共查询到20条相似文献，搜索用时 31 毫秒
1.
This paper considers, in a linear approximation, the instability of a low-density electron beam propagating along the magnetic field of the solar corona, with respect to the excitation of electromagnetic (transverse) waves at cyclotron harmonics. It is shown that the extraordinary mode is excited at harmonics, beginning with the second and higher. Results of calculations check well with observed spectra of groups of solar spike radio bursts in the decimetric wavelength range. 相似文献
2.
A. Moreira 《Planetary and Space Science》1983,31(10):1165-1170
Whistler mode wave emissions in the magnetosheath, known as lion roars, are thought to be generated by an electron cyclotron instability. Using reported satellite data we model a magnetosheath medium where lion roars emissions occurred and we study the character, absolute or convective, of the associated electron cyclotron instability. We use a linear hot plasma dispersion equation for parallel and oblique propagation to the static magnetic field and apply Derfler's frequency cusp criterion to discriminate between absolute and convective instability. Our results show that an absolute instability is compatible with experimental data. From the linear temporal growth rate we extrapolate the saturated wave magnetic field and find a good agreement with the measurements. 相似文献
3.
Jorick S. Vink Janet E. Drew Tim J. Harries René D. Oudmaijer Yvonne Unruh 《Monthly notices of the Royal Astronomical Society》2005,359(3):1049-1064
We present Hα spectropolarimetry observations of a sample of 10 bright T Tauri stars, supplemented with new Herbig Ae/Be star data. A change in the linear polarization across Hα is detected in most of the T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret in terms of a compact source of line photons that is scattered off a rotating accretion disc. We find consistency between the position angle (PA) of the polarization and those of imaged disc PAs from infrared and millimetre imaging and interferometry studies, probing much larger scales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find the polarization PA to be perpendicular to the imaged disc, which is expected for single scattering. On the other hand, the polarization PA aligns with the outer disc PA for the T Tauri stars DR Tau and SU Aur and FU Ori, conforming to the case of multiple scattering. This difference can be explained if the inner discs of Herbig Ae stars are optically thin, whilst those around our T Tauri stars and FU Ori are optically thick. Furthermore, we develop a novel technique that combines known inclination angles and our recent Monte Carlo models to constrain the inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, we consider the connection of the inner disc structure with the orientation of the magnetic field in the foreground interstellar medium: for FU Ori and DR Tau, we infer an alignment of the stellar axis and the larger magnetic field direction. 相似文献
4.
It is shown that the existing theory of type II bursts, based on a model of the emission from the shock wave front, has difficulties when compared with observational data. We suggests a new model for type II bursts. According to this model, in an expanding magnetic loop a cluster of energetic electrons acts to excite the cyclotron instability of plasma waves. The waves are excited on surfaces where the cyclotron resonance condition is satisfied, and are then transformed into electromagnetic emission by merging. Our proposed model may be useful to explain some observational facts, such as the narrow-band character of the emission and the space-time relationship between the harmonics. Some tests to check the validity are proposed. 相似文献
5.
Ion cyclotron waves are generated in the solar wind when it picks up freshly ionized planetary exospheric ions. These waves grow from the free energy of the highly anisotropic distribution of fresh pickup ions, and are observed in the spacecraft frame with left-handed polarization and a wave frequency near the ion’s gyrofrequency. At Mars and Venus and in the Earth’s polar cusp, the solar wind directly interacts with the planetary exospheres. Ion cyclotron waves with many similar properties are observed in these diverse plasma environments. The ion cyclotron waves at Mars indicate its hydrogen exosphere to be extensive and asymmetric in the direction of the interplanetary electric field. The production of fast neutrals plays an important role in forming an extended exosphere in the shape and size observed. At Venus, the region of exospheric proton cyclotron wave production may be restricted to the magnetosheath. The waves observed in the solar wind at Venus appear to be largely produced by the solar-wind-Venus interaction, with some waves at higher frequencies formed near the Sun and carried outward by the solar wind to Venus. These waves have some similarity to the expected properties of exospherically produced proton pickup waves but are characterized by magnetic connection to the bow shock or by a lack of correlation with local solar wind properties respectively. Any confusion of solar derived waves with exospherically derived ion pickup waves is not an issue at Mars because the solar-produced waves are generally at much higher frequencies than the local pickup waves and the solar waves should be mostly absorbed when convected to Mars distance as the proton cyclotron frequency in the plasma frame approaches the frequency of the solar-produced waves. In the Earth’s polar cusp, the wave properties of ion cyclotron waves are quite variable. Spatial gradients in the magnetic field may cause this variation as the background field changes between the regions in which the fast neutrals are produced and where they are re-ionized and picked up. While these waves were discovered early in the magnetospheric exploration, their generation was not understood until after we had observed similar waves in the exospheres of Mars and Venus. 相似文献
6.
From optical photometry the cataclysmic variable RX J0944.5+0357 is shown to have a double-peaked pulse profile with a period
∼ 2160 s. The two peaks vary rapidly in relative amplitude. Often most of the optical power is concentrated in the first harmonic
of the 2160 s modulation; RX J0944.5+0357therefore probably belongs to the relatively rare class of two-pole accreting intermediate
polars exemplified by YY Dra and V405 Aur.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
R. K. Konstantinova‐Antova A. P. Antov B. E. Zhilyaev I. A. Verlyuk Ya. O. Romaniuk O. A. Svyatogorov S. Avgoloupis J. H. Seiradakis M. E. Contadakis 《Astronomische Nachrichten》2005,326(1):38-42
V390 Aur is classified as a G8 spectral class active single giant by Fekel and Marshall (1991). Multi‐site cooperative high‐speed photoelectric monitoring was carried out for it in the period 1998‐2003 as part of a program developed in Bulgaria recently to study chromospherically active evolved stars. Astronomers from Greece and Ukraine joined the flare activity study of V390 Aur in 2000. As a result, several flares with a duration from seconds to minutes were detected on this star at 3 observatories. Taking into account all the events detected, the flare rate of the star is estimated to 0.205 flares per hour. Comparison with the active dwarfs AD Leo and EV Lac indicates that the flare rate of V390 Aur is lower than that of AD Leo but comparable with the well‐known flare star EV Lac. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
8.
Up to now, only a very small number of dwarf novae have been studied during their outburst state (∼30 per cent in the Northern hemisphere). In this paper we present the first comprehensive atlas of outburst spectra of dwarf novae. We study possible correlations between the emission and absorption lines seen in the spectra and some fundamental parameters of the binaries. We find that out of the 48 spectra presented, 12 systems apart from IP Peg show strong He ii in emission: SS Aur, HL CMa, TU Crt, EM Cyg, SS Cyg, EX Dra, U Gem, HX Peg, GK Per, KT Per, V893 Sco, IY UMa, and seven others less prominently: FO And, V542 Cyg, B i Ori, TY Psc, VZ Pyx, ER UMa and SS UMi. We conclude that these systems are good targets for finding spiral structure in their accretion discs during outburst if the models of Smak and Ogilvie are correct. This is confirmed by the fact that hints of spiral asymmetries have already been found in the discs of SS Cyg, EX Dra and U Gem. 相似文献
9.
Magnetosonic waves near the harmonics of proton cyclotron frequency can become unstable in the presence of oxygen ions in the ring current. For cos θ = 0 (θ being the angle between the wave vector and the geomagnetic field) the growth rates are peaked at some optimum value of the oxygen ion density, whereas for cos θ ≠ 0 they are reduced with the increase of oxygen ion density. The presence of hot oxygen ions can generate instability near the harmonics of oxygen cyclotron frequency. The growth rates are enhanced with the increase of cos θ. This mechanism can generate discrete spectrum of ELF hiss beyond the plasmapause. 相似文献
10.
V. V. Zheleznyakov 《Astrophysics and Space Science》1983,97(2):229-259
The influence of radiation on the electron velocity distribution in a hot nonrelativistic plasma localized near the surface of magnetic white dwarfs is investigated. The part played by the plasma in the formation of cyclotron features in the optical spectrum of these stars is studied. The region of parameters where the transverse temperature of plasma is defined by the brightness temperature of extraordinary radiation at the gyrofrequency is found. When escaping from the plasma in a homogeneous magnetic field, this component forms a cyclotron line in absorption. The ordinary radiation at the gyrofrequency and both modes at higher cyclotron harmonics are in emission or absorption depending on the magnetic field strength and hot plasma density. Possible interpretation of the observed spectral features of magnetic white dwarfs in terms of the developed theory is discussed. 相似文献
11.
The south residual cap of Mars is commonly described as a thin and bright layer of CO2-ice. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is a low-frequency radar on board Mars Express operating at the wavelength between 55 and 230 m in vacuum. The reflection of the radar wave on a stratified medium like the residual cap can generate interferences, causing weaker surface reflections compared to reflections from a pure water ice surface. In order to understand this anomalous low reflectivity, we propose a stratified medium model, which allows us to estimate both the thickness and the dielectric constant of the optically thin slab. First, we consider the residual cap as single unit and show that the decrease in the reflected echo strength is well explained by a mean thickness of 11 m and a mean dielectric constant of 2.2. This value of dielectric constant is close to the experimental value 2.12 for pure CO2-ice. Second, we study the spatial variability of the radar surface reflectivity. We observe that the reflectivity is not homogeneous over the residual cap. This heterogeneity can be modeled either by variable thickness or variable dielectric constant. The surface reflectivity shows that two different units comprise the residual cap, one central unit with high reflectivity and surrounding, less reflective units. 相似文献
12.
Spectral and polarization characteristics of radio emission from hot X-ray kernels are considered. It is shown that the frequency spectrum of thermal cyclotron radio emission and bremsstrahlung from these regions may contain a set of lines at cyclotron harmonics and the maximum at plasma frequency. This makes possible the diagnostics of the X-ray kernel plasma and magnetic fields according to spectral and polarization observations of microwave radio emission in solar flares. 相似文献
13.
Noble P. Abraham Samuel George G. Sreekala Sijo Sebastian Chandu Venugopal G. Renuka 《Earth, Moon, and Planets》2014,111(3-4):115-125
We have studied the stability of the electrostatic electron cyclotron wave in a plasma composed of hydrogen, oxygen and electrons. To conform to satellite observations in the low latitude boundary layer we model both the ionic components as drifting perpendicular to the magnetic field. Expressions for the frequency and the growth rate of the wave have been derived. We find that the plasma can support electron cyclotron waves with a frequency slightly greater than the electron cyclotron frequency ω ce ; these waves can be driven unstable when the drift velocities of both the ions are greater than the phase velocity of the wave. We thus introduce another source of instability for these waves namely multiple ion beams drifting perpendicular to the magnetic field. 相似文献
14.
Huang Guangs LI 《Solar physics》1987,114(2):363-373
A theoretical analysis of electron-cyclotron maser instabilities indicates that the distribution function of non-thermal electrons influences millisecond radio spikes in solar flares, and that a hollow beam distribution is more likely than a loss-cone distribution. The restrictions of classical theories of cyclotron resonant absorption are discussed and a formula is derived for the absorption coefficient near the resonant frequency. Finally, the computations show that for typical coronal parameters, the growth rates of the fundamental of fast extraordinary modes are much faster than those of their second harmonics; and because the directional angle of the fundamental is smaller, its resonant absorption may be neglected. Moreover, the band-width of the fundamental is consistent with observation of radio spikes; therefore, we claim that the millisecond radio spikes in the decimetric range are composed mainly of fundamentals of the fast extraordinary modes. The second harmonics of fast extraordinary modes may be generated for directions near to the vertical to the magnetic field, but it is impossible to observe both fundamental and second harmonics in the same direction. 相似文献
15.
16.
Dastgeer Shaikh 《Monthly notices of the Royal Astronomical Society》2009,395(4):2292-2298
Non-linear, three-dimensional, time-dependent fluid simulations of whistler wave turbulence are performed to investigate role of whistler waves in solar wind plasma turbulence in which characteristic turbulent fluctuations are characterized typically by the frequency and length-scales that are, respectively, bigger than ion gyrofrequency and smaller than ion gyroradius. The electron inertial length is an intrinsic length-scale in whistler wave turbulence that distinguishably divides the high-frequency solar wind turbulent spectra into scales smaller and bigger than the electron inertial length. Our simulations find that the dispersive whistler modes evolve entirely differently in the two regimes. While the dispersive whistler wave effects are stronger in the large-scale regime, they do not influence the spectral cascades which are describable by a Kolmogorov-like k −7/3 spectrum. By contrast, the small-scale turbulent fluctuations exhibit a Navier–Stokes-like evolution where characteristic turbulent eddies exhibit a typical k −5/3 hydrodynamic turbulent spectrum. By virtue of equipartition between the wave velocity and magnetic fields, we quantify the role of whistler waves in the solar wind plasma fluctuations. 相似文献
17.
It was shown by Zheleznyakov and Zlotnik (1980a, b) that in complex configurations of solar magnetic fields (in hot loops above the active centres, in neutral current sheets in the preflare phase, in hot X-ray kernels in the initial flare phase) a system of cyclotron lines in the spectrum of microwave radiation is likely to be formed. Such a line was obtained by Willson (1985) in the VLA observations at harmonics of the electron gyrofrequency. This communication interprets these observations on the basis of an active region model in which thermal cyclotron radiation is produced by hot plasma filling the magnetic tube in the corona above a group of spots. In this model the frequency of the recorded 1658 MHz line corresponds to the third harmonic of electron gyrofrequency, which yields the magnetic field (196 ± 4) G along the magnetic tube axis. The linewidth f/f 0.1 is determined by the 10% inhomogeneity of the magnetic field over the cross-section of the tube; the line profile indicates the kinetic temperature distribution of electrons over the tube cross-section with the maximum value 4 × 106 K. Analysis shows that study of cyclotron lines can serve as an efficient tool for diagnostics of magnetic fields and plasma in the solar active regions and flares. 相似文献
18.
Coordinated observations involving ion composition, thermal plasma, energetic particle, and ULF magnetic field data from GEOS 1 and 2 often reveal the presence of electromagnetic ion cyclotron and magnetosonic waves, which are distinguished by their respective polarization characteristics and frequency spectra. The ion cyclotron waves are identified by a magnetic field perturbation that lies in a plane perpendicular to the Earth's magnetic field B0 and propagate along B0. They are associated with the abundance of cold He+ in the presence of anisotropic pitch angle distributions of ions having energies E > 20 keV, and were observed at frequencies near the He+ gyrofrequency. The magnetosonic waves are characterized by a magnetic field perturbation parallel to B0 and thus seem to be propagating perpendicular to the Earth's magnetic field. They often occur at harmonics (not always including the fundamental) at the proton gyrofrequency and are associated with phase-space-density distributions that peak at energies E ～ 5–30 keV and at a pitch angle of 90°. Such a ring-like distribution is shown to excite instability in the magnetosonic mode near harmonics of the proton gyrofrequency. Magnetosonic waves are associated in other cases with sharp spatial gradients in energetic ion intensity. Such gradients are encountered in the early afternoon sector (as a consequence of the drift shell distortion caused by the convection electric field) and could likewise constitute a source of free energy for plasma instabilities. 相似文献
19.
Solar radio emission observations in the microwave frequency range show fine structures consisting of a number of almost parallel narrow-frequency bands. We interpret these bands as plasma emission at cyclotron harmonics. This emission is generated by the anisotropic electron beam, which excites longitudinal waves at a normal Doppler effect resonance. Subsequently, the longitudinal waves convert to radio emission at the second harmonic of the longitudinal wave frequency, and sometimes to the fundamental harmonic. The magnetic field strength is estimated on the basis of such a model in the microwave burst sources at 100–200 G. Estimates of the density variations are also made. 相似文献
20.
This paper examines the small-scale solar wind turbulence driven in view of the Alfvén waves subjected to ponderomotive nonlinearity. Filamentation instability is known to take place for the case of dispersive Alfvén wave (DAW) propagating parallel to the ambient magnetic field. The ponderomotive force associated with DAW is responsible for wave localization and these webs of filaments become more intense and irregular as one proceeds along the spatial domain. The ponderomotive force associated with pump changes with pump parameters giving rise to different evolution patterns. This paper studies in detail the nonlinear evolution of filamentation instability introduced by dispersive Alfven waves (DAWs) which becomes dispersive on account of the finite frequency of DAW i.e., pump frequency is comparable to the ion cyclotron frequency. We have explicitly obtained the perturbation dynamics and then examined the impact of pump magnitude on the driven magnetic turbulence using numerical simulation. The results show steepening at small scales with increasing pump amplitude. The compressibility associated with acoustic fluctuations may explain the variation in spectral scaling of solar wind turbulence as observed by Alexandrova et al. (Astrophys. J. 674:1157, 2008). 相似文献