Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science最新文献

Satellite observations of plasma waves are often analyzed within linear theory of an infinite, homogeneous and magnetized plasma. Important aspects of wave propagation, such as dispersion and polarization, can then be analyzed. However, even for this simplified plasma model, the structure of the eigenmodes is sometimes complex. For instance, several wave modes with different polarization may be allowed at the same frequency. One way of distinguishing between wave modes in real data is by using the polarization information contained in observed frequency spectra to reconstruct the energy density in wave vector space, also called the wave distribution function (WDF). The purpose of this presentation is to show how WDF analysis can be used for identifying wave modes in a case with two overlapping wave modes, and to compare the results obtained with two independently developed methods for reconstructing the WDF. For this purpose Freja observations of waves around the local proton gyrofrequency are analyzed.

Instantaneous mapping methods for hourly observations were developed within the COST 251 European Action. The hourly maps of foF2 for eclipse day are available on the web page http://www.cbk.waw.pl/rwc/idce.html of the Ionospheric Despatch Centre in Europe. However, the eclipse effect on the ionosphere requires more frequent time scale for the observations than 1 hour. This paper presents the results of maps of foF2, foF1, foE with the use of 5-minute measurements during the eclipse of 11 August 1999. The instantaneous maps are shown be a useful tool to study the effect of the solar eclipse upon the ionosphere.

High spatial and temporal resolution observations of auroral forms made by the University of Calgary Portable Auroral Imager are reviewed. Observations include auroral forms with apparent widths of on the order of tens to hundreds of meters and lifetimes of less than a second to several tens of seconds. Included here is a class of multiple auroral arc systems exhibiting a strikingly asymmetric internal topology. It is shown that such systems of multiple linear arcs may be explained by mode conversion of field-line resonance shear Alfvén waves to inertial Alfvén waves. A survey of small-scale, short-lived auroral vortices (curls) occurring on thin auroral forms within the active aurora has also been performed, with some results being reviewed here. The Kelvin-Helmholtz instability as a responsible mechanism is discussed in light of the observationally acquired statistical data. Observations of small-scale “black” auroral arcs and vortices are briefly reviewed as well.

We study how Alfvén waves propagate in the presence of sharp density gradients in the direction perpendicular to the ambient magnetic field. A fully electromagnetic electron guiding centre code is used for the simulation. During the propagation, initially parallel (k_∥ = 0), transverse scales of the order of c/ω_pe are quickly reached which contributes to the creation of a significant parallel component of the electric field in the region of density inhomogeneity. The effects of this field on the velocity distribution functions are then discussed. In particular, we show that they can present a strong deviation from their initial Gaussian shape (global shift in energy) due to the action of the parallel electric field. Evidences are then given for a net energy gain of the electrons, to the expense of the wave, during this process. This energy transfer mechanism may be relevant in order to explain the particle acceleration in the auroral plasma cavities.

Shear Alfvén wave dispersion produced by electron inertia, ion gyro-kinetic and electron thermal pressure is modeled using two-fluid MHD and kinetic theory. In a dipolar magnetosphere, dispersion and non-linearity determine the spatial structure, temporal evolution and amplitude of parallel electric fields and large amplitude density fluctuations near to the polar ionosphere. Deep auroral density cavities are found to have a strong influence on auroral electric field generation. Many features of satellite and ground based observations of discrete arcs are predicted using two-fluid MHD, but large parallel electric fields (mV/m) and keV electron precipitation cannot easily be explained. To explain the observed electric fields it is necessary to evaluate the non-local kinetic electron response to standing shear Alfvén waves on dipolar magnetic field lines. It is shown that electron trapping leads to a significant reduction of the collisionless electron conductivity and a large enhancement of parallel electric fields in the 1 – 4 mHz frequency range of observed field line resonances.

In order to examine the self-consistent formation of large-scale quasi-static parallel electric fields in the auroral zone on a micro/meso scale, a particle in cell simulation has been developed. The code resolves electron Debye length scales so that electron micro-processes are included and a variable grid scheme is used such that the overall length scale of the simulation is of the order of an Earth radii along the magnetic field. The simulation is electrostatic and includes the magnetic mirror force, as well as two types of plasmas, a cold dense ionospheric plasma and a warm tenuous magnetospheric plasma. In order to study the formation of parallel electric fields in the auroral zone, different magnetospheric ion and electron inflow boundary conditions are used to drive the system. It has been found that for conditions in the primary (upward) current region an upward directed quasi-static electric field can form across the system due to magnetic mirroring of the magnetospheric ions and electrons at different altitudes. For conditions in the return (downward) current region it is shown that a quasi-static parallel electric field in the opposite sense of that in the primary current region is formed, i.e., the parallel electric field is directed earthward. The conditions for how these different electric fields can be formed are discussed using satellite observations and numerical simulations.

Studying the variations of the within-the-hour relative deviations of daily 5-minutes measurements of the critical frequencies f_oF2, f_oE and the propagation factor M(3000)F2, it is shown that the deviations of these parameters follow a different statistical distribution in different percentages of the time in each month. Latitudinal dependences of the within-the-hour variability are investigated. Finally comparisons between distributions lead to preliminary quantitative specifications of the within-the-hour variability for each of the examined ionospheric parameters.

Magnetic-field-aligned potential drops in the auroral zone have recently been interpreted in terms of “quasi-steady” states which are evaluated for a fixed, given ion density profile. Here we present experimental studies of such states, observed when a voltage drop is suddenly applied to an inhomogeneous plasma column with a homogeneous, axial magnetic field. The quasi-steady potential drops have a spatial extension related to the gradient length of the initial ion density and the potential profile steepens slowly to a U-shaped double layer on the ion time scale. They exist only when the applied voltage drop does not exceed a critical value which depends on the maximum difference in ion density. Higher voltage drops concentrate in a cathode sheath. Results from PIC-simulations and from a theoretical model, based on steady electron motion, are also presented. They agree excellently for times up to about an ion plasma period (ω_pi⁻¹). The theoretical current-voltage characteristic, which is determined by electron reflection at a virtual cathode formed in the plasma, agrees with the experiments up to about one ω_pi⁻¹. However, already at about 4ω_pi⁻¹ the measured currents have dropped much below the theoretical values, and also the measured average potential profiles are inconsistent with steady acceleration of the electrons. The increased resistivity is associated with strong fluctuations.