrons appeared to be associated with a lower plasma density exhibiting no obvious tailward plasma flow (<20 keV). On the other hand, no electrons or only spike-like electron events have been observed in the cusp region during southward IMF. At that time, the plasma density was as high as that in the magnetosheath and was associated with a clear tailward flow. The fact that no stable energetic electron fluxes were observed during southward IMF indicates that the cusp has an open field line geometry. The observations indicate that both the South and North high latitude magnetospheric boundary regions (including both North and South cusp) can be energetic particle trapping regions. The energetic electron observations provide new ways to investigate the dynamic cusp processes. Finally, trajectory tracing of test particles has been performed using the Tsyganenko 96 model; this demonstrates that energetic particles (both ions and electrons) may be indeed trapped in the high latitude magnetosphere. 相似文献
16.
A guided propagation of magnetoacoustic wave in the plasma sheet located between two lobes of the magnetotail is investigated. The dispersion equation for the wave and equation connecting a disturbance of plasma pressure inside the plasma sheet and amplitude of the plasma sheet boundary oscillations are obtained. For some value of plasma pressure disturbance, the displacement of the plasma sheet boundaries becomes of order of the half-thickness of the plasma sheet. In the case of symmetrical oscillations of the boundaries (“sausage-like” mode), it creates the favorable conditions for reconnection of the magnetic field lines in the magnetotail and may lead to triggering of a substorm. The magnetoacoustic wave may be generated by sudden impulse of the solar wind plasma pressure. 相似文献
17.
A. A. Lyubchich A. G. Demekhov E. E. Titova A. G. Yahnin 《Geomagnetism and Aeronomy》2017,57(1):40-50
Using two-hour (from 2300 UT January 25, 2013 to 0100 UT January 26, 2013) measurement data from Van Allen Probes on fluxes of energetic particles, cold plasma density, and magnetic field magnitude, we have calculated the local growth rate of electromagnetic ion–cyclotron and whistler-mode waves for field-aligned propagation. The results of these calculations have been compared with wave spectra observed by the same Van Allen Probe spacecraft. The time intervals when the calculated wave increments are sufficiently large, and the frequency ranges corresponding to the enhancement peak agree with the frequency–time characteristics of observed electromagnetic waves. We have analyzed the influence of variations in the density and ionic composition of cold plasma, fluxes of energetic particles, and their pitch-angle distribution on the wave generation. The ducted propagation of waves plays an important role in their generation during the given event. The chorus VLF emissions observed in this event cannot be explained by kinetic cyclotron instability, and their generation requires much sharper changes (“steps”) for velocity distributions than those measured by energetic particle detectors on Van Allen Probes satellites. 相似文献
18.
19.
Fu Huishan Grigorenko Elena E. Gabrielse Christine Liu Chengming Lu San Hwang K. J. Zhou Xuzhi Wang Zhe Chen Fang 《中国科学:地球科学(英文版)》2020,63(2):235-256
In this paper, the particle acceleration processes around magnetotail dipolarization fronts(DFs) were reviewed. We summarize the spacecraft observations(including Cluster, THEMIS, MMS) and numerical simulations(including MHD, testparticle, hybrid, LSK, PIC) of these processes. Specifically, we(1) introduce the properties of DFs at MHD scale, ion scale, and electron scale,(2) review the properties of suprathermal electrons with particular focus on the pitch-angle distributions,(3)define the particle-acceleration process and distinguish it from the particle-heating process,(4) identify the particle-acceleration process from spacecraft measurements of energy fluxes, and(5) quantify the acceleration efficiency and compare it with other processes in the magnetosphere(e.g., magnetic reconnection and radiation-belt acceleration processes). We focus on both the acceleration of electrons and ions(including light ions and heavy ions). Regarding electron acceleration, we introduce Fermi,betatron, and non-adiabatic acceleration mechanisms;regarding ion acceleration, we present Fermi, betatron, reflection, resonance, and non-adiabatic acceleration mechanisms. We also discuss the unsolved problems and open questions relevant to this topic, and suggest directions for future studies. 相似文献