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1.
P. A. Robinson 《Solar physics》1991,134(2):299-314
A new model is developed for electron-cyclotron maser emission from flaring loops, which incorporates competition between driving of the instability and maser-induced relaxation, together with interactions between small neighboring regions of unstable plasma. This results in a picture in which radiation is emitted in bursts from regions whose length scale is determined self-consistently by previous bursts, while the unstable plasma fluctuates about the point, close to marginal stability, at which driving of the instability is balanced by relaxation due to maser-induced electron diffusion. Under the conditions applicable to flaring loops, time scales of fundamental x-mode (x1) driving and saturation are approximately equal at 1 ms, resolving a (104–106)-fold discrepancy in previous models and agreeing with the observed time scales of microwave spike bursts. Saturation effects are found to be especially effective in suppressing amplification of the most strongly growing modes. This suppression enables fundamental o-mode (o1) and second-harmonic x-mode (x2) emission to compete more effectively against x1 emission for the available free energy than has previously been estimated. Consideration of mode competition, burst time scales, suppression of growth due to overlap between amplification and absorption bands, and escape of radiation through absorption layers to the observer, implies that the observed radiation probably escapes from the corona principally in the o-mode, either emitted directly as o1 radiation or mode converted from x1 emission.  相似文献   

2.
D. B. Melrose 《Solar physics》1989,119(1):143-156
The possibility is explored that fundamental plasma emission in solar radio bursts of types I, II, and III is depolarized due to scattering off low-frequency waves. Three ways in which depolarization might occur are identified: (1) one or several large-angle scatters, (2) several small-angle scatters close to the plasma level, and (3) many small-angle scatters well above the plasma level. It is pointed out that the degree p of polarization (p = 1 initially) may be approximated by p() = cos after one large-angle scatter through an angle , and that for backscatter ( > /2) the sense of polarization changes (from o-mode to x-mode senses). Possibility (2) involves coupling between the o- and x-mode components through their longitudinal parts, and is explored in some detail. The wave vectors k required for the scatterings are identified, and it is suggested that ion-sound waves are suitable for possibility (1) and whistlers for possibility (2). The whistlers may be generated by the streaming electrons themselves.Large-angle scattering is favourable for depolarizing type I emission, as proposed by Wentzel, Zlobec, and Messerotti (1986). Scattering by whistlers near the plasma level is favourable for depolarizing type III bursts. Several predictions are made based on these possibilities.  相似文献   

3.
P. A. Robinson 《Solar physics》1991,136(2):343-360
It is shown that small magnetic perturbations can significantly alter the rates of cyclotron growth, absorption, mode conversion, and refraction because of the sensitive dependence of these processes on the field strength in narrow layers. In particular, growth lengths are increased, absorption depths decreased, mode conversion becomes more effective, and turbulent refraction leads to isotropization of the emission. The criteria for significant effects to occur are derived and it is shown that they can be met by the few-percent field perturbations observed in coronal loops. Relative to the theory of cyclotron-maser emission in smoothly varying plasmas, perturbations enable fundamental o-mode (o1) and second-harmonic x-mode (x2) radiation to saturate more effectively, increase the chance of x1, o1, and x2 radiation escaping to infinity through absorption and mode-coupling windows, and partially isotropize radiation emitted near the x-mode cutoff. It is concluded that o1 and x1 emission are both likely to be present, and that x2 emission is possible under some circumstances. However, x1 radiation can escape only at near-parallel propagation ( 0) or via mode conversion to the o-mode at 90°, whereas o1 and x2 emission can escape for a wide range of angles around = 0 and, under many circumstances, near = 90°.  相似文献   

4.
The electron-cyclotron maser is believed to be the source of microwave spike bursts often observed during solar and stellar flares. Partial absorption of this radiation as it propagates through the corona can produce plasma heating and soft X-ray emission over an extended region. In this paper, the propagation and absorption of the maser radiation during solar flares are examined through linear theory and electro-magnetic particle simulations. It is shown using linear theory that strong absorption of the radiation should occur as it propagates towards the second harmonic layer where the magnetic field is half as strong as in the emission region. Only radiation propagating nearly parallel to the magnetic field in a low-temperature plasma may be able to escape under certain, limited conditions. Finite temperature effects can cause radiation propagating nearly perpendicular to the magnetic field to refract, causing enhanced absorption. Particle simulations are then used to evaluate the nonlinear response of the plasma as the maser radiation propagates through the absorption layer. It is shown that some of the maser radiation is able to escape through a process of absorption below the second harmonic of the local gyrofrequency and re-emission above it. The fraction able to escape is much higher than that predicted by linear theory, although the amount of escaping energy is only a small fraction of the incident energy. The bulk of incident energy goes into the perpendicular heating of the ambient electrons, with the rate of energy absorption showing no signs of leveling off during the simulations. This indicates that the absorption layer does not become optically thin after continuous heating by the maser radiation. A few electrons are accelerated to several tens of keVs as a result of the heating.  相似文献   

5.
When coronal mass ejections (CMEs) interact with the solar corona and the interplanetary medium, emissions at different wavelengths occur. On the basis of study of the various radiation mechanisms of space plasma in the case of absence of CMEs, the radio radiation mechanisms of the plasma close to the Lagrange point L1 and affected by large CMEs from February to August 1999 are statistically analyzed. As shown by the results, the main radiation mechanisms are the Bremsstrahlung, a small amount of cyclotron radiation and a still weaker recombination radiation. Also, solar microwave bursts which are associated with CMEs in the same period are investigated. The results show that the microwave bursts are of the gradual type as well as spike bursts, and that the chief radiation mechanisms are the Bremsstrahlung, cyclotron resonance radiation, plasma radiation and electronic cyclotron maser radiation.  相似文献   

6.
We analyze high-resolution, one-dimensional observations of simple microwave bursts, obtained at 4.9 GHz with the Westerbork Synthesis Radio Telescope in 1980, together with H photographs of the associated flares from the Observatories of Athens and Meudon. In most cases the polarization structure can be interpreted in terms of extraordinary mode emission, taking into account the polarity of the underlying magnetic field and propagation effects, which may lead to inversion of the sense of polarization in the limbward part of the flaring loop. We found evidence for ordinary mode emission in two classes of events. In one class theo-mode comes from regions overlying strong magnetic field, which we interpret in terms of thermal gyroresonance absorption of the extraordinary mode at the third harmonic of the gyrofrequency. In the other class the entire burst emits in theo-mode, which may be attributed to high gyrosynchrotron optical depth.  相似文献   

7.
The shifts of Fraunhofer lines of different chemical elements in a homogeneous medium with a plane monochromatic progressive adiabatic sound waves are derived. The calculations indicate that lines of neutral elements (6 0 14) with lower excitation potentials 0 i= 0–2 eV are red shifted, those with excitation potential 0 i= 4–12 eV are blue shifted, and with 0 i= 3 eV are both blue and red shifted. The lines of ions are shifted toward the blue. The shifts of Fraunhofer lines are found to decrease from the centre of the solar disk to the limb. These results agree qualitatively and quantitatively with observations.  相似文献   

8.
A fully relativistic electron maser is proposed for the explanation of certain non-thermal solar and stellar radio bursts. This mechanism (maser synchrotron) is based on a gyroresonant interaction between waves and electrons of high energies and uses the free energy contained in an electronic distribution function that peaks for energies around 1 MeV.By a calculation of the growth rates of the three electromagnetic modes, we show that the X-mode prevails for values of p/ cup to 2 or 3. This result is very different from the standard cyclotron maser case where such values of p/ clead to quench the X-mode growth. Hence, the synchrotron maser instability appears to be a direct and efficient amplification process for considerably larger physical conditions than the cyclotron maser. In addition, the radiation, emitted over the second gyroharmonic, freely propagates without a strong reabsorbtion at the 2 clayer. All these points can constitute major advantages of this mechanism in an astrophysical context.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.  相似文献   

9.
By combining two two-dimensional subspaces, closed into themselves due to curvature, it is possible to create a model of three-dimensional space of the same properties. If the Universe is a space of this type, its effect is that of a monstrous lens. Close objects are observed to diminish according to the normal law of perspective; however, the remote galaxies are seen to be very highly magnified.The apparent angular size2 of a galaxy is more than the size1 in flat space according to relation:2 =1 cosec , where is the angular distance from the observer to the galaxy. The diameter2 d of a galaxy in curved space must be in the same relation to a diameter1 d with no curvature of space:2 d=1 d cosec . The apparent angular size2 and diameter2 d are distorted shapes in consequence of an optical illusion caused by the spatial curvature.It is necessary to distribute the multitude of galaxies into two parts in accordance with their location on the close or reverse hemihypersphere of the Universe. The minimum of apparent angular size2 of a galaxy of diameter1 d is at the equatorial zone.The most likely candidates for location in the reverse hemi-hypersphere are cD's of apparent supergiant sizes due, probably, to the curvature of space. The existence of supergiant sizes of galaxies is the second indirect proof, besides superluminal velocities, that the Universe is closed into itself through curvature. The third indirect evidence, i.e., inductive confirmation of the same fact, is the superposition of galaxies which need not inevitably be a new alternative to the present theories of collisions, cannibalism, merger, etc.The fourth indirect proof of the positive curvature of the Universe is the occurrence of background radiation, because that must vanish in hyperbolic space irrespective of its origin. The gravitational lens effect acquires another theoretical form, as usual, in the case of remote galaxies, because it is impossible to distinguish between gravitator and lensing image.  相似文献   

10.
Some properties of solar radio bursts observed at the Earth are mainly due to propagation effects in the corona. A radio echo of short-time narrow-band bursts is observed by a decameter radioheliograph on the basis of UTR-2 antenna. Propagation effects are manifested in the marked regular change of the burst intensity-time profile at 25 MHz during a half-rotation of the Sun. A displacement of limb diffuse bursts deep into the solar atmosphere of 1.5 - 2R has been also found during the burst lifetime.  相似文献   

11.
The occurrences of 5772 microwave bursts recorded by the Sagamore Hill and Manilla Solar Radio Observatories over the period January 1968 to July 1970, covering the maximum phase of the current solar cycle at frequencies 2695, 4995 and 8800 MHz and their energy excesses have been examined in relation to the S-component of solar radio emission. The average slowly varying component has been determined by the superposed epoch method commonly known as the Chree analysis. Similar treatment of the bursts, data, mentioned above has been made to examine any probable 27-day variation and the results obtained have been compared with that of the S-component. Further, spectra of the microwave bursts under the so-called spectral type - inverted U, particularly those having a peak at 4995 MHz, have also been examined and compared with the average spectrum of the S-component. Some of the important results obtained from the present analysis are: (1) the nature of variation of both the average number of occurrences and energy excesses of the microwave bursts follow in general the average 27-day variation of the S-component, (2) the number of occurrences and energy excesses of the microwave bursts are comparatively greater in the ascending phase of the 27-day cycle than those in the descending phase, (3) bursts at progressively higher frequencies originate at lower levels in the solar atmosphere than those of the associated S-component, and (4) the average spectrum of the microwave bursts of inverted U spectral type having a peak at 4995 MHz is quite identical in nature to that of the S-component.  相似文献   

12.
Wentzel  Donat G. 《Solar physics》1984,90(1):139-159
The fundamental of type III bursts is only partially polarized, yet all theory for emission near the plasma frequency predicts pure o-mode emission. I argue depolarization is inherent in the burst itself. The o-mode radiation is intensely scattered and mode-converted when it temporarily falls behind its own source and finds itself in the medium that is already disturbed by the electron beam. In particular, mode conversion is very efficient and yet causes only modest angular scattering at the height were p + 0.5.The predicted minimum polarization nearly equals the polarization of the harmonic, as observed. Spike polarization is naturally explained by the earlier arrival of the scattered o-mode. Additional residual polarization depends on the refraction at the site of emission; larger beam velocities imply higher polarization, as observed, because a larger fraction of the radiation escapes without mode-conversion. The polarization at the frequencies where U-bursts reverse is of particular interest.Support is acknowledged from the NSF Solar-Terrestrial Research Program.  相似文献   

13.
We analyze hard and soft X-ray, microwave and meter wave radio, interplanetary particle, and optical data for the complex energetic solar event of 22 July 1972. The flare responsible for the observed phenomena most likely occurred 20° beyond the NW limb of the Sun, corresponding to an occultation height of 45 000 km. A group of type III radio bursts at meter wavelengths appeared to mark the impulsive phase of the flare, but no impulsive hard X-ray or microwave burst was observed. These impulsive-phase phenomena were apparently occulted by the solar disk as was the soft X-ray source that invariably accompanies an H flare. Nevertheless essentially all of the characteristic phenomena associated with second-stage acceleration in flares - type II radio burst, gradual second stage hard X-ray burst, meter wave flare continuum (FC II), extended microwave continuum, energetic electrons and ions in the interplanetary medium - were observed. The spectrum of the escaping electrons observed near Earth was approximately the same as that of the solar population and extended to well above 1 MeV.Our analysis of the data leads to the following results: (1) All characteristics are consistent with a hard X-ray source density n i 108 cm–3 and magnetic field strength 10 G. (2) The second-stage acceleration was a physically distinct phenomenon which occurred for tens of minutes following the impulsive phase. (3) The acceleration occurred continuously throughout the event and was spatially widespread. (4) The accelerating agent was very likely the shock wave associated with the type II burst. (5) The emission mechanism for the meter-wave flare continuum source may have been plasma-wave conversion, rather than gyrosynchrotron emission.  相似文献   

14.
Type III solar radio bursts observed from 3.0 to 0.45 MHz with the ATS-II satellite over the period April–October 1967 have been analyzed to derive two alternative models of active region streamers in the outer solar corona. Assuming that the bursts correspond to radiation near the electron plasma frequency, pressure equilibrium arguments lead to streamer Model I in which the streamer electron temperature derived from collision damping time falls off much more rapidly than in the average corona and the electron density is as much as 25 times the average coronal density at heights of 10 to 50 solar radii (R ). In Model II the streamer electron temperature is assumed to equal the average coronal temperature, giving a density enhancement which decreases from a factor of 10 close to the Sun to less than a factor of two at large distances (> 1/4 AU). When the burst frequency drift is interpreted as resulting from the outward motion of a disturbance that stimulates the radio emission, Model I gives a constant velocity of about 0.35c for the exciting disturbance as it moves to large distances, while with Model II, there is a decrease in the velocity to less than 0.2c beyond 10 R .  相似文献   

15.
C. Sawyer 《Solar physics》1977,51(1):203-215
Optical activity near the time of two microwave negative bursts is distinctive enough to demonstrate a real relation, but does not occur simultaneously with the radio events. Such a loose association between microwave flux decreases and H prominence or filament activity is typical of a sample of 23 negative bursts. A model of a microwave occulting cloud differs from that of an H absorber: the H dark flocculus is relatively low, dense, and compact; the microwave occulter is higher in the corona, larger, and more completely ionized. The two types of absorption are not expected to be seen simultaneously, although they could be separate phases of an ejection of chromospheric material into the corona. The association of microwave decreases with H activity in some cases, and their usual non-simultaneity, is consistent with interpretation of the decrease as absorption, but it does not rule out alternative interpretations such as an intrinsic change in the emission of the microwave source.The location of decrease-associated activity has a suggestive, though not statistically significant, bias toward east limb that leads one toward an absorption interpretation, with asymmetry introduced by a tendency for ejected material to move from the leading part of an active region toward the following part, agreeing with Liszka's observation of asymmetry in the line-of-sight velocities of prominences.  相似文献   

16.
High time-resolution data observed in two periods, respectively, by three frequencies (1.42, 2.84, and 3.67 GHz) or four frequencies (1.42, 2.00, 2.84, and 4.00 GHz) of fast sampling radiotelescopes were processed. Obtained were some significant results showing that during the obviously rising or maximum phases of solar cycle 22, the occurrence frequency of millisecond radio spikes at three or four frequencies decreased with the frequency increase and the highest occurrence frequency was at 1.42 GHz. If we assume the secondx-mode is pre-dominant in the growth rate of ECM instability, we calculate the magnetic intensity of source regions with spike bursts at the four frequencies and interpret the occurrence frequency of millisecond radio spikes on long centimetre and short decimetre wavelengths. Finally, this paper suggests that, owing to the Razing effect, whenf126 MHz the occurrence frequency of millisecond radio spikes starts to decrease.  相似文献   

17.
A new model for solar spike bursts is considered based on the interaction of Langmuir waves with ion-sound waves: l+st. Such a mechanism can operate in shock fronts, propagating from a magnetic reconnection region. New observations of microwave millisecond spikes are discussed. They have been observed in two events: 4 November 1997 between 05:52–06:10 UT and 28 November 1997 between 05:00–05:10 UT using the multichannel spectrograph in the range 2.6–3.8 GHz of Beijing AO. Yohkoh/SXT images in the AR and SOHO EIT images testify to a reconstruction of bright loops after the escape of a CME. A fast shock front might be manifested as a very bright line in T e SXT maps (up to 20 MK) above dense structures in emission measure (EM) maps. Moreover one can see at the moment of spike emission (for the 28 November 1997 event) an additional maximum at the loop top on the HXR map in the AR as principal evidence of fast shock propagation. The model gives the ordinary mode of spike emission. Sometimes we observed a different polarization of microwave spikes that might be connected with the depolarization of the emission in the transverse magnetic field and rather in the vanishing magnetic field in the middle of the QT region. Duration and frequency band of isolated spikes are connected with parameters of fast particle beams and shock front. Millisecond microwave spikes are probably a unique manifestation of flare fast shocks in the radio emission.  相似文献   

18.
The variation in the polarization of radiation propagating in a magnetoactive plasma due to the Faraday effect and differential absorption of ordinary and extraordinary waves is considered. This problem is especially important for polarization studies of the distributed cosmic emission, the radiation of discrete sources, etc. An Equation (1.10) describing the variation of the polarization tensor (1.2) (or (1.2a)) along the direction of propagation is formulated. This equation correctly accounts for the effect of absorption in distinction to the corresponding equation ofKawabata (1964). Equation (1.10), which was obtained for a homogeneous medium, is also true for an inhomogeneous plasma when the geometrical optics approximation is valid for the radiation, the difference between the refractions of ordinary and extraordinary waves is negligible, and inequalities (1.13) are satisfied. In this case, however, the tensorsS iq ,R iqlm , andK iqlm in (1.10) will depend on the coordinate.The case of quasi-longitudinal propagation for circularly polarized ordinary and extraordinary waves is treated in detail by means of (1.10). In this case, which is frequently realized in a cosmic plasma, the equations of transfer written in terms of the Stokes parameters (1.3) take the form of (2.3). Their solution for the case of a uniform plasma is obtained as (2.8)–(2.10). From the analysis of these solutions it follows that, if absorption is neglected, the orientation of the polarization ellipse of the radiation emitted in a layer of thicknessz of a magnetoactive plasma varies according to (2.20), i.e. twice as slowly as the angle of radiation incident on the layer (see (2.15)). In the presence of absorption the polarization ellipse ceases to rotate at a distance from the beginning of the layer (K e, 0 is the amplitude of the absorption coefficient of the extraordinary wave). If the Faraday effect is not important (see (2.24)), the angle is close to the ellipse orientation of sources in the plasma S . For a strong Faraday effect (2.24a) the angle is displaced relative to S by ±/4.The character of polarization of radiation in a plasma changes abruptly if the conditions for negative re-absorption are satisfied (K e, o<0). For strong amplification within a source of dimensionsL and a marked difference in re-absorption of ordinary and extraordinary waves , the radiation emitted by the source belongs entirely to one type of wave; the polarization of this radiation is completely defined by the polarization of waves of this type in a cosmic plasma and does not depend directly on the polarization of radiation emitted by individual electrons of this source. The latter circumstance is of great importance for a treatment of the polarization characteristics of radio emission from cosmic sources with negative re-absorption.Translated from the Russian by Dean F. Smith.  相似文献   

19.
S. R. Kane 《Solar physics》1972,27(1):174-181
Observations of impulsive solar flare X-rays 10 keV made with the OGO-5 satellite are compared with ground based measurements of type III solar radio bursts in 10–580 MHz range. It is shown that the times of maxima of these two emissions, when detectable, agree within 18 s. This maximum time difference is comparable to that between the maxima of the impulsive X-ray and impulsive microwave bursts. In view of the various observational uncertainties, it is argued that the observations are consistent with the impulsive X-ray, impulsive microwave, and type III radio bursts being essentially simultaneous. The observations are also consistent with 10–100 keV electron streams being responsible for the type III emission. It is estimated that the total number of electrons 22 keV required to produce a type III burst is 1034. The observations indicate that the non-thermal electron groups responsible for the impulsive X-ray, impulsive microwave, and type III radio bursts are accelerated simultaneously in essentially the same region of the solar atmosphere.  相似文献   

20.
Doppler dimming of the Ovi resonance lines (1032 Å, 1037 Å) in an expanding corona is calculated including the pumping effect on the Ovi 1037.61 Å of both Cii lines at 1036.34 Å and 1037.02 Å, and the effect of the width of the absorption profiles of the coronal oxygen ions along the incident radiation. The pumping effect of the Cii line at 1036.34 Å allows us to extend to approximately 450 km s–1 the measurement of solar wind velocities with the Ovi line ratio technique. Since the emissivity ratio of the Ovi doublet depends on the width of the oxygen coronal absorbing profiles, this ratio can provide an accurate measurement of the solar wind velocity in the case that the width of the absorbing profile along the direction of the incident radiation is independently determined. However, if on the one hand the ratio of the emissivities of the Ovi doublet has limitations in probing the wind velocity, on the other hand it can be used as a diagnostics for inferring the velocity distribution of the coronal Ovi ions along the radial, and detecting possible velocity anisotropies. This diagnostics, applied to recent observational results, allows us to infer that the velocity distribution of the oxygen ions is much broader in the direction perpendicular to the magnetic field direction, and that the acceleration of the fast solar wind in the first 2 solar radii is high.  相似文献   

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