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.     

Hard Burst Emission from the Soft Gamma Repeater SGR 1900+14

We present evidence for burst emission from SGR 1900+14 with a power-law high-energy spectrum extending beyond 500 keV. Unlike previous detections of high-energy photons during bursts from soft gamma repeaters (SGRs), these emissions are not associated with extraordinarily bright flares. Not only is the emission hard, but the spectra are better fitted by D. Band's gamma-ray burst (GRB) function rather than by the traditional optically thin thermal bremsstrahlung model. We find that the spectral evolution within these hard events obeys a hardness/intensity anticorrelation. Temporally, these events are distinct from typical SGR burst emissions in that they are longer ( approximately 1 s) and have relatively smooth profiles. Despite a difference in peak luminosity of greater, similar1011 between these bursts from SGR 1900+14 and cosmological GRBs, there are striking temporal and spectral similarities between the two kinds of bursts, aside from spectral evolution. We outline an interpretation of these events in the context of the magnetar model.     

Energy of solar noise-storm radio bursts

Solar noise storms (NS) are analyzed by an algorithm which separates a random signal into pulses. The burst duration distribution is shown to be inversely proportional to the squared duration of bursts. The distribution ordinates are proportional to the average pulse repetition frequency, and the distribution maximum corresponds to the limiting pulse duration equal to 0.4–0.6 s. The aggregate lifetime of all short-lasting bursts is approximately equal to the aggregate lifetime of bursts of any other duration. The energy of short-lasting bursts with a duration of 0.2–0.4 s is five times smaller than the energy of longer bursts, and it constitutes only 2–5 percent of the energy of the NS burst component. The power of bursts increases as their duration changes from 0.2 to 1.2 s until it reaches some limit at a duration of 1.2–1.4 s. The power of longer bursts remains almost unchanged up to the end of the investigated duration interval (up to durations of 300 s). Solar burst chains can be some superposition of short-lasting bursts on one longer burst. Thus, the burst energy measurements do not support the widespread point of view that solar noise storms consist of short-lasting type I bursts.     

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.     

A Study of Solar Radio Bursts with Fine Structures during Flare/CME Events

We have selected 27 solar microwave burst events recorded by the Solar Broadband Radio Spectrometer (SBRS) of China, which were accompanied by M/X class flares and fast CMEs. A total of 70.4% of radio burst events peak at 2.84 GHz before the peaks of the related flares’ soft X-ray flux with an average time difference of about 6.7 minutes. Almost all of the CMEs start before or around the radio burst peaks. At 2.6?–?3.8 GHz bandwidth, 234 radio fine structures (FSs) were classified. More often, some FSs appear in groups, which can contain several individual bursts. It is found that many more radio FSs occur before the soft X-ray maxima and even before the peaks of radio bursts at 2.84 GHz. The events with high peak flux at 2.84 GHz have many more radio FSs and the durations of the radio bursts are independent of the number of radio FSs. Parameters are given for zebra patterns, type III bursts, and fiber structures, and the other types of FSs are described briefly. These radio FSs include some special types of FSs such as double type U bursts and W-type bursts.     

Mathematical Modeling of the Formation of Type IIId Solar Decameter Radio Bursts with Echo Components

Using numerical simulation, we investigate the possibility that echo components of type IIId solar decameter bursts can be produced through refraction of radio emission in a coronal plasma with large-scale regular electron density inhomogeneities. It is shown that the observed time profiles of radio burst intensity with three or four maxima can be related to the presence in the middle corona of streamers and a localized regular nonuniformity whose electron density exceeds background electron density by a factor of more than 3 – 4. The nonuniformity scale in this case is comparable with the optical diameter of the solar disk. The observed radio burst echo components with delays longer than 3 s are explained by the production of additional radio emission propagation modes within a “transverse" refraction waveguide arising between the localized electron density nonuniformity and deeper layers in the corona. As these additional modes are reflected from the streamers, they can reach the Earth. Calculations of the time profile of radio burst intensity take into consideration the influence of scattering by turbulent coronal inhomogeneities and of collisional absorption. Comparison of the modeling results with the observational data shows that the calculated values of some profile parameters differ from the observed values. One of the possible reasons is that the method used does not take into account the diffraction leakage of radio emission through the large-scale nonuniformity.     

Circular polarization of solar bursts at 22 GHz

The circular polarization of complex solar bursts was measured at short microwaves (22 GHz, × 1.35 cm) with high sensitivity (0.03 s.f.u. r.m.s.) and high time resolution (5 ms). The polarization shows up as soon as an excess burst emission is measured. Two components are found in the time development of the degree of circular polarization: (1) a steady level, sometime changing smoothly with time; (2) superimposed faster polarization time structures, small compared to the basic steady degree of polarization, and often not clearly related to the burst flux time structures. The observed degrees may range from 10% to more than 85%.In memoriam (1942–1981).     

Gamma-ray bursts: the time domain

Temporal aspects of the gamma-ray burst phenomenon are reviewed in a hierarchical schema. The macrocosm - burst profiles taken as a whole - is fairly well characterized. The bimodal duration distribution can be framed in terms of discretization of pulse structures. The average burst envelope is slightly asymmetric, an aspect possibly related to spectral softening. Burst durations are longer for dim BATSE bursts, an effect explainable by either cosmic time dilation or a luminosity function governed by special relativistic beaming, or a combination. GeV emission, persisting up to thousands of seconds after burst cessation at keV-MeV energies is one of the most challenging features of bursts. On the timescale of pulses structures (the mesocosm), some properties mirror the macrocosm: rise/decay asymmetry; wider pulses and longer intervals between pulses in dim bursts than in bright ones; and the tendency of pulses to soften with time. A central clue to the burst mechanism may be the organization in time and energy, manifest as pulses, for both long and short bursts. Burst profiles appear to be well represented by pulses, accounting for the vast majority of emission in the BATSE energy band. In the microcosm, existence of a higher frequency component - with properties possibly unlike those of pulses - has not been well addressed.     

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.     

Impulsive bursts of relativistic electrons discovered during Ulysses' traversal of Jupiter's dusk-side magnetosphere

During its flyby of Jupiter in February 1992, the Ulysses spacecraft passed through the Southern Hemisphere dusk-side Jovian magnetosphere, a region not previously explored by spacecraft. Among the new findings in this region were numerous, sometimes periodic, bursts of high energy electrons with energies extending from less than 1.5 MeV to beyond 16 MeV. These bursts were discovered by the High Energy Telescope (HET) and the Kiel Electron Telescope (KET) of the COSPIN Consortium. In this paper we provide a detailed analysis of observations related to the bursts using HET measurements. At the onset of bursts, the intensity of > 16 MeV electrons often rose by a factor of > 100 within 1 min, and multiple, pulsed injections were sometimes observed. The electron energy spectrum also hardened significantly at the onset of a burst. In most bursts anisotropy measurements indicated initial strong outward streaming of electrons along magnetic field lines that connect to the southern polar regions of Jupiter, suggesting that the acceleration and/or injection region for the electrons lies at low altitudes near the South Pole. The initial strong outward anisotropies relaxed to strong field-aligned bidirectional anisotropies later in the events. The bursts sometimes appeared as isolated events, but at other times appeared in quasi-periodic series with a period of 40 min. For smaller events shorter periods of the order 2–3 min were also observed in a few cases. For large events, multiple injections were sometimes observed in the first few minutes of the event. Radio bursts identified by the Ulysses URAP experiment in the frequency range 1–50 kHz were correlated with many of the electron bursts, and comparison of the time-intensity profiles for radio and electrons shows that the radio emission typically started several minutes before the electron intensity increase was observed. For the strongest electron bursts, small increases in the low energy (> 0.3 MeV) proton counting rates were also observed. Using a computerized identification algorithm to pick out bursts from the data record using a consistent set of criteria, 121 events were identified as electron bursts during the outbound pass, compared to only three events that satisfied the same criteria during the inbound pass through the day-side magnetosphere. No similar electron burst events have been found outside the magnetopause. Estimates of the electron content of a typical large burst (> 10²⁷ electrons) suggest that these bursts may make significant contributions to the fluxes of electrons observed in Jupiter's outer magnetosphere, and in interplanetary space.     

Statistical Properties of SGR 1806-20 Bursts

We present statistics of SGR 1806-20 bursts, combining 290 events detected with the Rossi X-Ray Timing Explorer/Proportional Counter Array, 111 events detected with the Burst and Transient Source Experiment, and 134 events detected with the International Cometary Explorer. We find that the fluence distribution of bursts observed with each instrument are well described by power laws with indices 1.43, 1.76, and 1.67, respectively. The distribution of time intervals between successive bursts from SGR 1806-20 is described by a lognormal function with a peak at 103 s. There is no correlation between the burst intensity and either the waiting times until the next burst or the time elapsed since the previous burst. In all these statistical properties, SGR 1806-20 bursts resemble a self-organized critical system, similar to earthquakes and solar flares. Our results thus support the hypothesis that the energy source for soft gamma repeater bursts is crustquakes due to the evolving, strong magnetic field of the neutron star, rather than any accretion or nuclear power.     

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.     

Formation Of Coronal Mhd Shock Waves – I. The Basic Mechanism

The formation and evolution of a large amplitude MHD perturbation propagating perpendicular to the magnetic field in a perfectly conducting low plasma is studied. The perturbation is generated by an abrupt expansion of the source region. Explicit expressions for the time and the distance needed for the transformation of the perturbation's leading edge into a shock wave are derived. The results are applied to coronal conditions and the dynamic spectra of the radio emission excited by the shock are synthesized, reproducing metric and kilometric type II bursts. The features corresponding to the metric type II burst precursor and the moving type IV burst in the case of kilometric type II bursts are identified. A specific radio signature that is sometimes observed at the onset of a metric type II burst is found to appear immediately before the shock wave formation due to the associated growth of the magnetic field gradient. Time delays and starting frequencies of bursts' onsets are calculated and presented as a function of the impulsiveness of the source-region expansion, using different values of the ambient Alfvén velocity and various time profiles of the expansion velocity. The results are confronted with the observations of metric and kilometric type II solar radio bursts.     

太阳米波和分米波Ⅱ型、Ⅲ型射电暴及其精细结构观测研究进展

太阳米波和分米波的射电观测是对太阳爆发过程中耀斑和日冕物质抛射现象研究的重要观测手段。米波和分米波的太阳射电暴以相干等离子体辐射为主导,表现出在时域和频域的多样性和复杂性。其中Ⅱ型射电暴是激波在日冕中运动引起电磁波辐射的结果。在Ⅱ型射电暴方面,首先对米波Ⅱ型射电暴的激波起源问题和米波Ⅱ型射电暴与行星际Ⅱ型射电暴的关系问题进行了讨论;其次,结合Lin-Forbes太阳爆发理论模型对Ⅱ型射电暴的开始时间和起始频率进行讨论:最后,对Ⅱ型射电暴信号中包含的两种射电精细结构,Herringbone结构(即鱼骨结构)和与激波相关的Ⅲ型射电暴也分别进行了讨论。Ⅲ型射电暴是高能电子束在日冕中运动产生电磁波辐射的结果。在Ⅲ型射电暴方面,首先介绍了利用Ⅲ型射电暴对日冕磁场位形和等离子体密度进行研究的具体方法;其次,对利用Ⅲ型射电暴测量日冕温度的最新理论进行介绍;最后,对Ⅲ型射电暴和Ⅱ型射电暴的时间关系、Ⅲ型射电暴和粒子加速以及Ⅲ型射电暴信号中包含的射电精细结构(例如斑马纹、纤维爆发及尖峰辐射)等问题进行讨论并介绍有关的最新研究进展。     

Kilometric shock-associated events and microwave bursts

The peak times of impulsive microwave bursts are compared with those of shock-associated (SA) kilometric radio events. The first peaks in these two frequency regimes are usually well-correlated in time, but the last peaks of the SA events observed at 1 MHz occur an average of 20 min after the last impulsive microwave peaks. In some cases, the SA events overlap in time with the post-burst increases of microwave bursts; sometimes there is general correspondence in their intensity time profiles. These observations suggest that the earlier components of the SA events are usually caused by electrons accelerated in or near the microwave source region. We discuss the possibility that the later components of some SA events could be associated with nonthermal electrons responsible for microwave post-burst increases, although they have traditionally been attributed to electrons accelerated at type II burst producing shocks in the upper corona.     

On the origin of pulsar ‘nulling’

We have discovered a correlation between the Venera spacecraft locations and the gamma-ray burst positions reported in the KONUS catalog (Mazetset al., 1981a). The reason for the correlation is not clear, but it could be due to spatial selection effects and/or large localization errors for weak of soft bursts. Whatever the cause, it seems likely that this systematic bias might significantly affect catalog results pertaining to gamma burst locations, intensity distributions, and spectra. For example, it can explain why the KONUS galactic latitude distribution is peaked significantly south of the galactic equator. Apparent discrepancies between the KONUS and Los Alamos gamma burst data bases are noted.     

The self absorption of gyro-synchrotron emission in a magnetic dipole field: Microwave impulsive burst and hard X-ray burst

The gyro-synchrotron emission from a model source with a non-uniform magnetic field is computed taking into account the self absorption. This model seems adequate not only to interpret the radio spectrum and its time variation of microwave impulsive bursts but also to solve the discrepancy between the numbers of non-thermal electrons emitting radio burst and those emitting hard X-ray burst.The decrease of flux of radio burst with decreasing frequency at low microwave frequencies is due to the self absorption and/or the thermal gyro-absorption. In this frequency range, the radio source is optically thick even at weak microwave bursts. The weakness of the bursts may be rather due to the small size of the radio source and/or the weakness of the magnetic field than the small number density of the non-thermal electrons.The time variation of the flux of radio burst may be mainly attributed to the variation of source size in a horizontal direction ( direction) instead of the variation of the number density of non-thermal electrons itself, implying that the acceleration region progressively moves in the horizontal direction leaving the non-thermal electrons behind during the increasing phase of the radio burst.     

Homology of X-ray bursts

Conclusions The present paper demonstrates on the basis of 2 series of events that one can extend the homology so far known for optical and radio flares also to the hard and soft X-ray bursts.The studied homologous X-ray flares occurred in the same active region and their time-intensity profiles were very similar. It has been found that the detected homologous X-ray bursts are associated with radio bursts that also are homologous. The time profile of centimeter radio bursts frequently is repeated in detail when compared with the time profile of X-ray bursts as one can see in Figure 1. This very close correspondence suggests that the centimeter radio bursts and X-ray bursts are generated simultaneously during flares, probably in the same region (Sengupta, 1968). Arnoldy et al. (1968) have found a detailed correlation between the time-intensity profiles of hard X-ray bursts and 3 or 10 cm radio bursts. This close correlation between the hard X-ray bursts and centimeter radio bursts leads to a suggestion that the hard X-ray and centimeter radio bursts are generated by the same electrons. On the basis of these considerations one can more easily understand the homology of both the X-ray bursts and the radio bursts. The occurrence of homologous bursts then can be explained by an existence of regions on the sun in which for a certain time (48 h after Fokker) the same conditions are maintained in the acceleration of the electrons generating the X-ray and radio bursts.     

Selection effects on the size and frequency distribution of cosmic gamma-ray bursts

We investigate selection effects on the size and frequency relation (logN-logS curve) of cosmic gamma-ray bursts. After analyzing the published data, we find an indication that an effect is caused by the different time profiles of the bursts. The effect is important for small bursts and causes significant changes in the logN-logS curve. in order to avoid this selection effect, we mention that it is essential to use the logN-logP relation of the logN-logS relation, as already suggested by other authors. Here,P is the peak flux of the burst, which is free from bias due to the difference in time profiles. After an analysis of the published data, we find a distribution nearP ^–3/2 in the range above a peak flux of 40 counts/0.25 s.We also show that the relation between the logN-logP curve and the observed celestial distribution for the bursts can easily be explained by our general arguments on a disk-like spatial distribution of burst sources in the Galaxy.     

Multiple radio echoes in the solar corona

Echo-type solar radio bursts are associated with preceding short-lived bursts in double events. The peculiar and rather rare decameter echoes are observed with a UTR-2 antenna. The initial narrow-band burst is followed, some 5 to 10 s later, by an echo-like burst at the same frequency. The observational data obtained for decameter echo evens are, on the whole, consistent with the model of a pulsed source emitting radio signals at the plasma-frequency harmonic, which is placed in a non-uniform corona and rotates together with the Sun.Intensity-time profiles of 25 MHz echo bursts are of an unusual shape, featuring an extended leading edge and an abrupt decay at the trailing edge and also showing some fine structures in the form of an additional, weakly pronounced maximum or a step at the final stage of the burst. Time parameters characterizing the profiles are evaluated. The step is delayed with respect to the main pulse at about two times longer than the principal echo maximum. At the same time, the time delays depend essentially on the heliolongitude of the active region and achieve their maximum values at the meridian. The step height does not exceed 0.5 of the echo maximum. At this level, the echo-decay time almost coincides with the initial burst duration but is about 1.7 times less than the echo-rise time. The feature at the echo tail can be interpreted as a result of a repeated reflection of the burst from the source region. The causes and conditions for the formation of multiple echoes are discussed. The extended leading edge of the echo permits us to assume a quasi-radial fibrous structure of the corona, capable of back-scattering the incident radiation.