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.     

Are Homologous Radio Bursts Driven by Solar Post-Flare Loops？

Three particularly complex radio bursts (2001 October 19, 2001 April 10 and 2003 October 26) obtained with the spectrometers (0.65-7.6GHz) at the National Astronomical Observatories, Chinese Academy of Sciences (NAOC, Beijing and Yunnan) and other in- struments (NoRH, TRACE and SXT) are presented. They each have two groups of peaks occurring in different frequency ranges (broad-band microwave and narrow-band decimeter wavelengths). We stress that the second group of burst peaks that occurred in the late phase of the flares and associated with post-flare loops may be homologous radio bursts. We think that they are driven by the post-flare loops. In contrast to the time profiles of the radio bursts and the images of coronal magnetic polarities, we are able to find that the three events are caused by the active regions including main single-bipole magnetic structures, which are associated with multipole magnetic structures during the flare evolutions. In particular, we point out that the later decimetric radio bursts are possibly the radio counterparts of the homologous flares (called "homologous radio bursts" by us), which are also driven by the single-bipole mag- netic structures. By examining the evolutions of the magnetic polarities of sources (17GHz), we could presume that the drivers of the homologous radio bursts are new and/or recurring appearances/disappearances of the magnetic polarities of radio sources, and that the triggers are the magnetic reconnections of single-bipole configurations.     

Statistical Analysis of Decimetric Radio Bursts,Flares, and Coronal Mass Ejections

A statistical analysis of decimetric radio bursts (RBs), X-ray flares and coronal mass ejections (CMEs) is carried out. We consider all radio bursts recorded by the Cracow Solar Radio Telescope from the beginning of 1996 until the end of 2004. It is found that the decimetric radio bursts are associated and strongly correlated with X-ray flares. Correlation coefficients between RBs durations and the maximal fluxes of the radio bursts and flares are found to be 0.60 and 0.87, respectively. We also demonstrated that a significant population of the decimetric radio bursts are associated with CMEs. The correlation coefficient between the maximal radio flux density multiplied by the duration of the RBs versus velocity multiplied by width of CMEs is found to be 0.55.     

Detailed analysis of flares,magnetic fields and activity in the sunspot group of Sept. 13–26, 1963

We analyze large-scale H-alpha movies of the large spot group of Sept. 13–26, 1963, together with radio, ionospheric and magnetic field data as well as white light pictures. The evolution of the group and associated magnetic fields is followed, and the positions of solar flares relative to the fields are noted, along with their morphology. Although the magnetic field is deformed in time, characteristic field structures may be traced through the deformation as the seat of recurrent homologous flares.We find that most flares are homologous, and some are triggered by disturbances elsewhere in the region. We note events produced by surges falling back to the surface, and one flare initiated by a bright bead seen to fly across the region. In almost every case of an isolated type III radio burst, a corresponding H-alpha brightening could be found, but not all flares produced bursts. Flares close to the sunspots are most likely to produce radio bursts. Flare surface waves in the region all travel out to the west, because of more open magnetic field structure there. In one case (Sept. 25) a wave is turned back by the closed field structure to the east.In almost all cases the time association of radio or ionospheric events is with the beginning of the flare or with the flash phase.Several morphological classes of flares are noted as recurrent types.     

Studies on metric–decimetric Type II bursts in relation to soft X-ray flares

The relationship between metric type II radio bursts and soft X-ray (SXR) flares is studied. Type II bursts are highly associated with SXR flares. The duration and drift rate of type II bursts are found to depend on the duration, asymmetry in duration (ratio of rise time to duration), as well as on the peak flux of SXR bursts. Important results obtained are: (i) the durations of type II bursts are linearly correlated with the durations of associated SXR bursts in case of long-lived events (duration >40 min), whereas in short-lived flares such a correlation is not found, (ii) the durations of type II bursts do not depend upon the SXR peak flux, (iii) more durable type II radio bursts are correlated with more symmetric SXR bursts, (iv) average drift rates of type II bursts are larger in the events associated with more powerful and more symmetric SXR bursts.     

Characteristics of Type-II Radio Bursts Associated with Flares and CMEs

We present a statistical study of the characteristics of type-II radio bursts observed in the metric (m) and deca-hectometer (DH) wavelength range during 1997–2008. The collected events are divided into two groups: Group I contains the events of m-type-II bursts with starting frequency ≥ 100 MHz, and group II contains the events with starting frequency of m-type-II radio bursts < 100 MHz. We have analyzed both samples considering three different aspects: i) statistical properties of type-II bursts, ii) statistical properties of flares and CMEs associated with type-II bursts, and iii) time delays between type-II bursts, flares, and CMEs. We find significant differences in the properties of m-type-II bursts in duration, bandwidth, drift rate, shock speed and delay between m- and DH-type-II bursts. From the timing analysis we found that the majority of m-type-II bursts in both groups occur during the flare impulsive phase. On the other hand, the DH-type-II bursts in both groups occur during the decaying phase of the associated flares. Almost all m-DH-type-II bursts are found to be associated with CMEs. Our results indicate that there are two kinds of shock in which group I (high frequency) m-type-II bursts seem to be ignited by flares whereas group II (low frequency) m-type-II bursts are CME-driven.     

Solar Type II Radio Bursts Recorded by the Compound Astronomical Low-Frequency Low-Cost Instrument for Spectroscopy in Transportable Observatories in Brazil

It is well established that solar Type-II radio bursts are signatures of magnetohydrodynamical (MHD) shock waves propagating outward through the solar corona. Nevertheless, there are long-standing controversies about how these shocks are formed; solar flares and the coronal mass ejections (CMEs) are considered to be the most likely drivers. We present the results of the analysis of four solar Type-II bursts recorded between 20 January 2010 and 17 November 2011 by the Compound Astronomical Low-frequency Low-cost Instrument for Spectroscopy in Transportable Observatories (CALLISTO-BR) (in Brazil), which operates in the frequency range of 45?–?870 MHz. For all four solar Type-II radio bursts, which consisted of one event without band splitting and three split-band variants, the outcomes are consistent with those reported in the literature. All four Type-II radio bursts were accompanied by both solar flares and CMEs, which are associated with the impulsive phase of the flares and, very likely, with the acceleration phase of the CMEs.     

Relationship of great soft X-ray flares with other solar activity phenomena

We present study of relationship of GSXR flares with Hα flares, hard X-ray (HXR) bursts, microwave (MW) bursts at 15.4 GHz, type II/IV radio bursts, coronal mass ejections (CMEs), protons flares (>10 MeV) and ground level enhancement (GLE) events we find that about 85.7%, 93%, 97%, 69%, 60%, 11.1%, 79%, 46%, and 23%% GSXR flares are related/associated with observed Hα flares, HXR bursts, MW bursts at 15.4 GHz, type II radio bursts, type IV radio bursts, GLE events, CMEs, halo CMEs, and proton flares (>10 MeV), respectively. In the paper we have studied the onset time delay of GSXR flares with Hα flares, HXR, and MW bursts which shows the during majority GSXR flares SXR emissions start before the Hα, HXR and MW emissions, respectively while during 15–20% of GSXR flares the SXR emissions start after the onset of Hα, HXT and MW emissions, respectively indicating two types of solar flares. The, onset time interval between SXR emissions and type II radio bursts, type IV radio bursts, GLE events CMEs, halo CMEs, and protons flares are 1–15 min, 1–20 min, 21–30 min, 21–40 min, 21–40 min, and 1–4 hrs, respectively. Following the majority results we are of the view that the present investigations support solar flares models which suggest flare triggering first in the corona and then move to chromospheres/ photosphere to starts emissions in other wavelengths. The result of the present work is largely consistent with “big flare syndrome” proposed by Kahler (1982).     

Homologous type II radio bursts and coronal transients

The homology of seven successive type II solar radio bursts, which occurred at the times of flares from an active region near the solar west limb on 1980, July 27–29, is described, together with evidence for coronal mass outflows accompanying these bursts. It is argued that homologous type II bursts imply that the corona is restructured in a similar manner by successive coronal transients.     

Center-to-limb variation of the peak flux spectra of type IV radio bursts associated with solar proton flares

Type IV radio bursts with wide band from microwave to metric-wave frequency are generally associated with solar proton flares. Recently, Castelli et al. (1967, 1968) have shown that the type IV radio bursts associated with solar proton flares show the U-shaped peak flux spectra with the minimum flux at decimetric frequencies. In this paper, the center-to-limb variation of such peak flux spectra is investigated in order to examine the effect of decrease of the peak flux at metric frequencies with increase of the angular distance from the central meridian of the Sun. It is shown that the U-shaped spectra are obtained independent of the position of proton flares, although the spectral form changes significantly in the case of the flares near the limb. It is further suggested that the U-shaped spectra consist of the two essentially independent components for microwave and metric-wave frequencies, respectively.     

Homology of solar flare-associated radio events

The concept of homology, introduced by Ellison, Mc Kenna, and Rceid (1960) for optical flares, can be extended to flare-associated radio events. Successive flares within the same centre of activity sometimes produce radio events that are remarkably similar. The similarity amounts to the fact that they extend over about the same range(s) in frequency, producing at each frequency responses of comparable intensity and duration. On some occasions the intensity curves at individual frequencies show even detailed resemblance. The occurrence of homologous radio events is commonly restricted to periods of less than 48 hours.Without being homologous, radio events that occur in the same centre of activity may present a common characteristic that is typical for that centre. Two such characteristics are (1) the production of radio responses at centimetric, decimetric and metric frequencies, and (2) the impulsiveness of microwave outbursts. The distribution of time intervals during which such bursts occur is compared with the same distribution for homologous events (Figure 3).     

Statistics and investigation of white light solar flares

We discuss the properties of white light flares on the basis of the published accounts of these events, together with the associated H flares, radio bursts, X-ray bursts, proton events and ionosperic distrubances. In addition, spectral plates taken at Purple Mountain Observatory since 1962 have been examined. We found that 5% of the spectrograms of solar flares show variable white-light emission. A minority of the white light flares are associated with H flare of small importance classes. We think these may be caused by perturbations originating in the convective zone below, while the majority accompanied by high-energy events are caused by the bombardment of energetic particles from above.     

Survey on Solar X-ray Flares and Associated Coherent Radio Emissions

The radio emission during 201 selected X-ray solar flares was surveyed from 100 MHz to 4 GHz with the Phoenix-2 spectrometer of ETH Zürich. The selection includes all RHESSI flares larger than C5.0 jointly observed from launch until June 30, 2003. Detailed association rates of radio emission during X-ray flares are reported. In the decimeter wavelength range, type III bursts and the genuinely decimetric emissions (pulsations, continua, and narrowband spikes) were found equally frequently. Both occur predominantly in the peak phase of hard X-ray (HXR) emission, but are less in tune with HXRs than the high-frequency continuum exceeding 4 GHz, attributed to gyrosynchrotron radiation. In 10% of the HXR flares, an intense radiation of the above genuine decimetric types followed in the decay phase or later. Classic meter-wave type III bursts are associated in 33% of all HXR flares, but only in 4% are they the exclusive radio emission. Noise storms were the only radio emission in 5% of the HXR flares, some of them with extended duration. Despite the spatial association (same active region), the noise storm variations are found to be only loosely correlated in time with the X-ray flux. In a surprising 17% of the HXR flares, no coherent radio emission was found in the extremely broad band surveyed. The association but loose correlation between HXR and coherent radio emission is interpreted by multiple reconnection sites connected by common field lines.     

The coronal disturbance of 1972, August 12

Using the observed data for metric and hectometric type III radio bursts, the dependence of burst characteristics on the solar longitude has been examined over a wide frequency range. It is found that there exists an east-west asymmetry for the extension of metric type III bursts into hectometric wavelength range. In particular, hectometric bursts are rarely observed for solar flares associated with metric bursts eastward solar longitude 60°E. Furthermore, for eastern longitudes, the low frequency radio observations show a large dispersion in drift time interval.     

Coronal Shocks Associated with Impulsive and Decaying Phases of Solar Flares

We have analyzed a set of 147 metric Type II radio bursts observed by Culgoora radio spectrograph from November 1997 to December 2006. These events were divided into two sets: The first subset contains Type II events that started during the impulsive phase of the associated solar flares and the second subset contains those starting during the decaying phase of flares. Our main aim is to differentiate the metric Type IIs, flares and coronal mass ejections (CMEs) of these two subsets. It is found that while Type II burst characteristics of both subsets are very similar, there are significant differences between flare and CME properties for these two subsets. Considering all analyzed relationships between the characteristics of Type IIs, flares and CMEs in these two Type II subsets, we conclude that most of the coronal shocks causing metric Type II bursts are driven by CMEs, but that a fraction of events are probably ignited by solar flares.     

Particle Acceleration and Propagation in Strong Flares without Major Solar Energetic Particle Events

Solar energetic particles (SEPs) detected in space are statistically associated with flares and coronal mass ejections (CMEs). But it is not clear how these processes actually contribute to the acceleration and transport of the particles. The present work addresses the question why flares accompanied by intense soft X-ray bursts may not produce SEPs detected by observations with the GOES spacecraft. We consider all X-class X-ray bursts between 1996 and 2006 from the western solar hemisphere. 21 out of 69 have no signature in GOES proton intensities above 10 MeV, despite being significant accelerators of electrons, as shown by their radio emission at cm wavelengths. The majority (11/20) has no type III radio bursts from electron beams escaping towards interplanetary space during the impulsive flare phase. Together with other radio properties, this indicates that the electrons accelerated during the impulsive flare phase remain confined in the low corona. This occurs in flares with and without a CME. Although GOES saw no protons above 10 MeV at geosynchronous orbit, energetic particles were detected in some (4/11) confined events at Lagrangian point L1 aboard ACE or SoHO. These events have, besides the confined microwave emission, dm-m wave type II and type IV bursts indicating an independent accelerator in the corona. Three of them are accompanied by CMEs. We conclude that the principal reason why major solar flares in the western hemisphere are not associated with SEPs is the confinement of particles accelerated in the impulsive phase. A coronal shock wave or the restructuring of the magnetically stressed corona, indicated by the type II and IV bursts, can explain the detection of SEPs when flare-accelerated particles do not reach open magnetic field lines. But the mere presence of these radio signatures, especially of a metric type II burst, is not a sufficient condition for a major SEP event.     

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.     

太阳射电爆发的起因：耀斑或／和日冕物质抛射

本文分析了近二十年来的地面和空间太阳有关观测资料,得出太阳射电爆发的起因为耀斑和／ 或日冕物质抛射（ＣＭＥ） 而不仅仅是耀斑,这将有利于更深刻地了解太阳射电爆发和共生高能现象的物理过程     

Comparative Analysis of Type ii Bursts and of Thermal and non-Thermal Flare Signatures

The relationship between metric type II radio bursts and solar flares is studied. Well-defined correlations between the properties of type II bursts and the characteristics of associated microwave and soft X-ray bursts are established in two entirely independent data sets. It is shown that the correlations are strongly affected by the wide range of coronal Alfvén velocities involved, comprising values from only 150 up to 800 km s^–1, with a typical value of 400 km s^–1. After careful data analysis it was inferred that type II bursts are more closely related to the soft X-ray bursts than they are to microwave bursts. The correlations indicate that type II burst shocks are preferably generated by flares with a relatively strong thermal component, and that the shocks are probably ignited by the plasma expansion associated with the 'evaporation' process in the transition region. Although the results imply that the majority of metric type II bursts are caused by flares, a simple geometrical consideration shows that a fraction of non-flare type II bursts cannot be explained by behind-limb events and that roughly 10% of metric type II bursts should be attributed to non-flare coronal mass ejections.     

The relevance of solar flares to astrophysics

The physical mechanisms associated with solar flares are reviewed. The relevance of flare mechanisms to other astrophysical phenomena is discussed. In this context, specific models of quasars and radio galaxies, Sco X-1 and gamma-ray bursts are examined.