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1.
High-resolution observations of solar radio bursts made simultaneously with multi-element compound interferometers at 3.75 and 9.4 GHz are presented.Preliminary results are: (1) The burst of December 16, 1967 showed a change in polarization distribution in the radio source, which suggests a magnetic field change in the source. (2) The existence of the multi-source burst is also confirmed at 3.75 GHz. (3) The source size of the impulsive burst is estimated to be 0.'5. (4) Among the GRF bursts there seem to be two kinds; one has a large angular size and the other has a small one.A brief discussion is given of the burst of December 16, 1967. 相似文献
2.
Hard X-ray and radio observations lead to the conclusion that production of non-thermal electrons is a common phenomenon of the active Sun. A preliminary analysis of three hard X-ray bursts observed with the OGO-5 satellite and the radio observations reported in the literature indicates that non-thermal particles are present in the flare region prior to the impulsive (flash) phase and also during the gradual rise and fall (GRF) bursts which are usually explained in terms of purely thermal radiation. The principal difference between the non-thermal electrons observed before the flash phase and during the flash phase appears to be in their total number rather than in the hardness of their energy spectrum. This indicates that the basic characteristics of the two acceleration processes are probably similar although the total energy converted into non-thermal electrons is considerably larger in the flash phase than in the build-up phase. Transient absorbing H features and filament activations are discussed in terms of their ability to produce energetic particle events and magnetic energy release.Presently at the Space Sciences Laboratory, University of California, Berkeley and Institute of Plasma Research, Stanford University, Stanford, California. 相似文献
3.
Gradual rise-and-fall (GRF) microwave bursts and long duration soft X-ray events (LDEs) are generally accompanied by solar coronal mass ejections (CMEs). We use reports from the Ottawa and Penticton stations to examine the annual variations from 1965 to 1985 of 10.7 cm GRF bursts with total durations of at least 4 hr. The annual numbers of such bursts are well correlated with the quiet-Sun 10.7 cm flux densities. This result is in contrast with the finding of Koomen et al. (1985) that the annual numbers of 4 hr GOES soft X-ray events are not well correlated with sunspot numbers. We show that the latter result is biased by the large variation of the quiet-Sun X-ray background throughout the solar cycle. Four-hour events are more easily detected in X-ray data than in 10.7 cm data at solar minimum, but, conversely, these events are much more easily detected in 10.7 cm data around solar maximum. About 70% of the most energetic CMEs are associated with 4 hr X-ray or 10.7 cm bursts. A one-to-one relationship does not exist between CMEs and either LDEs or GRF bursts viewed in full-Sun detectors. 相似文献
4.
Peak flux spectra of solar radio bursts in a wide frequency band have been statistically determined for different morphological types of bursts, for various ranges of magnetic field of the burst-associated sunspots and also for the bursts occurring in the central and limb region of the solar disk. Important results obtained are: (i) The generalised spectra have two peaks, one near to meter-wave and the other in the centimeter-wave region, the former peak being more pronounced than the latter; (ii) identical spectral shape is observed for the great and impulsive types and also for GRF and PBI types of bursts; (iii) the radio emission intensity is relatively higher in the central part than that in the limb part of the solar disk for frequencies 1–10 GHz, while the reverse is true for frequencies 0.245–1 GHz and 10–35 GHz; (iv) the optical depth of the absorbing layer above the source of a burst is found to be the same for meter to centimeter-wavelength bursts, implying that the radio sources in this wide band have uniform characteristics with respect to optical thickness; (v) in case of simultaneous emission in the dekameter to X-ray band, most of the decimetric bursts are seen to be very prompt and coincident with the associated flare's starting time. The interpretations of the obtained spectra give an insight into the possible generation mechanisms, pointing to the location of the source region in the solar atmosphere. 相似文献
5.
K. Kai H. Nakajima T. Kosugi R. T. Stewart G. J. Nelson S. R. Kane 《Solar physics》1986,105(2):383-398
Evidence for a delayed acceleration process in solar flares is presented in the form of an analysis of simultaneous observations in microwaves, decimetre and metrewaves, and hard X-rays of six delayed gradual bursts which appear 0.5–1 hr after the strong main bursts have faded. The observed characteristics of the delayed bursts are: (a) similarity of flux time profiles at all the wavelengths, (b) low turn-over frequency (4 GHz) of the microwave spectrum, (c) moderately strong circular polarization (30–40%) and low altitude of the microwave source (which is displaced toward the disk centre by a projected distance of 10–20 from that of the preceding main burst), and (d) low spectral index of the energy spectrum of hard X-rays.From these observations it is suggested that (i) electrons are accelerated up to MeV even some tens of minutes after the impulsive phase acceleration has almost ceased, (ii) the delayed acceleration occurs in a large magnetic structure extending to a height of at least 2 × 105 km, and (iii) the radio source has columnar structure with the microwave source predominantly near a leg or legs and the metrewave source near the top of the magnetic structure. The present observations of the delayed bursts do not seem to be consistent with the classical second-phase acceleration mechanism proposed in the past for normal hard X-ray gradual (extended) bursts.Minamimaki-mura, Minamisaku-gun, Nagano-ken 384-13, Japan.Greenbelt, MD 20771, U.S.A., NASA/NRC Research Associate, on leave from Tokyo Astronomical Observatory.P.O. Box 76, Epping, N.S.W. 2121, Australia.Berkeley, CA 94720, U.S.A. 相似文献
6.
A new magnetodynamic model for loop flares is proposed to explain the following observational facts obtained from space during the last solar activity maximum: (i) Blueshifted lines of Ca xix and Fe xxv appear in some cases a minute or so before the initiation of impulsive bursts and relax into the unshifted lines with large width by the time of the onset of impulsive bursts, (ii) the hot source is formed by that time at the top of a loop-like structure, and confined there for a considerable time, and (iii) -ray line enhancement occurs at about the same time as hard X-ray spikes.In our model, the supply of energy to the loop top comes from below the chromosphere immediately before the flare (30 s-1 min before the hard X-ray impulsive bursts) in the form of the relaxing fronts of magnetic twist of opposite sign. These packets are thought to be built up in the process of loop emergence, stored at the footpoints of the loop below the photosphere, and released when the part of the feet floats up further. These released packets of magnetic twist drive the mass in the high chromosphere and transition zone into helical flows with pinch heating, and when these collide at the top of the loop, a very hot region appears there with a violent unwinding of the twists, resulting in the rapid dynamical annihilation of the magnetic energy,
. Electrons and ions, raised to medium energies in the pinch at the incidence of the packets to the loop, are accelerated further by the Fermi-I mechanism between the approaching fronts of magnetic twist, and when B
is weakened by unwinding they are released towards the chromosphere, and cause simultaneous -ray and hard X-ray bursts. 相似文献
7.
Analyzing 205 radio bursts observed by the Ondejov radiospectrograph in the 1–4 GHz frequency range during 1992 and 1993, we found 6 examples of type II-like radio bursts coinciding with impulsive phases of solar flares. These bursts were interpreted as radio manifestations of MHD (shock) waves generated during impulsive phases of flares in the vicinity of the transition region. Assuming a magnetic-field perturbation origin of these waves, we studied pinch processes in the current sheet near the transition region. In the 2-D MHD numerical model of this current sheet we demonstrated that 2-D pinch processes induced by radiative losses can trigger the impulsive phase of some flares and so generate the observed high-frequency type II-like radio bursts. 相似文献
8.
A morphological study is made for the hard X-ray images (25–50 keV) of nine impulsive bursts observed by Hinotori. Most of them revealed single sources, either extended or compact, during the whole duration of the bursts. The sources of all of four spike bursts in the present sample are compact. After the main phase of the impulsive bursts, generally the source size becomes smaller accompanying a shift of position. The X-ray source size is much greater than that of the Hα kernel in two events out of three. Four possible explanations for the X-ray source to be single are suggested. One of these is the strong electric field along the magnetic field as demonstrated to be produced at the decay of force-free current. 相似文献
9.
Keizo Kai 《Solar physics》1979,61(1):187-199
Thirty-one moving type IV (IV(M)) bursts recorded with the Culgoora radioheliograph are examined to deduce their characteristic features, such as spatial distribution, projected velocity, etc., and their relation to other phenomena. The distribution of the projected velocity suggests that less than 15% of the total IV(M) bursts have fast velocities (>1000 km s–1), almost equal to MHD shock velocity, and that the remaining IV(M) bursts have slower velocities (400 km s–1) and are probably not associated with MHD shock waves. Most of the slow IV(M) bursts (and 70% of the total IV(M) bursts) are of an isolated plasmoid type. Even if they are associated with minor H flares, IV(M) bursts of the isolated-plasmoid type have 1031 ergs in the form of magnetic energy. They are in many cases closely associated with extended flare-continuum sources; this seems plausible if the flare continuum is interpreted as an interaction of a plasmoid with a large-scale magnetic arch.The association of IV(M) bursts with energetic proton events seems to be poor - contrary to expectation. 相似文献
10.
All four large EUV bursts (peak 10–1030 Å flux enhancements 2 ergs cm–2 s–1 at 1 AU as deduced from sudden frequency deviations), for which there were available concurrent white light observations of at least fair quality, were detected as white light flares. The rise times and maxima of the white light emissions coincided with rise times and maxima of the EUV bursts. The frequency of strong EUV bursts suggests that white light flares may occur at the rate of five or six per year near sunspot maximum. All of the white light flare areas coincided with intense bright areas of the H flares. These small areas appeared to be sources of high velocity ejecta in H. The white light flares occurred as several knots or patches of 2 to 15 arc-sec diameter, with bright cores perhaps less than 2 arc-sec diameter (1500 km). They preferred the outer penumbral borders of strong sunspots within 10 arc-sec of a longitudinal neutral line in the magnetic field. The peak continuum flux enhancement over the 3500–6500 Å wavelength range is about the same order of magnitude as the peak 10–1030 Å flux enhancement. 相似文献
11.
On 27 September 1998, Big Bear Solar Observatory (BBSO) and Transition Region and Coronal Explorer (TRACE) coordinated observations from 16:00 to 19:00 UT to study properties of microflares in AR NOAA No. 8340. Fortuitously, a C5.2 flare occurred at 16:30 UT in this active region. H and magnetograph movies were obtained at BBSO; Civ 1550 Å, Feix 171 Å, and Fexii 195 Å movies were obtained by TRACE; both with a cadence about 1 min. In this paper, we concentrate on the study of magnetic properties of 70 Civ microflares, as well as their relationship to the C5.2 flare. We obtained the following results: (1) We found two kinds of microflares: microflares of transient brightenings with a time scale of 1 to 5 min (impulsive events) and microflares lasting half an hour or longer (persistent events). Ninety percent of the microflares are impulsive events. Most of the event in this category are associated with well defined magnetic neutral lines, but some are found in non-neutral line areas. All of seven persistent events are found at parasitic magnetic configurations with inclusions of small magnetic flux within dominant magnetic flux of opposite polarity. (2) More than a third of the impulsive microflares occurred near the C5.2 flare site indicating that a local instability is responsible for both the C5.2 flare and microflares. This indirectly supports the avalanche theory of flare energy release, which implies that a big flare may be spatially associated with many small flares. 相似文献
12.
Tatsuo Takakura 《Solar physics》1972,26(1):151-175
The gyro-synchrotron emission from a model source with a non-uniform magnetic field is computed taking into account the self absorption. This model seems adequate not only to interpret the radio spectrum and its time variation of microwave impulsive bursts but also to solve the discrepancy between the numbers of non-thermal electrons emitting radio burst and those emitting hard X-ray burst.The decrease of flux of radio burst with decreasing frequency at low microwave frequencies is due to the self absorption and/or the thermal gyro-absorption. In this frequency range, the radio source is optically thick even at weak microwave bursts. The weakness of the bursts may be rather due to the small size of the radio source and/or the weakness of the magnetic field than the small number density of the non-thermal electrons.The time variation of the flux of radio burst may be mainly attributed to the variation of source size in a horizontal direction ( direction) instead of the variation of the number density of non-thermal electrons itself, implying that the acceleration region progressively moves in the horizontal direction leaving the non-thermal electrons behind during the increasing phase of the radio burst. 相似文献
13.
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
14.
Recent observations show that the rapid fluctuations in radio, hard X-ray, and H emissions are closely associated with type III and microwave (or decimetric) bursts during the impulsive and/or preimpulsive phases of solar flares.In order to clarify the physical processes of these observed phenomena, this paper proposes a tentative model of two acceleration regions A (source of type III bursts) and B (source of microwave or decimetric bursts) formed in the neutral sheet and at the top of a flaring loop, respectively; and also suggests that the electron beams streaming from region A and/or region B downward to the chromosphere are responsible for the rapid fluctuations in the different emissions mentioned above during the impulsive and/or pre-impulsive phases of solar flares. 相似文献
15.
The source positions of solar radio bursts of spectral types I, III(U) and III(J) and V observed by the Culgoora radioheliograph are found to lie almost radially above soft X-ray loops on pictures taken by the S-056 telescope aboard Skylab. The radio source positions and the X-ray loops occur near magnetic loops on computed potential field maps. However, the magnetic induction required to explain the radio observations is much greater than the computed potential field value at that height. Dense current-carrying magnetic flux tubes emanating from active regions on the Sun and extending to 1.5R
above the photosphere provide a satisfactory model for the radio bursts. 相似文献
16.
Radio-silent -ray flares are solar flares that lack any significant emission in the (non-thermal) radio wave band during their impulsive hard X-ray and -ray emission phases. Flares with extremely suppressed long-wavelength spectra have previously been reported by White et al. (1992) and have been discussed in different context by Hudson and Ryan (1995). A striking example of a radio-silent flare was observed by SMM during the onset of the 6 March 1989 energetic -ray flare. We argue that the absence of radio emission at wavelengths longer than microwave wavelengths is an indication of the compactness of the flare rather than that the flare did not exhibit non-thermal properties. Probably the flare site was restricted to altitudes above the photosphere in a newly emerging loop configuration lower than the equivalent altitude corresponding to an emission frequency of 1.4 GHz. This implies the presence of a dense and highly magnetized closed field configuration confining the electron component which causes the impulsive -ray continuum. Reconnection in such a configuration did not lead to open magnetic fields and streamer formation. Acceleration of particles in the and hard X-ray bursts was restricted to closed field lines. Thermal expansion of the loop system may subsequently lead to the generation of radially propagating blast waves in the solar corona which are accompanied by type II solar radio bursts and decimetre emissions. The emission during the onset of the flare was dominated by a continuum originating from electron bremsstrahlung at X-ray and -ray energies with only little evidence for the presence of energetic ions. It is, therefore, concluded that energetic electrons have been primary and not secondary products of the particle acceleration process. 相似文献
17.
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
18.
We review high-spatial-resolution observations of the Sun which reflect on the role of mode coupling in the solar corona, and present a number of new observations. We show that typically polarization inversion is seen at 5 GHz in active region sources near the solar limb, but not at 1.5 GHz. Although this is apparently in contradiction to the simplest form of mode coupling theory, in fact it remains consistent with current models for the active region emission. Microwave bursts show no strong evidence for polarization inversion. We discuss bipolar noise storm continuum emission in some detail, utilizing recent VLA observations at 327 MHz. We show that bipolar sources are common at 327 MHz. Further, the trailing component of the bipole is frequently stronger than the leading component, in apparent conflict with the leading-spot hypothesis. The observations indicate that at 327 MHz mode coupling is apparently strong at all mode-coupling layers in the solar corona. The 327 MHz observations require a much weaker magnetic field strength in the solar corona to explain this result than did earlier lower-frequency observations: maximum fields are 0.2 G. This is a much weaker field than is consistent with current coronal models.On leave from the Indian Institute for Astrophysics, Bangalore, India. 相似文献
19.
Keizo Kai 《Solar physics》1978,56(2):417-427
Two-dimensional, high-resolution observations of about 30 moving type IV bursts allow us to compare the polarization structure of the radio sources high in the corona with the distribution of magnetic fields measured at the photospheric level. Left- and right-handed circularly polarized moving type IV bursts are associated with active regions dominated by magnetic fields of plus and minus polarity respectively. The result suggests that the polarity of magnetic fields within the type IV source which moves high in the corona ( 1R
above the photosphere) is closely related to the polarity of local magnetic fields at the photosphere. The above relation between the sense of polarization and the polarity of magnetic field is contrary to what would be expected from the generally accepted synchroton hypothesis. One way of resolving this conflict is to postulate that the magnetic field within the radio source has the opposite polarity to that of the ambient magnetic fields. 相似文献
20.
A. Böhme F. Fürstenberg J. Hildebrandt O. Saal A. Krüger P. Hoyng G. A. Stevens 《Solar physics》1977,53(1):139-155
A two-component (core-halo) emission model has been applied reconciling hard and soft X-ray burst emissions with the microwave burst radiation. The core region is represented by a nonthermal energy distribution (Maxwellian+power law tail) and assumed to be surrounded by a thermal halo. Parameters characterizing the energy distribution and emission measures have been derived numerically from soft and hard X-ray measurements. Using an artificial magnetic field model the microwave flux spectrum has been calculated on the basis of gyro-synchrotron emission and absorption by solving the equation of radiation transfer along the ray trajectories. Open parameters were used to adapt the spectrum to the radio measurements.Thus probable informations about the most appropriate magnetic field parameters as well as about the time- and frequency- dependent source diameters (yielding growth velocities of the core region during the impulsive phase) are deduced for the burst of 1972 May 18 as an example. A fit of the observed spectrum at the burst maximum is consistent with a magnetic field of 150O G at the core centre decreasing up to about 40 G at the top of the halo at a height of 50 000 km above the centre, a core density of 1010 cm–3 decreasing to 109 cm–3 at the outer halo boundary, and a core diameter of 15 000 km (]20).Due to the simple geometry and emission process adopted,- the model refers primarily to special impulsive bursts. For the representation of broad band microwave bursts, e.g. type IV , events, a more complex source geometry and/or other variants of the emission mechanism must be invoked. 相似文献