Solar Bursts Research Articles

The features of CME-associated microwave burstst

Features of solar microwave bursts (SMBs) associated with coronal mass ejections (CMEs) are analyzed, including the duration, peak flux, type, spectral index and so on. 136 events in the period 1999 Nov–2003 Sep (60 associated with partial/full halo CMEs and 76 with normal/narrow CMEs) are selected for study. It is found (1) that the SMBs associated with normal/narrow CMEs usually have short durations, while those associated with partial/full halo MEs have both short and long durations over a rather broad range, (2) that the SMBs associated with slow CMEs usually have short durations, while those associated with fast CMEs have durations that cover a rather large range, (3) that the SMBs associated with normal/narrow CMEs or slow CMEs have small peak fluxes, while those associated with partial/full halo CMEs have peak fluxes that cover a rather large range, (4) that most of the SMBs associated with normal/narrow CMEs are S (simple) type, while most of the SMBs associated with halo CMEs are C (complex) or GB (great burst) type, (5) that the spectra of most CME-associated events are rather flat in the high-frequency part. The statistical results indicate that some intrinsic physical relationship exists between CME/flare events and SMBs, and that the SMBs may provide information on CME/flare events.

Discovery of a 500 Parsec Shell in the Nucleus of Centaurus A

Spitzer Space Telescope mid-infrared images of the radio galaxy Centaurus A reveal a shell-like, bipolar, structure 500 pc to the north and south of the nucleus. This shell is seen in 5.8, 8.0 and 24 micron broad-band images. Such a remarkable shell has not been previously detected in a radio galaxy and is the first extragalactic nuclear shell detected at mid-infrared wavelengths. We estimate that the shell is a few million years old and has a mass of order million solar masses. A conservative estimate for the mechanical energy in the wind driven bubble is 10^53 erg. The shell could have created by a small few thousand solar mass nuclear burst of star formation. Alternatively, the bolometric luminosity of the active nucleus is sufficiently large that it could power the shell. Constraints on the shell's velocity are lacking. However, if the shell is moving at 1000 km/s then the required mechanical energy would be 100 times larger.

Observing the Sun at 20–650 MHz at Thermopylae with Artemis

Fine structure of type IV radio solar bursts with a great variety and complexity often give much information in different ways and enable estimation of various coronal characteristics. In this work, we expose our new method for fine structure revealing and separation of two basic kinds of type IV fine structure, as fibers and pulsations. We also estimate frequency drift of fibers from dynamic spectra, clean from continuous background, with a prototype method using 2-D Fourier transform and we estimate periodicities of fibers as well as pulsations with continuous wavelet transform. Working with the last method we found periodicities close to 3 min umbral oscillations and 5 min global solar oscillations.

AN EVALUATION OF THE SOLAR RADIO BURST LOCATOR (SRBL) AT OVRO

The Solar Radio Burst Locator (SRBL) is a spectrometer that can observe solar microwave bursts over a wide band (0.1-18 GHz) as well as detect the burst locations without interferometry or mechanical scanning. Its prototype has been operated at Owens Valley Radio Observatory (OVRO) since 1998. In this study, we have evaluated the capability of the SRBL system in flux and radio burst location measurements. For this, we consider 130 microwave bursts from 2000 to 2002. The SRBL radio fluxes of 53 events were compared with the fluxes from USAF/RSTN and the burst locations of 25 events were compared with the optical flare locations. From this study, we found: (1) there is a relatively good correlation (r

Radiation mechanisms of the plasma near point L1 affected by CMEs and associated microwave bursts

When coronal mass ejections (CMEs) interact with the solar corona and the interplanetary medium, emissions at different wavelengths occur. On the basis of study of the various radiation mechanisms of space plasma in the case of absence of CMEs, the radio radiation mechanisms of the plasma close to the Lagrange point L1 and affected by large CMEs from February to August 1999 are statistically analyzed. As shown by the results, the main radiation mechanisms are the Bremsstrahlung, a small amount of cyclotron radiation and a still weaker recombination radiation. Also, solar microwave bursts which are associated with CMEs in the same period are investigated. The results show that the microwave bursts are of the gradual type as well as spike bursts, and that the chief radiation mechanisms are the Bremsstrahlung, cyclotron resonance radiation, plasma radiation and electronic cyclotron maser radiation.

Wide‐Band Spectroscopy of Two Radio Bursts on AD Leonis

We report high-time-resolution, broadband spectroscopic observations of two radio bursts on the classical flare star AD Leonis. The observations were acquired by the 305 m telescope at Arecibo Observatory on 2003 June 13-14. Using the Wideband Arecibo Pulsar Processor, these observations sampled a total bandwidth of 400 MHz, distributed over a 500 MHz frequency range, 1120--1620 MHz, with a frequency resolution of 0.78 MHz and a time resolution of 10 ms. A radio burst observed on June 13 is characterized by the presence of multitudes of short duration ($\Delta t \sim$30 ms), high brightness temperature ($T_{b}>10^{14}$K), highly circularly polarized, fast-drift radio sub-bursts, with median bandwidths $\Delta \nu/\nu \sim$5%. The inverse drift rates are small, and have a symmetric distribution (both positive and negative frequency drifts) with a Gaussian FWHM inverse drift rate of 4.5$\times10^{-4}$ s/MHz. The fast-drift sub-bursts occur at a mean rate of 13 s$^{-1}$ and show no evidence for periodic recurrence. The fast-drift radio events on AD Leo are highly reminiscent of solar decimetric spike bursts. We suggest the emission is due to fundamental plasma radiation. A second highly circularly polarized radio burst, recorded June 14, has markedly different properties: a smoothly varying intensity profile characterized by a slow drift in frequency with time (-52 MHz s$^{-1}$). Under the assumption that the source is due to a disturbance propagating through the low corona, a source size of 0.1--1 R$_{\star}$ is inferred, implying a brightness temperature range 6$\times10^{11}$--6$\times10^{13}$K: another example of a coherent radio burst.

Solar Drift Pair Bursts in the Decameter Range

The results of observations of solar decametric drift pair bursts are presented. These observations were carried out during a Type III burst storm on July 11–21, 2002, with the decameter radio telescope UTR-2, equipped with new back-end facilities. High time and frequency resolution of the back-end allowed us to obtain new information about the structure and properties of these bursts. The statistical analysis of more than 700 bursts observed on 13–15 July was performed separately for “forward” and “reverse” drift pair bursts. Such an extensive amount of these kind of bursts has never been processed before. It should be pointed out that “forward” and “reverse” drift pair bursts have a set of similar parameters, such as time delay between the burst elements, duration of an element, and instant bandwidth of an element. Nevertheless some of their parameters are different. So, the absolute average value of frequency drift rate for “forward” bursts is 0.8 MHz s−1, while for “reverse” ones it is 2 MHz s−1. The obtained functional dependencies “drift rate vs. frequency” and “flux density vs. frequency” were found to be different from the current knowledge. We also report about the observation of unusual variants of drift pairs, in particular, of “hook” bursts and bursts with fine time and frequency structure. A possible mechanism of drift pairs generation is proposed, according to which this emission may originate from the interaction of Langmuir waves with the magnetosonic waves having equal phase and group velocities.

Radio emission observed in decimetric waves associated with the onset of CMEs

Since the first observations by Skylab and SMM satellites coronal mass ejections (CME) have been more and more investigated. However, until now their origin and trigger mechanism remain an open question no matter if they are associated to flares or not. Recent observations over a broad spectrum suggest that flare energy is released in regions from where the decimetric emission is coming. Then, investigations of decimetric radio emission observed in association with CME phenomena may give clues to solve the previously mentioned questions. Using the Brazilian solar spectroscope (BSS), observations of solar bursts dynamic spectra with high time (100, 50, 20 ms) and frequency (50–100 channels) resolutions have been carried out daily (∼11–19 UT) within the range of 1000–2500 MHz. A sample of 274 CMEs were recorded by the large angle spectroscopic coronagraph (LASCO) instrument, on board the solar and heliospheric observatory (SOHO) satellite, within 11–19 UT, during the period of 1999–2002. From those, 42 CMEs are associated to BSS data and selected for analysis. It is interesting to note that in about half of the cases only one type of burst radio emission was recorded while in the remaining cases either two or more types were observed. There is a dominance of either continuum and/or pulsations. Here, we describe the association of burst radio emission with the starting time of CME phenomena.

Features of the solar X-ray bursts related to solar energetic particle events

Solar X-ray bursts appear to be most convincing signatures of particle acceleration during solar flares. It is not clear what part of this population escapes the flare region. Majority of solar energetic particles observed in the space after powerful and long lasting solar flares are probably not connected with flares and assumed to be accelerated by the shocks related to the coronal mass ejections. However, there is a significant correlation between intensity of the X-ray bursts and solar energetic particle energy. The paper considers characteristics of the X-ray bursts followed by solar energetic particle occurrences and those bursts not associated with solar energetic particles. It is shown that correlation between X-ray bursts and solar energetic particles increases with growing of X-ray burst class and solar energetic particle energy. In our opinion, it emphasizes the role of processes in the flare region for solar energetic particles occurrence.

Microwave emission from the trapped and precipitated electrons in solar bursts

We analyzed the microwave spectra of a sample of 13 solar flares out of 40 events observed by the Nobeyama Radio Polarimeter in the period of August 8, 1998 through November 24, 2001. The solar burst time profiles were filtered to separate the slow component from the fast structures which were associated with the emission from trapped particles and to the emission from the minor precipitating population, respectively. The spectra of both components suggest a trap in a region with a mean magnetic field of about 600 G and a minor emission from the loop legs with a magnetic field of 1100 G, resulting in a mirror ratio of about two. The spectral indices of the optically thin emission from these two regions indicate pitch angle diffusion by Coulomb collisions.

Frequency and time profiles of metric wave isolated Type I solar noise storm bursts at high spectral and temporal resolution

Type I noise storms constitute a sizeable fraction of the active Solar radio emission component. Observations of isolated instances of such bursts, in the swept-frequency mode at metric wavelengths, have remained sparse, with several unfilled regions in the frequency coverage. Dynamic spectra of the burst radiation in the 30–130 MHz band, obtained from the recently commissioned digital High Resolution Spectrograph at the Gauribidanur Radio Observatory, have unravelled in explicit detail the temporal and spectral profiles of isolated bursts thanks to the instrument's superior frequency and time resolution. Apart from presenting details of their fundamental emission features, the time- and frequency-profile symmetry, with reference to custom-specific Gaussian distributions, has been chosen as the nodal criterion to statistically explain the state of the source regions in the vicinity of magnetic reconnections, the latent excitation agent that contributes to plasma-wave energetics, and the quenching phenomenon that causes damping of the burst emission.

Evidence for Resonant Transition Radiation in Decimetric Continuum Solar Bursts

We investigate the properties of radio bursts observed in the 1–18 GHz frequency range that display two distinct spectral components peaking at decimetric (dm) and centimetric (cm) ranges. Thedmemission is relatively smooth in both frequency and time, with timescales comparable to those of the cm component. The two spectral components display specific correlations in their temporal and spectral behavior. Through detailed analysis, we find the following: (1) A large ratio of plasma frequency to gyrofrequency is a characteristic of all of these events. (2) The dm component generally displays a high degree of o-mode polarization. (3) The dm component is produced by an incoherent emission mechanism. (4) The dm and cm components are generated by the same electron distribution. (5)Thecharacteristicenergyofthefastelectronsproducingthedmcontinuumissignificantlylowerthantheenergy of the electrons generating the microwave gyrosynchrotron component. (6) The spectral shape of the dm component is not well correlated with the fast electron distribution over energy. These findings cannot easily be explainedwithstandardmechanismsfordmsolarburstemissionbutagreewellwiththepredictionsofthetheoryof resonant transition radiation. We conclude that these two-component bursts represent a distinctive subclass of events, comprising about 10% of all bursts, with the dm continuum component generated most probably by resonant transition radiation produced by interaction of fast electrons with small-scale inhomogeneities of the background plasma. We discuss new possibilities for flaring plasma diagnostics using these two-component bursts.

Observations of “zebra” pattern in cm-range with spatial resolution

We present the results of the first observations of the solar microwave burst with fine spectral structure of zebra type at the frequency about 5.7 GHz. The burst has been detected simultaneously by the Siberian Solar Radio Telescope and by the spectropolarimeter of the National Astronomical Observatory of China. Zebra pattern consisted of three parallel stripes with complex frequency drift. The degree of circular polarization of emission reached 100%, the polarization sense corresponded to the extraordinary wave (X-mode). We have determined the plasma parameters in the emission source: plasma density about 10 11 cm −3, magnetic field strength 60–80 G. We argue that in the given event the most probable mechanism of the zebra pattern generation is non-linear coupling of harmonics of Bernstein modes.

An experiment to study solar flare effects on radio-communication signals

The occurrence of radio signal fading events caused by ionospheric absorption plays an important role in the performance of radio-communication systems. It is necessary to know the magnitude and time-scale of such events in order to specify technical parameters of the communication system to be used. Generally, fading events are associated with solar flares, which are characterized by sudden increase in the solar X-ray flux that causes an increase in the ionization in the lower ionosphere. The abrupt increase of ionization causes the absorption of radio waves propagating in the Earth–ionosphere wave-guide and is reported as radio signal fading events. A simple experiment to monitor the behavior of lower ionosphere has been carried out at the Southern Space Observatory-SSO/INPE (29.43°S, 53.8°W), located in southern Brazil. The experiment is basically a computer controlled radio receiver that records the received signal strength of Amplitude Modulated (AM) radio signals in the HF (High Frequencies) range. We analyzed data of the 6 MHz beacon signal that has been transmitted by a broadcasting radio station located about 400 km from the observation site. In this work we present initial results of daily variation of the received signal strength and fading events associated with solar flares observed in the 6 MHz signal monitored by the experiment during 2001. X-ray solar flux data from the GOES-8 satellite were used to identify X-ray solar bursts associated with solar flares. Based on the one-year data collected by the experiment, a statistical summary of fading occurrences and their correlation with solar flares, as well as the distributions of time-scales and magnitudes of such events are presented.

LOFAR antenna development and initial observations of solar bursts

We are developing and testing active baluns and electrically short dipoles for possible use as the primary wide band receiving elements in the low-frequency array (LOFAR) for long wavelength radio astronomy. Several dipoles of various designs and dimensions have been built and tested. Their useful range occurs when the dipole arms are approximately 1 8 to one wavelength long and the feedpoint is less than 1 2 wavelength above ground. An eight-element NRL LOFAR test array (NLTA) interferometer has been built and fringes have been observed from the brightest celestial sources in the frequency range from 10 to 50 MHz. The antenna temperatures vary from about 10% to 100% of the average brightness temperature of the galactic background. With these parameters it is easy to make the amplifier noise levels low enough that final system temperature is dominated by the galactic background.

Correlated Dispersionless Structure in Suprathermal Electrons and Solar Energetic Ions in the Solar Wind

Dispersionless modulations in particle intensity are commonly observed in impulsive solar energetic ion events and are occasionally observed within low-energy (less than 1.4 keV) solar electron bursts. The electron burst modulations commonly occur in direct association with discontinuous changes in the intensity of the solar wind electron strahl. Both the energetic ion and the suprathermal electron burst modulations have been interpreted in terms of spatially limited burst source regions and magnetic field-line footpoint motions in the solar atmosphere. Concentrating on impulsive ion modulation events previously reported, we show that there is generally a close connection between the dispersionless modulations in energetic ions in those events and the simultaneous dispersionless modulations in low-energy solar electron bursts and in the electron strahl. This demonstrates that dispersionless modulations in both particle species have a common cause, which we associate with relatively abrupt changes in magnetic connection to the Sun. We find that some of these abrupt connection changes occur at structural boundaries in the solar wind flow. More often, the connection changes appear to arise from field-line footpoint motions or, possibly, solar wind turbulence. To a first approximation, when dispersionless structure is present in both particle species, the electrons and ions appear to be accelerated on the same field lines in spatially confined regions in the corona. Overall, however, the data suggest that electron burst source regions usually are more spatially uniform, have considerably broader spatial extents than the ion sources, or both.

Characteristics of Solar Microwave Bursts Associated with CMEs: a Statistical Study

We selected 133 solar microwave bursts (SMBs) recorded by SGD and 133 CMEs observed by SOHO/LASCO from November 1999 through September 2003. These SMBs are associated with CMEs and flares. We analyzed the characteristics of the SMBs, including duration, flux peak, burst type, and spectral index. Correlated events were distinguished by time and the location of flare associated with the SMB. We find that (1) The duration of SMBs associated with narrow (0 degrees f(max). A statistical result suggest that CME/flares and SMBs is probably a different manifestation of the same physical process. CME/flare and SMB have intrinsically a physical relationship.

The Evolving Features of the Source Region of a Fast Halo CME with Strong Geo-effects

A fast halo CME and correspondingly strong solar burst that occurred in NOAA9684 led to intense geo-effectiveness. The analysis and study in this paper were focused on the evolution of this active region.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

A CME and Related Phenomena on 2003 October 26

We present the observational results of the solar bursts on the band of 1-80 GHz (NORH) associated with both a CME and a flare on Oct. 26 2003. This event shows two parts of radio bursts in the time profile. The first part is associated with an X1.2 flare. However, the following part seams related to both the flare and the CME, as the radio emission is enhanced while the is decreasing. Thus, these two parts of radio bursts may originate from different physical processes, i.e., flare and CME shock. A primary study is performed on the difference between this two parts.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

A New Solar Broadband Radio Spectrometer (SBRS) in China

A new radio spectrometer, Solar Broadband Radio Spectrometer (SBRS) with characteristics of high time resolution, high-frequency resolution, high sensitivity, and wide frequency coverage in the microwave region is described. Its function is to monitor solar radio bursts in the frequency range of 0.7-7.6 GHz with time resolution of 1-10 ms. SBRS consists of five 'component spectrometers' which work in five different wave bands (0.7-1.5 GHz, 1.0-2.0 GHz, 2.6-3.8 GHz, 4.5-7.5 GHz, and 5.2-7.6 GHz, respectively). A combination of multi-channel and scanning techniques is adopted. The component spectrometers are attached to different antennas which are separately located at Beijing, Kunming, and Nanjing. Close attention was paid to solve the problems of sensitivity, dynamic range, interference-resistance, data acquisition, and handling a large amount of data. The SBRS was put into operation in the 23th solar maximum activity period, and has proved itself to be a valuable instrument for the study of solar bursts in microwaves.