Statistical Analysis of Decimetric Type III Bursts

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.     

On the observation of linear polarization of solar microwave bursts

It is known that mode coupling may occur in quasi-transverse magnetic field regions of the solar corona, which produces linear polarization at microwave frequencies. A microwave polarimeter measuring all 4 Stokes parameters at 8.918 GHz simultaneously at three different highfrequency bandwidths (40 kHz, 400 kHz and 5 MHz) has been developed in order to observe the linear component and its Faraday rotation. The respective minimum detectable changes of the Stokes parameters I, Q, U and V are 9, 3 and 1 solar flux unit at an integration time of 1 s. For burst intensities greater than 300 solar flux units, the minimum detectable degree of linear and circular polarization is 1 %–3 %, depending on the bandwidth. Observations of 68 bursts showed that most of the bursts were circularly polarized. No linear polarization could be found within the limits of accuracy of our polarimeter. Two possible explanations for this result are discussed. The possibility of mode coupling however cannot be excluded from these first observations.     

A Statistical Analysis of the Type-III Bursts in Centimeter and Decimeter Wavebands

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.     

Fast time structures superimposed to impulsive solar microwave bursts with slowly varying or stationary polarization degree

The ultimate definition of fast time structures superimposed on an impulsive solar microwave burst is limited by instrumental time resolution and sensitivity. We analysed 7 GHz bursts with a time constant of 100 ms. The fast time structures seem to be common to all events, although the resolution so far attained might still be smoothing out structures with finer scale. The polarization degree does not show corresponding fast changes. When the degree of circular polarization is referred to the burst's excess flux, it may show a slowly varying time development. When it is referred to the total active center contribution, the polarization degree might become nearly unchanged during the burst development. The polarization degree is set by the large scale magnetic field strength and morphology over the active center and the burst source. The present results suggest that the microwave fast component burst source might remain nearly stationary in relation to the polarizing medium, occupying the same position as the active center hot spot previous to the event. The absence of fast time structures in polarization degree indicate negligible fast changes in the large scale magnetic field which pervades the burst source. Slow changes in polarization degree are sometimes associated with the slow component of impulsive events, and might be representative of secondary accelerations interpreted in terms of trap models. We discuss qualitatively some energy conversion mechanisms based on turbulent processes which may account for the fast burst components.Formerly Centro de Rádio-Astronomia e Astrofisica Mackenzie, now absorbed by CNPq and being re-organized in connection to Observatio Nacional.     

Polarization of loop-top and footpoint sources in microwave bursts

The polarization is analyzed in four microwave bursts with one loop-top and two footpoint sources observed at 17 GHz with the Nobeyama Radioheliograph (NoRH). The loop-like structure of the four events is confirmed by simultaneous SOHO/MDI magnetograms and TRACE/EUV images or Yohkoh/SXT images. The heliocentric distance of the four events is greater than 30°. The three microwave sources in each given burst are polarized in the same sense. This may be interpreted in terms of extraordinary mode emission, taking into account the polarity of the underlying magnetic field and propagation effects, which may lead to inversion of the sense of polarization in the limbward foot and loop-top source of the flaring loop.     

Circular Polarization in a Solar Filament

Integrating 26 624 pairs of video frames, the authors have mapped the circular polarization in an active-region filament against the solar disk by using a traditional magnetograph working at the Hβ line. This filament, offset the disk center, appeared at the boundary of three decayed active regions. It was quiet and away from any strong enhanced network. The mapped circular polarization in the filament has an average polarization degree of 1.1×10⁻³ with a measurement precision of 4×10⁻⁴. The mapping of circular polarization in a filament may provide a supplementary diagnosis of the filament magnetic field, in addition to the mapping of linear polarization via the Hanle effect. However, the interpretation of the circular polarization requires treatment of the full quantum problem of Zeeman and non-Zeeman effects of Stokes line profiles.     

Observations of Powerful Type III Bursts in the Frequency Range 10 – 30 MHz

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.     

Polarization of solar active regions at 3.5 millimeter wavelength

A study of the circular polarization structure of solar active regions has been made from data obtained at 3.5 mm wavelength, using the 36 ft diameter radio telescope of the National Radio Astronomy Observatory at Kitt Peak, Arizona. The angular resolution of the telescope at this wavelength is 1.2. All important active regions observed at 3.5 mm are bipolar in nature; the degree of polarization ranges from 1 to about 2%. These oppositely polarized components correspond with the Mt. Wilson magnetic regions of opposite polarity; the line of zero polarization delineates the neutral line between the regions of opposite polarity on magnetograms. The longitudinal magnetic fields at the level of 3.5 mm emission computed from the degree of polarization are found to be several hundred gauss.     

Calculations of Coronal Magnetic Field Parallel and Perpendicular to Line-of-Sight in Microwave Bursts

Based on the approximations of the non-thermal gyro-synchrotron radiation used by G.A. Dulk, and K.A. Marsh, Astrophs. J. 259, 350, 1982, the author analyses calculations of the propagation angle and coronal magnetic field self-consistently with brightness temperature, spectral index, frequency, turnover frequency, and polarization degree in solar microwave bursts. Hence, the coronal magnetic fields parallel and perpendicular to the line-of-sight in the sources of microwave bursts are calculated theoretically, and discussed in an event observed by the Nobeyama Radio Polarimeters (NoRP).     

Observation of the magnetic structure of a type IV solar radio outburst

A continuous record of the 80 MHz image and polarization of a type IV solar outburst has been made with the Culgoora radioheliograph from which the magnetic structure of the event can be directly inferred. The first (‘moving’) part of the event appears beyond the limb as an expanding magnetic arch along which three concentrated sources develop: one unpolarized source near the peak, attributed to synchrotron radiation; and two polarized sources of opposite polarity near the feet, attributed to plasma radiation. The radio-emitting arch appears to lie above an eruptive prominence seen in Hα. The second (‘stationary’) part is seen later as a separate highly polarized source on the disk above the projected position of the flare that had previously triggered the prominence activity.     

The microwave spectrum of solar millisecond spikes

The microwave radiation from solar flares sometimes shows short and intensive spikes which are superimposed on the burst continuum. In order to determine the upper frequency limit of their occurrence and the circular polarization, a statistical analysis has been performed on our digital microwave observations from 3.2 to 92.5 GHz. Additionally, fine structures have been investigated with a fast (5 ms) 32-channel spectrometer at 3.47 GHz. We found that 10% of the bursts show fine structures at 3.2 and 5.2 GHz, whereas none occurred above 8.4 GHz. Most of the observed spikes were very short ( 10 ms) and their bandwidth varied from below 0.5 MHz to more than 200 MHz. Simultaneous observations at two further frequencies showed no coincident spikes at the second and third harmonic. The observations can be explained by the theory of electron cyclotron masering if the observed bandwidths are determined by magnetic field inhomogeneities or if the rise times are independent of the source diameters. The latter would imply source sizes between 50 and 100 km.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

Polarization of Microwave Radio Emission of Flare-Producing Solar Active Regions

On the basis of our multiwavelength observations made with the one-dimensional RATAN-600 radio telescope, we study the inversion of the circular polarization in the solar microwave emission at different frequencies. The inversion is detected in the emission of flare-producing active regions (FPARs) at various stages of their development, starting from the pre-flare stage. During the latest 23rd solar cycle maximum, numerous FPARs revealed spectral inhomogeneities in their polarized microwave radiation (Bogod and Tokhchukova, 2003, Astron. Lett. 29, 263). Here, we discuss a particular case of such inhomogeneities, the frequency-dependent double inversion of the sign of circular polarization, which probably reflects some essential processes in FPARs. We consider several mechanisms for the double inversion: linear interaction of waves in the region of a quasitransverse magnetic field, the propagation of waves through a region of zero magnetic field, the scattering of radio waves on waves of high-frequency plasma turbulence, the influence of the current fibrils on the propagation of the radio emission, and the magnetic “dips,” in which the direction of magnetic field lines changes the sign relative to the observer. All of them have shortcomings, but the last mechanism explains the observations the best.     

Analysis of spike emission data at 2545/2645 MHz in the peak year of the 22nd solar cycle

We briefly discuss the observed features including the high flux density, short duration, narrow emission band, fast frequency drift, quasi-periodic oscillation and fast variation of polarized components, of 51 spike emission events observed at 2545/2645 MHz in the solar activity peak year, 1991 January–December, and carry out correlation analysis between these events and optical flares, magnetic field intensity and configuration of flare regions, and sunspot evolution types of active regions. In view of the fact that the observed and statistical characteristics of the spike emissions are very different from those of known types of solar radio burst and known solar radio components, we think that the spike emission in the peak years is probably a new type of radio burst excited by electron cyclotron maser instability under wave-particle resonance, or a new solar radio component.     

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.     

A broadband spectrometer for decimeter and microwave radio bursts

Observations of solar microwave bursts with high temporal and spectral resolution have shown interesting fine structures (FSs) of short duration and small bandwidth which are usually superimposed on the smooth continuum. These FSs are very intense (up to 10¹⁵ K) and show sometimes a high degree of circular polarization (up to 100%). They are believed to be generated by electron cyclotron maser emission (ECME) in magnetic loops. Another type are the microwave type III bursts, which are drifting microwave FSs, and are probably the signatures of travelling electron beams in the solar atmosphere. The exact emission mechanisms for these phenomena, in particular the source configuration, the plasma parameters and the distribution of radiating electrons are not clear. For a detailed study of these problems new observations of intensity and polarization with high resolution in time and in frequency in decimeter and microwave wavebands are essential. In order to investigate these features in greater detail, spectrometers with high temporal and spectral resolution are being developed by the solar radio astronomy community of China (Beijing Astronomical Observatory (BAO), Purple Mountain Observatory (PMO), Yunnan Astronomical Observatory (YAO), and Nanjing University (NJU)). The frequency range from 0.7 to about 12 GHz is covered by about five spectrometers in frequency ranges of 0.7–1.4 GHz, 1–2 GHz, 2.4–3.6 GHz, 4.9–7.3 GHz, and 8–12 GHz, respectively. The radiospectrometers will form a combined type of swept-frequency and multi-channel receivers. The main characteristics of the solar radio spectrometers are: frequency resolution: 1–10 MHz; temporal resolution: 1–10 ms; sensitivity: better than 2% of the quiet-Sun level. We pay special attention to the sensitivity and the accuracy of polarization. Now, the 1–2 GHz radiospectrometer is being set up. The full system will be set up in 3–4 years.Presented at the CESRA-Workshop on Coronal Magnetic Release at Caputh near Potsdam in May 1994.     

Observations of Microwave Type-U Bursts

Two microwave type-U bursts observed with the 2.6–3.8 GHz spectrometer of Beijing Astronomical Observatory (BAO) are described and analysed in this paper. The microwave type-U bursts have very short durations (about 200 ms), narrow bandwidth, high-frequency drift rates of the ascending and descending branches, and a high degree of circular polarization (80%). The sense of polarization remains the same from the ascending to the descending branch.     

Relation between circular polarization of moving type IV bursts and polarity of photospheric magnetic fields

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.     

Source structure of gradual rise and fall bursts at 17 GHz

From 200 GRF (gradual rise and fall) bursts which have been recorded with the 17 GHz interferometer at Nobeyama, we deduce the following characteristics of GRF bursts: (1) Sources of GRF bursts are broader, less circularly polarized than those of impulsive bursts. (2) The sources are potentially of bipolar structure and have the peak brightness near the position at which the sense of circular polarization changes. (3) The association of GRF bursts with type III bursts, which are indicative of nonthermal electron acceleration, is significantly poorer than that of impulsive bursts.It is suggested that the sources of GRF bursts or generally of thermal bursts lie relatively high in the solar atmosphere possibly near the top of magnetic loops or arches which divide two regions of opposite magnetic polarity.     

Solar S-bursts at Frequencies of 10 – 30 MHz

Solar S-bursts observed by the radio telescope UTR-2 in the period 2001 – 2002 are studied. The bursts chosen for a detailed analysis occurred in the periods 23 – 26 May 2001, 13 – 16 and 27 – 39 July 2002 during three solar radio storms. More than 800 S-bursts were registered in these days. Properties of S-bursts are studied in the frequency band 10 – 30 MHz. All bursts were always observed against a background of other solar radio activity such as type III and IIIb bursts, type III-like bursts, drift pairs and spikes. Moreover, S-bursts were observed during days when the active region was situated near the central meridian. Characteristic durations of S-bursts were about 0.35 and 0.4 – 0.6 s for the May and July storms, respectively. For the first time, we found that the instantaneous frequency width of S-bursts increased with frequency linearly. The dependence of drift rates on frequency followed the McConnell dependence derived for higher frequencies. We propose a model of S-bursts based on the assumption that these bursts are generated due to the confluence of Langmuir waves with fast magnetosonic waves, whose phase and group velocities are equal.