Variations of type III burst parameters during a decametric solar storm

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.     

Decameter storm radiation,II

The physical properties of six decametric storms, observed at Clark Lake Radio Observatory are studied. The height of the storm continuum sources was determined from the rotation rate. Assuming that the radiation originates at the plasma frequency we computed the gradient of electron density for the regions where the storms originate. The mean angular size of the decametric continuum sources is large; it increases with decreasing frequency. The storm continuum is found to be strongly directive toward the disk center. The east-west asymmetry, well observed at meter wavelengths is also observed at decameter wavelengths. The occurrence of two distinct classes of type III bursts in storms is discussed: ‘off-fringe’ and ‘onfringe’ type Ill's. The ‘off-fringe’ Ill's are found to be displaced in position from the continuum source ; on the other hand, the ‘on-fringe’ ones coincide in position with the continuum. These two kinds of bursts differ in other properties as well. A model of the storm region is proposed. The continuum radiation and the ‘on-fringe’ type III's are believed to originate above closed magnetic loops, in regions of diverging field lines; the ‘offfringe’ type Ill's are thought to be excited by energetic electron streams, having access to open magnetic field lines at the base of the loops.     

Solar radio continuum storms

Radio noise continuum emissions observed in metric and deca-metric wave frequencies are, in general, associated with actively varying sunspot groups accompanied by the S-component of microwave radio emissions. It is known that these continuum emission sources, often called type I storm sources, are often associated with type III burst storm activity from metric to hectometric wave frequencies. This storm activity is, therefore, closely connected with the development of these continuum emission sources.It is shown that the S-component emission in microwave frequencies generally precedes by several days the emission of these noise continuum storms of lower frequencies. In order for these storms to develop, the growth of sunspot groups into complex types is very important with the increase of the average magnetic field intensity and area of these groups. In particular, the types of these groups such as and are very important on the generation of noise continuum storm sources and sharp increase of the flux of these continuum emissions. This fact suggests that sunspot magnetic configuration and its variation, both space and time, are very effective on the growth of the sources for these noise continuum emissions.Although we have not known yet the true mechanism of these emissions, it is very likely that energetic electrons, 10 to 100 keV, accelerated in association with the variation of sunspot magnetic fields, are responsible as the sources of those radio emissions. Furthermore it seems that these electrons are contributing to the emission of type III burst storms, which are associated with the noise continuum storm sources. In explaining the origin of these storms, some plasma processes must be taken into consideration. Furthermore, it should be remarked that the storage mechanism of the electrons mentioned above plays an important role in generating both the noise continuum emissions and type III burst storms, because on-fringe type III bursts are all generated above these noise continuum storms sources. After reviewing the theories of these noise continuum storm emissions, a model is briefly considered to explain the relation between these continuums and type III bursts, and a discussion is given on the role of energetic electrons on these two emissions. It is pointed out that instabilities associated with these electrons and their relation to their own stabilizing effects are important in interpreting both of these storm emissions.Astrophysics and Space Science Review Paper.     

Initial Results of the Ichon Solar Radio Spectrograph

A new solar radio spectrograph to observe solar radio bursts has been installed at the Ichon branch of the Radio Research Laboratory, Ministry of Information and Communication, Korea. The spectrograph consists of three different antennas to sweep a wide band of frequencies in the range of 30 MHz ∼ 2500 MHz. Its daily operation is fully automated and typical examples of solar radio bursts have been successfully observed. In this paper we describe briefly its hardware and data processing methods. Then we present coronal shock speeds estimated for 34 type II bursts from May 1998 to November 2000 and compare them with those from other observatories. We also present the close relationship between onset time of type II bursts and X-ray flares as well as their associations with coronal mass ejections.     

Statistics and Analysis of Microwave Type III Bursts at Low Frequencies

68 groups of type III bursts recorded with the dynamic spectrograph (from1 to 2 GHz) at the Beijing Astronomical Observatory are statistically analyzed in this present article, with the frequency drift, duration, bandwidth and polarization of each of these events being analyzed in detail. These observational features are different from those of metric, decimetric and microwave high frequency type III bursts, showing that there possibly exist transition phenomena in the microwave low frequency band and the microwave type III bursts at low frequencies are possibly produced by the harmonic plasma radiation caused by nonthermal electronic beams and by the electron gyro-maser radiation. This revised version was published online in July 2006 with corrections to the Cover Date.     

AR6659区伴随米波射电爆发的光学形态和磁场速度场特征

利用云南天文台声光频谱议１９９１年６月观测到的来自６６５９活动区的米波射电爆发记录,及其光学活动、磁场速度场的资料,作了分析研究。得到了一些有意义的结果     

A search of a connection between the polarization of decam-type III bursts and magnetic fields in different heights of the solar atmosphere

Polarization measurements of type III bursts at 23.5 and 29.5 MHz have been compared for several years with indicators of magnetic fields in different height levels such as sunspot data, S-component characteristics, and noise storm data. By applying the Mount-Wilson and Brunner types of the related spot groups there results a positive relationship between the average degree of type III burst polarization and the magnitude or complexity of photospheric magnetic fields. For other parameters (leading spot area, peak intensity of the S-component at 9.1 cm wavelength) such a clear monotonic relation has not been found. Possibly the degree of polarization is influenced by height variations of the emitting level of the type III bursts at a fixed frequency due to variable electron densities. No connection has been detected between the type III burst polarization and noise storm fluxes which may be due to the local distance of the origin of both emissions.     

The relation between the surges and solar radio emission

The 120 limb surges which have been observed by means of Wrocaw Observatory coronagraph from September 1966 to November 1977 are investigated. The evolution of surges was compared with the radio data during the surges. A correlation between radio bursts and the surges was found, particularly with chains of type I radio bursts, which is the first reliable correlation found of these bursts with non-radio events. The type I correlation only applied for surges without accompanying flare, of which 43% are correlated with this type of radio emission. In 23 of 30 associated events the start of a surge coincided within 5 minutes with the start or an enhancement of the type I storm. If flares were present, the association was not significant.We also compared the maximum height reached by a surge with the frequencies of the radio bursts emitted at the same time and the maximum velocity of the rising surge with the frequency drift of type I chains. No such a correlation was however found.We discuss the possibility that surges are the result of a sudden energy input into the chromosphere related to the type I source in the corona.     

A study of the spectrum and the polarization of the noise storm continua between 234 and 40 MHz

Intense noise storm continua at low frequencies are mostly observed in the later phase of solar cycles (Böhme, 1989). Radioheliographs may be needed to determine whether this effect is mainly caused by type I or type III continua. However, based on single-frequency records from the Tremsdorf Observatory a method to discriminate type I and type III continua even from polarization measurements was derived. Intense 40 MHz continua identified by spectral criteria as a type III continuum are more weakly polarized than type I continua. The increase in the number of intense continua at frequencies 64 MHz during the second, compared to the first, activity maximum of cycle No. 20 was due to an enhanced number of continua with a significant contribution of type I continuum. Relations between the parameters of the continua and the concurrent storm bursts confirm the validity of the above-mentioned ideas and may be useful to test models which try to explain the generation of type I and type III storms under common aspects.     

Spatial and Temporal Scales of Coronal Magnetic Restructuring in the Development of Coronal Mass Ejections

It has been commonly accepted that coronal mass ejections (CMEs) result from the restructuring or reconfiguring of large-scale coronal magnetic fields. In this paper, we analyzed four CME events using Nançay Radioheliograph (NRH) images and the experiments onboard the Solar and Heliospheric Observatory (SOHO) spacecraft to understand the coronal restructuring leading to CME initiation. We investigated the onset, duration, and position of the radio emissions in relation to EUV dimming and the inferred CME onset. It has been identified that the early CME development on the solar disk is characterized by a series of distinct radio bursts. These nonthermal radio sources appeared in phase with coronal dimming shown by SOHO-EIT images and are located within the EUV dimming or ongoing dimming regions. Three time scales are identified: the duration, the separation of individual radio bursts, and the overall time scale of all of the nonthermal sources. They fall in the ranges of approximately tens of seconds to three minutes, one to three minutes, and 15 – 20 minutes, respectively. The individual radio emission seems to shift and expand at the speed of the fast magnetoacoustic waves in the corona; while the nonthermal radio emissions as a whole appear episodically to correspond to the successive coronal restructuring. If we define the triggering speed by dividing the overall spatial scale by the temporal scale of all the radio bursts, then the triggering speed falls in the range of 300 – 400 km s^?1. This implies that the general process of coronal restructuring and reconfiguring takes place at a speed slower than either the Alvfenic or acoustic speed in the corona. This is a type of speed of “topology waves,” i.e., the speed of successive topology changes from closed to open field configuration.     

Spatially resolved spectrophotometry of comet P/Stephan-Oterma

Observations of Comet P/Stephan-Oterma were made with an Intensified Dissector Scanner spectrograph on the McDonald Observatory 2.7-m telescope during the period from July 1980 to February 1981. These spectra cover a range of heliocentric distances from 2.3 AU preperihelion to 1.8 AU postperihelion. A small aperture was used to map the spatial distributions of the gases in the coma. Column densities of the observed cometary emissions (CN, C₃, CH, and C₂) were calculated and it is shown that Stephan-Oterma appeared nearly spherically symmetric. These date are used by Cochran (1985, Icarus62, 82–92) to constrain chemical models of Stephan-Oterma.     

Fine structures in time profiles of type II bursts at frequencies above 200 MHz

We studied the properties of fine structures in 23 type II bursts recorded at the Trieste Astronomical Observatory at frequencies above 200 MHz.The lifetime of a single fine structure is a fraction of a second. The ratio of fine structures intensity vs bulk flux density is different in different type II bursts and it changes during the evolution of a single event; the reported maximum ratio is 3. The polarization of fine structures is nearly the same during the lifetime of an event. There is also no essential difference in polarization between fine structures and bulk emission; this holds also for an example of high-polarization (about 80%) event.At frequencies lower than 200 MHz the analogy between herringbone structure and type III bursts is frequently mentioned in the literature. From the observations we studied, it results, however, that the time profile of single fine-structure elements and their polarization are substantially different from the morphology of type III bursts.The observed fine structures and their characteristics are discussed in the framework of the model by Holman and Pesses (1983).Paper presented at the 4th CESRA Workshop in Ouranopolis (Greece) 1991.     

Occurrence of bidirectional type III bursts in solar flares

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.     

A study of hard X-ray associated meter-decameter bursts observed on December 19, 1979

We present the results of a study of the relationship of a complex meter-decameter wavelength radio burst observed with the Clark Lake E-W and N-S interferometers, with a hard X-ray burst observed with the X-ray spectrometer aboard ISEE-3. The radio burst consisted of several type III's, reverse drift type III's, a U burst, and type II and type IV bursts. The X-ray emission was also complex. The radio as well as hard X-ray emissions were observed before the flash phase of the flare; they were not always associated and we conjecture that this may constitute evidence for acceleration of electrons high in the corona. On the other hand, all components of the reverse drift burst were associated with hard X-ray subpeaks, indicating multiple injection of electron beams along field lines with different density gradients. While the type II burst appeared to be related to the hard X-ray burst, a detailed correspondence between individual features of the radio and hard X-ray burst emissions could not be found. The type IV burst started after all hard X-ray emissions ceased. Its source appeared to be a magnetic arch, presumably containing energetic electrons responsible for the gyrosynchrotron radiation of type IV.Presently at INPE/CRAAM, São Paulo, Brazil.     

Radio and visible-light observations of a coronal arcade transient

We discuss simultaneous visible-light and radio observations of a coronal transient that occurred on 9 April, 1980. Visible-light observations of the transient and the associated erupting prominence were available from the Coronagraph/Polarimeter carried aboard SMM, the P78-1 coronagraph, and from the Haleakala Observatory. Radio observations of the related type III-II-IV bursts were available from the Clark Lake and Culgoora Observatories. The transient was extremely complex; we suggest that an entire coronal arcade rather than just a single loop participated in the event. Type III burst sources observed at the beginning of the event were located along a nearby streamer, which was not disrupted, but was displaced by the outmoving loops. The type II burst showed large tangential motion, but unlike such sources usually do, it had no related herringbone structure. A moving type IV burst source can be associated with the most dense feature of the white-light transient.     

The characteristics of impulsive solar EUV bursts

We examine a number of high time resolution intensity-time profiles of EUV impulsive bursts as observed by the Harvard College Observatory EUV Spectroheliometer carried aboard the Skylab Apollo Telescope Mount. These bursts are found to be synchronous (to within the instrumental time resolution of 5.5 s) in all wavelengths observed, corresponding to emissions from temperatures ranging from upper chromospheric to coronal. The distribution with temperature of a suitably defined emission measure parameter is also examined as a function of time throughout the bursts and a marked similarity in the shape of this distribution, both between different events and throughout the time history of any particular event, is noted. The significance of these observations for physical processes associated with EUV bursts is briefly discussed.On leave from Dept. of Astronomy, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K.     

Association between gradual hard X-Ray emission and metric continua during large flares

X-ray radiation is used to study coronal phenomena in conjunction with meter wave observations during some large solar flares. It is found that metric flare continua and moving type IV bursts are associated with gradual and long lasting (a few tens of minutes) microwave and hard X-ray emissions. The detailed temporal analysis reveals that although metric and hard X-ray sources are located at very different heights, both kinds of emission result from a common and continuous/repetitive injection of electrons in the corona. The late part of the metric event (stationary type IV burst) is only associated with soft X-ray radiation. This indicates that the mean energy of the radiating electrons is lower during stationary type IV bursts than during the earlier parts of the event.     

Filament eruption and storm radiation at meter-decameter wavelengths

We report the study of a weak noise storm observed by the Clark Lake multifrequency radioheliograph at four frequencies. The noise storm onset was associated with a filament eruption and a gradual rise and fall in soft X-rays. We compare the noise storm emission with related emissions in other wavelengths to develop a composite scenario of the event. Using the properties of the quiet corona inferred from the simultaneously observed quiet-Sun radiation, we estimate the brightness temperature of the storm continuum, which seems to be consistent with the observations reported in Solar Geophysical Data. Superthermal particles with a temperature that is ten times the coronal electron temperature and a density of 10^–3 times the coronal density are adequate to explain the observed radiation. Since the noise storm observations were made at four frequencies, we were able to obtain a brightness temperature spectrum of the storm radiation. If the storm radiation is affected in the same way as the quiet-Sun emission by inhomogeneities, the observed spectrum can be interpreted as due to propagation effects. Since the Clark Lake instrument can observe both quiet Sun and weak bursts simultaneously, we were able to estimate the propagation effects from the quiet-Sun observations and use it to correct the brightness temperature of storm radiation.     

Results of observation of spectra and polarization of meter solar radio emission with high time resolution: May–June, 1969

Simultaneous observations of spectra and polarization of two noise storms with high time resolution have been performed in IZMIRAN during the periods: May 17–23 and June 7–13, 1969. The results of the analysis show that for different noise storms Type I bursts and chains of Type I bursts possess different spectral and polarization characteristics and different tendencies in variation of these characteristics from day to day. In particular, the first stage of the noise storm in May presented some Type I bursts which displayed a varying degree of polarization within their individual lifetimes. In addition, 112 Type III bursts with weak or moderate polarization were observed.     

On the possibility of radio emission from quasi-parallel and quasi-perpendicular propagation of shocks

A set of 21 solar type II radio bursts observed using Hiraiso radio spectrograph have been analysed to study the direction of propagation of coronal shocks. A simple analysis is carried out to find the approximate angle between the shock normal and magnetic field by solving the Rankine-Hugoniot MHD relation with assumption of Alfven speed and plasma beta. From this analysis, it is suggested that both quasi-parallel shocks (favourable) and quasi-perpendicular shocks can generate type II bursts depending upon the circumstances of the corona.