[en] The measurements recently performed by the Viking spacecraft have shown that, in addition to being cold plasma depleted, the source regions of the Auroral Kilometric Radiation (A.K.R.) are characterized by a relatively denser, more energetic electron component. In order to properly study the Cyclotron Maser Instability (C.M.I.) which is thought to be responsible for the A.K.R. generation, it is thus necessary to include relativistic corrections in both the hermitian and the antihermitian parts of the dielectric tensor characterizing the linear properties of the plasma. Here one presents an analytical study of the corresponding dispersion equation which aims to describe stable and unstable waves having frequencies lying very close to the electronic gyrofrequency and propagating across the geomagnetic field with a perpendicular refractive index less than a few units (n_{perpendicular} < 5). The electronic population of the auroral plasma is supposed to have two components. The distribution function of the non-thermal, energetic component is modelled by a Dory-Guest-Harris (DGH) distribution function and those of the tenuous thermal one by either a Dirac's function or a maxwellian. The calculations allow the authors to simply perform a parametric study of the various regimes presented by the CMI. For the range of parameters inferred from measurements of the Swedish spacecraft Viking: (P > 1 and χ small), the growth rate could maximize at the cut-off frequency of the relativistic X mode. Moreover, for small χ, the relativistic X mode is connected to freely propagating modes which guarantees an easy access of the electromagnetic energy to free space

[en] A theoretical model is developed here in order to determine an envelope for the average spectrum of the Saturnian kilometric radiation (SKR). The microscopic generation mechanism is supposed to be the so-called synchrotron (or cyclotron) maser instability. As in recent works on the terrestrial kilometric radiation, the effect of the magnetic field inhomogeneity on the generation process must be taken into account. Then, assuming that the emission is nonlinearly saturated by trapping, the calculation allows the authors to put an upper limit on the SKR spectral intensity very simply: the maximum level of the wave electric field within the source region is indeed linked to a few macroscopic plasma parameters, which can be derived from the observations (the structure of the magnetospheric magnetic field, the cold plasma density, and the characteristic energy of the hot emitting electrons). They have used a dipolar magnetic field model, while the plasma distribution results from the superposition of two components, an ionospheric population and a plasma disc, whose scale heights have been roughly determined from Voyager measurements. The energetic electrons responsible for the emission are supposed to precipitate along the high-latitude magnetic field lines where SKR emission is known to take place. The width of the source region can be self-consistently estimated from the model. A very good agreement is obtained between the theoretical spectrum and the observational radio data. The calculated spectral intensities exceed the most intense observed intensities by up to 1 order of magnitude, suggesting that the SKR emission is only marginally saturated by nonlinear processes

[en] It has recently been shown that radio emission from the Earth, the so-called auroral kilometric radiation, is emitted inside cold plasma depleted regions. These observations have motivated a new interest in direct generation processes. Since it allows the generation of an X mode just above the cutoff frequency of this mode (in agreement with observations) the maser synchrotron instability is a promising candidate. A detailed analytical approach to the understanding of this instability is proposed. The complete dispersion relation is derived in a situation where the cold plasma density is small (epsilon/sub c/ = ω²/sub p/e/ω²/sub c/<<1). The physical significance of this instability is discussed; in particular, the difference between the maser synchrotron instability and the classical Weibel instability is stressed. Depending upon the ratio between ''cold'' and energetic electron components, two regimes, the ''kinetic'' and the monoenergetic ''ring-like'' distribution, are considered and the results are compared with the numerical solution of the dispersion relation. Finally the saturation process of this instability is discussed in a homogeneous infinite medium; it is shown that trapping effects can insure an efficient saturation of the maser synchrotron instability

[en] Reference is made to an investigation by Lavergnat and Pellat (1979), who related the transient nature of some of the high-frequency observations in ionospheric electron beam injection experiments to spatial particle bunching at the beam's front. To interpret the continuous part of both the low-frequency and high-frequency waves, an investigation is made of the electrostatic instabilities of a radially limited electron beam flowing through a magnetized plasma and of certain nonlinear consequences of these instabilities. The usual Bers procedure (1972) is used in seeking an eventual absolute character of these instabilities, in particular in the higher frequency range, where the negative parallel group velocity of the plasma waves allows a feedback mechanism. 8 references

[en] For a better understanding of the high-frequency waves observed during active experiments of electron-beam injection into the ionospheric plasma, we do a complete investigation of the electrostatic-beam--plasma instabilities that do occur in such a situation. The electron beam is cold, it is aligned with the vacuum magnetic field, and has a finite radius; the background plasma is cold and homogeneous. We study both analytically and numerically the dispersion equation of the different branches and follow the dependence of the temporal and spatial growth rates with the beam and plasma parameters. Simultaneously we investigate the different types of instabilities (radiated or confined to the beam) in the various frequency ranges which are of interest. We give an analytical criterion for the convective or absolute character of these instabilities, taking into account the two-dimensional wave propagation along and across the vacuum magnetic field

[en] During injection into the ionospheric plasma of an electron beam, very low-frequency electrostatic noise has been detected far away from that beam. Owing to the characteristics of this emission a direct generation mechanism based, for instance, on a linear instability seems irrelevant. A different interpretation is proposed where the observed very low-frequency noise is related to a nonlinear antenna mechanism. Large-amplitude plasma waves which are strongly confined inside the beam, at frequencies close to the plasma frequency, are nonlinearly coupled and the corresponding beating produces a lower-frequency wave which can freely escape from the beam. A detailed comparison with observations is made to support this model

[en] For a better understanding of the high and low frequency waves observed during the Araks experiments we present a complete investigation of the electron-beam plasma instabilities which may occur under the Araks experimental conditions

[en] Using the conceptual framework of the Maser Synchrotron Instability (M.S.I.) process, one gives a description of the different manners the Auroral Kilometric Radiation (A.K.R.) could be generated. A general emphasis is put on the role of the various kinds of inhomogeneities that are encountered into/or near the source regions. In particular, the strong inhomogeneity of the geomagnetic field allows a description of the sources, a determination of the emission beam and an explanation for some observed features of A.K.R. (Spectral bandwidth structures of the dynamical spectrum including the presence of harmonic emission). This inhomogeneous theory also predicts necessary conditions for the generation of the fundamental emission, that could be of observational significance and lead to experimental test of the theory. The role of eventual density gradients is also discussed. Finally, the non linear mechanisms leading to a limitation of the emitted intensity are presented

[en] Through the nonlinear beating of high frequency confined waves (excited near the plasma frequency by beam plasma interaction) low frequency electrostatic forced waves are produced which may explain the characteristic features of the low frequency disturbances observed during the Araks experiments

[en] This paper considers the development process used to select the shield/antenna material satisfying the design requirements of the Solar Probe mission that will encounter a flux of 3000 suns at perihelion. A joint U.S. - French testing program was devised that would screen samples of carbon/carbon to determine the fabrication process that would produce the best thermal-optical and thermal-mechanical properties, lowest mass loss, and acceptable RF properties for temperatures up to 2400 K during shield operation. The U.S. testing program relied on three facilities to measure thermal-optical properties, emissivity, and mass loss. The facility for optical property testing consisted of a high intensity halogen lamp whose beam passed through a window in a vacuum chamber containing the sample. As the beam intensity was increased, the increasing temperature of the sample was measured by contact thermocouples for various angles of incidence.