The suppressing effect of field-aligned currents on VLF emissions in the magnetosphere

The growth rate of whistler-mode waves is calculated analytically for a bi-Maxwellian plasma in the presence of a beam of cool electrons. This beam is moving in the same direction as the gyroresonant electrons and in the opposite direction to the waves which are considered to propagate parallel (or anti-parallel) to the imposed geomagnetic field. A somewhat surprising result is found. This is that even if the anisotropy is greater than a critical value, which is strongly frequency dependent, the beam reduces the growth of the waves near half the electron gyrofrequency. For a field-aligned current density ~ 1 μA m⁻², this mechanism can explain the lack of signals near 1.4 kHz on auroral (return current) flux tubes. It can also explain the observed absorption of signals at half the electron gyrofrequency, around 7 kHz on L = 4 flux tubes, near the equatorial plane and just outside the plasmapause.     

Mineral precipitation associated with vertical fault zones: the interaction of solute advection,diffusion and chemical kinetics

This article is concerned with chemical reactions that occur between two interacting parallel fluid flows using mixing in vertical faults as an example. Mineral precipitation associated with fluid flow in permeable fault zones results in mineralization and chemical reaction (alteration) patterns, which in turn are strongly dependent on interactions between solute advection (controlled by fluid flow rates), solute diffusion/dispersion and chemical kinetics. These interactions can be understood by simultaneously considering two dimensionless numbers, the Damköhler number and the Z‐number. The Damköhler number expresses the interaction between solute advection (flow rate) and chemical kinetics, while the Z‐number expresses the interaction between solute diffusion/dispersion and chemical kinetics. Based on the Damköhler and Z‐numbers, two chemical equilibrium length‐scales are defined, dominated by either solute advection or by solute diffusion/dispersion. For a permeable vertical fault zone and for a given solute diffusion/dispersion coefficient, there exist three possible types of chemical reaction patterns, depending on both the flow rate and the chemical reaction rate. These three types are: (i) those dominated by solute diffusion and dispersion resulting in precipitation at the lower tip of a vertical fault and as a thin sliver within the fault, (ii) those dominated by solute advection resulting in precipitation at or above the upper tip of the fault, and (iii) those in which advection and diffusion/dispersion play similar roles resulting in wide mineralization within the fault. Theoretical analysis indicates that there exists both an optimal flow rate and an optimal chemical reaction rate, such that chemical equilibrium following focusing and mixing of two fluids may be attained within the fault zone (i.e. type 3). However, for rapid and parallel flows, such as those resulting from a lithostatic pressure gradient, it is difficult for a chemical reaction to reach equilibrium within the fault zone, if the two fluids are not well mixed before entering the fault zone. Numerical examples are given to illustrate the three possible types of chemical reaction patterns.     

Analytic properties of the whistler dispersion function

The analytic properties of the dispersion function of a whistler are investigated in the complex frequency plane. It possesses a pole and a branch point at a frequency equal to the minimum value of the electron gyrofrequency along the path of propagation. An integral equation relates the dispersion function to the distribution of magnetospheric electrons along the path and the solution of this equation is obtained. It is found that the electron density in the equatorial plane is very simply related to the dispersion function. A discussion of approximate formulae to represent the dispersion shows how particular terms can be related to attributes of the electron density distribution, and a new approximate formula is proposed.     

The “drift dissipative instability”: a critique

This paper is a critique of Kadomtsev's fluid dispersion equation for electrostatic drift instabilities in a plasma with weak BGK charged-neutral collisions. The numerical solution of this equation is compared with the solution of a more exact kinetic dispersion equation. At maximum growth rate, the fluid results do not correspond to the kinetic results in either the collisionless or the collision-dominated limits. Thus in the BGK model of charged-neutral collisions, the “drift dissipative instability” is not a distinct mode, but only an approximation to the universal density drift instability in a limited range of parameter space.     

What determines the latitudinal extent of the auroral acceleration region?

Characteristic scales associated with auroral precipitation are investigated on the basis of quasistatic magnetotail models, resistive MHD simulations of magnetotail dynamics, and a general relation between parallel electric fields and velocity shear. Since the inverted-V precipitation region of discrete auroras (on the dusk side) is associated with upward flowing, region 1, currents, we investigate the distribution of these currents first. The overall distribution of region 1 type field-aligned currents and their dynamic changes can be explained by characteristic scales in the magnetotail and their mapping to the ionosphere. The quiet time region 1 currents are associated with the decrease of tail flaring. Their overall extent in the north-south direction is closely related to the scale height of the cross-tail current. Dynamic region 1 currents are related to the velocity shear of earthward flow, which can be generated by a tail instability. In that case the peaks of the enhanced region 1 currents are found to map closer to midnight and to lower latitudes than the quiet region 1 currents, consistent with average observations [Iijima and Potemra (1976a), J. geophys. Res.81, 2165]. On the basis of a general relation between parallel electric fields and ‘slippage’ in the plasma transport [Hesse and Schindler (1988), J. geophys. Res.93, 5559; Schindleret al. (1991), Astrophys. J.380, 293], we make estimates of the spatial extent of nonideal regions, where parallel electric fields may exist. For a plausible model of substorm reconfiguration, we find a latitudinal extent of about 7 km for a time scale of 1 min and a integrated parallel electric field of 5 kV. The length scale is proportional to the time scale. The sign of this parallel electric field is consistent with downward acceleration of electrons on the dusk side. The spatial extent of the parallel electric field region is independent of the microscopic generation mechanism if the time scale and the characteristic parallel potential difference (i.e. the integrated parallel electric field) are independent of this mechanism.     

Modifications introduced in the nonlinear whistler-electron interaction by parallel electric fields

Large scale magnetospheric parallel electric fields expected to occur beyond the plasmapause modify the nonlinear behaviour of the cyclotron resonance between electrons and coherent whistler mode waves. Enhancement of the effects originated in the interaction requires waves of varying frequency which might be generated in natural emissions, or artificially injected into the magnetosphere. Adopting a static electric field model based on a differential pitch-angle anisotropy in the geomagnetic mirror, we contrast the evolution of the resonant particles and the adequate ground transmitter frequency format with the corresponding results for the zero electric field case. The outcome, obtained for a field line with L = 6.2, demonstrates the strong influence of the parallel electric field on the nonlinear cyclotron resonance.     

Measurement of vertical phase and group velocities of atmospheric gravity waves by VHF radar

A systematic method of deriving from MST radar data the group velocity and phase velocity of the atmospheric wave along the radar beam direction is proposed and verified by a series of numerical simulations. We apply the method to two data sets measured by Chung-Li radar under different background wind conditions. It is found that the vertical group velocity and phase velocity are mostly in the opposite direction when the background wind is weak. The energy source of downgoing wave packets was evidently related to the instability in the upper height range (10.5–11.7 km) where strong wind shear existed. When the background wind and wind shear are stronger, the vertical group and phase velocities may propagate in the same direction. We also found from numerical simulation and data analysis that the wave packet of gravity waves following power law spectrum are short-lived. A by-product of the group velocity measurement is that the horizontal wavelength may also be deduced from a vertical radar beam measurement from the dispersion relation if it is valid.     

The effects of the resonance broadening of Farley-Buneman waves on electron dynamics and heating in the auroral E-region

Several theories have now been developed which deal with the saturation mechanism of the Farley-Buneman instability and also the heating effect of Farley-Buneman waves on E-region electrons. Various related phenomena, such as anomalous collisions, anomalous cross-field diffusion and non-zero aspect-angle propagation have been treated. In this paper, it is demonstrated how several of these features may be included in a single resonance broadening formalism. Using this approach, the effects of Farley-Buneman waves on the heating and dynamics of background ionospheric electrons is discussed. In addition the effects of anomalous parallel diffusion in the context of E-region Farley-Buneman waves is treated for the first time.     

Beam-generated electrostatic electron waves: relevance to EISCAT VHF radar measurements

The generation by an electron beam of (slowly) growing electrostatic waves, detectable with the EISCAT VHF radar, is investigated. A set of graphs is derived from which it is possible to estimate, by interpolation, frequency, growth rate and resonant beam velocity for most situations of interest. The dependence of the mentioned variables on propagation direction, beam temperature and density and on ionospheric density over the part of parameter space that is of practical interest in the auroral and subauroral ionosphere is presented and discussed.     

TheConvergence of the Italian Regions and Unemployment: Theory and Evidence

We explore the links between the halt of the convergence process of Italian regions at the beginning of the 1970s and the increase in regional unemployment dispersion. We consider a neoclassical exogenous growth model with an imperfect labor market and show that during the transitional dynamics the imperfections of the labor market negatively influence the output growth rate. In particular, the model implies that centralized bargaining is likely to set a national minimum wage that is too high with respect to the labor productivity of the less developed regions, resulting in a negative impact on their per capita output growth. We test the implications of the model on a regional panel data set using the GMM framework. Both our market distortion measure and the unemployment rate are found to significantly lower the growth rate of per capita output.     

The theory of ionospheric focused heating

Ionospheric modification by high power radio waves and by chemical releases are combined in a theoretical study of ionospheric focused heating. The release of materials which promote electron-ion recombination creates a hole in the bottomside ionosphere. The ionospheric hole focuses high power radio waves from a ground-based transmitter to give a 20 dB or greater enhancement in power density. The intense radio beam excites atomic oxygen by collisions with accelerated electrons. Airglow from the excited oxygen provides a visible trace of the focused beam. The large increase in the intensity of the radio beam stimulates new wave-plasma interactions. Numerical simulations show that the threshold for the two-plasmon decay instability is exceeded. The interaction of the pump electromagnetic wave with the backward plasmon produces a scattered electromagnetic wave at 3/2 the pump frequency. The scattered wave provides a unique signature of the two-plasmon decay process for ground-based detection.     

Characteristics and frequency of occurrence of Trimpi events recorded during 1982 at Sanae,Antarctica

‘Trimpi’ amplitude perturbations on VLF signals received at Sanae, Antarctica, have been identified using a new computerised technique. Our survey of 1982 data, taken during magnetically disturbed times, shows that events of short duration (<25 s) constitute 60% of all events detected and that all events found are amplitude attenuations with deviations from quiescent levels ranging up to 90%. It is unusual, at Sanae, to observe the causative whistler with a Trimpi event. This, together with further evidence from Trimpi occurrence statistics, may suggest that the gyroresonant interactions responsible for some of the events occur with non-ducted whistler mode waves. A method for estimating the extent of the precipitation region is presented.     

The Future of the Philosophy of Historiography

This article argues that the perception of decline among philosophers of history reflects the diffused weak academic status of the discipline, as distinct from the booming research activity and demand for philosophy of history that keeps pace with the growth rate of publications in the philosophies of science and law. This growth is justified and rational because the basic problems of the philosophy of history, concerning the nature of historiographical knowledge and the metaphysical assumptions of historiography, have maintained their relevance. Substantive philosophy of history has an assured popularity but is not likely to win intellectual respectability because of its epistemic weaknesses. I suggest focusing on problems that a study of historiography can help to understand and even solve, as distinct from problems that cannot be decided by an examination of historiography, such as the logical structure of explanation (logical positivism)and the relation between language and reality (post‐structuralism). In particular, following Quine's naturalized epistemology, I suggest placing the relation between evidence and historiography at the center of the philosophy of historiography. Inspired by the philosophy of law, I suggest there are three possible relations between input (evidence)and output in historiography: determinism, indeterminism, and underdeterminism. An empirical examination of historiographical agreement, disagreement, and failure to communicate may indicate which relation holds at which parts of historiography. The historiographical community seeks consensus, but some areas are subject to disagreements and absence of communication; these are associated with historiographical schools that interpret conflicting models of history differently to fit their evidence. The reasons for this underdetermination of historiography by evidence needs to be investigated further.     

A simplified approach to internal gravity wave damping by viscous dissipation

Internal gravity wave perturbation velocity subject to constant dynamic viscosity and constant kinematic viscosity are approximately derived based on an energy conservation principle. When the dynamic viscosity is assumed to be a constant, the velocity at any height relative to the velocity at the saturation height, Z_sat, is found to be solely dependent on the number of scale heights measured from Z_sat. Gravity wave energy dissipation due to constant dynamic viscosity primarily occurs from one scale height below to one scale height above the saturation height. When the kinematic viscosity is assumed to be a constant, the perturbation amplitude either increases or decreases monotonically with height depending on whether the energy dissipation rate due, to viscosity, can offset the effect of the decreasing atmospheric density with increasing height. The derivations are made simple by assuming that the non-dissipative dispersion relation is applicable to the dissipative situation. The condition for the assumption to be approximately valid is also given.     

On the similarity of tree‐ring patterns: Assessing the influence of semi‐synchronous growth changes on the Gleichläufigkeitskoeffizient for big tree‐ring data sets

The Gleichläufigkeitskoeffizient (GLK), or the percentage of parallel variation (%PV), is an often used non‐parametric similarity measure in dendrochronological research. However, when analysing big data sets using the GLK, this measure has some issues. The main problem is that it includes not only synchronous but also semi‐synchronous growth changes. These are years in which the growth in one of the compared series does not change in two subsequent years. This influences the GLK, often only slightly, but the larger the data set the stronger the effect. The similarity between tree‐ring series can be more objectively expressed by replacing the GLK with the synchronous (SGC) and semi‐synchronous growth changes (SSGC). The calculation is similar, since GLK = SGC + SSGC/2. Large values of the SSGC are indicative of possible anomalies or even errors. The SGC is much better suited than the GLK to describe similarity. The SGC should therefore be used to analyse big data sets, for clustering and/or dendroprovenance studies. It is recommended to combine the SGC with parametric measures.     

Interactions between whistler-mode waves and energetic electrons in the coupled system formed by the magnetosphere,ionosphere and atmosphere

The physical mechanism of a cyclotron resonance interaction between trapped energetic electrons and whistler-mode waves in the magnetosphere is discussed. Not only do the electrons have their pitch angles reduced in this interaction, so that they may be precipitated into the upper atmosphere, but also the waves can be amplified. Such a flux of precipitating electrons can, either by direct ionisation or via bremsstrahlung radiation, cause a pimple to be produced on the bottom of the ionosphere. That can significantly modify the amplitude and/or phase of very low frequency radio signals propagating in the Earth-ionosphere waveguide. Various experimental observations that demonstrate the reality of such effects are reviewed. The conditions necessary for a positive feedback situation are discussed, and some evidence for its existence assessed.     

An Application of Taylor's Power Law to Measure Overdispersion of the Unemployed in English Labor Markets

Taylor's power law (TPL) is the power relation between mean densities and variance of natural populations, and described as one of ecology's few ubiquitous laws. Although the power model has been increasingly applied in social systems modeling, including economics, this article, using English and Welsh economic data as an applied example, suggests that TPL ought to be imported more carefully. The article seeks to convince readers that ecological population methodologies can have an important role in analysis of human spatial behavior, and that this function should not be diminished in pursuit of quick interdisciplinary results. Through the production of “scale‐adjusted dispersion indicators,” the article proposes an application of TPL that is quite different from its use in ecological modeling.     

Non-neutral field-aligned current sheet and the auroral electric field

Equilibrium configurations of the non-neutral field-aligned electron current sheet in a Maxwellian plasma are obtained from solutions of the time-independent Vlasov-Maxwell equations. This is the first field-aligned non-neutral current sheet model in a hot plasma in which the electrons are allowed to be nonadiabatic. The current in this model has a perpendicular (i.e. diamagnetic) as well as a parallel component to the external magnetic field. The electric field of the current sheet is pointing normally toward the midplane of the sheet. This zeroth-order perpendicular electric field is identified as the primary electric field which gives rise to a few keV potential drop along auroral field lines.     

China's Demographic and Urban Trends for the 21st Century

This paper assesses remarkable and far-reaching changes under way as China enters the 21st century. Among the most significant are the rapid decline in fertility and birth rates, leading to a slowdown in the rate of population growth and a reduction in the growth rate of the labor force. This occurs against the backdrop of continuing structural shift in the economy from a rural-based economy to one increasingly urban-based with growing employment in manufacturing and services. Oshima's conceptual framework for the demographic and industrial transition in Asia is invoked to help explain conditions of change in China. Urbanization proceeds in parallel with these economic changes, and a key feature of transition will be the growth of cities and towns of all sizes. The regional pattern of urbanization will proceed at different rates, with coastal regions advancing most rapidly owing to stronger linkages to the global economy. Journal of Economic Literature, Classification Numbers: J11, O10, O18. 3 figures, 6 tables, 36 references.     

Controlled electron beam experiments in space and supporting laboratory experiments: a review

The injection of energetic charged-particle beams in space represents an exciting area for controlled experiments in the Earth's ionosphere and magnetosphere. Under the influence of a multitude of controlling parameters, a mono-energetic charged-particle beam might follow a well-defined singleparticle trajectory or it might undergo collective beam-plasma effects that substantially modify its energy and density distributions. In the latter case, one finds a multitude of possibilities for unstable plasma modes. If on the other hand the beam maintains single-particle characteristics, it can be a useful diagnostic tool for such applications as geomagnetic conjugate mapping and measurement of magneticfield-aligned potentials. The evolution of these concepts, along with the developing technology of spaceborne charged-particle beam accelerators, has ushered in an area of basic plasma and geoplasma physics previously not available to the scientific community. Over the past five years this activity has brought about important improvements in our fundamental understanding of beams and beam plasma processes. This progress has been brought about not by space experiments alone, but by the synergistic efforts of computational physicists, and the exchange of ideas born from laboratory simulations of space plasma processes involving energetic particle beams. It is the aim of this review paper to develop a synoptic perspective on recent accomplishments, and to summarize plans and recommendations for future experiments which will contribute to continued growth in the field.