The meteor radar response function: Theory and application to narrow beam MST radar

The meteor radar response function is an important tool for analyzing meteor backscatter observed by radar systems. We extend previous work on the development of the response function to include a non-uniform meteor ionization profile, provided by meteor ablation theory, in contrast to what has been assumed in the past. This has the advantage that the height distribution of meteors expected to be observed by a radar meteor system may be accurately modeled. Such modeling leads to meteor height distributions that have implications for the composition of those meteoroids ablating at high altitudes which may be observed by “non-traditional” meteor radars operating at MF/HF. The response function is then employed to investigate meteor backscatter observed by narrow beam MST radars which in recent years have been used increasingly to observe meteors.     

Auroral research

An auroral substorm, unique in that it occurred at the relatively low latitude of SANAE, is described based on the full range of observational techniques at SANAE together with satellite data. Some features fit the generally accepted picture of substorms, others do not. From ground-based television imagery of Giant Undulations the temporal evolution of their dynamics is obtained and possible theories for their extistence examined. Super fast auroral waves, with speeds of up to 1200 km s^–1, have been observed for the first time. Their characteristics can not, as yet, be accounted for by a single generation mechanism. The potential value of simultaneous recordings of aurorae by television and riometer is discussed     

Effect of finite parallel conductivity on the magnetospheric convection

The magnetospheric plasma convection is studied, taking into account the finite conductivity along magnetic field lines. Field-aligned currents flowing at the inner boundary of the magnetospheric plasma sheet give rise to parallel electric fields which insignificantly affect the convection on the ionospheric level but change drastically the convection system in the magnetosphere. Intense azimuthal convective streams arise along both sides of the plasma sheet boundary. A part of convection lines appears to be completely closed in the inner magnetosphere.     

The effect of realistic conductivities on the high-latitude neutral thermospheric circulation

The dynamics of the high latitude thermosphere are dominated by the ion circulation pattern driven by magnetospheric convection. The reaction of the neutral thermosphere is influenced by both the magnitude of the ion convection velocity and by the conductivity of the thermosphere. Using a threedimensional, time-dependent, thermospheric, neutral model together with different ionospheric models, the effect of changes in conductivity can be assessed. The ion density is described by two models: the first is the empirical model of Chiu (1975) appropriate for very quiet geomagnetic conditions, and the second is a modified version of the theoretical model of Quegan et al. (1982). The differences in the neutral circulation resulting from the use of these two ionospheric models emphasizes the need for realistic high latitude conductivities when attempting to model average or disturbed geomagnetic conditions, and a requirement that models should couple realistically the ionosphere and the neutral thermosphere. An attempt is made to qualitatively interpret some of the features of the neutral circulation produced at high latitudes by magnetospheric processes.     

火星轨道器次表层探测雷达数据处理技术与现状研究

火星是人类重要的地外天体探测目标之一,对火星表面进行的探测和研究表明,火星表面曾经存在液态水,水是生命存在的基础,因此,在次表层寻找不同形式的水是目前火星探测的重要科学目标之一.近17年来,欧洲火星快车(Mars Express)上搭载的火星次表层和电离层探测先进雷达(Mars Advanced Radar for S...     

The coupling of solar convection and rotation – (Invited Review)

In recent years, helioseismology has provided an unprecedented look at the dynamics of the solar interior. These new insights have been accompanied by tremendous advances in high-performance computing technology, prompting increasingly sophisticated and realistic numerical models of solar convection. Among the most important helioseismic constraints on global-scale convection models is the mean differential rotation profile of the solar envelope, which is established by convection under the influence of rotation. The highly turbulent nature of solar convection makes this rotational influence difficult to determine and model. I will begin this review by discussing the solar rotation profile inferred from helioseismic measurements and various theoretical and numerical approaches to account for it. Computational constraints limited early numerical models to relatively laminar flow regimes but more recent investigations have begun to explore the distinct nature of turbulent convection. After a brief overview of empirical and numerical results on the related Rayleigh-Bernard system, I will outline the current state of numerical modeling of turbulent convection in rotating, stratified fluids, first in Cartesian and then in spherical geometries. The emphasis throughout will be on how rotation influences the structure, evolution, and transport processes of turbulent convection and what type of differential rotation can result.     

Magnetospheric convection at a low level power ϵ

It is demonstrated that the magnetospheric convection becomes evident in terms of the AE index only when the power ? of the solar wind-magnetosphere dynamo becomes greater than ～ 10¹⁸ erg s^?1 or a slightly lower value. An enhanced conductivity is a crucial factor for the magnetospheric convection to manifest even in a low-level increase of the AE index of ～ 50–100 γ.     

The Jovian hydrogen bulge: Evidence for co-rotating magnetospheric convection

The hydrogen bulge is a feature in Jupiter's upper atmosphere that co-rotates with the planetary magnetic field (i.e. the hydrogen bulge is fixed in System III coordinates). It is located approximately 180° removed in System III longitude from the active sector, which has been identified as the source region for Jovian decametric radio emission and for release of energetic electrons into interplanetary space. According to the magnetic-anomaly model, the active sector is produced by the effect of the large magnetic anomaly in Jupiter's northern hemisphere. On the basis of the magnetic-anomaly model, it has been theoretically expected for some time that a two-cell magnetospheric convection pattern exists within the Jovian magnetosphere. Because the convection pattern is established by magnetic-anomaly effects of the active sector, the pattern co-rotates with Jupiter. (This is in contrast to the Earth's two-cell convection pattern that is fixed relative to the Sun with the Earth rotating beneath it.) The sense of the convection is to bring hot magnetospheric plasma into the upper atmosphere in the longitude region of the hydrogen bulge. This hot plasma contains electrons with energies of the order of 100keV that dissociate atmospheric molecules to produce the atomic hydrogen that creates the observed longitudinal asymmetry in hydrogen Lyman alpha emission. We regard the existence of the hydrogen bulge as the best evidence available thus far for the reality of the expected co-rotating magnetospheric convection pattern.     

Implications of a steady-state magnetospheric convection

Fundamental to the development of many magnetospheric theories is the assumption of a steady-state magnetospheric convection. It is shown for the first time that this basic assumption is implicitly related to the source of magnetospheric plasma. The existence of a steady-state electric field is consistent with plasma entry into the magnetosphere from the North—South direction. If plasma entry is through the flanks of the magnetosphere, the large-scale magnetospheric electric field is intrinsically time-dependent. Thus, magnetospheric theories with the underlying assumption that the large-scale electric field is derivable from a scalar potential cannot deal with such a situation. Therefore, the origin of magnetospheric plasma and the time-variability of magnetospheric convection are directly related.     

On the convective mechanism for formation of the plasma sheet in the magnetospheric tail

Calculation of stationary distributions of the most important plasma parameters (particle energy, density, field-aligned and transversal pressure) is performed for a model magnetotail plasma sheet which is formed by convecting plasma mantle particles injected into the closed geomagnetic field line tubes. Computations have been done for two convection models: (i) a model of completely adiabatic particle motion with conservation of the first two invariants and (ii) a model with a strong pitch-angle diffusion which maintains isotropy. It is found that in both cases the heating and compression of the plasma are somewhat more effective than is necessary to account for the observed gradients of magnetic field in the magnetospheric tail. A leakage of accelerated particles through the dawn and dusk edges of the plasma sheet is proposed as a possible mechanism for maintenance of stationary convection in the magnetotail. The question of the dependence of the stationary magnetotail parameters on the solar wind state is discussed briefly.     

Measuring the plasma environment at Mercury: The fast imaging plasma spectrometer

Abstract— The plasma environment at Mercury is a rich laboratory for studying the interaction of the solar wind with a planet. Three primary populations of ions exist at Mercury: solar wind, magnetospheric, and pickup ions. These pickup ions are generated through the ionization of Mercury's exosphere or are sputtered particles from the Mercury surface. A comprehensive mission to Mercury, such as MESSENGER (MErcury: Surface, Space ENvironment, GEochemistry, Ranging), should include a sensor that is able to determine the dynamical properties and composition of all these plasma components. An instrument to measure the composition of these ion populations and their three‐dimensional velocity distribution functions must be lightweight, fast, and have a very large field of view. The fast imaging plasma spectrometer (FIPS) is an imaging mass spectrometer, part of NASA's MESSENGER mission, the first Mercury orbiter. This versatile instrument has a very small footprint, and has a mass that is ?1 order of magnitude less than other comparable systems. It maintains a nearly full‐hemisphere field of view, suitable for either spinning or three‐axis‐stabilized platforms. The major piece of innovation to enable this sensor is a new deflection system geometry that enables a large instantaneous (?1.5π) field of view. This novel electrostatic analyzer system is then combined with a position sensitive time‐of‐flight system. We discuss the design and prototype tests of the FIPS deflection system and show how this system is expected to address one key problem in Mercury science, that of the nature of the radar‐bright regions at the Hermean poles.     

Solar radar astronomy with the low-frequency array

Initial studies of the Sun's corona using a solar radar were done in the 1960s and provided measurements of the Sun's radar cross-section at about 38 MHz. These initial measurements were done at a time when the large-scale phenomenon known as a coronal mass ejection was unknown; however, these data suggest that coronal mass ejections (CMEs) may have been detected but were unrecognized. That solar radar facility, which was located at El Campo, TX, no longer exists. New solar radar investigations are motivated by our modern understanding of CMEs and their effects on the Earth. A radar echo from an Earthward-directed coronal mass ejection may be expected to have a frequency shift proportional to velocity; thus providing a good estimate of arrival time at Earth and the possible occurrence of geomagnetic storms. Solar radar measurements may also provide new information on electron densities in the corona. The frequencies of interest for solar radars fall in the range of about 10–100 MHz, corresponding to the lower range planned for the low-frequency array. In combination with existing or new high-power transmitters, it is possible to use the low-frequency array to re-initiate radar studies of the Sun's corona. In this report, we review the basic requirements of solar radars, as developed in past studies and as proposed for future investigations.     

Dynamics of electrons and heavy ions in Mercury's magnetosphere

Several basic magnetospheric processes at Mercury have been investigated with simple models. These include the adiabatic acceleration and convection of equatorially mirroring charged particles, the current sheet acceleration effect, and the acceleration of Na⁺ and other exospheric ions by the magnetospheric electric and magnetic fields near the planetary surface. The current steady-state treatment of the magnetospheric drift and convection processes suggests that the region of the inner magnetosphere as explored by the Mariner 10 spacecraft during its encounter with Mercury should be largely devoid of energetic (>100 keV) electrons in equatorial mirroring motion. As for ion motion, the large gyroradii of the heavy ions permit surface reimpact as well as loss via intercepting the magnetopause. Because of the kinetic energy gained in the gyromotion, the first effect could lead to sputtering processes and hence generation of secondary ions and neutrals. The second effect could account for the loss of about 50% of Mercury's exospheric ions.     

Meteoroid orbital characteristics by radar surveilence

H.F. radars are a powerful diagnostic for examining the orbits and spatial characteristics of the dust-meteoroid complex. Data on those properties are essential in realistic studies of the dynamics of small bodies in the solar system. An outline is here presented of the various programmes and their techniques employed in radar operations over the last 40 years. The particular value and contribution of radar facilities and comparisons with other methods is emphasised. Some of the features of the only current routine survey are discussed and, to illustrate to power of the system, examples of the types of data presentation are given.     

Quiet auroral arcs: Ionosphere effect of magnetospheric convection stratification

A detailed study of the mechanism of electromagnetic stratification of the large-scale stationary magnetospheric convection due to a friction of the convective flow in the ionosphere layer was performed. Magnetosphere-ionosphere interaction was taken into account by means of the effective boundary conditions on the ionosphere top and bottom boundaries including the actual height profile of charge particles velocity in the ionosphere. It has been shown that the magnetospheric convection is stratified into small-scale current sheets which are respective in the linear approximation to an oblique Alfvén wave. The dispersion equation was deduced for the Alfvén mode and its solution obtained determining the space-time scales and the increment of instability. The maximum increment is realized for the disturbances stretched along the convection velocity that is correspondent to the actual orientation of the auroral arcs. In the conditions of rapid growth of Alfvén velocity above the maximum of the ionosphere F layer, it was shown that small-scale disturbances with the transverse scales l_⊥ ? 1 km are localized at the altitudes up to several thousand kilometers whereas the large-scale stratification penetrate into the equatorial plane of the magnetosphere. A mechanism is proposed to intensify the parallel electric field acting at that stratification stage when the field-aligned currents in the Alfvén wave are sufficient to form abnormal resistance along geomagnetic lines of force.     

Accurate approximate formulae for toroidal standing hydromagnetic oscillations in a dipolar geomagnetic field

Calculations of the toroidal eigenmodes of oscillation of the magnetospheric plasma have been important in explaining the nature of Pc3, Pc4 and Pc5 geomagnetic pulsations. In this paper perturbation solutions of the governing equations are presented. These are much more accurate than the WKB approximation which has often been used, and much simpler to compute than the numerical solutions which have been used. A method of including the finite ionospheric conductivity is also presented.     

VLBI相位校正信号提取的软件实现方法

从VLBI相关处理结果中提取的延迟值包括了天线、终端等设备的时延,必须对其加以修正,最终结果才能达到精度要求。提取相位校正信号,可以消除这些设备引入的时延,从而校正同一波前信号到达基线两端的几何时延。该文介绍了提取相位校正信号的原理、算法及软件实现方法。软件采用多线程和SSE技术,具备4台站多通道全部相位校正信号提取能力。     

Motion of energetic particles in a magnetospheric model including a convection electric field

A magnetospheric model including convection and corotation electric fields is developed. Drift shells and pitch-angle evolution are obtained through perturbation methods. The asymmetry of the shell is maximum in a plane different from the noon-midnight meridian. As a consequence the dip in fluxes is not maximum in this meridian and the amount of shift is shown to be directly proportional to the convection electric field intensity. It is inversely proportional to the energy or the pitch-angle of the particles.     

The role of the Hermanus Magnetic Observatory in geomagnetic field research

Geomagnetic field research carried out at the Hermanus Magnetic Observatory over the past decade is reviewed. An important aspect of this research has been the study of geomagnetic field variations, with particular emphasis on ULF geomagnetic pulsations. Features of geomagnetic pulsations which are unique to low latitude locations have been investigated, such as the cavity mode nature of low latitude Pi 2 pulsations and the role played by ionosphericO ⁺ ions in the field line resonances responsible for Pc 3 pulsations. A theoretical model has been developed which is able to account for the observed relationships between geomagnetic pulsations and oscillations in the frequency of HF radio waves traversing ionospheric paths. Other facets of the research have been geomagnetic field modelling, aimed at improving the accuracy and resolution of regional geomagnetic field models, and the development of improved geomagnetic activity indices.