Electron precipitations and polar auroras

In the first part (Sections I–III) a brief historical review of the progress of our knowledge of the precipitation of auroral electrons is given. Observations by different techniques, in terms of detectors aboard balloons, sounding rockets, and polar-orbiting satellites, are reviewed (Sections I). The precipitation morphology is examined in terms of synoptic statistical results (Section II) and of latitudinal survey along individual satellite passes (Section III). In the second part (Section IV), a large number of simultaneous observations of auroras and precipitating auroral electrons by DMSP satellites are examined in detail, and it is shown that precipitation characteristics of auroral electrons are distinctly different for the discrete aurora and the diffuse aurora. In the third part (Section V), the source region of auroral electrons is discussed by comparing the auroral electron precipitation at low altitudes observed by DMSP satellites with the simultaneous ATS-6 observations near the magnetospheric equatorial plane approximately along the same geomagnetic field line. It is shown that the diffuse aurora is caused by direct dumping of the plasma sheet electrons from the equatorial region, whereas discrete auroras require acceleration of electrons between the plasma sheet and the polar atmosphere. The parallel electric field along the geomagnetic field line above the ionosphere is a likely candidate for the acceleration mechanism.Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland 20810, U.S.A.     

Recent progress in studies of DMSP auroral photographs

A new auroral pattern, which indicates major auroral characteristics in all local time sectors, is presented. It has emerged as a result of extensive study of DMSP-8531 and -10533 auroral photographs. The paper presents also a brief summary of recent studies on the role of the north-south component of the interplanetary magnetic field on large-scale auroral dynamics and on the relationship between substorm energy and the size of the oval.     

Dynamic morphology of auroras

Simultaneous changes of auroral forms, brightness, and motions over the whole polar region are studied, using IGY all-sky camera records from widely distributed stations in eastern Siberia, Alaska, Canada and the northern United States. It is found that the auroral system centered in the midnight sector in the auroral zone repeatedly undergoes an expansion and subsequent contraction; during the maximum stage of the activity, the whole auroral system extends over a substantial portion of the darkened polar region. Such extensive auroral activity as a whole may be regarded as a single event, and is described in terms of the auroral substorm. The substorm has two characteristic phases, an expansive phase and a recovery phase. Characteristic auroral displays over the entire polar region during the substorm are described in detail. The basic physical processes involved for the auroral substorm are also discussed.Geomagnetic disturbances associated with the auroral substorm are also described in detail in terms of the polar magnetic substorm, and it is shown that both the auroral substorm and the polar magnetic substorm are different aspects of the manifestation of a large-scale plasma motion in the magnetosphere.The distribution of the aurora for different degrees of the geomagnetic activity is also discussed in terms of the auroral belt. It is shown that the center line of the auroral belt moves greatly with respect to its average location (namely the auroral zone), depending on the degree of the magnetic activity.     

The application of image orthicon techniques to auroral observation

The relatively high quantum efficiency of the photocathodes now used in photoelectric devices is an essential feature which makes these devices attractive for use in astronomical and auroral observations. In addition to possessing high sensitivity by virtue of the photocathode, certain types of photoelectric devices allow an image to be accumulated over some increment of time and then be read out in the form of an electrical signal which can be processed directly or used to produce a 2-dimensional image. One of these devices, the image orthicon television tube, has been found to be quite useful for obtaining auroral photographs and auroral and airglow spectra of very short exposure.     

Studies of polar current systems using the IMS Scandinavian magnetometer array

As a contribution to the International Magnetospheric Study (IMS, 1976–1979) a two-dimensional array of 42 temporary magnetometer stations was run in Scandinavia, supplementary to the permanent observatories and concentrated in the northern part of the region. This effort aimed at the time-dependent (periods above about 100 s) determination of the two-dimensional structure of substorm-related magnetic fields at the Earth's surface with highest reasonable spatial resolution (about 100 km, corresponding to the height of the ionosphere) near the footpoints of field-aligned electric currents that couple the disturbed magnetosphere to the ionosphere at auroral latitudes. It has been of particular advantage for cooperative studies that not only simultaneous data were available from all-sky cameras, riometers, balloons, rockets, and satellites, but also from the STARE radar facility yielding colocated two-dimensional ionospheric electric field distributions. In many cases it therefore was possible to infer the three-dimensional regional structure of substorm-related ionospheric current systems. The first part of this review outlines the basic relationships and methods that have been used or have been developed for such studies. The second short part presents typical equivalent current patterns observed by the magnetometer array in the course of substorms. Finally we review main results of studies that have been based on the magnetometer array observations and on additional data, omitting studies on geomagnetic pulsations. These studies contributed to a clarification of the nature of auroral electrojets including the Harang discontinuity and of ionospheric current systems related to auroral features such as the break-up at midnight, the westward traveling surge, eastward drifting omega bands, and spirals.     

Electrodynamic coupling of the magnetosphere and ionosphere

The auroral zone ionosphere is coupled to the outer magnetosphere by means of field-aligned currents. Parallel electric fields associated with these currents are now widely accepted to be responsible for the acceleration of auroral particles. This paper will review the theoretical concepts and models describing this coupling. The dynamics of auroral zone particles will be described, beginning with the adiabatic motions of particles in the converging geomagnetic field in the presence of parallel potential drops and then considering the modifications to these adiabatic trajectories due to wave-particle interactions. The formation of parallel electric fields can be viewed both from microscopic and macroscopic viewpoints. The presence of a current carrying plasma can give rise to plasma instabilities which in a weakly turbulent situation can affect the particle motions, giving rise to an effective resistivity in the plasma. Recent satellite observations, however, indicate that the parallel electric field is organized into discrete potential jumps, known as double layers. From a macroscopic viewpoint, the response of the particles to a parallel potential drop leads to an approximately linear relationship between the current density and the potential drop.The currents flowing in the auroral circuit must close in the ionosphere. To a first approximation, the ionospheric conductivity can be considered to be constant, and in this case combining the ionospheric Ohm's Law with the linear current-voltage relation for parallel currents leads to an outer scale length, above which electric fields can map down to the ionosphere and below which parallel electric fields become important. The effects of particle precipitation make the picture more complex, leading to enhanced ionization in upward current regions and to the possibility of feedback interactions with the magnetosphere.Determining adiabatic particle orbits in steady-state electric and magnetic fields can be used to determine the self-consistent particle and field distributions on auroral field lines. However, it is difficult to pursue this approach when the fields are varying with time. Magnetohydrodynamic (MHD) models deal with these time-dependent situations by treating the particles as a fluid. This class of model, however, cannot treat kinetic effects in detail. Such effects can in some cases be modeled by effective transport coefficients inserted into the MHD equations. Intrinsically time-dependent processes such as the development of magnetic micropulsations and the response of the magnetosphere to ionospheric fluctuations can be readily treated in this framework.The response of the lower altitude auroral zone depends in part on how the system is driven. Currents are generated in the outer parts of the magnetosphere as a result of the plasma convection. The dynamics of this region is in turn affected by the coupling to the ionosphere. Since dissipation rates are very low in the outer magnetosphere, the convection may become turbulent, implying that nonlinear effects such as spectral transfer of energy to different scales become important. MHD turbulence theory, modified by the ionospheric coupling, can describe the dynamics of the boundary-layer region. Turbulent MHD fluids can give rise to the generation of field-aligned currents through the so-called -effect, which is utilized in the theory of the generation of the Earth's magnetic field. It is suggested that similar processes acting in the boundary-layer plasma may be ultimately responsible for the generation of auroral currents.     

The Freja Hot Plasma Experiment—Instrument and first results

The Hot Plasma Experiment, F3H, on boardFreja is designed to measure auroral particle distribution functions with very high temporal and spatial resolution. The experiment consists of three different units; an electron spectrometer that measures angular and energy distributions simultaneously, a positive ion spectrometer that is using the spacecraft spin for three-dimensional measurements, and a data processing unit. The main scientific objective is to study positive ion heating perpendicular to the magnetic field lines in the auroral region. The high resolution measurements of different positive ion species and electrons have already provided important information on this process as well as on other processes at high latitudes. This includes for example high resolution observations of auroral particle precipitation features and source regions of positive ions during magnetic disturbances. TheFreja orbit with an inclination of 63° allows us to make detailed measurements in the nightside auroral oval during all disturbance levels. In the dayside, the cusp region is covered during magnetic disturbances. We will here present the instrument in some detail and some outstanding features in the particle data obtained during the first months of operation at altitudes around 1700 km in the northern hemisphere auroral region.     

Primary sources of large-scale Birkeland currents

We review generation mechanisms of Birkeland currents (field-aligned currents) in the magnetosphere and the ionosphere. Comparing Birkeland currents predicted theoretically with those studied observationally by spacecraft experiments, we present a model for driving mechanism, which is unified by the solar wind-magnetosphere interaction that allows the coexistence of steady viscous interaction and unsteady magnetic reconnection. The model predicts the following: (1) the Region 1 Birkeland currents (which are located at poleward part of the auroral Birkeland-current belt, and constitute quasi-permanently and stably a primary part of the overall system of Birkeland currents) would be fed by vorticity-induced space charges at the core of two-cell magnetospheric convection arisen as a result of viscous interaction between the solar wind and the magnetospheric plasma, (2) the Region 2 Birkeland currents (which are located at equatorward part of the auroral Birkeland-current belt, and exhibit more variable and localized behavior) would orginate from regions of plasma pressure inhomogeneities in the magnetosphere caused by the coupling between two-cell magnetospheric convection and the hot ring current, where the gradient-B current and/or the curvature current (presumably the hot plasma sheet-ring current) are forced to divert to the ionosphere, (3) the Cusp Birkeland currents (which are located poleward of and adjacent to the Region 1 currents and are strongly controlled by the interplanetary magnetic field (IMF)) might be a diversion of the inertia current which is newly and locally produced in the velocity-decelerated region of earthward solar wind where the magnetosphere is eroded by dayside magnetic reconnection, (4) the nightside Birkeland currents which are connected to a part of the westward auroral electrojet in the Harang discontinuity sector might be a diversion of the dusk-to-dawn tail current resulting from localized magnetic reconnection in the magnetotail plasma sheet where plasma density and pressure are reduced.     

Electrodynamics of the magnetosphere: Geomagnetic storms

Geomagnetic and auroral storms provide a great deal of detailed information on the interaction between the solar plasma flows and the magnetosphere. Vast numbers of observations have been accumulated, and many theories have been developed to explain them. However, many of the most vital features of the interaction remain unsolved. The purpose of this paper is to provide the background for future work by summarizing fundamental morphological data and by reviewing critically the proposed theories.The paper consists of four sections. In the first section, the structure of the solar plasma flows and the magnetosphere are briefly discussed. Effects of the direct impact of the plasma flows on the magnetosphere are described in Section 2. Both Sections 3 and 4 are devoted to the discussion of the major phase of geomagnetic storms, namely the formation of the asymmetric ring current belt and the development of the auroral and polar magnetic substorms, respectively.Research supported in part by grants from the National Aeronautics and Space Administration to the University of Alaska (NsG 201-62) and to the University of Iowa (NsG 233-62).     

Role of ionospheric effects and plasma sheet dynamics in the formation of auroral arcs

At the ionospheric level, the substorm onset (expansion phase) is marked by the initial brightening and subsequent breakup of a pre-existing auroral arc. According to the field line resonance (FLR) wave model, the substorm-related auroral arc is caused by the field-aligned current carried by FLRs. The FLRs are standing shear Alfvén wave structures that are excited along the dipole/quasi-dipole lines of the geomagnetic field. The FLRs (that can cause auroral arc) thread from the Earthward edge of the plasma sheet and link the auroral arc to the plasma sheet region of 6–15 R _E. The region is associated with magnetic fluctuations that result from the nonlinear wave-wave interactions of the cross-field current-instability. The instability (excited at the substorm onset) disrupts the cross-tail current which is built up during the growth phase of the substorms and results in magnetic fluctuations. The diversion of the current to polar regions can lead to auroral arc intensification. The current FLR model is based on the amplitude equations that describe the nonlinear space-time evolution of FLRs in the presence of ponderomotive forces exerted by large amplitude FLRs (excited during substorms). The present work will modify the FLR wave model to include the effects arising from magnetic fluctuations that result from current disruption near the plasma sheet (6–15 R _E). The nonlinear evolution of FLRs is coupled with the dynamics of plasma sheet through a momentum exchange term (resulting from magnetic fluctuations due to current disruption) in the generalized Ohm's law. The resulting amplitude equations including the effects arising from magnetic fluctuations can be used to study the structure of the auroral arcs formed during substorms. We have also studied the role of feedback mechanism (in a dipole geometry of the geomagnetic field) in the formation of the discrete auroral arc observed on the nightside magnetosphere. The present nonlinear dispersive model (NDM) is extended to include effects arising from the low energy electrons originating from the plasma sheet boundary layer. These electrons increase the ionospheric conductivity in a localized patch and enhance the field-aligned current through a feedback mechanism. The feedback effects were studied numerically in a dipole geometry using the the NDM. The numerical studies yield the magnitude of the field-aligned current that is large enough to form a discrete auroral arc. Our studies provide theoretical support to the observational work of Newell et al. that the feedback instability plays a major role in the formation of the discrete auroral arcs observed on the nightside magnetosphere.     

Relationship Between Substorm Auroras and Processes in the Near-Earth Magnetotail

Although the auroral substorm has been long regarded as a manifestation of the magnetospheric substorm, a direct relation of active auroras to certain magnetospheric processes is still debatable. To investigate the relationship, we combine the data of the UV imager onboard the Polar satellite with plasma and magnetic field measurements by the Geotail spacecraft. The poleward edge of the auroral bulge, as determined from the images obtained at the LHBL passband, is found to be conjugated with the region where the oppositely directed fast plasma flows observed in the near-Earth plasma sheet during substorms are generated. We conclude that the auroras forming the bulge are due to the near-Earth reconnection process. This implies that the magnetic flux through the auroral bulge is equal to the flux dissipated in the magnetotail during the substorm. Comparison of the magnetic flux through the auroral bulge with the magnetic flux accumulated in the tail lobe during the growth phase shows that these parameters have the comparable values. This is a clear evidence of the loading–unloading scheme of substorm development. It is shown that the area of the auroral bulge developing during substorm is proportional to the total (magnetic plus plasma) pressure decrease in the magnetotail. These findings stress the importance of auroral bulge observations for monitoring of substorm intensity in terms of the magnetic flux and energy dissipation.     

Solar Wind Control of the Magnetospheric and Auroral Dynamics

A dependence of the polar cap magnetic flux on the interplanetary magnetic field and on the solar wind dynamic pressure is studied. The model calculations of the polar cap and auroral oval magnetic fluxes at the ionospheric level are presented. The obtained functions are based on the paraboloid magnetospheric model calculations. The scaling law for the polar cap diameter changing for different subsolar distances is demonstrated. Quiet conditions are used to compare theoretical results with the UV images of the Earth’s polar region obtained onboard the Polar and IMAGE spacecrafts. The model calculations enable finding not only the average polar cap magnetic flux but also the extreme values of the polar cap and auroral oval magnetic fluxes. These values can be attained in the course of the severe magnetic storm. Spectacular aurora often can be seen at midlatitude during severe magnetic storm. In particularly, the Bastille Day storm of July 15–16, 2000, was a severe magnetic storm when auroral displays were reported at midlatitudes. Enhancement of global magnetospheric current systems (ring current and tail current) and corresponding reconstruction of the magnetospheric structure is a reason for the equatorward displacement of the auroral zone. But at the start of the studied event the contracted polar cap and auroral oval were observed. In this case, the sudden solar wind pressure pulse was associated with a simultaneous northward IMF turning. Such IMF and solar wind pressure behavior is a cause of the observed aurora dynamics.     

The plasma physics of shock acceleration

The notion that plasma shocks in astrophysical settings can and do accelerate charged particles to high energies is not a new one. However, in recent years considerable progress has been achieved in understanding the role particle acceleration plays both in astrophysics and in the shock process itself. In this paper we briefly review the history and theory of shock acceleration, paying particular attention to theories of parallel shocks which include the backreaction of accelerated particles on the shock structure. We discuss in detail the work that computer simulations, both plasma and Monte Carlo, are playing in revealing how thermal ions interact with shocks and how particle acceleration appears to be an inevitable and necessary part of the basic plasma physics that governs collisionless shocks. We briefly describe some of the outstanding problems that still confront theorists and observers in this field.     

产品绿色设计方法研究现状及展望--DFX方法研究

可持续制造是实现可持续发展对制造业的必要要求。绿色设计是实施可持续制造的关键技术。国内外有关绿色设计方法的研究取得了重要进展。本论文的上篇对绿色设计的一般理论和方法进行了介绍,本文则对绿色设计中面向可拆卸性的设计、面向回收的设计、面向可维护性的设计等具体技术进行了详细分析,并对全文进行总结,指出了绿色设计理论及其方法的研究方向和发展趋势。     

Structure,luminosity, and dynamics of the Venus thermosphere

We review here observations and models related to the chemical and thermal structures, airglow and auroral emissions and dynamics of the Venus thermosphere, and compare empirical models of the neutral densities based in large part on in situ measurements obtained by the Pioneer Venus spacecraft. Observations of the intensities of emissions are important as a diagnostic tool for understanding the chemical and physical processes taking place in the Venus thermosphere. Measurements, ground-based and from rockets, satellites, and spacecraft, and model predictions of atomic, molecular and ionic emissions, are presented and the most important sources are elucidated. Coronas of hot hydrogen and hot oxygen have been observed to surround the terrestrial planets. We discuss the observations of and production mechanisms for the extended exospheres and models for the escape of lighter species from the atmosphere. Over the last decade and a half, models have attempted to explain the unexpectedly cold temperatures in the Venus thermosphere; recently considerable progress has been made, although some controversies remain. We review the history of these models and discuss the heating and cooling mechanisms that are presently considered to be the most important in determining the thermal structure. Finally, we discuss major aspects of the circulation and dynamics of the thermosphere: the sub-solar to anti-solar circulation, superrotation, and turbulent processes.     

Auroral Radio Emissions, 1. Hisses, Roars, and Bursts

The Earth's auroral electrons produce copious non-thermal radio emissions of various types, including auroral kilometric radiation (AKR), whistler mode auroral hiss, mode conversion radiation such as auroral roar and MF-burst, and possibly HF/VHF emissions. In some cases, mechanisms have been identified and quantitatively described, whereby the energy of the auroral electrons is converted into electromagnetic radiation. In many other cases, the radiation mechanism, or the relative significance of several possible mechanisms, remains uncertain. This review covers fairly comprehensively experimental and theoretical research on types of auroral radiation other than AKR, concentrating on emissions with frequency higher than about 1kHz and treating only emissions which are unique to the auroral zone. The review covers both ground-based and in-situ observations. It covers a wide range of theoretical approaches, emphasizing those which at present appear most important for producing non-AKR auroral radiations.     

Cusp dynamics and ionospheric outflow

One of the IMAGE mission science goals is to understand the dayside auroral oval and its dynamic relationship to the magnetosphere. Two ways the auroral oval is dynamically coupled to the magnetosphere are through the injection of magnetosheath plasma into the magnetospheric cusps and through the ejection of ionospheric plasma into the magnetosphere. The ionospheric footpoints of the Earth's magnetospheric cusps are relatively narrow regions in invariant latitude that map magnetically to the magnetopause. Monitoring the cusp reveals two important aspects of magnetic reconnection at the magnetopause. Continuous cusp observations reveal the relative contributions of quasi-steady versus impulsive reconnection to the overall transfer of mass, energy, and momentum across the magnetopause. The location of the cusp is used to determine where magnetic reconnection is occurring on the magnetopause. Of particular interest is the distinction between anti-parallel reconnection, where the magnetosheath and magnetospheric field lines are strictly anti-parallel, and component merging, where the magnetosheath and magnetospheric field lines have one component that is anti-parallel. IMAGE observations suggest that quasi-steady, anti-parallel reconnection is occurring in regions at the dayside magnetopause. However, it is difficult to rule out additional component reconnection using these observations. The ionospheric footpoint of the cusp is also a region of relatively intense ionospheric outflow. Since outflow also occurs in other regions of the auroral oval, one of the long-standing problems has been to determine the relative contributions of the cusp/cleft and the rest of the auroral oval to the overall ionospheric ion content in the Earth's magnetosphere. While the nature of ionospheric outflow has made it difficult to resolve this long-standing problem, the new neutral atom images from IMAGE have provided important evidence that ionospheric outflow is strongly controlled by solar wind input, is `prompt' in response to changes in the solar wind, and may have very narrow and distinct pitch angle structures and charge exchange altitudes.     

The hot ion component of the magnetospheric plasma and some relations to the electron component-observations and physical implications

The observations of hot ions in the high altitude ionosphere, at IR _e along the auroral zone magnetic field lines, near the equatorial plane in the inner magnetosphere, in the distant tail, and in the magnetospheric boundary regions are reviewed with particular regard to the relations of the ions to the electrons. The physical knowledge obtained from the observations is summarized.