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1.
We present 4.9 GHz observations of an impulsive radio burst observed at the Very Large Array on 1981 May 16. The flare occurred in a complex active region containing several spots. The radio burst lay at the edge of an active-region microwave source, close to a neutral line. The compact burst showed morphological evidence for the presence of two loops in the rise phase, with the subsequent burst peak lying between these loops. This suggests that interaction between the loops played some role in the initiation of the flare. The flare spectrum is consistent with thermal gyrosynchrotron emission. The main microwave peak was displaced from the nearest H kernels by about 10, but there is strong evidence for post-flare loops coincident with the H kernels during the later stages of the event. 相似文献
2.
We present two large flares which were exceptional in that each produced an extensive chain of H emission patches in remote quiet regions more than 105 km away from the main flare site. They were also unusual in that a large group of the rare type III reverse slope bursts accompanied each flare.The observations suggest that this is no coincidence, but that the two phenomena are directly connected. The onset of about half of the remote H emission patches were found to be nearly simultaneous with RS bursts. One of the flares (August 26, 1979) was also observed in hard X-rays; the RS bursts occurred during hard X-ray spikes. For the other flare (June 16, 1973), soft X-ray filtergrams show coronal loops connecting from the main flare site to the remote H brightenings. There were no other flares in progress during either flare; this, along with the X-ray observations, indicates that the RS burst electrons were generated in these flares and not elsewhere on the Sun. The remote H brightenings were apparently not produced by a blast wave from the main flare; no Moreton waves were observed, and the spatially disordered development of the remote H chains is further evidence against a blast wave. From geometry, time and energy considerations we propose: (1) That the remote H brightenings were initiated by direct heating of the chromosphere by RS burst electrons traveling in closed magnetic loops connecting the flare site to the remote patches; and (2) that after onset, the brightenings were heated by thermal conduction by slower thermal electrons (kT1 keV) which immediately follow the RS burst electrons along the same loops. 相似文献
3.
H observations of two-ribbon flares often show secondary brightenings which are not directly spatially connected with the main center of activity but which are correlated in time with the primary impulsive flare. We present here a mechanism which explains these secondary brightenings via the reconnection of magnetic loops which are tied to only one of the two ribbons, in contrast with the loops responsible for the main flare which are tied to both ribbons. The distant footpoint is then interpreted as the site of the secondary brightening. We apply our model to the two-ribbon flare of 17:52 UT, 23 June, 1988, which started during the rocket flight of the Normal Incidence X-ray Telescope.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. Partial support for the National Solar Observatory is provided by the USAF under a Memorandum of Understanding with the NSF. 相似文献
4.
We study the active region NOAA 6718 and the development of a (2N, M3.6) flare in radio and H. Due to our knowledge of the magnetic field structure in the active region we are able to associate the different radio flare burst components with the stages in the H flare evolution. A discussion of the data in terms of chromospheric flare kernel heating reveals that in the present case the observed flare-related radio burst continuum switch-off is caused by the penetration of hot, ablated gas into the coronal radio source. 相似文献
5.
A mm-wave thermal burst has been observed at 73 GHz. The simultaneous observation at 17 GHz revealed that this mm-wave burst was quite a different component from the non-thermal burst co-existing at a cm-wavelength range. Since the source of this burst seemed to be opaque or nearly opaque, the temperature became several tens of thousands degrees. Considering also the similarity between time profiles of the 73 GHz intensity and the H light curve, it is concluded that this mm-wave burst is situated very close to the H flare region. 相似文献
6.
Two distinct types of centimeter solar bursts, classified simple, have been identified and related to the position (with respect to a sunspot) of the related flare. Type S in which the flare occurs directly over the spot has a radio spectrum with a maximum beyond 10 GHz, type P which occurs away from the spot has its maximum occurring near 3 GHz. Considering the structure of the spot magnetic field and invoking the synchrotron effect, it is shown that the origin of the radio burst may be attributed to a burst of energetic electrons with an energy peak near 3 MeV. 相似文献
7.
Gallagher Peter T. Williams David R. Phillips Kenneth J.H. Mathioudakis Mihalis Smartt Raymond N. Keenan Francis P. 《Solar physics》2000,195(2):367-380
We report on observations of a large eruptive event associated with a flare that occurred on 27 September 1998 made with the Richard B. Dunn Solar Telescope at Sacramento Peak Observatory (several wave bands including off-line-center H), in soft and hard X-rays (GOES and BATSE), and in several TRACE wave bands (including Feix/x 171 Å, Fexii 195 Å, and Civ 1550 Å). The flare initiation is signaled by two H foot-point brightenings which are closely followed by a hard X-ray burst and a subsequent gradual increase in other wavelengths. The flare light curves show a complicated, three-component structure which includes two minor maxima before the main GOES class C5.2 peak after which there is a characteristic exponential decline. During the initial stages, a large spray event is observed within seconds of the hard X-ray burst which can be directly associated with a two-ribbon flare in H. The emission returns to pre-flare levels after about 35 min, by which time a set of bright post-flare loops have begun to form at temperatures of about 1.0–1.5 MK. Part of the flare plasma also intrudes into the penumbra of a large sunspot, generally a characteristic of very powerful flares, but the flare importance in GOES soft X-rays is in fact relatively modest. Much of the energy appears to be in the form of a second ejection which is observed in optical and ultraviolet bands, traveling out via several magnetic flux tubes from the main flare site (about 60° from Sun center) to beyond the limb. 相似文献
8.
M. R. Kundu 《Solar physics》1985,100(1-2):491-514
Over the past decade two large arrays — the Westerbork Synthesis Radio Telescope (WSRT) and the Very Large Array (VLA) built primarily for sidereal radio astronomy have been used for solar radio astronomical studies with spatial resolution of a few seconds of arc. In this review, we discuss some results obtained at Maryland using these instruments.The quiet Sun observations made with the WSRT have premitted us to produce synthesized maps of supergranulation network at 6 cm wavelength. The brightness temperatures of typical network elements and cells are respectively 2.5 × 104 K and 1.5 × 104 K; thus the contrast is 1.71 which compares with 1.31 for Ca + K and 20 for L networks. Limb profiles in both equatorial and polar directions have been obtained; limb brightening is observed at both west and south limbs, peak limb temperature being about 40% higher than disk temperature. We have produced synthesized maps of disk filaments which correspond well to H disk filaments and regions of reduced emission in He i 10 830 Å spectroheliograms. Using the WSRT synthesized maps at 6 cm, we have compared the structure of a sunspot associated source with model computations. Using a new method of analysis we have been able to map the vertical as well as the horizontal component of the sunspot magnetic field at specific locations in the low corona. Using the VLA, we have mapped coronal loops at 20 cm; the radio emission is attributed to bremsstrahlung near the loop footpoints whereas gyroresonance process dominates near the loop top. Using the VLA, we have carried out simultaneous observations of a microwave burst at 2 and 6 cm. The 6 cm burst source is apparently located near the top of a flaring loop, while the 2 cm emission originates from the loop footpoints. The 6 cm emission is attributed to gyrosynchrotron radiation of thermal electons in the bulk heated plasma at 4 × 107 K, while the 2 cm emission is due to nonthermal particles released and accelerated during the flare process. A DC electric field flare model appears to explain the observed delay between the peaks at the two wavelengths. From the delay, the strength of the electric field in the flaring region is estimated. 相似文献
9.
Due to the emission of shock-accelerated electrons, broadband radio observations display propagating super Alfvénic shock waves in the low corona ('type II bursts'). We study the 9 July 1996 flare (AR NOAA 7978) focusing on the aspect of shock generation. This event's radio spectrogram shows two different type II bursts in sequence. Radio imaging data (Paris, Meudon Observatory) reveal that both bursts appear at different sites above the H flare. The driver of the first type II burst seems to propagate with twice the speed of the second one. The projected source site of the first type II burst (seen earlier and at higher frequencies) is spatially situated further away from the H flare site than the source of the second type II burst. We try to understand this by comparing with Yohkoh soft X-ray images. The first shock source occurs near the top of high soft X-ray loop structures. Its driver can be a guided fast mode magnetic disturbance. The second type II source appears in-between two high soft X-ray loop systems. This might be a piston-driven disturbance powered by an evaporation front. We get a consistent picture only by assuming a very inhomogeneous Alfvén speed in the active region's atmosphere. 相似文献
10.
The impulsive phase of a large solar limb flare of June 20, 1989 总被引：1，自引：0，他引：1
On 1989 June 20, we observed in H the impulsive phase of a 3B/X1.6 limb flare with high temporal resolution. Line profiles have been acquired every 2.3 s with an imaging spectrograph. During the eruption of a filament we observed in H a moving plasma blob from what we believe to be a second loop which correlated spatially and temporally with a microwave source at 1.4 GHz observed by VLA. A magnetodynamic model is used to understand the development of the moving plasma blob. 相似文献
11.
Takeo Kosugi 《Solar physics》1982,75(1-2):293-304
A proton-event-associated microwave burst occurred on November 10, 1978 and was observed with the 17 GHz interferometer at Nobeyama. The burst had a very broad extent of about 4.5 arc and consisted of at least four separate sources. The time evolutions of the individual sources were almost independent of each other. We suggest that the sources are fallen into two distinct types as follows: (i) The two-ribbon-associated sources are characterized by the source expansion in size and the relatively flat microwave spectrum, both of which can be explained by thermal emission from hot condensed plasma in the magnetic arcades whose legs are seen as the two-ribbon H flare, and (ii) the spot-related sources are characterized by the high polarization degree with a compact unipolar structure, the rapid time variation, and the inverted-U shape microwave spectrum. The intimate relation of the latter sources to the evolution of the associated type IVm-dm burst with spectral fine features is also discussed. 相似文献
12.
We describe observations of three flares made at 5 and 15 GHz with the VLA, two subflares near the limb on 1981 November 21 and 22, and an M7.7 flare on 1981 May 8. Even though the time histories of the November flares indicated simple impulsive bursts, the VLA observed no 5 GHz radiation at all from one flare, and from the other, the 15 GHz radiation emanated from a source which was smaller, lower and displaced from the 5 GHz source. Without the spatial information, we would have derived incorrect results from the assumption that photons of different energy (both at X-ray and radio wavelengths) arose from one homogeneous volume. The 1981 May 8 flare was intense and complex, having two. or more sources at both 5 and 15 GHz. Prior to the peak of the flare, the sources grew in size to > 20″ to 40″, after which they were not visible to the VLA; only (weak) subsources could be seen. These were located between or at the edge of the Hα ribbons and the two hard X-ray sources imaged by the Hinotori. Highly polarized, bursty radiation observed at Toyokawa at 1 and 2 GHz, indicated that an electron-cyclotron maser operated during the flare. We derive 360 to 660 gauss as the maximum field strength in flaring loops. 相似文献
13.
Very-Large-Array (VLA) observations of the Sun at 20, 91 and 400 cm have been combined with data from the SOHO, TRACE and Wind solar missions to study the properties of long-lasting Type I noise storms and impulsive metric and decimetric bursts during solar flares and associated coronal mass ejections. These radio observations provide information about the acceleration and propagation of energetic electrons in the low and middle corona as well as their interactions with large-scale magnetic structures where energy release and transport takes place. For one flare and its associated CME, the VLA detected impulsive 20 and 91 cm bursts that were followed about ten minutes later by 400 cm burst emission that appeared to move outward into the corona. This event was also detected by the Waves experiment on Wind which showed intense, fast-drifting interplanetary Type III bursts following the metric and decimetric bursts detected by the VLA. For another event, impulsive 91 cm emission was detected about a few minutes prior to impulsive bursts at 20.7 cm, suggesting an inwardly propagating beam of electrons that excited burst emission at lower levels and shorter wavelengths. We also find evidence for significant changes in the intensity of Type I noise storms in the same or nearby active region during impulsive decimetric bursts and CMEs. These changes might be attributed to flare-initiated heating of the Type I radio source plasma by outwardly-propagating flare ejecta or to the disruption of ambient magnetic fields by the passage of a CME. 相似文献
14.
An impulsive flare October 24, 1969 produced two bursts with virtually identical time profiles of 8800 MHz emission and X-rays above 48 keV. The two spikes of hard X-rays correspond in time to the times of sharp brightening and expansion in the H flare. The first burst was not observed at frequencies below 3000 MHz. This cut off is ascribed to plasma cutoff above the low-lying flare.A model of the flare based on H observations at Big Bear shows that the density of electrons with energy above 10 keV is 5 × 107 if the field density is 1011. The observed radio flux would be produced by this electron distribution with the observed field of 200 G. The H emission accompanying the hard electron acceleration is presumed due to excitation of the field atoms by the hard electrons. 相似文献
15.
Long-enduring quasi-periodic oscilations (1.5s) superimposed upon a solar burst have for the first time been observed simultaneously at two different mm-wavelengths (22 GHz and 44 GHz). The oscillations were present throughout the burst duration (about 10 min), and were delayed at 44 GHz with respect to 22 GHz by 0.3 s. The relative amplitude of the oscillation was of about 20% at 44 GHz and of about 5% at 22 GHz. Interferometer measurements at 10.6 GHz indicated the burst source position stable within 1 arc sec. An He i D3 line flare showed two persistent small spots separated by about 10 arc sec. The 22/44 GHz burst position corresponds well with the location of the He i D3 spots. The oscillations display features which distinguish them from ultrafast time structures found in other bursts. One possible interpretation is a modulation of the synchrotron emission of trapped electrons by a variable magnetic field on a double burst source, optically thin at 44 GHz and with optical thickness 0.3 at 22 GHz. 相似文献
16.
We present the observation and interpretation of a solar radio burst whose evolution of the source position at 48 GHz has been correlated with microwave spectral observations from 3.1 to 19.6 GHz and H imaging spectrograms. The event of November 19, 1990 showed 4 impulsive peaks in microwaves and 2 H kernels. There exists strong evidence that the impulsive emission has originated from nonthermal electrons including an electron beam during the rising phase of the third microwave peak. The complex evolution of the source position at 48 GHz is attributed to two inhomogeneous and spatially separated sources with changing relative brightness. 相似文献
17.
Microwave observations with exceptionally high spectral resolution are described for a set of 49 solar flares observed between May and October 1981. Total power data were obtained at 40 frequencies between 1 and 18 GHz by the Owens Valley frequency-agile interferometer with 10 s time resolution. Statistical analysis of this sample of microwave bursts established the following significant characteristics of their microwave spectra: (i) Most ( 80%) of the microwave events displayed complex spectra consisting of more than one component during some or all of their lifetime. Single spectral component bursts are rare. It is shown that the presence of more than one component can lead to significant errors when data with low spectral resolution are used to determine the low-side spectral index. (ii) The high-resolution data show that many bursts have a low-side spectral index that is larger than the maximum value of about 3 that might be expected from theory. Possible explanations include the effect of the underlying active region on the perceived burst spectrum and/or the necessity for more accurate calculations for bursts with low effective temperatures, (iii) the peak frequencies of the bursts are remarkably constant during their lifetimes. This is contrary to expectations based on simple models in which the source size and ambient field remain constant during the evolution of a burst.Swiss National Science Foundation Fellow from the University of Bern. 相似文献
18.
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
19.
Repeated raster images of solar active regions taken at the line centers of the Si iv and C iv resonance lines using the University of Colorado (CU) ultraviolet spectrometer aboard OSO-8 reveal dramatic transient brightenings of up to factors of 10. These brightenings last several minutes and frequently show a repetitive character. Inspection of simultaneous H flare patrol records show that these transition zone events are often associated with subflare-like brightenings in the chromosphere. These observations indicate that direct excitation or heating of material already at transition zone temperatures caused by non-thermal particle streams is inadequate to explain the degree of brightening of these lines. The measurements suggest that some process that enhances density of material at 105 K is occurring during these events. 相似文献
20.
A high-resolution microwave spectrometer has been developed by converting the Owens Valley solar interferometer to frequency-agile operation. The system uses 27 m antennas equipped with phase-locked receivers which can change their observing frequency in 25 or 50 ms. Microwave spectra between 1 and 18 GHz are obtained in a few seconds by successive observations at up to 86 discrete frequencies. At each frequency the data are equivalent to the total power from each antenna and the interferometric amplitude and phase. All data are fully calibrated with respect to cosmic sources.The instrument was motivated by the need for better microwave spectral resolution for the study of plasma parameters, non-thermal electrons and coronal magnetic field strengths in solar flares and active regions. Early observations with the system are illustrated by a sequence of flare spectra featuring cases with exceptionally narrow continuum bandwidths. 相似文献