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.     

The origin of solar energetic particles and type II meter radio bursts

The relationship between the proton intensity in the interplanetary space and radio bursts of type II for 78 proton events for the period of 1989–2005 is studied based on the data of the Radio Solar Telescope Network. Two families of events have been revealed in plots describing the dependence of the intensity of protons with different energies and the rate of the frequency drift of meter-decameter radio bursts. This suggests the generation of shock waves both in the region of flare energy release and at the fronts of coronal mass ejection.     

On the origin of gamma-ray bursts

A model for gamma-ray bursters which naturally explains the main observational features of GRB is proposed. The GRB turned out to be linked by evolution with a well-known type of astrophysical object.     

Statistical analysis of high-frequency decimetric type III bursts

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 t_1/2=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 2^ndharmonic, the beam parameters of type III bursts are determined.     

On the source conditions for herringbone structure in type II solar radio bursts

We investigate the correlation of the occurrence of the herringbone phenomenon in type II solar radio bursts with various flare properties. We show that herringbone is strongly correlated with the intensity of the type II burst: whereas about 21% of all type II bursts show herringbone, about 60% of the most intense bursts contain herringbone. This fact can explain most of the correlations between herringbone and other properties such as intense type III bursts, type IV emission, and high type II starting frequencies. We also show that when this is taken into account, there is no need to postulate two classes of type II burst in order to explain why there appears to be a difference in herringbone occurrence between the set of type II bursts associated with the leading edges of coronal mass ejections, and those not so associated. We argue that the data are consistent with the idea that all coronal type II bursts are due to blast waves from flares.     

On the spatial directivity of solar radio bursts

In this paper, a new method of estimating the spatial directivity (in the form of center-to-limb variation) of microwave burst emission is proposed and derived. Estimations of radioemission directivity values vs observation frequency are obtained. Results are compared to the radio source model using an inhomogeneous magnetic field, source size and particle density, and show a high degree of agreement. Values of model parameters from earlier estimations are confirmed.     

On the stellar origin of gamma ray bursts

We show that our original suggestion that gamma-ray bursts (GRB) may be flares on Magnetically Active Stellar Systems (MASS) namely flare stars, RS CVn binaries and Cataclysmic variables agrees well with the new observations of CGRO. We make a multi component fit to the log(N) - log(S) distribution and the high degree of isotropy as observed by the previous generation of satellites as well as BATSE/CGRO using the second BATSE catalogue. We then discuss individual source association and optical transient observations and show that they favor the present suggestion. We discuss the physical mechanisms and gamma-ray production processes that can occur on such systems giving the GRB their characteristics. We predict increase of anisotropy in the BATSE/CGRO observations for bright GRB.     

On the superfine structure of solar microwave bursts

Solar microwave bursts with a zebra pattern commonly exhibit a superfine time structure: the zebra stripes consist of separate spike-like pulses. We investigate the superfine structure in the April 21, 2002 event. The emission pulses are shown to exhibit a high periodicity (with a period of about 30 ms); there is a clear correlation between the individual zebra stripes. This structure of the dynamic spectra most likely reflects a periodic injection of electron beams, which generate emission at the double plasma resonance levels.     

Deterministic behaviour in the dynamics of solar metric radio bursts with intermediate drifting patterns

The dynamic characteristics of solar metric radio bursts with intermediate drifting patterns (fiber bursts) as they evolve at fixed frequency are examined. The data were recorded using the radio polarimeter of the Trieste Astronomical Observatory. The aim is to determine if the underlying process can be described as a deterministic chaos. Correlation dimensions and Hurst exponent are estimated showing deterministic chaotic system of low dimension.     

On the spectrum of solar impulsive hard X-ray bursts

This article puts forward a new method for the theoretical analysis of the X-radiation spectrum of impulsive hard X-ray bursts. It points out that the electron density energy state function must obey the fundamental kinetic equation. In the case of several model source functions, the electron density energy spectra are deduced. This can serve as a basis for an analysis of the spectrum of X-radiaiton in impulsive hard X-ray bursts. The article also makes a preliminary discussion of these energy state functions which help to explain the phenomena of softening of the X-radiation spectrum.     

Comparable energy release rates at the origin of small and large solar bursts

The discovery that the simplest impulsive solar bursts at microwaves and hard X-rays have time scales in the range of 10–100 ms suggest that the energy release rates in such small events are comparable to rates attributed to large events. This condition may be reconciled to the concept that observed burst fluxes in solar events are the convolution of many fast primary energetic injections, at various repetition rates.     

A comparison of type III metric radio bursts and global solar potential field models

We compare evidence of coronal magnetic fields from polarized metric type III radio bursts with (a) global potential field models, (b) direct averages of the observed photospheric magnetic field, and (c) H synoptic charts. The comparison clearly indicates both that the principal aspects of type III burst radiation are understood and that global potential field models are a significantly more accurate representation of coronal magnetic field structure than either the large-scale photospheric field or H synoptic charts.     

Characteristics of flares producing metric type II bursts and coronal mass ejections

We attempt to study the origin of coronal shocks by comparing several flare characteristics for two groups of flares: those with associated metric type II bursts and coronal mass ejections (CMEs) and those with associated metric type II bursts but no CMEs. CMEs accompany about 60% of all flares with type II bursts for solar longitudes greater than 30°, where CMEs are well observed with the NRL Solwind coronagraph. H flare areas, 1–8 Å X-ray fluxes, and impulsive 3 cm fluxes are all statistically smaller for events with no CMEs than for events with CMEs. It appears that both compact and large mass ejection flares are associated with type II bursts. The events with no CMEs imply that at least many type II shocks are not piston-driven, but the large number of events of both groups with small 3 cm bursts does not support the usual assumption that type II shocks are produced by large energy releases in flare impulsive phases. The poor correlation between 3 cm burst fluxes and the occurrence of type II bursts may be due to large variations in the coronal Alfvén velocity.Sachs/Freeman Associates, Inc., Bowie, MD 20715, U.S.A.     

Interference origin of the fine structure in the dynamic spectra of solar radio bursts

The multibeam propagation of radio waves in the solar plasma is analyzed, because the emission from a solar flare passes through an inhomogeneous solar atmosphere on its way to the observer. A formula (a mathematical model) for calculating the structure of the dynamic spectrum for flare radio bursts has been obtained. Comparison of the calculated spectra with the observed ones shows that the results of interference explain the formation of a zebra structure and the separation of its stripes into individual spikes, describe the time profile of the spikes, and explain the properties of fibers, ropes of fibers, and chains of “point” bursts. The similarity of the dynamic spectra testifies that the fine structure of the spectra is formed not in the emission source but as a result of the propagation of waves through the solar corona and interplanetary space.     

Herringbone bursts associated with type II solar radio emission

We report detailed observations of the herringbone (HB) fine structure on type II solar radio bursts. Data from the Culgoora radiospectrograph, radiometer and radioheliograph are analyzed. We determine the characteristic spectral profiles, frequency drift rates and exciter velocities, fluxes, source sizes, brightness temperatures, and polarizations of individual HB bursts. Correlations between individual bursts within the characteristic groups of bursts and the properties of the associated type II bursts are examined. Our data are compatible with HB bursts being radiation at multiples of the plasma frequency generated by electron streams accelerated by the type II shock. We conclude that HB bursts are physically distinct phenomena from type II and type III bursts, differing significantly in emission processes and/or source conditions; this conclusion indicates that many of the presently available theoretical ideas for HB bursts are incorrect.Now at: Department of Physics and Astronomy, University of Iowa, U.S.A.Now at Anglo-Australian Observatory, Sydney, Australia.     

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.     

On the origin of solar flare X-rays

The origin of X-ray solar bursts is investigated on the basis of the theoretical model developed by Syrovatskii. According to this model (i) one of the most important manifestations of flares is the acceleration of charged particles (mainly of electrons) to subrelativistic and relativistic energies, and (ii) the two flare phases: stationary (soft) and nonstationary (hard) should be distinguished. The first phase is accompanied by the generation of the soft (2–8 Å) thermal X-rays and the second one by the generation of hard thermal and nonthermal X-rays in the 10 keV range. The thermal X-rays arise in both phases due to the heating of the ambient gas by accelerated particles. The possible mechanisms of non-thermal X-rays are investigated. Simple models of the emitting region are considered, taking into account the simultaneous observations in different regions of the electromagnetic spectrum.     

On the origin of solar cycle periodicity

Recently, the origin of the solar cycle is considered to be rooted in the dynamics of the solar core (Grandpierre, 1996). The dynamic solar core model requires macroscopic flow and magnetic field as basic inputs. The macroscopic flow cannot be generated by the quasistatic solar structure and it has to reach a larger than critical size (Grandpierre, 1984) in order to survive dissipation. Therefore the flow must be generated by outer agents. The most significant outer agents to the Sun are the planets of the Solar System. These theoretical arguments are supported by observations showing that planetary tides follow a pattern correlating with the solar cycle in the last three and a half centuries (Wood, 1972; Desmoulins, 1995). Therefore the pulsating-ejecting solar core model gives a firm theoretical basis for the interpretation of these largely ignored observations. In this paper a new and simple calculation is presented which enlightens the planetary origin of the eleven-year period and gives a physical basis for a detailed modelling of the dynamo and the solar cycle.     

On the polarization of solar microwave bursts observed at 17 GHz

The polarization distribution of 17 GHz bursts is studied observed within a period of 1 yr after maximum solar activity. The typical variation of polarization with time of impulsive bursts leads to the conclusion of a thermalization of the emission region in the post-burst phase. The fine structure of the polarization curve of complex bursts is shown and two possible interpretations of the observed inversion of the polarization at 17 GHz during a complex event are given.