Solar Bursts Research Articles

An Estimation of Spatial Variations of Magnetic Field and Superthermal Electron Distribution in cm-Radio Burst Sources

The paper presents a new method of the estimation of spatial variations of the magnetic field and superthermal electron distribution in solar cm-radio burst sources. The method is based on minimization of the difference between the theoretical and observed radio fluxes and on the analysis of several burst spectra recorded in different moments of time. Several solar cm-radio bursts are analyzed by this method. It is found that the measure of the spatial variations of the superthermal electron distribution in the radio source is always larger than that for the magnetic field.

STATISTICS OF FLARING LOOPS OBSERVED BY NOBEYAMA RADIOHELIOGRAPH. II. SPECTRAL EVOLUTION

The spectral evolution of solar microwave bursts is studied in 10 impulsive events with loop-like structures, which are selected in the flare list of Nobeyama Radioheliograph. Most events have a brighter and harder looptop (LT) with maximum time later than at least one of its two footpoints (FPs), and have a common feature of the spectral evolution in the LT and the two FPs. There are five simple impulsive bursts with a well known pattern of soft-hard-soft or soft-hard-harder (SHH). It is first found that the other five events have multiple subpeaks in their impulsive phase, and mostly have a new feature of hard-soft-hard (HSH) in each subpeak, but, the well known tendency of SHH is still maintained in the total spectral evolution of these events. All of these features in the spectral evolution of the 10 selected events are consistent with the full Sun observations of Nobeyama Radio Polarimeters in these events. The new feature of HSH may be explained by the thermal free-free emission before, during, and after these bursts, together with multiple injections of nonthermal electrons, while the SHH pattern in the total duration may be directly caused by the trapping effect.

Nonlinear analysis of decimetric solar bursts

AbstractThe solar radio emissions in the decimetric frequency range (above 1 GHz) are very rich in temporal and spectral fine structures due to nonlinear processes occurring in the magnetic structures on the corresponding active regions. In this paper we characterize the singularity spectrum, f(α), for solar bursts observed at 1.6, 2.0 and 3 GHz. We interpret our findings as evidence of inhomogeneous plasma turbulence driving the underlying plasma emission process and discuss the nonlinear multifractal approach into the context of geoeffective solar active regions.

Antenna performance analysis for decameter solar radio observations

Decameter wavelength radio emission is finely structured in solar bursts. For their research it is very important to use a sufficient sensitivity of antenna systems. In this paper we study an influence of the radiotelescope-antenna effective area on the results of decameter solar radio observations. For this purpose we compared the solar bursts received by the array of 720 ground-based dipoles and the single dipole of the radiotelescope UTR-2. It's shown that a larger effective area of the ground-based antenna allows us to measure a weaker solar emission and to distinguish a fine structure of strong solar events. This feature has been also verified by simultaneous ground- and space-based observations in the overlapping frequency range.

Effects of various solar indices on accuracy of Earth’s thermospheric neutral density models

Four kinds of solar indices F 10.7, E 10.7, S 10, Mg 10 and four thermospheric neutral density models, i.e., CIRA72, DTM94, NRLMSISE00 and JB2006, are discussed. The CHAMP accelerometer data are used to calculate thermospheric total mass density. Based on the comparison of the model densities with CHAMP observations, the effects of various indices on the model accuracy are detected. It is found that under quiet and moderate solar conditions (F 10.7<160), all of the models’ errors are reduced about 15% by using E 10.7 instead of F 10.7, while under active solar conditions (F 10.7>200) the error’s standard deviation using E 10.7 increases quickly and causes the models’ accuracy to fall down. With regard to S 10, Mg 10, their effects under quiet solar conditions are inconspicuous. If under active solar conditions, they can reduce the model error’s standard deviation by 5%–10%, implying that S 10, Mg 10 make the model error more stable. The JB2006 model, which was constructed by multi-solar-index (F 10.7, S 10, Mg 10), is compared with DTM94 and NRLMSISE00 based on single-solar-index. It is found that JB2006’s accuracy is better than DTM94’s, and is close to NRLMSISE00 under the quiet solar condition. During the solar burst occurring on October 26, 2003, JB2006 has been in best agreement with CHAMP observations. All in all, the new indices may improve thermospheric density models’ accuracy under some special conditions. Concretely, E 10.7 may reduce the average error of models and S 10, Mg 10 may prevent the error’s divergence.

A time profile of millisecond pulsations of solar microwave radio bursts

An hypothesis on the interference origin of millisecond pulsations of solar-burst microwave radio emissions based on the fact that the signal scintillation appears as a result of radio-wave propagation through an inhomogeneous turbulent corona is considered. It is shown that the time profile of pulsations depends on the phase difference of interfering waves and can either look like pulses of “emission” and “absorption” or it can have a sawtooth form with slow buildup and fast drop. The observed properties of pulsations were compared with predictions of this model; this comparison showed that the formation of pulsations and their observed properties are satisfactorily explained by multipath propagation, which takes place at traversal of the coronal plasma by radio waves.

Diagnostics of the Low-Cutoff Energy of Nonthermal Electrons in Solar Microwave and Hard X-Ray Bursts

Diagnostics of the Low-Cutoff Energy of Nonthermal Electrons in Solar Microwave and Hard X-Ray Bursts

RAPID PULSATIONS IN SUB-THz SOLAR BURSTS

A new solar burst emission spectral component has been found showing sub-THz fluxes increasing with frequency, spectrally separated from the well known microwave component. Rapid pulsations are found present in all events observed at the two frequencies of the solar submillimeter-wave telescope: 212 and 405 GHz. They were studied in greater detail for three solar bursts exhibiting the new THz spectral component. The pulse amplitudes are of about 5%–8% of the mean flux throughout the bursts durations, being comparable for both frequencies. Pulsations range from one pulse every few seconds to 8–10 per second. The pulse repetition rates (R) are linearly proportional to the mean burst fluxes (S), following the simple relationship S = kR, suggesting that the pulsations might be the response to discrete flare particle accelerator injections quantized in energy. Although this result is consistent with qualitative trends previously found in the GHz range, the pulse amplitude relative to the mean fluxes at the sub-THz frequencies appear to be nearly ten times smaller than expected from the extrapolation of the trends found in the GHz range. However there are difficulties to reconcile the nearly simultaneous GHz and THz burst emission spectrally separated components, exhibiting rapid pulsations with considerably larger relative intensities in the GHz range.

The Korean Solar Radio Burst Locator (KSRBL)

ABSTRACTThis paper describes the design and operation of the Korean Solar Radio Burst Locator (KSRBL). The KSRBL is a radio spectrometer designed to observe solar decimeter and microwave bursts over a wide band (0.245–18 GHz) as well as to detect the burst locations without interferometry or mechanical sweeping. As a prototype, it is temporarily observing at the Owens Valley Radio Observatory (OVRO), California, USA, and after commissioning will be operated at the Korea Astronomy and Space Science Institute (KASI), Daejeon, Republic of Korea. The control system can agilely choose four 500 MHz intermediate frequency (IF) bands (2 GHz instantaneous bandwidth) from the entire 0.245–18 GHz band, with a standard time resolution of 100 ms, although higher time resolution is possible subject to data-rate constraints. To cover the entire band requires 10 tunings, which are therefore completed in 1 s. Each 500 MHz band is sampled at a 1 GS s-1 (gigasample per second) rate, and 4096 time samples are Fast Fourier transformed (FFT) to 2048 subchannels for a frequency resolution of 0.24 MHz. To cover the entire range also requires two different feeds, a dual-frequency Yagi centered at 245 and 410 MHz, and a broadband spiral feed covering 0.5–18 GHz. The dynamic range is 35 dB over the 0.5–18 GHz band, and 55 dB in the 245 and 410 MHz bands, set by using switchable attenuators in steps of 5 dB. Each 500 MHz IF has a further 63 dB of settable analog attenuation. The characteristics of the spiral feed provide the ability to locate flaring sources on the Sun to typically 2′. The KSRBL will provide a broadband view of solar bursts for the purposes of studying solar activity for basic research, and for monitoring solar activity as the source of Space Weather and solar-terrestrial effects.

Plasma wave measurements with STEREO S/WAVES: Calibration, potential model, and preliminary results

The S/WAVES experiments on the two STEREO spacecraft measure waves, both in situ plasma waves and remotely generated waves such as Type II and Type III solar bursts. A part of the experiment is aimed at understanding the generation of electromagnetic waves from electrostatic Langmuir waves. For this, rapid measurements of plasma density, sufficiently rapid to be on the time scale of Langmuir wave fluctuations, are deemed necessary. Measurements of the potential of the antennas relative to the spacecraft can supply these rapid measurements. The antennas were not provided with a bias current, and so this unbiased technique has not been used previously. However, the cylindrical antennas of S/WAVES respond to temperature as well as the density of the ambient plasma, giving five quantities, ne, Te, and 3 components of E, to be determined from the three measurements of antenna potential. The work presented here discusses the analysis and interpretation of these measurements from the early part of the mission, when there were frequent observations of foreshock Langmuir waves to use for calibration. A model of the photoemission‐plasma equilibrium has been constructed, using these and other measurements. It is shown that the response to one or a few of the five quantities may be negligible, depending on the phenomenon observed, so that useful measurements are obtained of the others. Application to observation and analysis of various plasma wave phenomena will be discussed.

Sub-terahertz, Microwaves and High Energy Emissions During the 6 December 2006 Flare, at 18:40 UT

The presence of a solar burst spectral component with flux density increasing with frequency in the sub-terahertz range, spectrally separated from the well-known microwave spectral component, bring new possibilities to explore the flaring physical processes, both observational and theoretical. The solar event of 6 December 2006, starting at about 18:30 UT, exhibited a particularly well-defined double spectral structure, with the sub-THz spectral component detected at 212 and 405 GHz by the Solar Submilimeter Telescope (SST) and microwaves (1 – 18 GHz) observed by the Owens Valley Solar Array (OVSA). Emissions obtained by instruments onboard satellites are discussed with emphasis to ultra-violet (UV) obtained by the Transition Region And Coronal Explorer (TRACE), soft X-rays from the Geostationary Operational Environmental Satellites (GOES) and X- and γ-rays from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The sub-THz impulsive component had its closer temporal counterparts only in the higher energy X- and γ-rays ranges. The spatial positions of the centers of emission at 212 GHz for the first flux enhancement were clearly displaced by more than one arc-minute from positions at the following phases. The observed sub-THz fluxes and burst source plasma parameters were difficult to be reconciled with a purely thermal emission component. We discuss possible mechanisms to explain the double spectral components at microwaves and in the THz ranges.

Spike Decomposition Technique: Modeling and Performance Tests

We develop an automated technique for fitting the spectral components of solar microwave spike bursts, which are characterized by narrowband spectral features. The algorithm is especially useful for periods when the spikes occur in densely packed clusters, where the algorithm is capable of decomposing overlapping spike structures into individual spectral components. To test the performance and applicability limits of this data reduction tool, we perform comprehensive modeling of spike clusters characterized by various typical bandwidths, spike densities, and amplitude distributions. We find that, for a wide range of favorable combinations of input parameters, the algorithm is able to recover the characteristic features of the modeled distributions within reasonable confidence intervals. Having model-tested the algorithm against spike overlap, broadband spectral background, noise contamination, and possible malfunction of some spectral channels, we apply the technique to a spike cluster recorded by the Chinese Purple Mountain Observatory (PMO) spectrometer, operating above 4.5 GHz. We study the variation of the spike distribution parameters, such as amplitude, bandwidth, and related derived physical parameters, as a function of time. The method can be further applied to observations from other instruments and to other types of fine structures.

Peak Frequency Dynamics in Solar Microwave Bursts

We analyze the dynamics of the broadband frequency spectrum of 338 microwave bursts observed in the years 2001 – 2002 with the Owens Valley Solar Array. A subset of 38 strong microwave bursts that show a single spectral maximum are studied in detail. Our main goal is to study changes in spectral peak frequency νpk with time. We show that, for a majority of these simple bursts, the peak frequency shows a high positive correlation with flux density – it increases on the rise phase in ≈83% of 24 bursts where it could be cleanly measured, and decreases immediately after the peak time in ≈62% of 34 bursts. This behavior is in qualitative agreement with theoretical expectations based on gyrosynchrotron self-absorption. However, for a significant number of events (≈30 – 36%) the peak frequency variation is much smaller than expected from self-absorption, or may be entirely absent. The observed temporal behavior of νpk is compared with a simple model of gyrosynchrotron radio emission. We show that the anomalous behavior is well accounted for by the effects of Razin suppression, and further show how an analysis of the temporal evolution of νpk can be used to uniquely determine the relative importance of self-absorption and Razin suppression in a given burst. The analysis technique provides a new, quantitative diagnostic for the gyrosynchrotron component of solar microwave bursts. Applying this analysis technique to our sample of bursts, we find that in most of the bursts (60%) the spectral dynamics of νpk around the time of peak flux density is caused by self-absorption. On the other hand, for a significant number of events (≈70%), the Razin effect may play the dominant role in defining the spectral peak and dynamics of νpk, especially on the early rise phase and late decay phase of the bursts.

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.

A solar burst with a spectral component observed only above 100 GHz during an M class flare

Context. Since the installation of submillimeter solar radio telescopes, a new spectral burst component was discovered at frequencies above 100 GHz, creating the THz burst category. In all the reported cases, the events were X-class flares and the THz component was increasing. Aims. We report for the first time an M class flare that shows a different submillimeter radio spectral component from the microwave classical burst. Two successive bursts of 2 min duration and separated by 2 min occurred in active region NOAA 10226, starting around 13:15 UT and having an M 6.8 maximum intensity in soft X-rays. Methods. Submillimeter flux density measured by the Solar Submillimeter Telescope (SST) is used, in addition to microwave total Sun patrol telescope observations. Images with Hα filters, from the Hα Solar Telescope for Argentina (HASTA), and extreme UV observations, from the Extreme-ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory (SoHO), are used to characterize the flaring region. An extensive analysis of the magnetic topology evolution is derived from the Michelson Doppler Imager (SoHO, MDI) magnetograms and used to constrain the solution space of the possible emission mechanisms. Results. The submillimeter component is only observed at 212 GHz. We have upper limits for the emission at 89.4 and 405 GHz, which are less than the observed flux density at 212 GHz. The analysis of the magnetic topology reveals a very compact and complex system of arches that reconnects at low heights, while from the soft X-ray observations we deduce that the flaring area is dense (n ∼ 10 12 cm −3 ). The reconnected arches are anchored in regions with magnetic field intensity differing by an order of magnitude. Accordingly, we conclude that the microwave emission comes from mildly relativistic electrons spiraling down along the reconnected loops. A very small portion of the accelerated electrons can reach the footpoint with the stronger magnetic field (2000 G) and produce synchrotron emission, which is observed at submillimeter frequencies. Conclusions. The finding of a submillimeter burst component in a medium-size flare indicates that the phenomenon is more universal than shown until now. The multiwavelength analysis reveals that neither positron synchrotron nor free-free emission could produce the submillimeter component, which is explained here by synchrotron of accelerated electrons in a rather complex and compact magnetic configuration.

Polar rain gradients and field‐aligned polar cap potentials

ACE SWEPAM measurements of solar wind field‐aligned electrons have been compared with simultaneous measurements of polar rain electrons precipitating over the polar cap and detected by DMSP spacecraft. Such comparisons allow investigation of cross‐polar‐cap gradients in the intensity of otherwise‐steady polar rain. The generally good agreement of the distribution functions, f, from the two data sources confirms that direct entry of solar electrons along open field lines is indeed the cause of polar rain. The agreement between the data sets is typically best on the side of the polar cap with most intense polar rain but the DMSP f's in less intense regions can be brought into agreement with ACE measurements by shifting all energies by a fixed amounts that range from tens to several hundred eV. In most cases these shifts are positive which implies that field‐aligned potentials of these amounts exist on polar cap field lines which tend to retard the entry of electrons and produce the observed gradients. These retarding potentials undoubtedly appear in order to prevent the entry of low‐energy electrons and maintain charge quasi‐neutrality that would otherwise be violated since most tailward flowing magnetosheath ions are unable to follow polar rain electrons down to the polar cap. In more limited regions near the boundary of the polar cap there is sometimes evidence for field‐aligned potentials of the opposite sign that accelerate polar rain electrons. A solar electron burst is also studied and it is concluded that electrons from such bursts can enter the magnetotail and precipitate in the same manner as polar rain.

On spatial variations of magnetic field and superthermal electron distribution in cm-radio burst source

AbstractThe paper presents a new method of an estimation of spatial variations of the magnetic field and superthermal electron distribution in solar cm-radio burst sources. The method is based on the analysis of several burst spectra recorded in the different moments of time and on the minimization of the difference between the theoretical and observed radio fluxes. It is found that the measure of the spatial variations of superthermal electron distribution in the radio source is always greater than that for the magnetic field. In most cases this measure has a minimum at the impulsive phase of cm-radio bursts.

Formation of zebra patterns of solar microwave bursts as a result of propagation of radio waves through the inhomogeneous corona

The geoefficiency of solar bursts is diagnosed using the dynamic radio emission spectrum. At certain time intervals, the spectrum exhibits nearly parallel narrow-band emission strips termed the zebra pattern. Although there are many hypotheses of its origin, all of them do not take into account changes in the signal parameters upon signal propagation through the solar corona. Our analysis shows that the propagation effects form a dynamic spectrum that contains a zebra pattern. The properties of the modeled spectrum are shown to coincide with the basic properties of the observed spectrum. It is clarified that the spike structure of strips is a natural consequence of the interference of radio waves, and the occurrence of this structure is considered to be evidence in favor of the interference nature of the zebra pattern formation. Consequently, the zebra pattern can be formed not in the radiation source itself, but rather can arise as a result of propagation of radio waves through an inhomogeneous refracting medium of the solar corona.

Perpendicular scattering for electron beams by the electron/electron instability in solar electron bursts

Energy dependence of electron pitch angle distributions in solar electron bursts is studied using both particle‐in‐cell simulations of the electron/electron instability and a time‐of‐flight model for streaming electrons. The simulations show that the electron/electron instability leads to a maximum of electron pitch angle width in electron kinetic energy. The energy where the maximum appears varies with electron beam parameters. The simulations further show that the maximum electron pitch angle width increases with increasing electron beam density. The time‐of‐flight effect which leads to positive slopes in electron velocity distributions implies that the probability of electron/electron instability growth in the solar wind depends on electron kinetic energy because of the presence of an electron halo distribution. It is concluded that the electron/electron instability induced by the time‐of‐flight effect is likely to yield a maximum electron pitch angle width near 1 keV.

Resonant Transition Radiation and Solar Radio Bursts

We calculate the resonant transition radiation (RTR) using the most general formulas and obtain approximate analytical expressions describing the RTR dependence on the fast-particle velocity and magnetic-field intensity. Dependence of the RTR on the angle between the wave vector and the magnetic-field direction and also on the thermal velocity of background-plasma electrons is considered on the basis of approximate expressions for RTR, in which the longitudinal dielectric permittivity of the plasma is written neglecting the wave vector of transverse electromagnetic waves. The obtained dependences are combined into unified expressions for RTR coefficients. Comparison with the earlier found RTR coefficients is performed. It is shown that the obtained coefficients correspond better to the nature of the resonant transition radiation. Finally, the obtained RTR coefficients are used for interpretation of the radio emission of the solar burst of December 24, 1991.