Electrodynamics of the outer solar atmosphere

The structure of the outer solar atmosphere and its magnetic coupling to the photospheric motions indicate the existence of large-scale current systems. The heating and the dynamics of coronal structures is therefore governed by electrodynamic coupling of these structures to the underlying photosphere. In a structured corona, the heating is enhanced because of several processes such as resonance absorption of Alfvénic surface waves, anomalous Joule heating, reconnection and the related topological dissipation. The global thermal and dynamic behaviour of coronal structures can be fruitfully described in terms of equivalent electrodynamic circuits, taking into account the paramount role of the photospheric boundaries. Coronal current systems may be stable, as in the case of coronal loops, but occassionally they show catastrophic behaviour if the current intensity surpasses a critical threshold.     

Magnetic energy storage and conversion in the solar atmosphere

A review of the theoretical problems associated with preflare magnetic energy storage and conversion is presented. The review consists of three parts; preflare magnetic energy storage, magnetic energy conversion mechanisms, and preflare triggers. In Section 2, the relationship between magnetic energy storage and the electrodynamic coupling of the solar atmosphere is developed. By accounting for the electrodynamic coupling of the solar atmosphere, we are able to examine the fundamental problems associated with the concept of in situ versus remote magnetic-energy storage. Furthermore, this approach permits us to distinguish between the roles of local and global parameters in the storage process.Section 3 is focused on the conversion mechanisms that can explain, in principle, the rapid energy release of a flare. In addition, we discuss how electrodynamic coupling eventually dictates which mechanism(s) is responsible for releasing the stored magnetic energy, and how the global coupling dictates the final evolution of the relevant mechanism. Section 4 examines preflare triggers and Section 5, we examine the most promising directions for future research into the problem of magnetic-energy storage and conversion of the Sun.     

Phase coherence of MHD waves in the solar wind

Large amplitude MHD waves are commonly found in the solar wind. Nonlinear interactions between the MHD waves are likely to produce finite correlation among the wave phases. For discussions of various transport processes of energetic particles, it is fundamentally important to determine whether the wave phases are randomly distributed (as assumed in quasi-linear theories) or they have a finite coherence. Using a method based on a surrogate data technique and a fractal analysis, we analyzed Geotail magnetic field data (provided by S. Kokubun and T. Nagai through DARTS at the Institute of Space and Astronautical Science) to evaluate the phase coherence among the MHD waves in the earth's foreshock region. The correlation of wave phases does exist, indicating that the nonlinear interactions between the waves is in progress. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interplanetary shock waves generated by solar flares

Recent observational and theoretical studies of interplanetary shock waves associated with solar flares are reviewed. An attempt is made to outline the framework for the genesis, life and demise of these shocks. Thus, suggestions are made regarding their birth within the flare generation process, MHD wave propagation through the chromosphere and inner corona, and maturity to fully-developed coronal shock waves. Their subsequent propagation into the ambient interplanetary medium and disturbing effects within the solar wind are discussed within the context of theoretical and phenomenological models. The latter — based essentially on observations — are useful for a limited interpretation of shock geometric and kinematic characteristics. The former — upon which ultimate physical understanding depends — are used for clarification and classification of the shocks and their consequences within the solar wind. Classification of limiting cases of blast-produced shocks (as in an explosion) or longer lasting ejecta (or piston-driven shocks) will hopefully be combined with the study of the flare process itself.The theoretical approach, in spite of its contribution to clarification of various concepts, contains some fundamental limitations and requires further study. Numerical simulations, for example, depend upon a non-unique set of multi-parameter initial conditions at or near the Sun. Additionally, the subtle but important influence of magnetic fields upon energy transport processes within the solar wind has not been considered in the numerical simulation approach. Similarity solutions are limited to geometrical symmetries and have not exploited their potential beyond the special cases of the blast and the constant-velocity, piston-driven shock waves. These continuum fluid studies will probably require augmentation or even replacement by plasma kinetic theory in special situations when observations indicate the presence of anomalous transport processes. Presently, for example, efforts are directed toward identification of detailed shock structures (as in the case of Earth's bow shock) and of the disturbed solar wind (such as the piston).Further progress is expected with extensive in situ and remote monitoring of the solar wind over a wide range of heliographic radii, longitudes and latitudes.This paper is a revised and updated version of an invited review originally presented at the IUGG XV General Assembly, Moscow, U.S.S.R., 2–14 August 1971.     

Further studies of waves accompanying the solar wind pick-up of interstellar hydrogen

Data collected by the magnetometer onboard the Ulysses spacecraft are surveyed for the occurrence of waves generated during the pick-up of interstellar hydrogen. Thirty one wave events were found during a 640 day study period, between March 21, 1992 and December 20, 1993 (after the Ulysses encounter with Jupiter). It is found that observation of the waves does not depend on the magnitude of the background magnetic field, but is a strong function of the angle between the magnetic field and the solar wind flow direction, with small angles being favored.     

The importance of spectroscopy in the 80–800 å region for plasma diagnostics in the solar atmosphere

We discuss the importance of the spectral range from about 80 to 800 Å for determining physical conditions in different regions of the solar atmosphere. We give examples of line ratios that may be used to determine electron densities in quiet Sun regions, active regions, and flares. We discuss the possibility of determining electron temperatures from line ratios in the EUV.We show that profiles as well as intensities of spectral lines must be obtained for a proper interpretation of the spectra. We give approximate parameters for a solar grazing incidence spectrograph suitable for the study of the 80–800 Å wavelength region.Given as an invited review paper at the EGAS Meeting in Munich, Germany, 11–14 July 1978.     

MHD structures, waves and turbulence in the solar wind: Observations and theories

A comprehensive overview is presented of recent observational and theoretical results on solar wind structures and fluctuations and magnetohydrodynamic waves and turbulence, with preference given to phenomena in the inner heliosphere. Emphasis is placed on the progress made in the past decade in the understanding of the nature and origin of especially small-scale, compressible and incompressible fluctuations. Turbulence models to describe the spatial transport and spectral transfer of the fluctuations in the inner heliosphere are discussed, and results from direct numerical simulations are dealt with. Intermittency of solar wind fluctuations and their statistical distributions are briefly investigated. Studies of the heating and acceleration effects of the turbulence on the background wind are critically surveyed. Finally, open questions concerning the origin, nature and evolution of the fluctuations are listed, and possible avenues and perspectives for future research are outlined.     

Interaction of non-perpendicular/parallel solar wind shock waves with the earth's magnetosphere

The interaction of travelling interplanetary shock waves with the bow shock-magnetosphere system is considered. We consider the general case when the interplanetary magnetic field is oblique to the Sun-planetary axis, thus, the interplanetary shock is neither parallel nor perpendicular. We find that an ensemble of shocks are produced after the interaction for a representative range of shock Mach numbers. First, we find that the system S ₊ R _– CS _– S ₊ appears after the collision of travelling fast shock waves S ₊ (Mach number M = 2 to 7) with the bow shock. Here, S _– and R _– represent the slow shock wave and slow rarefaction wave, and C represents the contact surface. It is shown that in the presence of an interplanetary field that is inclined by 45° to the radial solar wind velocity vector, the waves R _– and S _– are weak waves and, to the first degree of approximation, the situation is similar to the previously studied normal perpendicular case. The configuration, R ₊ C _m S _– S ₊ or R ₊ C _m R _– S ₊ where C _m is the magnetopause, appears as the result of the fast shock wave's collision with the magnetopause. In this case the waves S _– and R _– are weak. The fast rarefaction wave reflected from the magnetosphere is developed similar to the case for the collision of a perpendicular shock. The shock wave intensity is varied for Mach numbers from 2 to 10. Thus, in the limits of the first approximation, the validity of the one-dimensional consideration of the nonstationary interaction of travelling interplanetary shock waves with the bow shock-magnetosphere system is proved. The appearance of the fast rarefaction wave, R ₄, decreasing the pressure on the magnetosphere of the Earth after the abrupt shock-like contraction, is proved. A possible geomagnetic effect during the global perturbation of the SSC or SI⁺ type is discussed.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

Amplification of the influence of solar flux variations on the winter stratosphere by planetary waves

Recent observational evidence has suggested that variations in solar activity may affect winter stratospheric polar ozone and temperature levels. The paucity of direct sunlight available during this season points strongly to a dynamical mechanism. We have carried out several large ensemble experiments within the middle atmosphere and the coupled middle atmosphere and lower thermosphere to simulate the radiative/dynamical coupling via planetary waves for a range of solar fluxes. In the former case, the model response in the winter stratosphere was linear and of the order of the summer stratopause forcing, whilst in the latter, the level of correlation in the winter stratosphere remained high, but was diluted over a wider volume. The inclusion of the upper atmosphere enhanced the winter polar stratospheric response by a factor of three.     

Thermospheric gravity waves: Observations and interpretation using the transfer function model (TFM)

Gravity waves are prominent in the polar region of the terrestiral thermosphere, and can be excited by perturbations in Joule heating and Lorents force due to magnetospheric processes. We show observations from the Dynamics Explorer-2 satellite to illustrate the complexity of the phenomenon and review the transfer function model (TFM) which has guided our interpretation. On a statistical basis, the observed atmospheric perturbations decrease from the poles toward the equator and tend to correlate with the magnetic activity index, Ap, although individual measurements indicate that the magnetic index is often a poor measure of gravity wave excitation. The theoretical models devised to describe gravity waves are multifaceted. On one end are fully analytical, linear models which are based on the work of Hines. On the other end are fully numerical, thermospheric general circulation models (TGCMs) which incorporate non-linear processes and wave mean flow interactions. The transfer function model (TFM) discussed in this paper is between these two approaches. It is less restrictive than the analytical approach and relates the global propagation of gravity waves to their excitation. Compared with TGCMs, the TFM is simplified by its linear approximation; but it is not limited in spatial and temporal resolution, and the TFM describes the wave propagation through the lower atmosphere. Moreover, the TFM is semianalytical which helps in delineating the wave components. Using expansions in terms of spherical harmonics and Fourier components, the transfer function is obtained from numerical height integration. This is time consuming computationally but needs to be done only once. Once such a transfer function is computed, the wave response to arbitrary source distributions on the globe can then be constructed in very short order. In this review, we discuss some numerical experiments performed with the TFM, to study the various wave components excited in the auroral regions which propagate through the thermosphere and lower atmosphere, and to elucidate the properties of realistic source geometries. The model is applied to the interpretation of satellite measurements. Gravity waves observed in the thermosphere of Venus are also discussed.     

Principles of Invariance in Radiative Transfer

We have reviewed the principle of invariance, its applications and its usefulness for obtaining the radiation field in semi-infinite and finite atmospheres. Various laws of scattering in dispersive media and the consequent radiation field are studied. The H-functions and X- and Y-functions in semi-infinite and finite media respectively are derived in a few cases. The Discrete Space Theory (DST) which is a general form of the Principle of Invariance is described. The method of addition of layers with general properties, is shown to describe all the properties of multiple scattering. A few examples of the application of DST such as polarization, line formation in expanding stellar atmospheres, etc., and a numerical analysis of DST are presented. Other developments in the theory of radiative transfer are briefly described. This revised version was published online in June 2006 with corrections to the Cover Date.     

Helium in the terrestrial atmosphere

After a brief historical review of the discovery of helium in the terrestrial atmosphere, the production mechanisms of the isotopes He⁴ and He³ are discussed. Although the radioactive production of He⁴ in the Earth is well understood, some uncertainty still exists for the degassing process leading to an atmospheric influx of (2.5 ±1.5) × 10⁶ atoms cm^–2 s^–1. Different production mechanisms are possible for He³ leading to an influx of (7.5±2.5) atoms cm^–2 s^–1. Observations of helium in the thermosphere show a great variability of this constituent. The different mechanisms proposed to explain the presence of the winter helium bulge are discussed. Since helium ions are present in the topside ionosphere and in the magnetosphere, ionization mechanisms are analyzed. Owing to possible variations and uncertainties in the solar UV flux, the photoionization coefficient is (8±4) × 10^–8 s^–1. Finally, the helium balance between production in the earth and loss into the interplanetary space is discussed with respect to the different processes which can play an effective role.     

约束阻尼型镗杆的优化及减振性能

孔加工过程中镗杆的切削颤振影响着表面加工质量和加工精度,约束阻尼型镗杆可有效抑制这种切削振动,但其作用机理未被完全研究清楚,导致其抑制振动的效果一般。对约束阻尼型镗杆的结构优化、材料优选及减振性能进行了理论和实验研究。首先,根据Kelvin-Voigt粘弹性力学模型理论建立了镗杆的动力学模型,研究证实增大镗杆的静刚度和结构损耗因子能提高其减振性能从而提高孔加工质量;其次,基于建立的约束阻尼型镗杆静刚度和结构损耗因子理论公式,对其进行结构优化、材料优选。结果显示:存在一个最佳尺寸范围可减小镗杆在主要工作频域段上的振动,同时所选用的阻尼层应具有较小的弹性模量和较大的材料损耗因子,约束层材料应具有较大的弹性模量;最后,设计制造4种不同材料的约束阻尼型镗杆,通过模态实验获得静刚度、结构损耗因子,并与理论计算结果进行对比分析,同时研究切削过程中约束阻尼型镗杆的材料及切削参数对减振性能的影响。结果显示:约束阻尼型镗杆能有效减小径向振动以提高加工质量,不同材料的约束阻尼型镗杆在切削过程中径向振动差别较大,优化后的钢-PMMA-硬质合金镗杆在不同切深及转速下的径向振动加速度较小且更加稳定。     

The solar wind in the outer solar system

The properties of the solar wind including magnetic fields, plasma, and plasma waves are briefly reviewed with emphasis on conditions near and beyond the orbit of Jupiter. An extrapolation of the steady-state wind to large distances, evolution of disturbances and structure, modulation of cosmic rays, interactions with planetary bodies (bow shocks and magnetosheaths), and interactions with interstellar neutral helium and hydrogen are briefly discussed. Some comments on instrumentation requirements to observationally define the above phenomena are also included.This is one of the publications by the Science Advisory Group.     

Radiative properties of pulsar magnetospheres

We discuss recent results of radius to frequency mapping of pulsars. This method shows that for 43 pulsars the radio emission originates near the polar cap for millisecond pulsars and a few hundred km away for longer period pulsars. If the magnetospheres of these object contain dipolar magnetic fields, the corresponding magnetic field strength in the emission region is about 10⁷ gauss, for all pulsars in the sample. We investigate possible physical reasons for the location of the radio emission.     

Radiative Forcing of Climate Change

Our current understanding of mechanisms that are, or may be, acting to cause climate change over the past century is briefly reviewed, with an emphasis on those due to human activity. The paper discusses the general level of confidence in these estimates and areas of remaining uncertainty. The effects of increases in the so-called well-mixed greenhouse gases, and in particular carbon dioxide, appear to be the dominant mechanism. However, there are considerable uncertainties in our estimates of many other forcing mechanisms; those associated with the so-called indirect aerosol forcing (whereby changes in aerosols can impact on cloud properties) may be the most serious, as its climatic effect may be of a similar size as, but opposite sign to, that due to carbon dioxide. The possible role of volcanic eruptions as a natural climate change mechanism is also highlighted.     

飞机管道颗粒碰撞阻尼器设计与试验验证

飞机管道振动超标是严重威胁飞机飞行安全的重要故障,降低飞机管道振动水平,对于提高飞机可靠性和安全性具有重要意义。针对难于施加管道卡箍约束的飞机管道结构的减振问题,设计了一种基于颗粒碰撞阻尼技术的管道减振器。该减振器通过特定的结构设计,在不影响现有管道结构的基础上,很方便地安装到管道上进行减振。其减振原理是基于减振器内部的颗粒碰撞而导致的能量耗散,从而提高管道结构的阻尼效应。因此,将此颗粒碰撞阻尼器安装在振动管道上,在管道发生共振的情况下,管道振动峰值将明显降低。本文基于所设计的管道减振器,利用振动台试验研究了颗粒填充率对减振效果的影响,发现改变阻尼器内部颗粒的填充率,管道的振动随颗粒填充率的增加有先减小后增大的趋势,同时利用EDEM颗粒流仿真软件计算了减振器振动过程中颗粒的能量耗散情况,发现颗粒能量耗散速率最大时所对应的颗粒填充率与试验过程中管道振动加速度降到最低时所对应的颗粒填充率达到了一致,仿真结果与试验结果取得了很好的一致性。最后,将所设计的颗粒阻尼减振器安装在液压动力源管道上进行实际减振试验,测试了在安装减振器前后,试验管道在X、Y、Z三个方向的振动加速度,经过对比分析,发现安装颗粒阻尼减振器后,液压管道的压力脉动频率下的振动水平得到了明显抑制,试验结果充分表明了本文所设计的飞机管道颗粒减振器的有效性和实用性。