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1.
The effect of collisions of suprathermal electrons with a thermal background plasma is investigated and is shown to cause flattening of a monotonically descending velocity distribution of fast particles. As a result flare-produced energetic electrons that are trapped in a coronal magnetic arch and that are initially distributed in energy according to a power-law, can give rise to an instability of Langmuir waves in the background plasma and the subsequent emission of continuum radiation as observed in type IV dm bursts.To explain the appearance of sudden reductions observed in type IV dm outbursts the effects both of magnetic compressions of the source region and of renewed injections of fast particles on the continuum source are investigated. It is found that the latter process can explain the observed reductions. 相似文献
2.
E. Y. Zlotnik 《Solar physics》2013,284(2):579-588
Solar radio emission is a significant source of information regarding coronal plasma parameters and the processes occurring in the solar atmosphere. High resolution frequency, space, and time observations together with the developed theory make it possible to retrieve physical conditions in the radiation source and recognize the radiation mechanisms responsible for various kinds of solar radio emission. In particular, the high brightness temperature of many bursts testifies to coherent radiation mechanisms, that is, to plasma instabilities in the corona. As an example, the fine structure of solar radio spectra looking like a set of quasi-harmonic stripes of enhanced and lowered radiation, which is observed against the type IV continuum at the post-flare phase of activity, is considered. It is shown that such emission arises from a trap-like source filled with a weakly anisotropic equilibrium plasma and a small addition of electrons which have a shortage of small velocities perpendicular to the magnetic field. For many recorded events with the mentioned fine spectral structure the instability processes responsible for the observed features are recognized. Namely, the background type IV continuum is due to the loss-cone instability of hot non-equilibrium electrons, and the enhanced striped radiation results from the double-plasma-resonance effect in the regions where the plasma frequency f p coincides with the harmonics of electron gyrofrequency f B ; f p=sf B . Estimations of the electron number density and magnetic field in the coronal magnetic traps, as well as the electron number density and velocities of hot electrons necessary to excite the radiation with the observed fine structure, are given. It is also shown that in some cases several ensembles of non-equilibrium electrons can coexist in magnetic traps during solar flares and that its radio signature sensitively depends on the parameters of the distribution functions of the various ensembles. 相似文献
3.
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
4.
We investigate the problem of determining the plasma composition of relativistic jets in blazars and microquasars from the polarization frequency spectra of their synchrotron radiation. The effect of plasma composition on this radiation is attributable to a change in the structure of the ordinary and extraordinary waves in plasma, depending on the presence of a nonrelativistic electron-proton component in it and on the type of relativistic particles (electrons, positrons). The structure of the normal waves determines the properties of the observed radiation and primarily the shape of the polarization frequency spectrum. Our analytic calculations of the polarization spectra for simple models of jets with a uniform magnetic field and with a magnetic-field shear revealed characteristic features in the polarization spectra. These features allow us to differentiate between the synchrotron radiation from an admixture of relativistic particles in a cold plasma and the radiation from a relativistic plasma. However, definitive conclusions regarding the relativistic plasma composition (electrons or electron-positron pairs) can be reached only by a detailed analysis of the polarization frequency spectra that will be obtained in future radioastronomical studies with high angular and frequency resolutions. 相似文献
5.
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
6.
J. P. Wild 《Solar physics》1969,9(2):260-264
A continuous record of the 80 MHz image and polarization of a type IV solar outburst has been made with the Culgoora radioheliograph
from which the magnetic structure of the event can be directly inferred. The first (‘moving’) part of the event appears beyond
the limb as an expanding magnetic arch along which three concentrated sources develop: one unpolarized source near the peak,
attributed to synchrotron radiation; and two polarized sources of opposite polarity near the feet, attributed to plasma radiation.
The radio-emitting arch appears to lie above an eruptive prominence seen in Hα. The second (‘stationary’) part is seen later
as a separate highly polarized source on the disk above the projected position of the flare that had previously triggered
the prominence activity. 相似文献
7.
An analysis of new observations showing fine structures consisting of narrowband fiber bursts as substructures of large-scale
zebra-pattern stripes is carried out. We study four events using spectral observations taken with a newly built spectrometer
located at the Huairou station, China, in the frequency range of 1.1 – 2.0 GHz with extremely high frequency and time resolutions
(5 MHz and 1.25 ms). All the radio events were analyzed by using the available satellite data (SOHO LASCO, EIT, and MDI, TRACE,
and RHESSI). Small-scale fibers always drift to lower frequencies. They may belong to a family of ropelike fibers and can
also be regarded as fine structures of type III bursts and broadband pulsations. The radio emission was moderately or strongly
polarized in the ordinary wave mode. In three main events fiber structure appeared as a forerunner of the entire event. All
four events were small decimeter bursts. We assume that for small-scale fiber bursts the usual mechanism of coalescence of
whistler waves with plasma waves can be applied, and the large-scale zebra pattern can be explained in the conventional double
plasma resonance (DPR) model. The appearance of an uncommon fine structure is connected with the following special features
of the plasma wave excitation in the radio source: Both whistler and plasma wave instabilities are too weak at the very beginning
of the events (i.e., the continuum was absent), and the fine structure is almost invisible. Then, whistlers generated directly at DPR levels
“highlight” the radio emission only from these levels owing to their interaction with plasma waves. 相似文献
8.
A. Böhme 《Solar physics》1972,24(2):457-474
The existence of a group of broad-band continua during the initial stage of some type IV bursts can be shown which are polarized in the extraordinary sense. Unfortunately interferometric records were available only for one of these continua. In this case no movement of the radiation source could be stated during the whole of its lifetime. It can be shown that the time behaviour of the broad-band continua is clearly different from that of the moving type IV bursts recorded directly by interferometers. Therefore, it must be concluded that both types of continua originate from different physical processes at the Sun, though both are caused by synchrotron radiation.Based on the author's thesis in Berlin. 相似文献
9.
G. P. Chernov 《Solar System Research》2008,42(5):434-447
Several recent phenomena with zebra patterns (ZPs) and fiber bursts on the dynamic spectra of solar type IV radio bursts have been complexly analyzed using all available ground-based and satellite data (SOHO, TRACE, RHESSI). ZPs and fiber bursts were observed at frequencies of 50–3800 MHz. The main relative spectral parameters and the degree of circular polarization of ZPs and fiber bursts are almost identical. The fine structure was observed in powerful and weak phenomena (and was more impressive in weak phenomena) during impulsive and decline phases at instants of recurring continuum bursts. The shape of the fine structure depends on that of the magnetic loops in a radio source, the type of fast particle acceleration (impulsive or prolonged), and the presence of shock waves and coronal mass ejections. Several new effects of the interaction between zebra stripes and fiber bursts have been detected. Specifically, up to 40 fiber bursts with different frequency ranges were simultaneously observed in the frequency range 1–2 GHz against a background of sudden absorptions. It has been indicated that different effects in the ZP stripe behavior can be explained within the scope of the model with whistlers, if the quasi-linear diffusion of fast particles on whistlers (which deforms the particle velocity distribution function) is taken into account. 相似文献
10.
Jan Kuijpers 《Solar physics》1975,44(1):173-193
The possible generation of intermediate drift bursts in type IV dm continua through coupling between whistler waves, traveling along the magnetic field, and Langmuir waves, excited by a loss-cone instability in the source region, is elaborated. We investigate the generation, propagation and coupling of whistlers. It is shown that the superposition of an isotropic background plasma of 106K and a loss-cone distribution of fast electrons is unstable for whistler waves if the loss-cone aperture 2α is sufficiently large (sec α?4); a typical value of the excited frequencies is 0.1 ω ce (ω ce is the angular electron cyclotron frequency). The whistlers can travel upwards through the source region of the continuum along the magnetic field direction with velocities of 21.5–28 v A (v A is the Alfvén velocity). Coupling of the whistlers with Langmuir waves into escaping electromagnetic waves can lead to the observed intermediate drift bursts, if the Langmuir waves have phase velocities around the velocity of light. In our model the instantaneous bandwith of the fibers corresponds to a frequency of 0.1–0.5 ω ce and leads to estimates of the magnetic field strength in the source region. These estimates are in good agreement with those derived from the observed drift rate, corresponding to 21.5–28 v A, if we use a simple hydrostatic density model. 相似文献
11.
E. A. Corbett A. Robinson D. J. Axon J. H. Hough 《Monthly notices of the Royal Astronomical Society》2000,311(3):485-492
We present a series of new spectroscopic observations of BL Lacertae. The observations were obtained over a period of 30 months and sample the violent continuum flare of 1997 June–August. The equivalent width of the recently discovered broad H α line varies approximately inversely with the optical continuum flux, implying that the broad-line-emitting gas does not respond significantly to large variations in the beamed synchrotron continuum which dominates the spectrum at optical wavelengths. The simplest and most plausible interpretation of this result is that the broad-line region in BL Lacertae is 'unaware' of the synchrotron beam and is photoionized by another source, a likely candidate being a hot accretion disc. Alternatively, similar behaviour would be observed if the broad-line region is indeed photoionized by the synchrotron beam but is predominantly matter-bounded so that it absorbs only a small fraction of the incident ionizing photon flux. 相似文献
12.
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
13.
Bhonsle and Degaonkar (1980) have presented an evidence for the Razin-cutoff frequency in the dkm range of type IV continuum observed by the Ahmedabad spectroscope on 27 April, 1979. They explained this cutoff as due to the Razin effect in the coronal medium. From the broad-band spectrograph at Weissenau, covering 1000 to 30 MHz, additional information on the initial features of type IV is furnished here. This record also confirms the preceeding type II burst, the shock front of which was assumed to produce the quasiperiodicity observed at the lower cut off frequency by interaction with the type IV radiation source. 相似文献
14.
G. P. Chernov M. L. Kaiser J.-L. Bougeret V. V. Fomichev R. V. Gorgutsa 《Solar physics》2007,241(1):145-169
The analysis of WIND/WAVES RAD2 spectra with fine structure in the form of different fibers in 14 events covering 1997?–?2005 is carried out. A splitting of broad bands of the interplanetary (IP) type II bursts into narrow band fibers of different duration is observed. The instantaneous-frequency bandwidth of fibers is stable: 200?–?300 kHz for slow-drifting fibers in type II bursts, and 700?–?1000 kHz for fast-drifting fibers in type II?+?IV (continuum). Intermediate drift bursts (IDB or fiber bursts) and zebra patterns with variable frequency drift of stripes, typical for the metric range, were not found. Comparison of spectra with the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph (SOHO/LASCO C2) images shows a connection of the generation of the fiber structures with the passage of shock fronts through narrow jets in the wake of Coronal Mass Ejections (CME). Therefore the most probable emission mechanism of fibers in IP type II bursts appears to be resonance transition radiation (RTR) of fast particles at the boundary of two media with different refractive indices. The same mechanism is also valid for striae in the type III bursts. Taking into account a high-density contrast in the CME wake and the actually observed small-scale inhomogeneities, the effectiveness of the RTR mechanism in IP space must be considerably higher than in the meter or decimeter wavelengths. For the most part the fibers in the type IV continuum at frequencies of 14?–?8 MHz were seen as the direct expansion of similar fine structure (as fibers or “herringbone” structure) in the decametric range observed with the Nançay and IZMIRAN spectrographs. 相似文献
15.
A moving Type IV burst, observed with the Culgoora radioheliograph on 1970 April 29, moved out to about 3 R
and attained high circular polarization before fading. The appearance of the moving Type IV source suggests an isolated, self-contained, synchrotron emitting plasmoid. Magnetic field maps of the corona derived from photospheric observations indicate that the plasmoid moved almost radially outward from the flare region along open field lines. To explain the observed source structure and high unipolar polarization, we suggest that a ring of electric current was ejected from the low corona and guided by coronal magnetic field lines; the radio emission was synchrotron radiation generated by mildly-relativistic electrons trapped in the poloidal magnetic field of the ring current.Part of the research reported here was carried out while the author was at the Division of Radiophysics, C.S.I.R.O., Sydney, Australia.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
16.
G. Ghisellini F. Haardt & R. Svensson 《Monthly notices of the Royal Astronomical Society》1998,297(2):348-354
The high-energy continuum in Seyfert galaxies and galactic black hole candidates is likely to be produced by a thermal plasma. There are difficulties in understanding what can keep the plasma thermal, especially during fast variations of the emitted flux. Particle–particle collisions are too inefficient in hot and rarefied plasmas, and a faster process is called for. We show that cyclo-synchrotron absorption can be such a process: mildly relativistic electrons thermalize in a few synchrotron cooling times by emitting and absorbing cyclo-synchrotron photons. The resulting equilibrium function is Maxwellian at low energies, with a high-energy tail when Compton cooling is important. Assuming that electrons emit completely self-absorbed synchrotron radiation and at the same time Compton scatter their own cyclo-synchrotron radiation and ambient UV photons, we calculate the time-dependent behaviour of the electron distribution function, and the final radiation spectra. In some cases, the 2–10 keV spectra are found to be dominated by the thermal synchrotron self-Compton process rather than by thermal Comptonization of UV disc radiation. 相似文献
17.
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
18.
E. R. Seaquist 《Astrophysics and Space Science》1972,15(2):284-292
It is shown that relativistic electrons in the presence of circularly polarized hydromagnetic waves emit synchrotron radiation which is partially circularly polarized. The relation between the degree of polarization of the radiation and the energy density and wavelength of the waves is derived, and the factors determining the sense of polarization are discussed. Waves of the type required are generated by pitch angle anisotropies in a relativistic electron gas. An application of the result to conditions expected in quasistellar objects shows that the degree of circular polarization of synchrotron radiation in these objects may be of order 1% or greater. 相似文献
19.
The physical properties of six decametric storms, observed at Clark Lake Radio Observatory are studied. The height of the storm continuum sources was determined from the rotation rate. Assuming that the radiation originates at the plasma frequency we computed the gradient of electron density for the regions where the storms originate. The mean angular size of the decametric continuum sources is large; it increases with decreasing frequency. The storm continuum is found to be strongly directive toward the disk center. The east-west asymmetry, well observed at meter wavelengths is also observed at decameter wavelengths. The occurrence of two distinct classes of type III bursts in storms is discussed: ‘off-fringe’ and ‘onfringe’ type Ill's. The ‘off-fringe’ Ill's are found to be displaced in position from the continuum source ; on the other hand, the ‘on-fringe’ ones coincide in position with the continuum. These two kinds of bursts differ in other properties as well. A model of the storm region is proposed. The continuum radiation and the ‘on-fringe’ type III's are believed to originate above closed magnetic loops, in regions of diverging field lines; the ‘offfringe’ type Ill's are thought to be excited by energetic electron streams, having access to open magnetic field lines at the base of the loops. 相似文献
20.
R. D. Robinson 《Solar physics》1978,60(2):383-398
Properties of 23 moving type IV bursts observed with the Culgoora Radioheliograph are summarized. Both shock and plasmoid models are examined. It is found that the theories invoking shocks have limited application and that plasmoid models have several problems with regard to plasmoid formation as well as with explanations for multiple sources and large values of circular polarization. While the synchrotron radiation mechanism is the most widely accepted for both shock and plasmoid models, it is possible that Langmuir wave emission processes may be important, at least in some events. To overcome some of the difficulties of the plasmoid theory, a new source model is proposed. This model involves synchrotron radiation from electror ; confined by rapid scattering through hydromagnetic wave particle interactions.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST-74-04129 with the National Science Foundation. 相似文献