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1.
Pohjolainen  S.  Valtaoja  E.  Urpo  S.  Aurass  H. 《Solar physics》1997,173(1):131-149
Two small radio flares following the great gamma-ray burst on 11 June 1991 are studied. We analyse the different association of emission features at microwaves, decimeter waves, and soft and hard X-rays for the events. The first flare has well-defined emission features in microwaves and soft and hard X-rays, and a faint decimetric signature well after the hard X-ray burst. It is not certain if the decimetric event is connected to the burst features. The second event is characterized by an almost simultaneous appearance of hard X-ray burst maxima and decimetric narrowband drift bursts, but soft X-ray emission is missing from the event. With the exception of the possibility that the soft X-ray emission is absorbed along the way, the following models can explain the reported differences in the second event: (1) Microwave emission in the second event is produced by 150 keV electrons spiraling in the magnetic field relatively low in the corona, while the hard X-ray emission is produced at the beginning of the burst near the loop top as thick-target emission. If the bulk of electrons entered the loop, the low-energy electrons would not be effectively mirrored and would eventually hit the footpoints and cause soft X-ray emission by evaporation, which was not observed. The collisions at the loop top would not produce observable plasma heating. The observed decimetric type III bursts could be created by plasma oscillations caused by electron beams traveling along the magnetic field lines at low coronal heights. (2) Microwave emission is caused by electrons with MeV energies trapped in the large magnetic loops, and the electrons are effectively mirrored from the loop footpoints. The hard X-ray emission can come both from the loop top and the loop footpoints as the accelerated lower energy electrons are not mirrored. The low-energy electrons are not, however, sufficient to create observable soft X-ray emission. The type III emission in this case could be formed either at low coronal heights or in local thick regions in the large loops, high in the corona.  相似文献   

2.
A. Gordon Emslie 《Solar physics》1989,121(1-2):105-115
We review the somewhat questionable concept of an isolated flare loop and the various physical mechanisms believed to be responsible, to some degree, for energy transport within the loop structure. Observational evidence suggests a predominant role for high-energy electrons as an energy transport mechanism, and we explore the consequences of such a scenario in some detail, focusing on radiation signatures in the soft X-ray, hard X-ray, and EUV wavebands, as observed by recent satellite observatories. We find that the predictions of flare loop models are in fact in excellent agreement with these observations, reinforcing both the notion of the loop as a fundamental component of solar flares and the belief that electron acceleration is an integral part of the flare energy release process.  相似文献   

3.
This study has been motivated by the detection of a small number of optically thin microwave bursts with maximum emission near the loop top, which is contrary to the prediction of isotropic gyrosynchrotron models. Using Nobeyama Radioheliograph (NoRH) high-spatial-resolution images at 17 and 34 GHz, we study the morphology at the radio peak of 104 flares that occurred relatively close to the limb. Using data from the Nobeyama Polarimeter we were able to determine whether the 17- and 34-GHz emissions came from optically thin or thick sources. We identified single-loop events, taking into account supplementary information from EUV and soft X-ray (SXR) images. We found optically thin emission from the top of the loop in 36% of single-loop events. In agreement with standard models, in this sample 46% and 18% of the events showed optically thin emission from the footpoints and optically thick emission from the entire loop, respectively. The derived percentage of events with gyrosynchrotron emission from isotropic populations of energetic electrons is possibly an upper limit. This point is illustrated by the analysis of an optically thin event that shows footpoint emission during the rise phase and loop-top emission during the decay phase. A model that takes into account both anisotropies in the distribution function of nonthermal electrons and time evolution can reproduce the observed transition from footpoint to loop-top morphology, if electrons with pitch-angle anisotropy are injected near one of the footpoints.  相似文献   

4.
The structure and evolution of 26 limb flares have been observed with a soft X-ray telescope flown on Skylab. The results are:
  1. One or more well defined loops were the only structures of flare intensity observed during the rise phase and near flare maximum, except for knots which were close to the resolution of the telescope in size (≈2 arc seconds) and whose structure can therefore not be determined.
  2. The flare core features were always sharply defined during the rise phase.
  3. For the twenty events which contain loops, the geometry of the structure near maximum was that of a loop in ten cases, a loop with a spike at the top in four cases, a cusp or triangle in four cases, and a cusp combined with a spike in another two cases.
  4. Of the fifteen cases in which sufficient data were available to allow us to follow a flare's evolution, five showed no significant geometrical deviation from a loop structure, one displayed little change except for a small scale short-lived perturbation on one side of the loop 10 seconds before a type III radio burst was observed, eight underwent a large scale deformation of the loop or loops on a time scale comparable to that of the flare itself and one double loop event changed in a complex and undetermined manner, with reconnection being one possibility.
Based on observation of the original film, it is suggested that the eight flares which underwent large scale deformations had become unstable to MHD kinks. This implies that these flares occurred in magnetic flux tubes through which significant currents were flowing. It is suggested that the high energy electrons responsible for type III bursts accompanying these flares could have been accelerated by the V x B electric field induced by a small scale short-lived perturbation of parts of a flaring flux tube, similar to the one perturbation which was observed having these characteristics.  相似文献   

5.
The magnetic field structure of five flares observed by HINOTORI spacecraft is studied. The double source structure of impulsive flares seems to indicate hard X-ray emission from the two footpoints of a flaring loop, but the potential field computation does not reproduce a loop connecting the two sources. Therefore the magnetic field could be in a sheared configuration and the force-free field modeling would be the next step to examine. On the other hand gradual flares are characterized by hard X-ray sources located in the corona, 2–4 x 104 km above the photosphere. The potential field modeling is found to give a reasonable fitting in this type of flares, and the hard X-ray sources are located at the top of the magnetic loop or arcade. This configuration is consistent with the thick-target trap model of the hard X-ray bursts.  相似文献   

6.
We present three-dimensional (3D) hydrodynamical simulations of ram pressure stripping of massive disc galaxies in clusters. Studies of galaxies that move face-on have predicted that in such a geometry the galaxy can lose a substantial amount of its interstellar medium. But only a small fraction of galaxies is moving face-on. In this work we focus on a systematic study of the effect of the inclination angle between the direction of motion and the galaxy's rotation axis.
In agreement with some previous works, we find that the inclination angle does not play a major role for the mass loss as long as the galaxy is not moving close to edge-on (inclination angle ≲60°). We explain this behaviour by extending Gunn & Gott's estimate of the stripping radius, which is valid for face-on geometries, to moderate inclinations.
The inclination plays a role as long as the ram pressure is comparable to pressures in the galactic plane, which can span two orders of magnitude. For very strong ram pressures, the disc will be stripped completely, and for very weak ram pressures, mass loss is negligible independent of inclination. We show that in non-edge-on geometries the stripping proceeds remarkably similar. A major difference between different inclinations is the degree of asymmetry introduced in the remaining gas disc.
We demonstrate that the tail of gas stripped from the galaxy does not necessarily point in a direction opposite to the galaxy's direction of motion. Therefore, the observation of a galaxy's gas tail may be misleading about the galaxy's direction of motion.  相似文献   

7.
Low energy protons (< 300 keV) can enter the field of view of X-ray telescopes, scatter on their mirror surfaces at small incident angles, and deposit energy on the detector. This phenomenon can cause intense background flares at the focal plane decreasing the mission observing time (e.g. the XMM-Newton mission) or in the most extreme cases, damaging the X-ray detector. A correct modelization of the physics process responsible for the grazing angle scattering processes is mandatory to evaluate the impact of such events on the performance (e.g. observation time, sensitivity) of future X-ray telescopes as the ESA ATHENA mission. The Remizovich model describes particles reflected by solids at glancing angles in terms of the Boltzmann transport equation using the diffuse approximation and the model of continuous slowing down in energy. For the first time this solution, in the approximation of no energy losses, is implemented, verified, and qualitatively validated on top of the Geant4 release 10.2, with the possibility to add a constant energy loss to each interaction. This implementation is verified by comparing the simulated proton distribution to both the theoretical probability distribution and with independent ray-tracing simulations. Both the new scattering physics and the Coulomb scattering already built in the official Geant4 distribution are used to reproduce the latest experimental results on grazing angle proton scattering. At 250 keV multiple scattering delivers large proton angles and it is not consistent with the observation. Among the tested models, the single scattering seems to better reproduce the scattering efficiency at the three energies but energy loss obtained at small scattering angles is significantly lower than the experimental values. In general, the energy losses obtained in the experiment are higher than what obtained by the simulation. The experimental data are not completely representative of the soft proton scattering experienced by current X-ray telescopes because of the lack of measurements at low energies (< 200 keV) and small reflection angles, so we are not able to address any of the tested models as the one that can certainly reproduce the scattering behavior of low energy protons expected for the ATHENA mission. We can, however, discard multiple scattering as the model able to reproduce soft proton funnelling, and affirm that Coulomb single scattering can represent, until further measurements at lower energies are available, the best approximation of the proton scattered angular distribution at the exit of X-ray optics.  相似文献   

8.
L. Györi 《Solar physics》1989,120(2):421-430
For a perfectly aligned Coudé heliograph the north direction of Sun's image in the image plane of the heliograph changes linearly with the hour angle of the Sun or in a common heliograph it is constant. But if the alignment is not perfect and there are instrumental errors the angle between the north direction of the Sun's image and a direction fixed in the image plane is a complicated function of the hour angle of the Sun. In this paper we derive this dependence.  相似文献   

9.
In this paper, the 3B flare of February 4, 1986 is studied comprehensively. The escape electrons accelerated to 10–100 keV at the top of coronal loop are confirmed by III type bursts. The energetic electron beams moved downward trigger the eruptions in the low layer of solar atmosphere. The radio and soft X-ray bursts are interpreted, respectively, by the maser mechanism and evaporation effect. Finally, the important role of energetic electron beams in solar flares is pointed out.  相似文献   

10.
Observations using the Bent Crystal Spectrometer instrument on the Solar Maximum Mission show that turbulence and blue-shifted motions are characteristic of the soft X-ray plasma during the impulsive phase of flares, and are coincident with the hard X-ray bursts observed by the Hard X-ray Burst Spectrometer. A method for analysing the Ca xix and Fe xxv spectra characteristic of the impulsive phase is presented. Non-thermal widths and blue-shifted components in the spectral lines of Ca xix and Fe xxv indicate the presence of turbulent velocities exceeding 100 km s-1 and upward motions of 300–400 km s-1.The April 10, May 9, and June 29, 1980 flares are studied. Detailed study of the geometry of the region, inferred from the Flat Crystal Spectrometer measurements and the image of the flare detected by the Hard X-ray Imaging Spectrometer, shows that the April 10 flare has two separated footpoints bright in hard X-rays. Plasma heated to temperatures greater than 107 K rises from the footpoints. During the three minutes in which the evaporation process occurs an energy of 3.7 × 1030 ergs is deposited in the loop. At the end of the evaporation process, the total energy observed in the loop reaches its maximum value of 3 × 1030 ergs. This is consistent with the above figures, allowing for loss by radiation and conduction. Thus the energy input due to the blue-shifted plasma flowing into the flaring loop through the footpoints can account for the thermal and turbulent energy accumulated in this region during the impulsive phase.On leave from Torino University, Italy.  相似文献   

11.
We explore the 3–8 keV X-ray source motion along the loop legs in two solar flares observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on August 12 and November 28, 2002. Firstly, an artificial loop is constructed to have an outline with a fixed width wide enough to cover the X-ray sources at an energy band between 3–60 keV and at various times. Secondly, RHESSI images are reconstructed at 15 energy bands with an 8 s integration window but 1 s cadence. Thirdly, the X-ray source motions are traced from the brightness distribution along the flare loop. We find that these two events tend to start as a single source at 3–8 keV around the loop top, and then separate into two which move downward along the loop legs. These two almost reach the feet of the loop at the hard X-ray (i.e. at 25–50 keV) peak. After that, the two sources move back upward to the loop top and merge together at the same position where they began. The typical timescale is about ～70 s, and the maximum speed can reach 1000 km?s?1. Such a downward-to-upward motion along the loop is rarely seen in the observations, and it seems to be consistent with the density evolution at the loop top, first decreasing after heating and then increasing due to evaporation.  相似文献   

12.
High-resolution images of the decay phase of a soft X-ray flare observed by the S-054 experiment on Skylab are compared with interferometric scans of the radio burst obtained simultaneously at 2.8 cm (Felli et al., 1975). The spatial resulution of the radio instrument in one direction, although lower than the X-ray telescope resolution, is high enough for a detailed comparison. The comparison clarifies the relationship between the sources of soft X-ray and thermal radio emission in solar flares. The X-ray emission is localized in a loop-like structure which appears spatially coincident with the rapidly varying component of the radio burst. The more stable components of the radio source, which do not appear to contribute substantially to X-ray emission, are found to be spatially associated with the extremes of the X-ray loop. A model of plasma-filled loops is suggested which accounts for the emissions in both spectral ranges and for their spatial location and temporal development.On leave from Osservatorio Astrofisico di Arcetri, Florence, Italy.  相似文献   

13.
We present a detailed classification of the X-ray states of Cyg X-3 based on the spectral shape and a new classification of the radio states based on the long-term correlated behaviour of the radio and soft X-ray light curves. We find a sequence of correlations, starting with a positive correlation between the radio and soft X-ray fluxes in the hard spectral state, changing to a negative one at the transition to soft spectral states. The temporal evolution can be in either direction on that sequence, unless the source goes into a very weak radio state, from which it can return only following a major radio flare. The flare decline is via relatively bright radio states, which results in a hysteresis loop on the flux–flux diagram. We also study the hard X-ray light curve, and find its overall anticorrelation with the soft X-rays. During major radio flares, the radio flux responds exponentially to the level of a hard X-ray high-energy tail. We also specify the detailed correspondence between the radio states and the X-ray spectral states. We compare our results to those of black hole and neutron star binaries. Except for the effect of strong absorption and the energy of the high-energy break in the hard state, the X-ray spectral states of Cyg X-3 closely correspond to the canonical X-ray states of black hole binaries. Also, the radio/X-ray correlation closely corresponds to that found in black hole binaries, but it significantly differs from that in neutron star binaries. Overall, our results strongly support the presence of a black hole in Cyg X-3.  相似文献   

14.
Processes leading to the excitation of soft X-ray line spectra are discussed in relation to their thermal or non-thermal nature. Through analysis of calcium spectra from the XRP experiment on SMM, it is shown that the ionization balance during the gradual phase of flares is effectively in the steady-state. A search of suitable complex flares with multiple impulsive features has shown indications of soft X-ray line intensity anomalies, consistent with the presence of a non-thermal electron component.  相似文献   

15.
We calculate the spatial structure of hard X-ray emission during the impulsive phase of electron-heated solar flares. Both direct non-thermal bremsstrahlung and the thermal bremsstrahlung arising from the heated plasma are considered. Our results indicate that the spread of non-thermal emission into the upper parts of the loop, through evaporation of the chromospheric target, may be more important than the appearance of a hot thermal source in the corona. The effects of varying the viewing angle to the flare loop, and of finite-size resolution element, are also considered, and we compare our results with observations from the Solar Maximum Mission Hard X-Ray Imaging Spectrometer. We also contrast the predicted structures with those predicted by other models of flare energy release, and it is found that the electron-heated model provides the most satisfactory agreement with the observations.On leave from: Department of Physics and Astronomy, The University, Glasgow G12 8QQ, Scotland, U.K.Presidential Young Investigator.  相似文献   

16.
To better understand long-term flare activity, we present a statistical study on soft X-ray flares from May 1976 to May 2008. It is found that the smoothed monthly peak fluxes of C-class, M-class, and X-class flares have a very noticeable time lag of 13, 8, and 8 months in cycle 21 respectively with respect to the smoothed monthly sunspot numbers. There is no time lag between the sunspot numbers and M-class flares in cycle 22. However, there is a one-month time lag for C-class flares and a one-month time lead for X-class flares with regard to sunspot numbers in cycle 22. For cycle 23, the smoothed monthly peak fluxes of C-class, M-class, and X-class flares have a very noticeable time lag of one month, 5 months, and 21 months respectively with respect to sunspot numbers. If we take the three types of flares together, the smoothed monthly peak fluxes of soft X-ray flares have a time lag of 9 months in cycle 21, no time lag in cycle 22 and a characteristic time lag of 5 months in cycle 23 with respect to the smoothed monthly sunspot numbers. Furthermore, the correlation coefficients of the smoothed monthly peak fluxes of M-class and X-class flares and the smoothed monthly sunspot numbers are higher in cycle 22 than those in cycles 21 and 23. The correlation coefficients between the three kinds of soft X-ray flares in cycle 22 are higher than those in cycles 21 and 23. These findings may be instructive in predicting C-class, M-class, and X-class flares regarding sunspot numbers in the next cycle and the physical processes of energy storage and dissipation in the corona.  相似文献   

17.
Morita  Satoshi  Uchida  Yutaka  Hirose  Shigenobu  Uemura  Shuhei  Yamaguchi  Tomotaka 《Solar physics》2001,200(1-2):137-156
In February 1992, three flares, which we consider constitute a homologous flare series (flares having basically the same configuration repeating in the same situation), occurred in the active region NOAA 7070 and were observed by Yohkoh SXT. In the present paper, we first discuss the homology of these three flares, and derive the 3D structure by making use of the information obtained from the three different lines of sight at common phases. The result of this analysis made clear for the first time that the so-called `cusped arcade' at the maximum phase in the well-known 21 February 1992 flare is, contrary to the general belief, an `elongated arch' created at the beginning of the flare, seen with a shallow oblique angle. It is not the `flare arcade' seen on axis as widely conceived. This elongated arch roughly coincides with a diagonal of the main body of the soft X-ray arcade that came up later. The magnetic structure responsible for the flare as a whole turned out to be a structure with quadruple magnetic sources – with the third and fourth sources also playing essential roles. The observationally derived information in our paper provides strong restrictions to the theoretical models of the process occurring in arcade flares.  相似文献   

18.
E. W. Cliver 《Solar physics》1983,84(1-2):347-359
Observations are presented for several large solar flares in which a timing association is observed between late (? 30 min after the flash phase) microwave peaks and late stationary decametric continua. It is suggested that the late microwave peaks are a phenomenon of the post flare loop (relaxation) stage of large flares and are caused by field line reconnections occurring above the Hα and soft X-ray emitting loops. A simple model to account for the association between the secondary radio peaks observed at discrete frequencies and the late decametric continua is proposed.  相似文献   

19.
X-ray observations of the solar corona show that it is comprised of three-dimensional magnetic structures which appear to be primarily in the form of fluxtubes or loops. Imaging the X-ray corona has led to a greater understanding of the dynamical behaviour of and the energy distribution in these magnetic structures. However, imaging observations, by their very nature, integrate along the line of sight resulting in a two-dimensional representation of the actual three-dimensional distribution. The optically thin nature of the solar corona to X-ray radiation makes the integrated images particularly difficult to interpret. The analysis of the two-dimensional observations must, therefore, inlcude the effect of the orientation of the coronal structure to the line-of-sight direction; a fact which is almost always ignored. In this paper we discuss the effect of loop orientation on the two-dimensional representation and argue that these effects may lead to a misinterpretation of the physics occurring in the structures observed. In particular, we discuss observations taken by the Soft X-ray Telescope (SXT) on board the Yohkoh satellite, taking account of the instrumental thermal response, spatial resolution, and point-spread-function.We test the effect of geometry on the determination of the loop pressure by considering equatorial loops at various longitudes and discuss the implications of this for studies of coronal soft X-ray loops.  相似文献   

20.
The power-law frequency distributions of the peak flux of solar flare X-ray emission have been studied extensively and attributed to a system having self-organized criticality(SOC).In this paper,we first show that,so long as the shape of the normalized light curve is not correlated with the peak flux,the flux histogram of solar flares also follows a power-law distribution with the same spectral index as the powerlaw frequency distribution of the peak flux,which may partially explain why power-law distributions are ubiquitous in the Universe.We then show that the spectral indexes of the histograms of soft X-ray fluxes observed by GOES satellites in two different energy channels are different:the higher energy channel has a harder distribution than the lower energy channel,which challenges the universal power-law distribution predicted by SOC models and implies a very soft distribution of thermal energy content of plasmas probed by the GOES satellites.The temperature(T) distribution,on the other hand,approaches a power-law distribution with an index of 2 for high values of T.Hence the application of SOC models to the statistical properties of solar flares needs to be revisited.  相似文献   

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