Solar Bursts Research Articles

Spikes in solar radio bursts at frequencies near 3 GHz

Many solar microwave bursts presenting fine structures were recorded at high temporal resolution (8 ms) by the 2.6–3.8 GHz spectrometer of National Astronomical Observatories of China (NAOC). Here we present data that were recorded on 15 April 1998. After data processing, more than one hundred spikes were detected in the interval 07:59:29.622–07:59:50.362 UT. Some of the spikes were single, while others were grouped in clusters. We report the observational characteristics including lifetime, frequency bandwidth, drift rate and polarization degree, as well as duration of spike clusters. Afterwards we discuss the difference between the lifetime of the spikes presented here (near 3 GHz) and those reported formerly at frequency up to 1 GHz, the probable source density and dimension, the brightness temperature and some other characteristics.

First detection of the impulsive and extended phases of a solar radio burst above 200 GHz

We present a detailed analysis of radio observations obtained at 212 and 405 GHz during the 2000 March 22 Hα 2N flare that occured in AR8910 at ~1834 UT. These data are compared with microwave, soft X-ray and hard X-ray measurements of this flare. While the flare emission is not clearly detected at 405 GHz, the time profile of the 212 GHz emission exhibits an impulsive burst, associated in time with the 1–18 GHz impulsive microwave burst and a long-lasting thermal burst which finishes at about the same time as the soft X-ray emission but reaches its maximum later. The 212 GHz impulsive emission and the lack of detection at 405 GHz are consistent with synchrotron radiation from a population of ultrarelativistic electrons in an average magnetic field of 400–600 G. This radiating population of electrons has a hard energy spectrum (power law index ≈2.7). The expected >1 MeV gamma-ray continuum emission from the radio emitting electrons is comparable to that detected for mid-size electron-dominated events and the hard X-ray flux they would produce at 100 keV is consistently lower than the upper limit inferred from the observations. It is shown that the 212 GHz thermal source has to be different from that radiating the soft X-ray and microwave thermal emission. The present observations of a solar burst provide the first evidence of the extension of the gyrosynchrotron spectrum of an impulsive radio burst in the synchrotron domain above 200 GHz.

Center-to-Limb Variation of Solar Microwave Bursts

Statistical studies of hard X-ray flares position on the solar disk have shown that the more energetic hard X-rays have a tendency to be more concentrated near the limb rather than at disk center, whereas lower-energy hard X-ray emission seems isotropic. Since the high-frequency radio emission is believed to be produced by the same energetic electron population responsible for the high-energy hard X-rays, we searched the microwave/millimeter emitting bursts for center-to-limb variation in their emission. A total of 499 bursts observed by the radio telescopes in Bern at the frequencies of 3.1, 5.2, 8.4, 11.8, 19.6, 35.0, and 50.0 GHz were analyzed. Simultaneous Hα flares were used for determination of the radio burst position on the solar disk. For each of the 7 frequencies, the peak flux and duration were studied as a function of heliocentric position. For 312 bursts, spectral parameters such as spectral index, peak frequency, and flux at spectral maximum were analyzed. For a subset of 43 bursts with emission at all frequencies, the emission and spectral parameters were analyzed. Center-to-limb variations of the spectral parameters for all bursts were sought. In order to interpret the observational results, we have performed a numerical simulation of gyrosynchrotron spectra. We find that high-frequency events, which are also the more energetic ones, have larger center-to-limb variations in their parameters than do the overall flares. Moreover, this behavior agrees with theoretical predictions.

Low Frequency VLBI Project

An international Low Frequency VLBI Network (LFVN) has been organized for observations at 327 MHz and 1665 MHz frequencies with MARK-II and S2 recording terminals. Data on the interplanetary medium and spike-like solar bursts were obtained in first preliminary experiments.

Estimate of coronal magnetic fields from contour maps of the intensity and polarization degree of the burst on 1992 October 27

In this paper contour maps at 17 GHz of the 1992 October 27 solar burst are analysed. Using a nonthermal inhomogeneous model of the radio source, variations of calculated brightness temperature with distance from the projected center of the source is in better agreement with the observed map at 01 : 45 : 29.889 UT. The calculation shows that the characteristic magnetic field gradient α B (magnetic field distribution is B m ( r/ r m ) α B = 2000( r/ r m ) −2) at the peak of the 17 GHz emission is about 2, magnetic strength B is about 737 G with the magnetic field B m of the core being 2000 G. On the other hand, from the polarization contour map and the “limiting polarization” concept, we obtain a magnetic moment d = 1.4 – 2.5 × 10 32 cgs units, and a coronal magnetic field B = 20 – 30 G at the height of 1.7 – 1.9 × 10 10 cm.

Observations of Solar Bursts Using the New Radio Spectrograph

This paper presents a detailed analysis of 80 type III radio bursts, two noise storms and 10 type II radio bursts observed in the 30-80 MHz range with the new radio spectrograph (Madurai Solar Radio Spectrograph) when it was operated on a daily basis during the periods December 1997 — April 1998 and August 1998. Results from statistical analysis of type III bursts are in agreement with earlier results. Employing a technique of separating the background continuum of noise storms show that the continuum is produced by electron accumulated from the preceding bursts. Also it is seen that the number of occurrences of type I bursts is maximum during the maximum of the background continuum. Comparison of records of type II bursts from MSRS with other spectrographs (Culgoora & Hiraiso) for the same bursts indicate that the sensitivity of the MSRS at low frequencies is much better than the other spectrographs.

New developments of MSRT’s working modes: One-dimensional radio heliograph and IPS telescope

This note introduces the newly developed working modes, i.e. one-dimensional meter-wave radio heliograph (MRH) and interplanetary scintillation (IPS) telescope, of meter-wave aperture synthesis radio telescope (MSRT) at Beijing Astronomical Observatory (BAO). The note describes briefly the scientific objectives, configurations of the hardware and software, and functions of the system. It presents the examples of observations on solar meter-wave bursts and IPS with the two new working modes. The results indicated that new modes not only can provide the information on the evolution of solar activities with space and time, but also can trace and monitor the propagation and spatial distribution of interplanetary plasma shock resulting from solar activities and the instability of the ionosphere, etc. Both modes are new facilities that could fill the gaps in scientific frontiers.

Anisotropic Beam Model for the Spectral Observations of Radio Burst Fine Structures on 1998 April 15

A fine structure consisting of three almost equidistant frequency bands was observed in the high frequency part of a solar burst on 1998 April 15 by the spectrometer of Beijing Astronomical Observatory in the range 2.6–3.8 GHz. A model for this event based on beam-anisotropic instability in the solar corona is presented. Longitudinal plasma waves are excited at cyclotron resonance and then transformed into radio emission at their second harmonic. The model is in accordance with the observations if we suppose a magnetic field strength in the region of emission generation of about 200 G.

Solar Hard X-Ray Bursts and Electron Acceleration Down to 8 [CLC]ke[/CLC

In 2 days that were typical of solar maximum (1991 October 1-2), the CGRO/BATSE Spectroscopy Detectors observed more than five solar X-ray bursts per hour above 8 keV [8-13 keV flux greater than 0.4 (cm2 s keV)-1]. Only approximately one-third of these were detected above 25 keV [25-59 keV flux greater than 1.7 × 10-3 (cm2 s keV)-1] by the BATSE Large Area Detectors (LADs). Most of the >8 keV bursts are impulsive and similar to solar hard X-ray bursts observed above 20 keV—the bursts occurred on the most rapid rise of the GOES thermal soft X-ray burst, and the ratio of 8-13 keV to greater than 13 keV flux for the impulsive bursts fitted a power-law spectrum [dJ/d(hν) ~ (hν)-γ] with γ = 3-7. More than half of the bursts that should be detected by the LADs if the power law extends to high energies are not detected, indicating that the spectrum steepens or cuts off above ~25 keV. These findings suggest that the acceleration of electrons occurs for essentially all impulsive energy releases but may only be detectable at low energies in smaller releases. The burst occurrence frequency varies with energy W(>8 keV) in accelerated electrons as dn/dW ~ W-1.6 above W ~ 1029 ergs, with an average rate of energy deposition of ~1026 ergs s-1, about 100 times that above 20 keV. Comparison with the occurrence frequency of the energy content in active region transient brightenings (ARTBs) detected by the Yohkoh soft X-ray telescope suggests that accelerated electrons may provide a significant fraction of the total energy in ARTBs.

Solar microwave combined type-III bursts

We call a “microwave combined type III burst” when there is a microwave common type III burst at the low frequency end and a simultaneous microwave continuous type U burst at the high frequency end. The latter is the result of further evolution of single microwave type U bursts in one and the same magnetic ring, and, as such, is still a subtype of type III bursts. Hence the whole combined type III burst is also a subtype of type III bursts. The source of radiation of the microwave combined type III burst (i.e. high-energy electron beams) comes from one and the same region of acceleration. But in its interaction with magnetic rings in low coronal regions an electron beam is divided into two beams, one of captured electrons and one of escaping electrons. These beams have different trajectories of motion and give rise to simultaneous continuous type III burst at the high frequency end and common type III burst at the low frequency end. The formation of the combined burst is closely related to the structure of magnetic rings in the low coronal regions, so it is peculiar to the microwave range.

Multi-resolution wavelet analysis of high time resolution millimeter wavelength observations of solar bursts

By means of the wavelet representation of multi-resolution analysis, we study millimeter wavelength (mm-w) radio solar bursts obtained at 48 GHz with high time resolution (≤ 8 ms); observed at Itapetinga (Brazil). The multi-resolution analysis decomposes the signal locally both in time and in frequency, allowing us to identify the different temporal structures which underlie the flux density time series and the transient phenomena characteristic of solar bursts. The analysis was applied to the flux time profile of four solar bursts observed at a mm-w, studying separately the pre-flare, impulsive and post-flare phases when possible. We find that a wide range of time scales contributes to the radio flux. The minimum time scale found for the impulsive phase is 32 ms, after which the noise dominates the emission; the maximum is 8 s. Pre- and post-flare phases have a minimum time scale of 256 ms and a maximum between 1 to 8 s. Multi-resolution spectral indexes, which are a measure of the self-similar behavior of the signal, are in some cases bigger for the impulsive phase than for the pre- and post-flare phases. We find that as the flux becomes higher the contribution from shorter time scales is enhanced.

Ion-sound model of microwave spikes with fast shocks in the reconnection region

A new model for solar spike bursts is considered based on the interaction of Langmuir waves with ion-sound waves: l+s→t. Such a mechanism can operate in shock fronts, propagating from a magnetic reconnection region. New observations of microwave millisecond spikes are discussed. They have been observed in two events: 4 November 1997 between 05:52–06:10 UT and 28 November 1997 between 05:00–05:10 UT using the multichannel spectrograph in the range 2.6–3.8 GHz of Beijing AO. Yohkoh/SXT images in the AR and SOHO EIT images testify to a reconstruction of bright loops after the escape of a CME. A fast shock front might be manifested as a very bright line in T e SXT maps (up to 20 MK) above dense structures in emission measure (EM) maps. Moreover one can see at the moment of spike emission (for the 28 November 1997 event) an additional maximum at the loop top on the HXR map in the AR as principal evidence of fast shock propagation. The model gives the ordinary mode of spike emission. Sometimes we observed a different polarization of microwave spikes that might be connected with the depolarization of the emission in the transverse magnetic field and rather in the vanishing magnetic field in the middle of the QT region. Duration and frequency band of isolated spikes are connected with parameters of fast particle beams and shock front. Millisecond microwave spikes are probably a unique manifestation of flare fast shocks in the radio emission.

Solar Microwave Bursts and Injection Pitch‐Angle Distribution of Flare Electrons

We calculate the time variation of the energy and pitch angle of high-energy electrons injected into a magnetic loop and subsequently trapped there because of magnetic mirroring. We use the evolving distribution in the calculation of gyrosynchrotron emission, as an aid to interpretation of a particular microwave burst observed using the Owens Valley Solar Array (OVSA) during a GOES class C2.8 flare on 1993 June 3. The electrons are assumed to have a Gaussian pitch-angle distribution, whose width and mean pitch angle are calculated as they evolve in time, taking into account the electron energy loss and a specific magnetic loop structure set as a model for the target active region. Various temporal behaviors of the microwave spectrum are found as a function of injection and trap conditions, which can be used to infer some of the injection properties directly from the observed microwave spectra. As a main result we found that initial pitch-angle distribution plays an important role in the microwave spectral evolution. This is largely due to the fact that pitch-angle diffusion of electrons under Coulomb collisions markedly differs at those electron energies to which the microwave spectrum is sensitive. This effect cannot be reproduced by adjusting the trap properties and therefore could be used to determine whether the initial pitch-angle distribution is isotropic or narrowly beamed. The microwave burst spectra observed during the 1993 June 3 flare are found to be most consistent with the hypothesis of an initially narrow beamed injection (≤30°) into a low-density (~4 × 109 cm-3) magnetic trap. This result explains the observed asymmetric microwave time curve consisting of a relatively short rise (~32 s) and a long decay (≥5 minutes) in terms of a transport effect rather than acceleration characteristics. The physical connection of the proposed microwave model to hard X-ray models in thin/thick targets is briefly discussed.

Study of the ionospheric TEC using GPS during the large solar flare burst on Nov. 6, 1997

Using the data from 15 global positioning system (GPS) stations scattered between 28° N—45° N and 90° W—77° E we obtained the total electron content (TEC) variations with time during a large solar flare. The results indicated that the flare makes the TEC of ionosphere increasing in a large spatial area. The amounts of the TEC enhancement are mainly determined by the local time of subionospheric point, and the largest TEC enhancement is about 2.5 TECU. It is also concluded that the TEC enhancement is not symmetrical about the local noon time, the TEC enhancement values during morning hours are larger than those of the afternoon hours.

Solar Radio Radiation Bursts on 1998 April 15

Abstract We present two remarkable “decimetric pulsation” events (DCIM) around the time 1998 0415 0800, detected by spectrometers (1.00–2.00 GHz and 2.60–3.80 GHz) at Beijing Astronomical Observatory (BAO). High-resolution dynamic spectra reveal various fine structures, such as spike groups, pulsations, type-III bursts, patches, and zebra patterns or evolving emission lines on the rising phase of the second DCIM around 3 GHz. It is the first time that such a wealth of fine features have been detected in a same type IV-DCIM event; it is also the first time that blips pairs simultaneously drifting towards the same frequency have been recorded; it is the second time to observe an evolving emission line, and the first above 1 GHz.

Mapping Analysis of Solar Radio Burst at 17 GHz on 1992 October 27

The contour maps at 17 GHz on the 1992 October 27 solar burst are analysed. Using a nonthermal inhomogeneous model of the radio source, the variations of calculated source size with time are basically consistent with the observation of maps. The variations of calculated brightness temperature with a distance ρ displaced from the project center are in better agreement with the observation of map at 01: 45: 29: 889 UT. The calculation shows that the characterizing magnetic field gradient αB at the peak of 17 GHz is about 2, magnetic strength B is about 737 G with the magnetic field Bm of the core being 2000 G. However in order to measure the coronal magnetic field accurately, it is necessary to obtain radio-imaging data at more frequencies.

A study of the fine structures superimposed on solar bursts in the 2.6–3.8 GHz frequency band

Microwave type III bursts and microwave spikes superimposed on solar bursts observed by Beijing Astronomical Observatory (BAO) are studied in this paper. The bursts and spikes are closely related and together they make up fine structure clusters; but the time resolution of the instrument plays an important role in their relation.

Flare Frequency‐Size Distributions for Individual Active Regions

It is well known that the size (peak count rate of emission, or inferred energy) distribution of —ares observed on the Sun follows a power law. However, the distributions that are constructed include —ares from diUerent active regions that have a variety of physical properties. In this paper cumulative peak count rate distributions are constructed for individual active regions using events listed in the WATCH solar X-ray burst catalog. The individual distributions are found to be consistent with the distribution constructed from events in all active regions named in the catalog. This result suggests that the —are power-law size distribution re—ects an intrinsic property of the —are mechanism and is not a product of the distributions of physical parameters between active regions. The result is consistent with the ava- lanche model for —ares and more generally argues against macroscopic —are models. Subject headings: MHDSun: activitySun: coronaSun: —ares

Radiation shielding of spacecraft in manned interplanetary flights

During the interplanetary flights the crewmembers will be exposed to cosmic ray radiation with great risk for their health. The absorbed dose due to CR depends on the galactic (GCR) or solar (SCR) origin. GCRs are isotropic and relatively high in energy and deliver a dose nearly constant with time that can be reduced only by means of “heavy” passive protection. The outer walls of the spacecraft usually shield the SCRs up to a few tens of MeV, but during some exceptional solar bursts, a great number of particles, mainly protons, are ejected at higher energies. In this case the dose delivered in a few hours by a solar burst can easily exceed 1 year cumulated dose by GCRs. The high-energy component of SCRs is quasi-directional so that a shielding system based on a superconductive magnetic lens can reduce the daily dose of SCRs to the level delivered by GCRs.

Magnetic Trapping and Electron Injection in Two Contrasting Solar Microwave Bursts

We study two microwave bursts (which occurred about 12 minutes apart in the same active region) that show contrasting characteristics in morphology and spectral variation in microwave data from the Owens Valley Radio Observatory (OVRO) Solar Array and the Nobeyama Radioheliograph (NoRH). The first flare (X-ray class C2.8, 23:22 UT) shows radio morphology that changes from a single source at a low frequency (5 GHz) to a double source at high frequencies (10-17 GHz), and shows a total power spectrum that hardens in the decay phase. In contrast, the second flare (X-ray class M1.0, 23:35 UT) shows a simple, single-source morphology at 7-17 GHz. This source is located at one of the two footpoints of a small loop identified in Yohkoh/HXT maps, and its spectrum evolves in a typical soft-hard-soft pattern. We infer the trap properties of these two loops from the microwave spectral behaviors and a coronal field extrapolation from the vector magnetogram of the active region obtained from Mees Solar Observatory, along with X-ray maps obtained from Yohkoh/SXT. It is shown that the radio characteristics of the first event are consistent with the hypothesis in which Coulomb collisions dominate in a highly anisotropic loop with low ambient electron density ~ 5 × 109 cm-3. The second flare involves a small loop in which a more uniform field strength allows rapid loss of electrons by precipitation, resulting in a spectral variation mainly controlled by the energy dependence and time profile of the injection. In spite of the significantly differing decay times (~8 and ~1 minutes, respectively), very similar injection times of ~30 s are inferred from the analysis of time profiles under the above hypotheses. We therefore present a view that the largely different morphologies and the corresponding spectral variations are consequences of the differing trap properties rather than dissimilar injection properties. The low trap density inferred for the first event (~ 5 × 109 cm-3) as compared with the second event (~ 8 × 1010 cm-3) also explains why this event showed richer microwave output in spite of weaker GOES activity.