摘要:

Sudden onsets of high‐voltage differential charging on spacecraft in an ambient environment may affect operations and the survival of on‐board electronics. Triple‐root jumps may be very sudden. We discuss two aspects, one theoretical and one observational. By using current balance, the theoretical parametric domain in which triple‐root jumps may occur for a surface material in a double Maxwellian electron environment has been calculated. The relation between the domain and the “critical” or “threshold” temperature for a material is revealed. We present an example of the prediction of the occurrence of a triple‐root spacecraft potential jump in a time‐varyin

ISSN:0148-0227    

DOI:10.1029/91JA01653

年代:1991

数据来源: WILEY

摘要:

First, we discuss the dynamics of a weakly charged dust grain orbiting in the equatorial plane of a planet surrounded by a rigidly corotating magnetosphere, where the magnetic field can be represented by a centered aligned dipole field. If the grain has a constant charge, we can introduce an effective one‐dimensional potential, from which we find that the perturbation due to electrostatic forces induces a motion of the pericenter, similar to the effect of the planetary oblateness. However, the electric charge on an orbiting dust grain is generally not constant. Accordingly, we explore the case where the charge varies periodically due to the modulation of the photoelectron current that occurs as the grain enters and leaves the planetary shadow. Thus the electromagnetic perturbation resonates with the orbital period and can modify the orbit's size and eccentricity. We demonstrate this effect both numerically and analytically for small grains comprising the Jovian ring. We show that the resulting changes are periodic, and their amplitude is much bigger than the amplitude of the periodic changes due to light‐pressure perturbation or the secular changes due to resonant charge variations that develop over a comparable time s

ISSN:0148-0227    

DOI:10.1029/91JA01982

年代:1991

数据来源: WILEY

摘要:

Observations of strongly enhanced ion acoustic shoulders of the incoherent scatter spectrum at 933 MHz at altitudes from 138 to 587 km have been obtained with the European Incoherent Scatter UHF radar. The enhancements can be up to 1 or 2 orders of magnitude in total backscattered power and can occur at either one or both of the ion acoustic shoulders. They show a variation of frequency with height of about 2 to 1, the same as the normal ion line spectral width and the ion temperature. These unusual spectra appear in two preferred height regions having different characteristics, one below 200 km and one above about 300 km. The enhancements are associated with geomagnetic disturbance, high electron temperatures, auroral arcs, and red aurora in theFregion. The observations, which are mainly along the magnetic field direction, indicate that field‐aligned thermal electron drifts are destabilizing the ion acoustic waves. They confirm and extend the one other publication reporting on similar echoes. We suggest that field‐aligned flows of soft electrons depositing their energy at horizontally poor conductingFregion heights are the cause of parallel electric fields in the ionosphere. These fields then produce the thermal electron motions that we argue have to be the cause of the observati

ISSN:0148-0227    

DOI:10.1029/91JA01188

年代:1991

数据来源: WILEY

摘要:

The measurements of electron spectra in the Martian magnetosphere by the HARP instrument on board the Phobos 2 orbiter are presented. The energy of the electrons (a few tens of electron volts) is sufficient for the impact ionization of the planetary neutral gas, and the characteristic flux of electrons (∼108cm−2s−1) could produce the nightside ionospheric layer with a peak density of a few thousands of electrons per cubic centimeter, which corresponds to densities observed earlier during radio occultations of the Mars 4 and 5 and Viking 1 and 2 spacecraft. The possibility of magnetospheric electron precipitation into the nightside atmosphere of Mars is in agreement with the mainly induced nature of the magnetic field in the planetary magnetotail (as at Venus), while the variability of the Martian nightside ionosphere may be explained by the partial screening of the atmosphere by a weak intrinsic magnetic field of the p

ISSN:0148-0227    

DOI:10.1029/91JA00924

年代:1991

数据来源: WILEY

摘要:

The measurements of proton spectra obtained by the TAUS spectrometer on board the Phobos 2 spacecraft in elliptical orbits near Mars are presented. A strong deceleration of the solar wind upstream of the Martian bow shock was revealed. It can be caused by the mass loading of the plasma flow by ions originating from the hot oxygen/hydrogen corona of Mars and/or by protons specularly reflected from the bow shock. In the first case the deceleration of the solar wind by about 100 km/s implies that the hot oxygen corona of Mars could be several times denser than it was anticipated to be (at least during the observation period that was close to solar cycle maximum). Furthermore, the loss of planetary oxygen through the corona appears to be the main process of oxygen loss from Mars. The upper limit of loss rate for such a process is determined to be 1026oxygen atoms or 2.5 kg of oxygen per second.

ISSN:0148-0227    

DOI:10.1029/90JA02561

年代:1991

数据来源: WILEY

摘要:

The isotopic composition (long‐term average) of solar flare (SF) Ne has been determined by three isotope correlation techniques applied to data measured on chemically etched pyroxene separates prepared from the Kapoeta meteorite, which is known to contain implanted solar gases. The SF20Ne/22Ne ratio obtained is 11.6±0.2 and confirms previous determinations of this SF ratio in lunar and meteoritic samples. The same SF Ne composition is also obtained by applying an ordinate intercept technique to the same data set. The ordinate intercept technique was also applied to the Ar and He data, on which the three‐isotope correlation technique cannot be applied. The isotopic composition of SF Ar and SF He so obtained are SF36Ar/38Ar=4.9±0.1 and SF4He/³He=3800±200, which are significantly different from the solar wind (SW) Ar and SW He values of ∼5.35 and ∼2500, respectively. Correlation between20Ne/22Ne and36Ar/38Ar for the same data set also gives a similar SF36Ar/38Ar ratio of 4.8 ± 0.2. The determined SF He, Ne and Ar isotopic ratios differ from those in SW by 52%, 17% and 9%, respectively, but the elemental compositions of4He/36Ar and20Ne/36Ar do not show obvious differences between SF and SW. The concentration of the SF component in Kapoeta pyroxenes is ∼20% that of the SW component, which is orders of magnitude higher than expected from SW and SF proton flux measurements. Variations in elemental and isotopic composition of He, Ne and Ar in SF relative to SW are found to correlate well with a (Z/A)² dependence, indicating a rigidity‐dependent particle spectru

ISSN:0148-0227    

DOI:10.1029/91JA01948

年代:1991

数据来源: WILEY

摘要:

The diffuse ionospheric resonancesDn, stimulated by topside sounders, have been studied for over 2 decades. These resonances are observed below the plasma frequency ƒNbetween the harmonics of the electron gyrofrequency ƒH. TheDnresonances are often accompanied by upper and lower subsidiary branches. The present paper concentrates on the classification of diffuse resonances, one motivation being the possible application to the interpretation of naturally occurring radio emissions in the magnetosphere. Osherovich has shown that theDnresonances are characterized by a nonequally spaced spectrum of frequencies ƒDn= ƒDsn1/2(n= 1, 2, 3, and 4), where ƒDs= 0.95(ƒNƒH)1/2, and that the upper subsidiary resonancesDn+can be considered as upper hybrid resonances ofDnwith frequencies given by ƒDn+= (ƒDn²+ ƒH²)1/2. This classification is here extended to include the lower subsidiary resonancesDn−(n= 1, 2, 3, and 4), and it is shown that their frequencies are related to ƒDnand ƒHby the expression ƒDn−= (ƒDn² − ƒH²)1/2. This result is based on a combination of previously published data and newly scaled ionograms from the Alouette 2 and ISIS 1 top

ISSN:0148-0227    

DOI:10.1029/91JA01961

年代:1991

数据来源: WILEY

摘要:

Electron plasma waves, commonly called Langmuir waves, are often observed upstream from planetary bow shocks and are widely accepted as being generated by electron beams. We have studied the self‐consistent nonlinear evolution of electron plasma waves excited by electron beams streaming along the ambient magnetic field in a long simulation systemLx= 2048λe. Only electrostatic waves are treated by the simulation code. The simulation results show the presence of a process moving wave energy from frequencies and wavenumbers predicted by linear theory to Langmuirlike frequencies during saturation of the instability. An explanation in terms of the characteristics of the growing waves changing with beam temperature is suggested. A decay process producing backward propagating (backscattered) Langmuirlike waves is observed. Low‐frequency waves are also produced. However, the simulation results indicate that the backscattering process is not the conventional Langmuir wave decay. Electrostatic waves near multiples of the electron plasma frequency are generated by wave‐wave coupling during the nonlinear stage of the simulations, thereby confirming the suggestions of Klimas (1983). However, the spectral width of the pumping waves and beam density are found to significantly affect the levels of harmonic waves. Care must therefore be taken to prevent unrealistically high levels of harmonic waves by including sufficient numbers of linearly unstable modes in the simulation

ISSN:0148-0227    

DOI:10.1029/91JA01738

年代:1991

数据来源: WILEY

摘要:

Many aspects of the relation between spatial and temporal structures in morningside aurorae can be explained with the aid of a nonlinear model that describes the interactions between energetic electrons and VLF waves. The simplest form of the model consists of three differential equations, with time derivatives generated in reference frames moving with either the energetic electrons, the cold plasma, or the VLF waves that interact with the energetic electrons. Although unstable solutions that could explain the origin of spatial structures are not found, the existence of spatial structures having sufficiently fine scales can provide a repetitive perturbation that maintains the temporal variations. An analysis of the behavior of solutions of the nonlinear system reveals that a strange attractor does not occur in the autonomous system; but when the repetitive perturbations are regarded as external periodic forcing terms, unusual forms of nonlinear behavior are revealed. If the frequencies of the forcing terms are not commensurate with the natural periods of the system, the solutions present some of the characteristics of deterministic chaos. The nearest analogy is a driven nonlinear oscillator with asymmetric damping. However, because of the stiffness of the system, even those solutions that are not truly chaotic (in the commonly accepted usage) will be indistinguishable from chaos over tens to hundreds of pulsation cycles. In certain cases, when damping terms are reduced to zero, the system displays a form of behavior with many of the important characteristics of chaos, but the phase trajectories do not appear to be bounded. Variation of the forcing amplitude produces regions of unbounded phase trajectories, which do not approach stable limit cycles, separated by narrow regions of classical period doubling.

ISSN:0148-0227    

DOI:10.1029/91JA01826

年代:1991

数据来源: WILEY

摘要:

Instability of laminar magnetospheric convection with respect to the strongly anisotropic Alfven waves which are of small scale in the horizontal plane is examined. The waves prove to be trapped in the ionospheric Alfven resonator, bounded from below by the ionosphericElayer and from above by a zone of rapidly increasing Alfven velocity at altitudes of up to ∼104km. The finite‐amplitude Alfven waves dissipate within a layer of anomalous resistance formed near the upper wall of the resonator. As a result, a high‐energy particle source appears in the upper ionosphere. Further evolution results in the transition of laminar convection to turbulent flow conditions and in the formation of a turbulent Alfven boundary layer in the polar ionosphere at altitudes from 10² to 104km. The energy status of the turbulent Alfven boundary layer is calculated. It has been shown that the accelerated‐electron energy flux density can reach ∼100 er

ISSN:0148-0227    

DOI:10.1029/91JA00376

年代:1991

数据来源: WILEY