Solar radio spectra between 160 and 320 MHz

Dynamic spectra of incremental solar radio emission reveal that (mostly faint) pulsating and morphologically similar structures, including patterns of bright spots, dominate between ca. 250 and 320 MHz. As individual bursts these bright spots are similar to Type I bursts but they appear in an entirely different environment. This kind of activity is indicative of Type IV emission and seems to favour the generation of Type III bursts. Often its boundary with storm activity at the lower side of the band is rather sharp.     

Quasi-periodic solar radio pulsations at decimetric wavelengths

Observations are reported of radio-pulsations at dm-wavelengths. The pulsations are quasiperiodic with a period of 0.5 s, they have a bandwidth > 300 MHz and show up to a 50% enhancement of the underlying type IV continuum.     

Some characteristics of pulsations in radio bursts at 9.375 GHz

A type of pulsation in a time scale of seconds superimposed on microwave burst at 9.375 GHz has been found during the twenty-second solar active maximum period by us. This phenomenon is quite different from radio spike emission at decimeter and long centimeter wavelengths. The flux level of the bursts rises as the repetition rate of pulsations increases, following an approximate linear relationship. This feature resembles that at mm wavelength, but some other features are different. Some mechanisms for interpretation have been proposed.     

Dual pulsations in solar radio bursts at short centimeter wavelengths

It is well known that the oscillating MHD waves drive periodic variations in the magnetic field. But how the MHD waves can be triggered in the flaring loops is not yet well known. It seems to us that this problem should be connected with the physical processes occurring in the flare loop during a solar flare. A peculiar solar flare event at 04:00–04:51 UT on May 23, 1990 was observed simultaneously with time resolutions 1 s and 10 ms by Nanjing University Observatory and Beijing Normal University Observatory, which are about 1000 km apart, at 3.2 cm and 2 cm wavelengths, respectively. Two kinds of pulsations with quasi-periods 1.5 s and 40 s were found in radio bursts at the two short centimeter waves; however, the shorter quasi-periodic pulsations were superimposed upon the longer ones. From the data analysis of the above-mentioned quasi-periodic pulsations and associated phenomena in radio and soft X-ray emissions during this flare event published in Solar Geophysical Data (SGD), the authors suggest that the sudden increase in plasma pressure inside (or underlying) the flare kernel due to the upward moving chromospheric evaporated gas, which is caused by the explosive collision heating of strong non-thermal electrons injected downwards from the microwave burst source, plays the important role of triggering agents for MHD oscillations (fast magneto-acoustic mode and Alfvén mode) of the flare loop. These physical processes occurring in the flare loop during the impulsive phase of the solar flare may be used to account for the origin and observational characteristics of quasi-periodic pulsations in solar radio bursts at the two short centimeter wavelengths during the flare event of 1990 May 23. In addition, the average physical parameters N, T, B inside or underlying the flare kernel can be also evaluated.     

A model for solar radio pulsations at short centimetric band

In this paper, the observed solar radio pulsations during the bursts at 9.375 GHz are considered to be excited by some plasma instability. Under the condition of the conservation of energy in the wave-particle interaction, the saturation time of plasma instabilities is inversely proportional to the initial radiation intensity, which may explain why the repetition rate of the pulsations is directly proportional to the radio burst flux at 9.375 GHz as well as 15 GHz and 22 GHz. It is also predicted that the energy released in an individual pulse increases with increasing the flux of radio bursts, the modularity of the pulsations decreases with increasing the flux of radio bursts, these predictions are consistent with the statistical results at 9.375 GHz in different events. The energy density of the non-thermal particles in these events is estimated from the properties of pulsation. For the typical values of the ambient plasma density (10⁹ cm^–3) and the ratio between the nonthermal and ambient electrons (10^–4), the order of magnitude of the energy density and the average energy of the nonthermal electrons is 10^–4 erg/cm³ and 10 kev, respectively. It is interesting that there are two branches in a statistical relation between the repetition rate and the radio burst flux in a special event on March 11–17, 1989, which just corresponds to two different orders of magnitude for the quasi-quantized energy released in these five bursts. This result may be explained by the different ratios between the thermal and the nonthermal radiations.     

Solar radio emission at 1.2 mm wavelength

Some properties of solar active regions at 1.2 mm wavelength are discussed. Equatorial and polar brightness distributions of the quiet Sun at 1.2 mm wavelength are also presented.     

Structure and evolution of solar radio bursts at 26.4 MHz

Two dimensional source brightness distributions at 26.4 MHz for solar bursts of spectral type II, III, IV, and V are derived from observations with a multiple-baseline, time-sharing interferometer system. It was designed explicitly to study the large angle (40 halo) component of low frequency solar bursts first reported by Weiss and Sheridan (1962). Thirty-two bursts occurring in the interval of June–August, 1975, were fit with a circular gaussian core and an elliptical gaussian halo component. Half-power halo diameters (E-W×N-S) averaged 30×28 for type III bursts and 42×27, 28×37, 30×25 for type V, II and IV bursts respectively. Typical core sizes fell in the range of 10±4 giving 31 halo to core size ratio. All burst types were found to have some large angle structure: the specific intensity was 10% compared to the core but the total power in each component was comparable. Two processes for producing the core-halo structure of type III bursts are compared: scattering and refraction of a point source and refraction from many sources over an extended region. It is concluded that the core can be explained by either model but the halo is more consistent with emission from an extended source region of 40° in longitude.     

Search for giant pulses of radio pulsars at frequency 111 MHz with LPA radio telescope

We have used the unique low frequency sensitivity of the Large Phased Array(LPA) radio telescope of Pushchino Radio Astronomy Observatory to collect a dataset consisting of single pulse observations of second period pulsars in the Northern Hemisphere. During observation sessions in 2011–2017, we collected data on 71 pulsars at a frequency of 111 MHz using a digital pulsar receiver. We have discovered Giant Radio Pulses(GRPs) from pulsars B0301+09 and B1237+25, and confirmed earlier reported generation of anomalously strong(probable giant) pulses from B1133+16 in a statistically significant dataset. Data for these pulsars and from B0950+08 and B1112+50, earlier reported as pulsars generating GRPs, were analyzed to evaluate their behavior over long time intervals. It was found that the statistical criterion(power-law spectrum of GRP distribution of energy and peak flux density) seems not to be strict for pulsars with a low magnetic field at their light cylinder. Moreover, spectra of some of these pulsars demonstrate unstable behavior with time and have a complex multicomponent shape. In the dataset for B0950+08, we have detected the strongest GRP from a pulsar with a low magnetic field at its light cylinder ever reported, having a peak flux density as strong as 16.8 kJy.     

Theory of radio pulsations in coronal loops

Pulsations include a wide range of phenomena from strictly sinusoidal oscillations up to quasiperiodic fine structures, observed in the radio, microwave and X-ray frequency range. The various versions of pulsation models are reviewed and classified in three groups according to their driver mechanisms: (1) Magnetic flux tube oscillations (the emissivity of trapped particles is modulated by a standing or propagating MHD wave), (2) cyclic self-organizing systems of plasma instabilities (wave-particle, wave-wave interactions), and (3) modulation of acceleration (acceleration/injection of particles into the source). Observational references illustrate the applicability of the models. In conclusion, discrimination criteria of models are discussed, in order to give a key for interpretation of observations.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

Coronal X-ray holes and the quiet radio Sun at 2800 MHz

Radio cool regions observed on strip scans of the Sun made at 2800 MHz with a 1.5 min arc fan beam are associated with X-ray coronal holes and are used to derive lower envelopes which are similar to spotless Sun drift curves. Fluxes are evaluated from the Ottawa-ARO solar patrol;. e.g. that of Coronal Hole 1 observed during the Skylab Mission with central meridian passage on July 25, 1973 is 66.5 s.f.u. ± 0.6%. This level is identified as that observed during sunspot minimum by comparison with the flux of 67.2 observed in July, 1964, and with the low daily values of 67.5 and 67.1 observed in April, 1975 and January, 1976. The enhancement of the quiet Sun of 3.0 s.f.u. for the optically inactive hemisphere of May 20, 1974 suggests that the radio quiet Sun may vary during the sunspot cycle.     

Polarization measurements of solar type III radio bursts at 25.3 MHz

We report the results of 1966, 1968, and 1969 polarization measurements of solar type III radio noise bursts made by recording the output of two orthogonally polarized receiving channels and subsequent digital processing of selected data. The processed data yield total intensity, degree of polarization, ellipticity, and polarization ellipse orientation at 1 second intervals. The measurements are made in a 100 Hz bandwith to minimize the influence of the propagating medium on the measurements. The mean degree of polarization was found to be about 65% in contrast to previous studies which indicated that type III events were more weakly polarized. By assuming that type III bursts are flare related we study the polarization characteristics of type III bursts as a function of the solar longitude of the related flares. The relation between type III event polarization characteristics and flare importance is also investigated. The significance of polarization measurements in studies of solar radio events is pointed out and suggestions for further theoretical research are given.     

A torsional wave model for solar radio pulsations

One of the widely accepted models for solar radio pulsations invokes radial oscillations of a magnetic flux tube. Due to acoustic, radiative damping, this theory does not easily explain the long length of the pulse trains, the large modulation depths or the great stability of the pulse repetition rate often observed. Torsional waves efficiently modulate synchrotron emission, and since they do not undergo radiative damping, can produce stable pulse repetition rates and long pulse trains.     

Solar radio emission at 9.1 cm and sector boundaries

A possible connection between solar radio emission from 1.0 to 9.4 GHz and the interplanetary sector boundaries has been previously reported in the literature. The present research does not support the previous work as expected. The 9.1 cm activity appears to be organized around sector boundaries only in a very limited sense in that the distribution of very strong active regions peaks near the –/+ boundaries. However, this phenomenon is only observed during the most active part of the solar cycle. A peculiar asymmetry is found regarding the length of the positive and negative sectors.     

Solar radio bursts with a spectral flattening at millimeter wavelengths

An analysis of solar radio burst spectra in the range 3–80 GHz is carried out using measurements of the observatories at Bern and Nobeyama supplemented by data from worldwide network stations. Special interest was focused on strong events at frequencies above 30 GHz. It is found that there exists an extended group of events with a flattening of the spectra at millimeter wavelengths. In particular, two types of flattening are observed: (i) a high-frequency flattening either following a monotonic spectral flux increase at cm-waves or forming a flat broad-band spectrum at mm-wavelengths ; (ii) a millimetric flattening as a decrease of the slope (i.e., a hardening) of the descending branch of the spectrum having a peak in the microwave range. Besides this, in complicated bursts a strong temporal evolution of millimeter spectra may occur resulting in either type of the flattening. Some factors capable of producing the millimeter flattening are considered: (1) superposition of multiple source regions of gyrosynchrotron radiation, (2) gyromagnetic radiation from a two-component energy spectrum of the accelerated electrons at high energies, or by a temporal hardening of the electron spectrum during extended flares, and (3) optically thin bremsstrahlung of evaporated plasma.Presented at the CESRA Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994.     

Short-period pulsations observed simultaneously by X-ray and radio waves

From simultaneous high-time-resolution observations of solar X-rays from Hinotori and the millimeter waves at Itapetinga Radio Observatory in Brazil during a solar flare on November 4, 1981 at 1827 UT, short period ( 300 ms) pulsations have been detected in five time intervals of 2 s each. Both a cross-correlation analysis between X-rays and microwaves and a Fourier analysis were made to verify the significance of the quasi-periodic pulsations. The cross-correlation is significant but the pulsations could not be periodic oscillation.on leave of absence from Physical Res. Lab., Ahmedabad, India