Characteristics of a type II solar radio burst on 22 March 1998

A solar radio type II burst (which was seen as two patches of emission, one during 07:00–07:13 UT and other one during 07:20–07:35 UT) was observed on 22 March 1998 using the Madurai radio spectrograph. A broad range of data (from Culgoora and Hiraiso spectrographs, white-light data from SOHO/LASCO and X-ray data from Yohkoh and GOES satellites) was also studied for this event, which was analyzed in comparison with these supplementary data. In addition, the conditions associated with this shock were analyzed quantitatively. From the above investigations, the following conclusions have been made. The temporal relationship between H-alpha flare and burst has shown that the active region AR 8185 is the source of this type II burst. A bright front feature observed with LASCO is also associated with this type II burst and active region AR 8185. The time profile of the shock derived from the first patch of this type II burst coincides with the flare starting time. Also, within error limits, the start time of the CME is same as the flare. Hence, it is not possible to decide whether the type II originated in the flare or was driven by CME. In addition, the investigations of the second patch alone has provided the following results. The inferred shock speed for the second patch of emission is lower than the first and closer to the CME speed. The emission occurred below 50 MHz. These conditions imply that this patch may be a separate burst which might have been produced by the CME alone.     

Shock-associated kilometric radio emission and solar metric type II bursts

We present statistics relating shock-associated (SA) kilometric bursts (Cane et al., 1981) to solar metric type II bursts. An SA burst is defined here to be any 1980 kHz emission temporally associated with a reported metric type II burst and not temporally associated with a reported metric type III burst. In this way we extend to lower flux densities and shorter durations the original SA concept of Cane et al. About one quarter of 316 metric type II bursts were not accompanied by any 1980 kHz emission, another quarter were accompanied by emission attributable to preceding or simultaneous type III bursts, and nearly half were associated with SA bursts. We have compared the time profiles of 32 SA bursts with Culgoora Observatory dynamic spectral records of metric type II bursts and find that the SA emission is associated with the most intense and structured part of the metric type II burst. On the other hand, the generally poor correlation found between SA burst profiles and Sagamore Hill Observatory 606 and 2695 MHz flux density profiles suggests that most SA emission is not due to energetic electrons escaping from the microwave emission region. These results support the interpretation that SA bursts are the long wavelength extension of type II burst herringbone emission, which is presumed due to the shock acceleration of electrons.Also: Department of Physics and Astronomy, University of Maryland, College Park, MD 20742, U.S.A.     

Harmonic structure of type IIIb and III bursts

A decameter solar radio storm of type IIIb and III bursts has been analysed, using single frequency records at frequencies 12.5 and 25.0 MHz.Several kinds of burst associations are classified. As a result it is shown that in double oblique burst-traces of type IIIb + III on the frequency-time plane the type III burst is shifted by an octave above the type IIIb burst at any moment of the IIIb + III pair's lifetime. In particular, the harmonic structure of the spectrum is peculiar to the event of type IIIb + III in the initial and the final stages. This property of the pair is clear if the type IIIb and III radiations occur at the fundamental coronal plasma frequency and its harmonic respectively. On the other hand, if it is assumed that a type IIIb burst is the precursor of a type III one, there is no reason why the two bursts should be harmonically related.     

A new method for reconstructing type III trajectories

A local density approximation (LDA) method is developed for reconstructing the trajectories of type III radio bursts through the interplanetary medium. The method uses the measured source directions and the measured frequency drift rates of the type III burst to determine the locations of the radio source in the interplanetary medium at consecutive frequency levels. The technique is used to reconstruct the trajectory of an actual type III burst and the results are compared to the trajectory obtained from the global density law method. The LDA method represents an improvement in that it utilizes more observed data on the type III burst and that it takes full account of the local density variations at the source locations.     

A type IV burst associated with a coronal streamer disruption event

A type IV burst was observed on February 17, 1985 with the Clark Lake Radio Observatory multifrequency radioheliograph operating in the frequency range 20–125 MHz. This burst was associated with a coronal streamer disruption event. From two-dimensional images produced at 50 MHz, we show evidence of a type II burst and a slow moving type IV burst. The observations of the moving type IV burst suggests that a plasmoid containing energetic electrons can result from the disruption of a coronal streamer.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

Chains of type I stormbursts

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.     

On the probability of occurrence of the type IIIb burst as a precursor

It is shown that a precursor type IIIb burst is really associated with a type III burst. The broad longitude distribution of occurrence of type IIIb bursts also suggests that these bursts are emitted at a large angle to the open magnetic field in the corona.     

A metric type III burst asymmetry relative to simple bipolar active regions

Metric type III solar radio burst positions are compared spatially and temporally to underlying active region geometry. The positions of these radio bursts have an asymmetric location distribution relative to simple bipolar regions. The type III bursts show a tendency to occur nearer the leading active region - an association shown before from type III burst and magnetic field polarity measurements. The type III bursts also generally occur to the left of the outward to inward directed magnetic field. The asymmetry relative to the outward directed magnetic field has a sense that is consistent with a mechanism of type III burst production that involves a pre-existing coronal current system situated between expanding closed and open magnetic field lines.     

Properties of metre-wavelength solar bursts associated with interplanetary type II emission

A statistical analysis is used to determine the properties of metre-wavelength events which are associated with interplanetary type II bursts. It is found that the likelihood of an interplanetary type II burst is greatly increased if: (a) an associated metre-wavelength type II has a starting frequency less than 45 MHz; (b) a strong metre-wavelength continuum is present; (c) the type II contains herringbone fine structure; and (d) the metre-wavelength activity is accompanied by strong, long-lasting H and soft X-ray events.Visiting scientist at Division of Radiophysics, January 1983; previous address - NASA/Goddard Space Flight Center, Greenbelt, Maryland.     

On the possibility of determining the coronal magnetic field from solar type III radio burst observations

A simple method of estimating the coronal magnetic field is suggested. It is based on the observational fact that the duration of the highly polarized part in type III bursts can be different, varying from a small fraction of the burst length to its total duration. We suggest that this difference is determined by the relation between the size of the region where only the ordinary wave can propagate and the size of the region where the burst is generated at a fixed frequency. The magnetic field is estimated at several tens of gauss in regions emitting highly polarized type III bursts at frequencies over 200 MHz. Density and magnetic field scales are estimated.     

Kilometer-wave type III burst: Harmonic emission revealed by direction and time of arrival

A type III solar burst was observed at seven frequencies between 3.5 MHz and 80 kHz by the Michigan experiment aboard the IMP-6 satellite. From the data we can determine burst direction-of-arrival as well as time-of-arrival. We predict these quantities using simple models whose parameters we vary to obtain a good fit to the observations. We find that between 3.5 MHz and 230 kHz the observed radiation was emitted at the fundamental of the local plasma frequency while below 230 kHz it was emitted at the second harmonic. The exciter particles that produced the burst onset and burst peak have velocities of 0.27 and 0.12, respectively, in units of the velocity of light.     

Time profile of type III bursts

On the hypothesis that the time profile of a type III burst corresponds directly to the flux of electron beam, the similarity of time profile is shown to be maintained even if the electron velocity decreases with distance provided that the time is normalized to unity at the time of maximum flux. The observed time profiles of type III bursts with simple shape seem to follow the similarity law in almost all frequency range. This evidence may indicate that the time profile, both the rising and decaying phases, of a type III burst should be attributed to a common origin, e.g., the time variation of exciter determined by the initial velocity distribution in the electron beam, instead of attributing the rising time to the beam length and the decay time to the damping of plasma waves after the passage of the electron beam.     

A decameter type II burst associated with a behind-the-limb flare

The characteristics of a decameter type II burst associated with a possible behind-the-limb flare are discussed. The burst source had an unusually high velocity. Assuming that the disturbance propagated as an MHD wave, the magnetic field strength at the 40 MHz plasma level is estimated to be 5.6 gauss.     

Spectral evidence of type ii shock influence on razin-cutoff frequency in the decametric type iv continuum

Bhonsle and Degaonkar (1980) have presented an evidence for the Razin-cutoff frequency in the dkm range of type IV continuum observed by the Ahmedabad spectroscope on 27 April, 1979. They explained this cutoff as due to the Razin effect in the coronal medium. From the broad-band spectrograph at Weissenau, covering 1000 to 30 MHz, additional information on the initial features of type IV is furnished here. This record also confirms the preceeding type II burst, the shock front of which was assumed to produce the quasiperiodicity observed at the lower cut off frequency by interaction with the type IV radiation source.     

On a strange recurring type I burst pattern

Two remarkable intensity-time patterns in the 113 and 64 MHz single-frequency radio flux records during a type I noise storm and/or a type IV burst on 31 July, 1983 are studied. A comparison of the patterns at both frequencies reveals a high degree of resemblance and inherent common structure although the 64 MHz pattern was seen 40 min later than the 113 MHz pattern. An interpretation is given assuming a slowly uprising and thereby expanding clumpy plasma-magnetic field configuration which is (via accompanying coronal loops) two times illuminated by energetic electrons coming from the soft X-ray flare precursor source region of the H-alpha flares F1 and F2 (see Figure 1).     

Interplanetary type III radiobursts and relativistic electrons

For the time periods 1979 April 22–May 17 and 1980 May 9–June 10, when the HELIOS spacecraft were located inside 0.5 AU, we compared the antenna temperature T _A of the 466 kHz type III bursts measured by the SBH instrument on ISEE 3 with the fluxes of 0.5 MeV electrons measured by HELIOS. For 51 flare-associated kilometric type III bursts (FAIII bursts) with log(T _A) > 10 we find: (1) 25 bursts (49%) are accompanied by a relativistic electron event in interplanetary space, (2) the probability for detection of an electron event decreases from more than 74% inside a cone of ± 20 ° to 56% inside a cone of ± 60° around the flare site, (3) there is only a small correlation between the brightness temperature of the radio burst and the size of the electron event, and (4) despite the broad scatter of these values there is a clear indication that for a given size of the relativistic electron event the intensity of the type III burst is about a factor of 5 higher if it is accompanied by a type II burst. These results give evidence (a) that at least part of the relativistic electrons frequently is accelerated together with non-relativistic electrons and (b) that the coronal shock associated with the metric type II burst has a weaker effect on relativistic than on non-relativistic electrons.Now at DFVLR, Oberpfaffenhofen, Germany.     

Decameter solar type III bursts: Data classification with use of cluster analysis

The many-parametric data on decametric type III bursts containing more than 1000 events were classified with use of cluster analysis, i.e., the pattern recognition procedure. For the classification such parameters have been used as intensity, duration and degree of circular polarization derived from burst time profiles. The automatic classification has resulted in division of daily samples of type III bursts into classes of bursts, more homogeneous statistically, which can be associated with different components of type III radiation distinguished by their physical origin. For the classes obtained, statistically reliable dependences of the mean intensity on source position can be found which allow one to conclude about the source structure and evolution, as well as about the characteristics defined by a burst generation mechanism and propagation effects.     

Directivity of low frequency solar type III radio bursts

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.     

Positions of type II fundamental and harmonic sources in the 30–100 MHz range

Observations of a type II burst with fundamental and harmonic structure were made, with the Clark Lake Radio Observatory's E-W and N-S sweep frequency interferometers operating in the range 120–20 MHz with time and frequency resolution of 1 s and 100 kHz, respectively. The type II burst was preceded by a type III-type V, and the associated flare of importance SN was located at S 20 W 73. The interferometric data show that the fundamental and harmonic were coincident in position. Further, the type III positions as a function of frequency were practically the same as those of the type II burst. The implications of these results are discussed.     

Solar type III burst profiles at decametre-wave frequencies

Observations of type III burst profiles at 18, 22, 26 and 36 MHz, by Barrow and Achong (1975), are used to calculate the form of the exciter function. The burst profile is treated as the convolution of an exciter function and an exponential decay function. The average form of the exciter profile is obtained directly from calculated profiles and further inferred from the first three statistical moments. Normalised average profiles of the burst and the exciter are presented for each frequency.The analysis shows that over the frequency range 18–36 MHz, (1) the exciter function possesses negative skewness, (2) the shapes of burst profiles and exciter profiles are approximately constant and, (3) burst peak time varies linearly with height in the corona. It is suggested that the passage of the exciting electron stream through field-dominated and flow-dominated coronal regions has different effects on the profiles.