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1.
D. McConnell 《Solar physics》1982,78(2):253-269
Observations of the solar radio spectrum have been made with high time and frequency resolution. Spectra were recorded over six 3-MHz bands between 30 and 82 MHz. The receivers used were capable of time and frequency resolutions of 1 ms and 2 kHz, respectively. A large number of radio bursts exhibiting a variety of find spectral structure were recorded.The bursts, referred to here as S bursts, were observed throughout the 30–82 MHz frequency range but were most numerous in the 33–44 MHz band and were very rare at 80 MHz. On a dynamic spectrum the bursts appeared as narrow sloping lines with the centre frequency of each burst decreasing with time. The rate of frequency drift was about 1/3 that of type III bursts. Most bursts were observed over only a limited frequency range (< 5 MHz) but some drifted for more than 10 MHz. The durations measured at a single frequency and the instantaneous bandwidths of S bursts were small; t = 49 ± 34 ms and f = 123 ± 56 kHz for bursts observed near 40 MHz. A significant number had t 20 ms. Flux densities of S burst sources were estimated to fall in the range 1023-5 × 1021 Wm–1 Hz–1.A small proportion (1–2%) of bursts showed a fine structure in which the burst source apparently only emitted at discrete, regularly spaced frequencies causing the spectrogram to exhibit a series of bands or fringes. The fringe spacing increased with wave frequency and was f - 90 kHz for fringes near 40 MHz. The bandwidths of fringes was narrow, often less than 30 kHz and in some cases down to 10–15 kHz.New address: Astronomy Program, University of Maryland, College Park, MD, U.S.A. 相似文献
2.
Here we report the statistical analysis of 160 isolated decimetric type III bursts, a majority of them (74%) having central frequency above 1000 MHz, observed in 8 flares by the spectrometer Phoenix. The most important finding of the detailed analysis is: 67% of all the bursts are reverse slope, 38% normal and 5% bi-directional. Also, we obtained the following results: (a) the best fits for the average half power duration and for the average drift rate as a function of frequency are given by t1/2=1.7×104f–0.60and /df/dt|=(0.09±0.03)f(1.35 ± 0.10), respectively; (b) the frequency range of most of the type III bursts is less than 250 MHz; (c) the number of bursts decreases with increasing starting frequency and flux; (d) peak flux decreases with increasing frequency. The relations obtained fit well for higher frequency observations. Assuming an improved density model and type III emission at 2ndharmonic, the beam parameters of type III bursts are determined. 相似文献
3.
The properties of powerful (flux >10−19 W m−2 Hz−1) type III bursts observed in July – August 2002 by the radio telescope UTR-2 at frequencies 10 – 30 MHz are analyzed. Most
bursts have been registered when the active regions associated to these bursts were located near the central meridian or at
40° – 60° to the East or West from it. All powerful type III bursts drift from high to low frequencies with frequency drift
rates 1 – 2.5 MHz s−1. It is important to emphasize that according to our observations the drift rate is linearly increasing with frequency. The
duration of the bursts changes mainly from 6 s at frequency 30 MHz up to 12 s at 10 MHz. The instantaneous frequency bandwidth
does not depend on the day of observations, i.e. on the disk location of the source active region, and is increasing with frequency. 相似文献
4.
The 13 pairs of type III bursts with the bidirectional drift structures recorded with the spectrograph in the frequency ranges of 230–300 MHz and 625–1500 MHz at the Yunnan Observatory and 2600–3800 MHz at the Beijing National Astronomical Observatories are analyzed in this present article and the outstanding characteristics of these events are obtained. These bursts respectively reveal that the separatrix frequency between the bursts with positive and negative drifts comes between 250 MHz and 3420 MHz, with a gap being between 0.6 MHz and 110 MHz; the duration is 53 ms–1880 ms and the frequency drift rate is between 45 MHz/s and 56000 MHz/s. The drift rate at metric wavelengths is relatively low, only a few decades of MHz while it is comparatively high at microwave wavelengths, reaching 56000 MHz/s. The qualitative explanation of these events is given in this paper. 相似文献
5.
From radio spectra between 160 and 320 MHz of chains of type I bursts it appears that their duration distributions allow an exponential fit, and that those of samples containing long and short chains respectively, taken from the same storm, have virtually the same characteristic time (logarithmic slope). On the average this figure decreases - as a function of the frequency - at about 1 s per 10 MHz. The high frequency cut-off of chain activity (noise storms) is mainly a consequence of the frequency dependence of the probability for the first burst of a chain to appear. Given the density of type I bursts in a chain, it is concluded that the probability of a type I burst to be followed by another one is at least 90% below 250 MHz and 70–80% at 300 MHz, which makes it essential for type I theories to include a mechanism to this effect. The drift rate distribution for chains is symmetrical with a peak at-10 MHz/s. The statistics is indicative of a correlation between drift rate and duration. No evidence has been found for the occurrence of chain pairs or frequency splitting in chains, nor for an association between chains and type III bursts. 相似文献
6.
Electron beams in the low corona 总被引：3，自引：0，他引：3
Selected high-resolution spectrograms of solar fast-drift bursts in the 6.2–8.4 GHz range are presented. The bursts have similar characteristics as metric and decimetric type III bursts: rise and decay in a few thermal collision times, total bandwidth 3% of the center frequency, low polarization, drift rate of the order of the center frequency per second, and flare association. They appear in several groups per flare, each group consisting of some tens of single bursts. Fragmentation is also apparent in frequency; there are many narrowband bursts randomly scattered in the spectrum. The maximum frequency of the bursts is highly variable.The radiation is interpreted in terms of plasma emission of electron beams at plasma densities of more than 1011 cm–-3. At this extremely high frequency, emission from the plasma level even at the harmonic is only possible in a very anisotropic plasma. The scale lengths perpendicular and parallel to the magnetic field can be estimated. A model of the source region and its environment is presented.Paper presented at the 4th CESRA Workshop in Ouranopolis (Greece) 1991. 相似文献
7.
Detailed statistics and analysis of 264 type III bursts observed with the 625–1500 MHz spectrograph during the 23rd solar
cycle (from July 2000 to April 2003) are carried out in the present article. The main statistical results are similar to those
of microwave type III bursts presented in the literature cited, such as the correlation between type III bursts and flares,
polarization, duration, frequency drift rate (normal and reverse slopes), distribution of type III bursts and frequency bandwidth.
At the same time, the statistical results also point out that the average values of the frequency drift rates and degrees
of polarization increase with the increase in frequency and the average value of duration decreases with the increase in frequency.
Other statistical results show that the starting frequencies of the type III bursts are mainly within the range from 650 to
800 MHz, and most type III bursts have an average bandwidth of 289 MHz. The distributions imply that the electron acceleration
and the place of energy release are within a limited decimetric range. The characteristics of the narrow bandwidth possibly
involve the magnetic configuration at decimetric wavelengths, the location of electron acceleration in the magnetic field
nearto the main flare, the relevant runaway or trapped electrons, or the coherent radio emission produced by some secondary
shock waves. In addition, the number of type III bursts with positive frequency drift rates is almost equal to that with negative
frequency drift rates. This is probably explained by the hypothesis that an equal number of electron beams are accelerated
upwards and downwards within the range of 625 to 1500 MHz. The radiation mechanism of type III bursts at decimetric wavelengths
probably includes these microwave and metric mechanisms and the most likely cause of the coherent plasma radiation are the
emission processes of the electron cyclotron maser. 相似文献
8.
M. L. Kaiser 《Solar physics》1975,45(1):181-187
Over 500 days of low-frequency (<1 MHz) radio observations from the IMP-6 spacecraft have been accumulated to produce a two-dimensional map (frequency vs elongation) of solar type III burst occurrences. This map indicates that most solar bursts in this frequency range are observed at the second harmonic of the plasma frequency rather than the fundamental. The map also shows that the solar wind electron density varies as R ?γ , where γ can be somewhat less than 2 to perhaps 3 or higher. 相似文献
9.
The occurrence rate of type III solar bursts in the frequency range 4.9 MHz to 30 kHz is analyzed as a function of burst intensity and burst arrival direction. We find that (a) the occurrence rate of bursts falls off with increasing flux, S, according to the power law S
–1.5, and (b) the distribution of burst arrival directions at each frequency shows a significantly larger number of bursts observed west of the Earth-Sun line than east of it. This western excess in occurrence rate appears to be correlated with the direction of the average interplanetary magnetic field, and is interpreted as beaming of the observed burst radiation along the magnetic field direction.Presently at the University of Maryland, College Park, Maryland. 相似文献
10.
An analysis has been made of type III bursts recorded during a decametric solar storm observed from July 29 to August 16, 1975 with the UTR-2 antenna (Kharkov, IRE Acad. Sci. Ukr. SSR). The bursts were recorded with a dynamic spectrograph and radiometers at 25.0, 20.0, 16.7, and 12.5 MHz. Daily observations have yielded histograms of the type III burst distribution with respect to the frequency drift rate in three subbands between 25.0 and 12.5 MHz. During the middle stage of the storm the drift rate was about twice as high as at the onset and the final stage of the storm. Abrupt changes in the mean frequency drift rate were registered some two to three days after the active region McMath 13790 had come onto the limb and also before it disappeared behind the solar disk. Sudden changes in the drift rates of the type III bursts were accompanied by sudden changes of their mean duration. The rather long burst durations observed at 25.0 MHz at the beginning and the end of the radio storm coincided with such at the twice lower frequency, i.e. 12.5 MHz, during the period when an increased drift rate was observed.Similar variations of type III burst parameters can be interpreted in the framework of the plasma mechanism of burst generation in the corona, assuming that at the middle stage of the storm the bursts observed in the 25.0–12.5 MHz range were emitted at the fundamental whereas when the emitting region was near the limb the bursts received corresponded to the second harmonic of the Langmuir oscillations in the range of 12.5 to 6.25 MHz excited at greater heights. 相似文献
11.
Radio and X-ray observations are presented for three flares which show significant activity for several minutes prior to the main impulsive increase in the hard X-ray flux. The activity in this ‘pre-flash’ phase is investigated using 3.5 to 461 keV X-ray data from the Solar Maximum Mission, 100 to 1000 MHz radio data from Zürich, and 169 MHz radio-heliograph data from Nançay. The major results of this study are as follows:
- Decimetric pulsations, interpreted as plasma emission at densities of 109–1010 cm?3, and soft X-rays are observed before any Hα or hard X-ray increase.
- Some of the metric type III radio bursts appear close in time to hard X-ray peaks but delayed between 0.5 and 1.5 s, with the shorter delays for the bursts with the higher starting frequencies.
- The starting frequencies of these type III bursts appear to correlate with the electron temperatures derived from isothermal fits to the hard X-ray spectra. Such a correlation is expected if the particles are released at a constant altitude with an evolving electron distribution. In addition to this effect we find evidence for a downward motion of the acceleration site at the onset of the flash phase.
- In some cases the earlier type III bursts occurred at a different location, far from the main position during the flash phase.
- The flash phase is characterized by higher hard X-ray temperatures, more rapid increase in X-ray flux, and higher starting frequency of the coincident type III bursts.
12.
The Source Regions of Impulsive Solar Electron Events 总被引：1，自引：0，他引：1
Benz Arnold O. Lin Robert P. Sheiner Olga A. Krucker Säm Fainberg Joe 《Solar physics》2001,203(1):131-144
Low-energy (2–19 keV) impulsive electron events observed in interplanetary space have been traced back to the Sun, using their interplanetary type III radiation and metric/decimetric radio-spectrograms. For the first time we are able to study the highest frequencies and thus the radio signatures closest to the source region. All the selected impulsive solar electron events have been found to be associated with an interplanetary type III burst. This allows to time the particle events at the 2 MHz plasma level and identify the associated coronal radio emissions. Except for 5 out of 27 cases, the electron events were found to be associated with a coronal type III burst in the metric wavelength range. The start frequency yields a lower limit to the density in the acceleration region. We also search for narrow-band spikes at the start of the type III bursts. In about half of the observed cases we find metric spikes or enhancements of type I bursts associated with the start of the electron event. If interpreted as the plasma emission of the acceleration process, the observed average frequency of spikes suggests a source density of the order of 3×108 cm–3 consistent with the energy cut-off observed. 相似文献
13.
G. P. Chernov A. A. Stanislavsky A. A. Konovalenko E. P. Abranin V. V. Dorovsky G. O. Rucker 《Astronomy Letters》2007,33(3):192-202
We have performed a comparative analysis of the fine structure of two decametric type II bursts observed on July 17 and August 16, 2002, with the 1024-channel spectrograph of the UTR-2 radio telescope in the frequency range 18.5–29.5 MHz and with the IZMIRAN spectrograph in the frequency range 25–270 MHz. The August 16 burst was weak, ～2–5 s.f.u., but exhibited an unusual fine structure in the form of broadband fibers (Δf e > 250–500 kHz) that drifted at a rate characteristic of type II bursts and consisted of regular narrow-band fibers (Δf e > 50–90 kHz at 24 MHz) resembling a rope of fibers. The July 17 burst was three orders of magnitude more intense (up to 4500 s.f.u. at 20 MHz) and included a similar fiber structure. The narrow fibers were irregular and shorter in duration. They differed from an ordinary rope of fibers by the absence of absorption from the low-frequency edge and by slow frequency drift (slower than that of a type II burst). Both type II bursts were also observed in interplanetary space in the WIND/WAVES RAD2 spectra, but without any direct continuation. Analysis of the corresponding coronal mass ejections (CMEs) based on SOHO/LASCO C2 data has shown that the radio source of the type II burst detected on August 16 with UTR-2 was located between the narrow CME and the shock front trailing behind that was catching up with the CME. The July 17 type II fiber burst also occurred at the time when the shock front was catching up with the CME. Under such conditions, it would be natural to assume that the emission from large fibers is related to the passage of the shock front through narrow inhomogeneities in the CME tail. Resonant transition radiation may be the main radio emission mechanism. Both events are characterized by the possible generation of whistlers between the leading CME edge and the shock front. The whistlers excited at shock fronts manifest themselves only against the background of enhanced emission from large fibers (similar to the continuum modulation in type IV bursts). The reduction in whistler group velocity inside inhomogeneities to 760 km s?1 may be responsible for the unusually low drift rate of the narrow fibers. The magnetic field inside inhomogeneities determined from fiber parameters at 24 MHz is ～0.9 G, while the density should be increased by at least a factor of 2. 相似文献
14.
C. Mercier 《Solar physics》1976,46(2):499-500
On 1 July 1971, about ten groups of type III bursts were observed with high time resolution (10?1 sec) with the 169 MHz Nançay radioheliograph. Each group consists of two or several bursts, appearing successively from E to W in all cases, with very short delays. The analysis of successive E-W profiles allowed us to show that, for each event:
- the delay between maximum times of the sources was in the range 0.3–0.8 s and that their time profiles were very similar.
- the mutual distance between sources was ～1.5 × 105 km.
15.
Several hundred radio bursts in the decimetric wavelength range (300–1000 MHz) have been compared with simultaneous soft and hard X-ray emission. Long lasting (type IV) radio events have been excluded. The association of decimetric emission with hard X-rays has been found to be surprisingly high (48%). The association rate increases with bandwidth, duration, number of structural elements, and maximum frequency. Type III-like bursts are observed up to the upper limit of the observed band. This demonstrates that the corona is transparent up to densities of about 1010 cm–3, contrary to previous assumptions. This can only be explained in an inhomogeneous corona with the radio source being located in a dense structure. The short decimetric bursts generally occur during the impulsive phase, i.e. simultaneously with hard X-rays. The times of maximum flux are well correlated (within 2 s). The HXR emission lasts 4 times longer then the radio emission in the average. This work finds a close relationship between decimetric and HXR emission with sufficient statistics offering additional information on the flare process. 相似文献
16.
V.N. Mel'nik A.A. Konovalenko H.O. Rucker A.A. Stanislavsky E.P. Abranin A. Lecacheux G. Mann A. Warmuth V.V. Zaitsev M.Y. Boudjada V.V. Dorovskii V.V. Zaharenko V.N. Lisachenko C. Rosolen 《Solar physics》2004,222(1):151-166
We present the results of radio telescope UTR-2 observations of solar Type II radio bursts in the 10–30 MHz frequency range. These events possess a fine structure consisting of fast drift sub-bursts similar to Type III bursts. The frequency drift rate of the Type II bursts at decameter wavelengths is smaller than 0.1 MHz s–1. One of these bursts with herringbone structure has a wave-like backbone that almost does not drift. The features of the observed bursts are discussed. 相似文献
17.
We recently investigated some of the hitherto unreported observational characteristics of the low frequency (85–35 MHz) type III–V bursts from the Sun using radio spectropolarimeter observations. The quantitative estimates of the velocities of the electron streams associated with the above two types of bursts indicate that they are in the range \({\gtrsim }0.13c\)–0.02c for the type V bursts, and nearly constant (\({\approx }0.4c\)) for the type III bursts. We also find that the degree of circular polarization of the type V bursts vary gradually with frequency/heliocentric distance as compared to the relatively steeper variation exhibited by the preceding type III bursts. These imply that the longer duration of the type V bursts at any given frequency (as compared to the preceding type III bursts) which is its defining feature, is due to the combined effect of the lower velocities of the electron streams that generate type V bursts, spread in the velocity spectrum, and the curvature of the magnetic field lines along which they travel. 相似文献
18.
D. B. Melrose 《Solar physics》1974,35(2):441-450
Type III bursts often have brightness temperatures at the fundamental greater than 109K. If the fundamental emission is due to scattering of Langmuir waves into transverse waves by thermal ions, this implies that induced scattering dominates over spontaneous scattering, which in turn requires that the energy density in Langmuir waves be greater than some minimum value, e.g. W
l > 3 × 10-10 erg cm-3 for bursts at f
p
= 100 MHz. Such Langmuir waves become isotropic on a time-scale shorter than the rise-time of type III bursts, e.g. < l s at f
p
= 100 MHz. Consequently, their coalescence, leading to emission at the second harmonic, proceeds. The above inequalities would imply a brightness temperature at the second harmonic in excess of 109K at f = 200 MHz.The predicted values of the brightness temperatures
T1
t
and
T2
t
(at the fundamental and second harmonic respectively) can be expressed in terms of an optical depth . After is eliminated a functional relation between
T1
t
,
T2
t
and the plasma frequency, f
p
, remains. The form of this relation is not dependent on a quantitative theory of how the Langmuir waves are generated by the stream of electrons. Consequently, comparison with observed quantities should provide further insight into the detailed properties of the emission processes. 相似文献
19.
Baselyan L. L. Goncharov N. Yu. Zaitsev V. V. Zinichev V. A. Rapoport V. O. Tsybko Ya. G. 《Solar physics》1974,39(1):223-231
The paper deals with the observations of the fine structure of type III bursts in the 12.5–25 MHz band using the UTR-2 (IRE AN UkSSR, Kharkov) radio telescope. A fine structure arises in the form of chains of short-lived narrow-band bursts. The chains have a frequency drift analogous to type III bursts. Observations allow two different-type chains to be singled out. Ordinary stria-bursts, split-pairs and triplets belong to the first type chains. They may also involve the echo-type phenomena The second type chains (IIId) involve diffusive stria-bursts, diffusive split-pairs and triplets. The analysis of a harmonic structure of chains incidates that the first type chains are generated at the frequencies close to the local plasma electron frequency
pe
. The second type chains and, consequently, diffusive stria-bursts correspond to the second harmonic of the plasma frequency 2
pe
. Experimental data evidence that the type III bursts with a fine structure are excited by the faster particle streams than the ordinary type III bursts with a diffusive character both of the fundamental and the second harmonic. 相似文献
20.
A statistical analysis of the type-III bursts observed by the spectrographs in the ranges of 625∼1500 MHz, 2600-3800 MHz, and 5200-7600 MHz during the 23rd solar cycle (from 2000 July to 2004 September) is carried out. The distribution of the type-III bursts, and their durations, frequency drift rates, polarization degrees and frequency bandwidths are given in this paper. The results indicate that the average values of the frequency drift rates and frequency bandwidths increase with the frequency. The average values of the durations and polarization degrees are neither constant nor uniformly varied over a broad frequency range. Most of type-III bursts are distributed in the range from 625 to 3800 MHz, and decrease with the frequency in number. This analysis shows that the places of electron acceleration and energy release are mainly in the decimetric range, and the characteristic of this frequency range is possibly related with the magnetic configuration at the decimeter wavelengths, as well as the electron acceleration in the magnetic reconnection site close to the main flare. However, there are also a considerable number of type-III bursts in the range of 5200-7600 MHz, it means that the sites of electron acceleration are widely distributed in the coronal region. The radiation mechanisms of type-III bursts at the centimeter-decimeter wavelengths include most probably the coherent plasma radiation and the emission process of electron cyclotron maser. 相似文献