隧道塌方的尖点灾变模型及应用

针对隧道塌方失稳问题，运用灾变理论建立隧道塌方失稳的尖点灾变模型。基于平衡曲面方程，可求得隧道围岩体内承载区介质刚度与对应的松驰区弱化介质本构曲线拐点处理刚度之比值K，并给出了塌方与否的判据，当K≤1时，隧道将发生塌方失稳；当K＞1时，隧道将不会发生塌方失稳。根据此模型，笔者解释了隧道中几种常见的灾变破坏机理，并提出了相应的治理方案。     

Magnetosheath Interaction with the High Latitude Magnetopause

We present both statistical and case studies of magnetosheath interaction with the high-latitude magnetopause on the basis of Interball-1 and other ISTP spacecraft data. We discuss those data along with recently published results on the topology of cusp-magnetosheath transition and the roles of nonlinear disturbances in mass and energy transfer across the high-latitude magnetopause. For sunward dipole tilts, a cusp throat is magnetically open for direct interaction with the incident flow that results in the creation of a turbulent boundary layer (TBL) over an indented magnetopause and downstream of the cusp. For antisunward tilts, the cusp throat is closed by a smooth magnetopause; demagnetized ‘plasma balls’ (with scale ∼ few R_E, an occurrence rate of ∼25% and trapped energetic particles) present a major magnetosheath plasma channel just inside the cusp. The flow interacts with the ‘plasma balls’ via reflected waves, which trigger a chaotization of up to 40% of the upstream kinetic energy. These waves propagate upstream of the TBL and initiate amplification of the existing magnetosheath waves and their cascade-like decays during downstream passage throughout the TBL. The most striking feature of the nonlinear interaction is the appearance of magnetosonic jets, accelerated up to an Alfvenic Mach number of 3. The characteristic impulsive local momentum loss is followed by decelerated Alfvenic flows and modulated by the TBL waves; momentum balance is conserved only on time scales of the Alfvenic flows (1/f_A ∼12 min). Wave trains at f_A∼1.3 mHz are capable of synchronizing interactions throughout the outer and inner boundary layers. The sonic/Alfvenic flows, bounded by current sheets, control the TBL spectral shape and result in non-Gaussian statistical characteristics of the disturbances, indicating the fluctuation intermittency. We suggest that the multi-scale TBL processes play at least a comparable role to that of macro-reconnection (remote from or in the cusp) in solar wind energy transformation and population of the magnetosphere by the magnetosheath plasma. Secondary micro-reconnection constitutes a necessary chain at the small-scale (∼ion gyroradius) edge of the TBL cascades. The thick TBL transforms the flow energy, including deceleration and heating of the flow in the open throat, ‘plasma ball’ and the region downstream of the cusp.     

Spatial and Temporal Cusp Structures Observed by Multiple Spacecraft and Ground Based Observations

Downward precipitating ions in the cusp regularly exhibit sudden changes in ion energy distributions, forming distinctive structures that can be used to study the temporal/spatial nature of reconnection at the magnetopause. When observed simultaneously with the Polar, FAST, and Interball satellites, such cusp structures revealed remarkably similar features. These similar features could be observed for up to several hours during stable solar wind conditions. Their similarities led to the conclusion that large-scale cusp structures are spatial structures related to global ionospheric convection patterns created by magnetic merging and not the result of temporal variations in reconnection parameters. The launch of the Cluster fleet allows cusp structures to be studied in great detail and during changing solar wind conditions using three spacecraft with identical plasma and field instrumentation. In addition, Cluster cusp measurements are linked with ionospheric convection cells by combining the satellite observations with SuperDARN radar observations that are used to derive the convection patterns in the ionosphere. The combination of satellite observations with ground-based observations during variable solar wind conditions shows that large-scale cusp structures can be either spatial or temporal. Cusp structures can be described as spatial features observed by satellites crossing into spatially separated flux tubes. Cusp structures can also be observed as poleward-traveling (temporal) features within the same convection cell, most probably caused by variations in the reconnection rate at the magnetopause.     

Energetic Particles Observed in the CUSP Region During a Storm Recovery Phase

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.     

The Magnetospheric cusps: A Summary

The polar cusps of the magnetosphere are key regions for the transfer of mass, momentum, and energy from the solar wind into the magnetosphere. Understaning these key regions and the dynamical interactions that occur there are fundamentally important to determining the physical nature of the magnetosphere. In this paper we try to summarize many of the conclusions reached in the papers of this special issue emphasizing the present concepts and definition of the cusp, what variations could be temporal structures and what could be spatial structures. We address the need for further measurements and the role of present and planned projects to address these needs.     

High-Altitude CUSP: The Extremely Dynamic Region in Geospace

The high-altitude dayside cusps (both northern and southern) are extremely dynamic regions in geospace. Large diamagnetic cavities with significant fluctuations of the local magnetic field strength have been observed there. These cusp diamagnetic cavities are always there day after day and are as large as 6 R_E Associated with these cavities are charged particles with energies from 20 keV up to 10 MeV. The intensities of the cusp energetic ions have been observed to increase by as much as four orders of the magnitude when compared with regions adjacent to the cusp which includes the magnetosheath. Their seed populations are a mixture of ionospheric and solar wind particles. The measured energetic ion fluxes in the high-altitude cusp are higher than that in both the regions upstream and downstream from the bow shock. Turbulent electric fields with an amplitude of about 10 mV/m are also present in the cusp, and a cusp resonant acceleration mechanism is suggested. The observations indicate that the dayside high-altitude cusp is a key region for transferring the solar wind mass, momentum, and energy into the Earth’s magnetosphere.     

Low-Frequency Plasma Waves in the Outer Polar CUSP: A Review of Observations from Prognoz 8, Interball 1, Magion 4, and Cluster

This paper presents a review of the most interesting observations of low-frequency plasma waves together with plasma particles which were made by the Interball 1, Magion 4 and Prognoz 8 satellites in the outer polar cusp. Accelerated plasma particles, hot electron populations and very strong wave activity, particularly at low frequencies, are observed. A detailed study of the wave spectra together with the distribution function for electrons indicate the correlation between the presence of lower-hybrid waves and the population of the particles with higher energy than in the surrounding space. These experimental facts suggest that strong coupling between waves and particles is responsible for plasma heating. During polar cusp crossings by Interball 1 and Prognoz 8, FFT analysis of the wave form indicates many bursts of ULF emissions in both electric and magnetic components. These waves have highly non-stationary characteristics. To study the dynamics of changes in the spectral characteristics of the waves wavelet analysis has been used. Nonlinear interactions are studied using bispectral methods of analysis. This presentation gives the results of such an analysis for selected cusp crossings at different altitudes. An example of wave activity registered by the STAFF instrument onboard the CLUSTER spacecraft in the polar cusp is also presented.     

On determining the noon polar cap boundary from SuperDARN HF radar backscatter characteristics

Previous work has shown that ionospheric HF radar backscatter in the noon sector can be used to locate the footprint of the magnetospheric cusp particle precipitation. This has enabled the radar data to be used as a proxy for the location of the polar cap boundary, and hence measure the flow of plasma across it to derive the reconnection electric field in the ionosphere. This work used only single radar data sets with a field of view limited to 2 h of local time. In this case study using four of the SuperDARN radars, we examine the boundary determined over 6 h of magnetic local time around the noon sector and its relationship to the convection pattern. The variation with longitude of the latitude of the radar scatter with cusp characteristics shows a bay-like feature. It is shown that this feature is shaped by the variation with longitude of the poleward flow component of the ionospheric plasma and may be understood in terms of cusp ion time-of-flight effects. Using this interpretation, we derive the time-of-flight of the cusp ions and find that it is consistent with approximately 1 keV ions injected from a subsolar reconnection site. A method for deriving a more accurate estimate of the location of the open-closed field line boundary from HF radar data is described.     

Nonlinear plasma wave processes observed in the polar cusp

We present observations of electric and magnetic field variations from proton (about few Hz) to electron cyclotron frequencies (about few kHz) obtained by STAFF instrument on Cluster satellites during two cusp crossings, at ∼6 R _E altitude, in September 2002. The cusp was identified by the presence of intensive fluxes of counter streaming electrons with low energies and broadband wave activity which is typical for this region. Special attention is given for the interval of measurements when the waveform of the magnetic field fluctuations was taken in this region by CLUSTER satellites. The wave has been processed using the wavelet and bispectral analysis. Results showing the cascade of turbulence and wave-wave interactions are presented in this paper. A three wave process can be responsible for the broadening of the wave spectra in the polar cusp.     

南极中山站与北京地磁中心台1991年3月24日磁暴的变化特征

本文分析了１９９１年３月２４日急始型磁暴的变化特征。分析表明，南极极隙区与中低纬区磁暴形态差异很大。中山站地磁台的磁暴急始为ＳＣ＊型，无明显磁暴主相，它是由一组强烈的磁亚暴所组成。北京地磁中心台的磁暴急始为ＳＣ型，磁暴主相持续时间较短，变化较大。这次磁暴是由磁暴前太阳黑子相对数增多、３月２２日２２ｈ４６ｍＵＴ有３ｂ级耀斑爆发等一系列太阳活动所引起的