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.     

Global dynamics of the magnetosphere for northward IMF conditions

Ground-based and spacecraft observations of polar cap geophysical phenomena during periods of northward interplanetary magnetic field (IMF) show specific patterns of electric fields, field-aligned currents, aurora and particle precipitation. These are basically different from those when the IMF is southward. The total combination of observational data for northward IMF indicates rather a closed magnetosphere. This topology has led to the formation of a specific convection pattern in the distant plasma sheet. As different theoretical studies show, the connection of the IMF to geomagnetic flux tubes poleward of the cusp region may serve as the driving mechanism for plasma sheet convection and as the dynamo of current systems. Unfortunately, the direct observations of processes in the distant magnetosphere are too scarce either to accept or reject the concept of a closed magnetosphere. There are also some experimental data that are inconsistent with the closed magnetosphere topology. Definitive open or closed models must await future measurements.     

The investigation of electromagnetic interactions between regions surrounding the Earth—1. Initial equations,the boundary conditions,the distribution of external currents and the methods of numerical solution

A numerical model is presented suitable for the study of the penetration of the global atmospheric electric field from the troposphere into the ionosphere. Equations are obtained in the plane-stratified approximation which permit us to use the same mathematical techniques in the atmosphere, the ionosphere and the magnetospheres. We take into account the anisotropy in the general case, the variation of conductivity of the medium and the temporal changes of the fields. The relationship between the electric and magnetic modes is taken into account by means of Ohm's law in anisotropc media, using the electrical conductivity sensor. The feasibility of the plane-stratified approximation is analysed by means of the analogous equations taking into account the spherical symmetry of the global electric field. Models of the external current distribution are discussed. The boundary conditions take into account the field attenuation into the conducting Earth and the field propagation into the magnetosphere as Alfven and magneto-acoustic waves. The methods of numerical solution of the equations obtained and their accuracy are described in brief.     

Auroral rays: filamentation in the auroral accelerator

In situ measurements of particles, fields and optical emissions from a rocket that encountered auroral rays are reported. The measurements give insight into the production of rays, as well as the production of large fluctuations in electric fields perpendicular to the magnetic field. The fine structure and rapid variations of the electron energy flux associated with the rays are apparently produced by modulation in the degree of electron acceleration. Rays are produced when the energy flux increases in localized regions to values even higher than those normally encountered in bright auroral forms. Close and consistent similarities in the variations of the electron energy flux, the light and the electric fields suggest that the field variations were produced as a direct result of the changes in the stream of accelerated electrons. In examining possible causes of the velocity changes that produce the rays, two acceleration processes are considered; acceleration as a consequence of a potential difference between the magnetosphere and the atmosphere and acceleration by waves.     

Whistler-mode propagation: results of model calculations for an inhomogeneous plasma

Waveforms computed using the new whistler model derived from Maxwell's equations were analysed. In the calculations, realistic model values for magnetospheric parameters were used. The results accurately described whistler-mode propagation in the magnetosphere and provide explanations for some features exhibited by real observed whistlers when they are analysed. In particular, solutions of the exact full-wave whistler model can explain the whistler fine structure, and may be used to develop a more accurate (matched filtering) fine-structure analysis method.     

Dayside high latitude magnetic impulsive events: their characteristics and relationship to sudden impulses

On 17 December 1990 a series magnetic impulsive events (MIEs) were observed at high latitudes near local noon. EISCAT, situated some 5 hours of MLT away from the noon sector, detected simultaneous impulsive electron density enhancements at heights between 90 and 120 km. The MIEs at noon were also associated with riometer absorption spikes. The correlated EISCAT and riometer observations indicate that there was an elongated electron precipitation region some 3000 km wide stretching from local noon to morning. In close association with the impulsive electron precipitation, VLF emissions were observed by groundbased stations in the morning side. We interpret the large scale electron precipitation and VLF emissions as signatures of a global compression of the Earth's magnetosphere. This is confirmed by the specific type of magnetic variations simultaneously recorded at the worldwide network of magnetometers. We conclude that the small scale MIEs with their drifting ionospheric current vortex structures can (but do not necessarily have to) occur in conjunction with large scale SIs. Moreover, MIEs and SIs have a common origin: the interaction of solar wind inhomogeneities with the Earth's magnetosphere. They do, however, represent different effects of the same primary agent.     

Periodic and quasiperiodic VLF emissions

A review of periodic and quasiperiodic VLF emissions observed at ground-based stations and in the Earth's magnetosphere is presented. Emissions with periods below 10 s are divided into three main groups: periodic emissions, hisslers and pulsing hiss, while the quasiperiodic emissions with periods above 10 s are divided into two main groups: QP1, which are definitely related to geomagnetic pulsations, and QP2, which are not obviously related to them, based on ground observations. However, it is pointed out that all types of quasiperiodic emissions with periods over 10 s observed onboard satellites show some association with geomagnetic pulsations which suggests that both QP1 and QP2 can have similar mechanisms for their generation. Different approaches to the theoretical modelling of periodic and quasiperiodic emissions are discussed.     

The influence of ionization and recombination processes on the dynamics of ionospheric plasma clouds

The dynamics of a one-dimensional ionospheric irregularity interacting with the magnetosphere is studied by numerical simulation. The polarization electric field produced by charge separation within the irregularity propagates along magnetic field lines with the Alfvén velocity V_A and drives polarizational and field-aligned currents in the magnetosphere. Their values and localization are controlled by motion and deformation of the irregularity resulting from its electrostatic coupling to the background ionosphere. The pattern of the field-aligned currents varies with time and depends primarily on gradients of the polarization electric field. The latter is controlled by the ambient electric field, diffusion, recombination process, intensity of the initial perturbation, etc. Feedback effect of the magnetospheric conductance on the development of the irregularity is examined.     

On the effect of a beam of energetic electrons on the whistler-mode growth rate due to gyroresonant interactions

The dispersion relation for whistler-mode waves in the presence of an idealised beam of energetic electrons that is moving either parallel or antiparallel to the gyroresonant electrons is analysed. The dependence of the real part of the dispersion relation on the velocity of the beam is shown to be responsible for an additional growth rate, for the beam antiparallel to the gyroresonant electrons, or for a reduced growth rate for the parallel condition. The effect could be important in explaining some features of VLF emissions such as hiss which propagate through the magnetosphere in the opposite direction to the gyroresonant electrons.     

On the development of folds in auroral arcs

The development of an auroral arc in the midnight sector, from diffuse to discrete with subsequent large scale folding, is studied with the aid of several ground-based observations, including incoherent scatter radar, and data from a HILAT satellite pass. Ion drift velocities in the F-region, as measured by EISCAT, were consistently eastward throughout and after the whole period of development, whilst the ion temperature showed two large enhancements just prior to the appearance of the main auroral fold. The fold moved eastwards and crossed the EISCAT antenna beam, appearing as a short-lived spike in electron density at altitudes between about 100 km and 400 km. The spike in electron density came progressively later at higher altitudes. The observations are interpreted as the result of enhanced convection in the ionosphere and in the magnetosphere. The auroral arc folding is suggested to be caused by the Kelvin-Helmholtz instability in a velocity shear zone in the magnetosphere.     

About the origin of high energy electrons in the inner radiation belt

A large flux of > 100 MeV electrons were registered in the inner radiation belt on low-altitude satellites. The origin of that flux is discussed. It appears that slow radial diffusion (D_o = 10⁻¹³ 1/s) gives a low probability for penetration of these electrons to small L from the boundary of magnetosphere because of synchrotron radiation energy losses. It is found that they can enter to the inner belt region without such losses after great magnetic storms when high speed radial diffusion sometimes takes place. Two great storms on 8–9 Feb.] 986 and 24 March 1991 are examples when one can directly observe a penetration of energetic electron fluxes into magnetosphere. The assumption about their Jovian origin is discussed.     

Voltage and current sources of high latitude thermospheric movements

In studies of the high latitude thermospheric movements caused by electromagnetic fields, the driving source in the magnetosphere is usually treated as a voltage source. There may be situations where the driving source can be treated as a current source. The transient behaviours of the thermospheric movement due to the two sources are quite different. We illustrate this.     

Energetic particles in the environment of the Earth's magnetosphere

This review summarizes the work in the field of magnetospheric energetic particles during the years 1987–1989. Out of a wealth of contributions it concentrates on a few topics. First it follows the path of ions extracted out of the polar ionosphere and their acceleration parallel and perpendicular to the magnetic field, as well as their subsequent transport into the equatorial magnetosphere and tail region. Then it focuses on acceleration of ions in the magnetotail and the related characteristics in the boundary layers including consequences for current substorm modeling. In the ring current region, the AMPTE and VIKING missions have made possible detailed studies of charge state and pitch-angle distributions as well as their variations during magnetospheric storms and substorms, from which conclusions on the transport and loss processes can be drawn. Recently, observations of energetic particles from orbiting nuclear reactors in the magnetosphere, which can be used as tracers for these satellites, have been made public. However, this may also constitute a serious background problem for future γ-astronomy missions with an increased sensitivity of the instrumentation. Finally, leading beyond the boundaries of the magnetosphere, attention is drawn to the still ongoing debate on the source of energetic particles upstream of the Earth's bow-shock and the respective importance of particle leakage and/or acceleration at the magnetospheric boundaries.     

Convecting towards the catastrophe of substorm onset

A nonlinear model of substorm evolution is constructed which self-consistently unifies the directly driven and the loading-unloading systems. Using only the principle that the magnetosphere minimizes its potential energy within the constraints imposed by external conditions, substorms are described with reference to trajectories on a cusp catastrophe surface. The model predicts the conditions required for substorm onset and the amount of energy explosively released at the beginning of the expansive phase.     

A review of observational,theoretical and numerical studies of VLF triggered emissions

We review theoretical and numerical studies of VLF triggered emissions generated by manmade signals propagating in the whistler mode in the magnetosphere. Instead of listing all the past studies on the subject we select some of those works which give important ideas and the theoretical basis for future studies. The main purpose of the paper is to clarify what problems remain unresolved and to make suggestions for future studies.We first describe the phenomenon of VLF triggered emissions and give a brief review of experiments and observations. We then summarize the basic physics of VLF wave-particle interactions, which is indispensable for an understanding of the triggering mechanisms. We review important ideas and theories, as well as numerical models so far employed, summarize the present understanding of the phenomenon and consider briefly those problems which need further study. Especially, we focus our attention on numerical simulations whose capabilities in nonlinear physics are increasing with the increasing power of supercomputers.     

On modeling component processes in the Earth's global electric circuit

There are many component processes interacting simultaneously in the Earth's global electric circuit. The three main generators in the circuit are thunderstorms, the ionosphere wind dynamo, and the solar wind/magnetosphere dynamo. This paper reviews the progress made in recent years in modeling the various generators and their mutual interactions. The progress made in modeling various component processes that affect the distribution of electric fields and currents is also reviewed.     

An alternative interpretation of auroral precipitation and luminosity observations from the DE,DMSP, AUREOL,and Viking satellites in terms of their mapping to the nightside magnetosphere

We present an interpretation, which differs from that commonly accepted, of several published case studies of the patterns of auroral electron precipitation into the high-latitude upper atmosphere in the near-midnight sector based on their mapping to the nightside magnetosphere. In our scheme bright discrete auroral structures of the oval and respective precipitation are considered to be on the field lines of the Central, or Main, Plasma Sheet at distances from 5–10 to 30–50 R_E, depending on activity. This auroral electron precipitation pattern was discussed in detail by Feldstein and Galperin [(1985) Rev. Geophys.23, 217] and Galperin and Feldstein [(1991) Auroral Physics, p. 207. Cambridge University Press. It is applied and shown to be consistent with the results of case studies based on selected transpolar passes of the DE, DMSP, AUREOL-3 and Viking satellites.A diagram summarising the polar precipitation regions and their mapping from the magnetospheric plasma domains is presented. It can be considered as a modification of the Lyons and Nishida (1988) scheme which characterizes the relationship between the gross magnetospheric structure and regions of nightside auroral precipitation. The modification takes into account non-adiabatic ion motions in the tail neutral sheet, so that the ion beams characteristic of the Boundary Plasma Sheet (BPS) originate on closed field lines of the distant Central Plasma Sheet (say, at distances more than ~30 R_E).     

Trimpi events on low latitude paths: An investigation of gyroresonance interactions at low L-values

We report on Trimpi events observed at Durban (L = 1.69, 29°53′S, 31°00′E) and investigate the efficacy of gyroresonance scattering in precipitating electrons into the atmosphere at low L (<2). The rate of occurrence of Trimpis at Durban is less than one per day. Our observations include a number of daytime events on OMEGA signals from La Reunion. Using the full relativistic equations of motion, a test particle simulation is employed to find the region in parameter space where large pitch angle scattering occurs. We find that at low L the conditions for pitch angle scattering are less favourable than at higher L (L ∼ 4). Resonant electrons have high (relativistic) energies, interaction times are of the order of milliseconds (T_i ∼ 5 ms) and large wave amplitudes (B_w ∼ 200 pT) are required at whistler frequencies to produce pitch angle changes of greater than 1°. Large pitch angle scattering is needed near Durban since particles near the loss cone will have been lost in the South Atlantic Geomagnetic Anomaly. We note that the radio frequencies transmitted into the magnetosphere from lightning are too low to give effective electron scattering at low L. We suggest an explanation for the low rate of occurrence of Trimpis at Durban.     

Theoretical models of polar-cap convection under the influence of a northward interplanetary magnetic field

The unexpected patterns of high-latitude auroral luminosity and ionospheric convection that are observed when the interplanetary magnetic field (IMF) has a northward orientation have inspired a variety of theoretical interpretations. The existing models, all referring to steady-state conditions, can be classified according to the topology of the polar magnetic field lines and of the polar-cap convection streamlines. The classes of model include: (1) a closed magnetosphere model, (2) a conventional open model with a distorted, but topologically unchanged, polar-cap boundary, (3) a conventional open model with distorted, but topologically unchanged, polar-cap convection cells, (4) a modified open model with ‘lobe convection cells’ contained wholly on open magnetic-field lines, and (5) a modified open model with a bifurcated polar cap. The third and fourth types require significant regions of sunward flow on open polar-cap field lines, a concept that presents serious theoretical difficulties. The other three types appear equally viable from a theoretical point of view, and the comparison against observations is an ongoing enterprise. Outstanding theoretical questions include (a) how do observed structures in the polar ionosphere map along magnetic field lines into the magnetosphere?, (b) what is the mechanism that drives the observed sunward convection at highest latitudes on the day side?, and (c) what role does time dependence play in the observed phenomena?     

Monthly simulations of the lunar semi-diurnal tide

Although having a much smaller amplitude than solar tides, lunar tides are also present in the atmosphere. Lunar tides are attractive for theoretical and observational studies because their frequencies and forcing are better determined than for any other atmospheric waves. Lunar tides are generated by the lunar tidal potential which itself is very well determined. However, this potential also affects the Earth and oceans, modifying their mass distributions and elevations, and creating secondary tidal potentials as well as periodic movements of their interfaces with the atmosphere. The periodic load of ocean over the Earth crust due to the tides also induces a secondary modification of tidal potential as well as movement of the atmosphere interface. In our present work we seek to provide a comprehensive model of atmospheric M2 lunar tidal oscillations from the surface to the lower thermosphere (c. 105 km), taking account of the above-mentioned effects i.e. Earth, ocean and load tides. This study is motivated by two facts. First, we now have good determinations of ocean tides using satellite altimetry whereas previous studies of lunar tides were based only on numerical simulations of these ocean tides. Second, some reliable analyses of lunar tides in the lower thermosphere are now available as radars are now operating over a sufficiently long period. Their number should increase in the near future leading to a need for a theoretical study of lunar tides. In this paper, we develop a numerical model and analyse the effects of the different primary and secondary forcings on the lunar tide. Monthly simulations are discussed, and some comparisons with available data (at ground level as well as in the lower thermosphere) are presented.