共查询到20条相似文献，搜索用时 15 毫秒
1.
W. K. Yip 《Solar physics》1972,24(1):197-209
Combination scattering of the Cerenkov plasma waves generated by a fast electron beam on the electron density fluctuations in a magnetoactive plasma is assumed to be the cause of the emission of the drift pair (or the hook burst) from the solar corona. The features of the combination emission are studied numerically with parameters appropriate to the solar corona condition. It is found that the major properties of the drift pair and the hook burst can be accounted for. 相似文献
2.
The problem of strong polarization of the zebra-type fine structure in solar radio emission is discussed. In the framework of the plasma mechanism of radiation at the levels of the double plasma resonance, the polarization of the observed radio emission may be due to a difference in rates of plasma wave conversion into ordinary and extraordinary waves or different conditions of escaping of these waves from the source. In a weakly anisotropic plasma which is a source of the zebra-pattern with rather large harmonic numbers, the degree of polarization of the radio emission at twice the plasma frequency originating from the coalescence of two plasma waves is proportional to the ratio of the electron gyrofrequency to the plasma frequency, which is a small number and is negligible. Noticeable polarization can therefore arise only if the observed radio emission is a result of plasma wave scattering by ions (including induced scattering) or their coalescence with low-frequency waves. In this case, the ordinary mode freely leaves the source, but the extraordinary mode gets into the decay zone and does not exit from the source. As a result, the outgoing radio emission can be strongly polarized as the ordinary mode. Possible reasons for the polarization of the zebra pattern in the microwave region are discussed. 相似文献
3.
T. Takakura 《Solar physics》1979,61(1):161-186
A simulation of normal type III radio bursts has been made in a whole frequency range of about 200 MHz to 30 kHz by the usage of the semi-analytical method as developed in previous papers for the plasma waves excited by a cloud of fast electrons. Three-dimensional plasma waves are computed, though the velocities of fast electrons are assumed to be one-dimensional. Many basic problems about type III radio bursts and associated solar electrons have been solved showing the following striking or unexpected results.Induced scattering of plasma waves, by thermal ions, into the plasma waves with opposite wave vectors is efficient even for a solar electron cloud of rather low number density. Therefore, the second harmonic radio emission as attributed to the coalescence of two plasma waves predominates in a whole range from meter waves to km waves. Fundamental radio emission as ascribed to the scattering of plasma waves by thermal ions is negligibly small almost in the whole range. On the other hand, third harmonic radio emission can be strong enough to be observed in a limited frequency range.If, however, the time integral of electron flux is, for example, 2 × 1013 cm–2 (>5 keV) or more at the height of 4.3 × 1010 cm (
p = 40 MHz) above the photosphere, the fundamental may be comparable with or greater than the second harmonic, but an effective area of cross-section of the electron beam is required to be very small, 1017 cm2 or less, and hence much larger sizes of the observed radio sources must be attributed to the scattering alone of radio waves.The radio flux density expected at the Earth for the second harmonic can increase with decreasing frequencies giving high flux densities at low frequencies as observed, if x-dependence of the cross-sectional area of the electron beam is x
1.5 or less instead of x
2, at least at x 2 × 1012 cm.The second harmonic radio waves are emitted predominantly into forward direction at first, but the direction of emission may reverse a few times in a course of a single burst showing a greater backward emission at the low frequencies.In a standard low frequency model, a total number of solar electrons above 18 keV arriving at the Earth orbit reduces to 12% of the initial value due mainly to the collisional decay of plasma waves before the waves are reabsorbed by the beam electrons arriving later. However, no deceleration of the apparent velocity of exciter appears. A change in the apparent velocity, if any, results from a change in growth rate of the plasma waves instead of the deceleration of individual electrons.Near the Earth, the peak of second harmonic radio flux as emitted from the local plasma appears well after the passage of a whole solar electron cloud through this layer. This is ascribed to the secondary and the third plasma waves as caused in non-resonant regions by the induced scattering of primary plasma waves in a resonant region. 相似文献
4.
N. Gopalswamy 《Solar physics》1990,126(2):367-370
A new mechanism has been proposed for the continuum and burst components of solar storm radiation by Genkin, Erukhimov, and Levin (1989a, b). In this paper, we point out that while bursts can be explained by the proposed mechanism of scattering on plasma turbulence generated density fluctuations, the continuum cannot be explained by sattering on thermal ion density fluctuations. The reason is, under the same coronal conditions, second harmonic emissions will dominate over the fundamental emission due to scattering on thermal ion density fluctuations in contradiction to observations. We also note that the range of plasma wave densities needed for this mechanism may not be realistic for the case of plasma wave generation due to loss cone instability. It is further argued that coalescence of plasma waves with low-frequency waves still seems to be the plausible mechanism. 相似文献
5.
Low frequency turbulence in the solar corona and fundamental radiation of type III solar radio burst
T. Takakura 《Solar physics》1979,62(2):375-382
On the basis of the previous numerical simulations, a new mechanism for the emission of the fundamental radio waves of solar radio type III bursts is presented. This hypothesis is to attribute the fundamental radio emission to the coalescence of the plasma waves with the low frequency turbulence, whistler or ion acoustic waves, pre-existing on the way of the electron beam which excite the plasma waves.It is estimated that ion acoustic waves could be occasionally unstable in the solar corona due to that drifting bi-Maxwellian distribution of electrons as observed in the solar wind, which is probably caused by collision-less heat conduction.It is also suggested that the reduced damping of the ion acoustic waves in such a distorted electron distribution in the corona may decrease the threshold electric current to cause the anomalous resistivity to be the onset of the solar flares. 相似文献
6.
A.M. Uralov 《Solar physics》1998,183(1):133-155
Possible scattering regimes of the emission from a solar radio source due to dielectric permitivity fluctuations of an extended coronal plasma co-rotating with the Sun are discussed. The exact and approximate expressions are given for the spectrum of temporal intensity fluctuations in the regime of weak scattering. The frequency, at which the spectrum shows a bend, is determined by the location of the effective scattering screen if the source size is not too large. In the regime of strong scattering of the emission from a broadbanded nonimpulsive radio source, the formation of random intensity spikes, namely millisecond, narrowbanded spike bursts is a possibility. Their apparent size can be quite significant. However, the sources with very small true sizes are required in order to produce strong spikes. 相似文献
7.
V. G. Ledenev 《Solar physics》2008,253(1-2):191-198
If plasma waves propagate in the direction of the plasma density decrease, their spectrum shifts to large wave numbers (to small phase velocities). This means that the spectrum of plasma waves excited by an electron beam concentrates near the distribution function (“plateau”) border, which shifts in the region of low velocities in the process of quasilinear relaxation. As the spectrum of excited plasma waves shifts in the region of large wave numbers, their frequency grows in accordance with the dispersion equation, which describes these waves. When the growth of the plasma wave frequency exceeds the decrease of the frequency owing to the regular inhomogeneity in the corona, the branch with positive frequency drift appears on the dynamic spectrum of the radio emission. Our computations allow us to estimate the density and energy of electron beams generating type U bursts. 相似文献
8.
Nobuki Kawashima Kazunori Akai Yukio Murasato Susumu Sasaki 《Astrophysics and Space Science》1984,106(1):117-123
In an electron beam emission experiment on board the EXOS-B (JIKIKEN) satellite (200 V, 1 mA-maximum), several types of waves are strongly excited by the beam such as plasma frequency, upper hybrid frequency, electron cyclotron frequency, their harmonics and nonlinear coupling of these waves. Measurements of these waves give information on local plasma density and magnetic field strength and it is revealed that the electron beam emission from the spacecraft is a powerful diagnostic tool in the magnetosphere. A long term observation in this electron beam experiment has provided us with the average plasma density profile in the magnetosphere. It is also useful for the detection of the plasmapause. Plasma density measurements down to the order of 10 cm–3 are possible. The instrument itself is very simple and compact, so that it will be a powerful plasma diagnostic tool in future magnetospheric and planetary explorations. 相似文献
9.
It is usually assumed that the ions of cosmic rays contribute nothing to the observable electromagnetic radiation. However, this is true only when these ions are moving in a vacuum or a quiet (nonturbulent) plasma. In the case of fast ions in a turbulent plasma, there is an effective nonlinear mechanism of radiation which is discussed in this paper. The fast ion (relativistic or nonrelativistic) moving in the plasma creates a polarization cloud around itself which also moves with the particles. The turbulent plasma waves may scatter on the moving electric field of this polarization cloud. In the process of this scattering an electromagnetic wave with frequency (2.7) is generated. Let 1 and k1 be the frequency and wave vector of turbulent plasma waves,V is the velocity of the ion, and is the angle between the wave vector of electromagnetic radiation and the direction of the ion velocity. The method of calculating the probability of the conversion of plasma waves (k1) into electromagnetic waves (k) by scattering on an ion with velocityV is described in detal in Section 2 (Equation (2.14)).The spectral coefficients of spontaneous radiation in the case of scattering of plasma waves on polarization clouds created by fast nonrelativistic ions are given in (3.6) for an ion energy distribution function (3.4) and in (3.8) for more general evaluations. The Equations (3.9)–(3.13) describe the spectral coefficients of spontaneous emission for different modes of plasma turbulence (Langmuir (3.9), electron cyclotron in a weak (3.10) or strong (3.11) magnetic field and ion acoustic (3.12)–(3.13) waves). The coefficients of reabsorption or induced emission are given by Equations (3.14) and (3.16)–(3.19). There is a maser effect in the case of scattering of plasma waves on a stream of ions. The effective temperature of the spontaneous emission is given by Equation (3.15). The spectral coefficients of radiation due to scattering of plasma waves on relativistic ions are calculated in the same manner (Equations (4.14)–(4.15)). The total energy loss due to this radiation is given in Equations (4.23)–(4.25). The coefficients of induced emission are given in (4.26)–(4.28).The results are discussed in Section 5. It is shown that the loss of energy by nonlinear plasma radiation is much smaller than the ionization loss. However, the coefficients of synchrotron radiation of electrons and nonlinear radiation of ions under cosmic conditions may be comparable in the case of a weak magnetic field and fairly low frequencies (5.5)–(5.6). Usually the spectrum of nonlinear plasma radiation is steeper than in the case of synchroton radiation. Equation (5.10) gives the condition for nonlinear radiation to prevail over thermal radiation.Translated by D. F. Smith. 相似文献
10.
A general theory of scattering of waves in a magnetoactive plasma by particles of arbitrary energy is presented. The cross-section
for the scattering of magnetoionic waves by thermal particles is derived and discussed. Conditions under which the effect
of the spiralling motion of the scattering electron can be neglected in treating inverse Compton radiation are found. 相似文献
11.
The possibility is investigated that the plasma turbulence used in many recent models of the primary energy release and acceleration in solar flares should be detectable by radiation near the fundamental and second harmonic of the plasma frequency. Formulae are derived for fundamental emission due to the combination of ion-acoustic and Langmuir plasma turbulence and for second harmonic emission due to the combination of two Langmuir waves. These results are applied to recent primary energy release and acceleration models which shows that either such radiation should be detectable and possibly distinguishable with suitable microwave interferometers or that its absence places fairly stringent constraints on the possible level of Langmuir or Langmuir and ion-acoustic waves in these models.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
12.
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
13.
K. G. McClements 《Solar physics》1987,109(2):355-363
The conditions required for the stability of a steady-state electron beam propagating in the solar corona are determined using the quasi-linear theory. The growth rate for electron plasma waves in a magnetized plasma is evaluated, with the electron distribution function being given by an analytic solution of the linearized Fokker-Planck equation. It is shown that, when the gyrofrequency is less than the plasma frequency, the instability has a narrow angular range, with the maximum growth rate occuring along the magnetic field. A stability boundary in parameter space is determined, indicating that electron beams must be highly collimated at injection to be Langmuir unstable at any point in space. The implications of the results for alternative models of hard X-ray bursts are discussed and it is argued that Langmuir instability will not occur on either the trap model or the thermal model. Such models would, therefore, be refuted by the detection of a large flux of plasma microwave radiation associated with hard X-ray emission. 相似文献
14.
Numerical analysis has been carried out on the one-dimensional quasi-linear relaxation of a group of fast electrons travelling through the plasma. It is demonstrated that the electron velocity distribution of fast electrons tends to be a plateau form exciting the electron plasma waves and that the plasma waves are almost completely reabsorbed later by electrons arriving later. Both the velocity range and time interval in which quasi-plateau distribution is formed increase with distance from the origin of the fast electrons. There is no net energy loss of the electron cloud during the travel through the plasma if we neglect both the collisional losses and the scattering of plasma waves. Although the present computation is preliminary and limited to rather low beam density, we can see that the characteristics of both the electron beam and the plasma waves tend, with distance, to those of the analytical solution given by Ryutov and Sagdeev; though a modification to set a low velocity cutoff on the plasma waves due to the thermal electrons is necessary. 相似文献
15.
It is shown that “zebra-pattern” in solar continuum events (in type IV bursts) can be formed as a result of interference between
direct and reflected rays coming from a source of small size in a stratified atmosphere. The emission is generated by plasma
mechanism. Full emission flux is contributed from a great number of narrow-band short-lived sources of small sizes, which
are formed by plasma waves captured in density minima of background plasma fluctuations. 相似文献
16.
在中子星磁轴吸积柱的上部,少数高能电子通过磁镜点反射,可使部份电子的速度分布形成非热分布,由此激发激射(Maser)不稳定性。波被放大,发射出频率近似为电子迴旋频率及其倍频的相干辐射。用此模型计算了HerX-1的迴旋线发射。发现不稳定性增长率与吸积柱中电子数密度成正比,因而比非相干散射产生的连续辐射随电子数密度增长更快;而且发射线的强度和能量均与脉冲相位关联。这个理论可解释近期的HerX-1观测结果。 相似文献
17.
We report on a new investigation of microbursts at meter-decameter wavelengths observed using the Broad Band Array at Gauribidanur Radio Observatory. This is an independent set of observations of microbursts: previous observations had been obtained only by the Clark Lake multifrequency radioheliograph. We confirm several properties of microbursts reported earlier. In addition, we have studied some new properties of microbursts such as time profile characteristics, flux density and energy spectra for comparison with the corresponding properties of normal type III bursts. The present study supports the idea that the microbursts and the normal type III bursts are generated by electron beams of similar characteristics. We interpret the low brightness temperature of microbursts as follows: plasma waves generated by the electron beams through beam-plasma instability are quickly isotropized as they scatter on the density fluctuations in the corona. The resulting low levels of plasma waves are converted into transverse radiation of low brightness temperature. One important consequence of the isotropization is that the second harmonic plasma emission dominates the fundamental and hence the microbursts are expected to be predominantly a harmonic plasma emission. 相似文献
18.
A. A. Kuznetsov 《Solar physics》2006,237(1):153-171
The plasma mechanism of radio emission generation in an inhomogeneous medium is investigated. In the model under study, the
electron beam with loss-cone distribution generates upper-hybrid waves that, in turn, are transformed into radio emission.
It is shown that the influence of the plasma density inhomogeneity limits the plasma waves’ intensity considerably due to
variation in their wave vector. The results are used to interpret the intermediate drift (IMD) bursts. A model is proposed
in which these bursts are reflections of propagating small-scale (with amplitudes of about 1% and sizes of hundreds of kilometers)
magnetohydrodynamic (MHD) disturbances of magnetic tubes. It is shown that this model allows us to explain the spectral parameters
of the bursts in question. At present, the lack of precise and independent data about the magnetic field does not allow us
to decide definitively between the existing models (whistler or MHD waves) of the IMD bursts; nevertheless, if the proposed
model is correct, it can be used to determine the characteristics of the coronal MHD waves. 相似文献
19.
A model for the source of microwave bursts from the Crab pulsar in the form of a current sheet with a transversemagnetic field has been investigated. The emission generation mechanism is based on the excitation of plasma waves at the double plasma resonance frequencies in a nonrelativistic nonequilibrium plasma followed by their scattering into electromagnetic waves that escape from the current sheet into the neutron star magnetosphere. The basic parameters of the source explaining the observed characteristics of quasi-harmonic bursts in the interpulses of radio emission from this pulsar have been established. 相似文献
20.
David Tsiklauri 《Solar physics》2010,267(2):393-410
1.5D Vlasov – Maxwell simulations are employed to model electromagnetic emission generation in a fully self-consistent plasma
kinetic model for the first time in the context of solar physics. The simulations mimic the plasma emission mechanism and
Larmor-drift instability in a plasma thread that connects the Sun to Earth with the spatial scales compressed appropriately.
The effects of spatial density gradients on the generation of electromagnetic radiation are investigated. It is shown that
a 1.5D inhomogeneous plasma with a uniform background magnetic field directed transverse to the density gradient is aperiodically
unstable to the Larmor-drift instability. The latter results in a novel effect of generation of electromagnetic emission at
plasma frequency. The generated perturbations consist of two parts: i) non-escaping (trapped) Langmuir type oscillations, which are localised in the regions of density inhomogeneity, and are
highly filamentary, with the period of appearance of the filaments close to electron plasma frequency in the dense regions;
and ii) escaping electromagnetic radiation with phase speeds close to the speed of light. When the density gradient is removed (i.e. when plasma becomes stable to the Larmor-drift instability) and a low density super-thermal, hot beam is injected along the domain, in the direction perpendicular to the magnetic field, the plasma emission
mechanism generates non-escaping Langmuir type oscillations, which in turn generate escaping electromagnetic radiation. It
is found that in the spatial location where the beam is injected, standing waves, oscillating at the plasma frequency, are
excited. These can be used to interpret the horizontal strips (the narrow-band line emission) observed in some dynamical spectra.
Predictions of quasilinear theory are: i) the electron free streaming and ii) the long relaxation time of the beam, in accord with the analytic expressions. These are corroborated via direct, fully-kinetic
simulation. Finally, the interplay of the Larmor-drift instability and plasma emission mechanism is studied by considering
a dense electron beam in the Larmor-drift unstable (inhomogeneous) plasma. The latter case enables one to study the deviations from
the quasilinear theory. 相似文献