Natural spectral bandwidth of electron cyclotron maser emission

We studied the spectral properties of the electron cyclotron maser emission generated by anisotropic distributions of fast electrons with power-law magnitude distributions in solar flares. The natural bandwidth of the generated spectral line depends significantly on the angle between the direction of the wave emission and the direction of the magnetic field in the source. In addition, the line bandwidth depends on the parameters of the momentum and pitch-angle distribution functions for fast electrons. The typical spectral bandwidths are shown to lie within the range 0.1–0.4%, in agreement with the minimum observable bandwidths of millisecond solar radio spikes.     

晚型恒星宁静微波辐射的一种产生机制

本文提出具幂津谱分布的非热电子在多重偶极子磁场中产生回旋同步辐射是晚型恒星宁静微波辐射的一个可能机制。文中假设有10—20个磁偶极子随机地分布在恒星光球之下,非热电子与背景热电子数密度之比<10~(-3),并且在非热电子分布中引入了与其寿命有联系的因子。由此计算并分析了回旋同步辐射谱和偏振性质,并获得了辐射源的空间分布特性。     

Pitch-angle scattering of energetic particles in the current sheet of the magnetospheric tail and stationary distribution functions

An investigation of pitch-angle scattering of energetic particles in magnetic field configurations with a current sheet similar to that observed in the geomagnetotail has been performed. The magnetic field model is specified by two parameters which are the current sheet thickness in units of particle gyroradius and the angle between the magnetic field lines and the sheet plane. Computations of a considerable number of trajectories (about 20,000 for each model case) has provided the possibility of obtaining the matrix of pitch-angle scattering and the corresponding kernel function of the integral equation for the stationary particle distribution function. Solution of this equation shows that isotropic distributions are formed only in the case of a sufficiently thick current sheet. Particle scattering in a thin field reversal region leads to the formation of an anisotropic stationary distribution. The results can be used for interpretation of the data on the spatial distribution of energetic particle fluxes in the near part of the magnetospheric tail and in the vicinity of the outer boundary of the radiation belt.     

A Statistical Study of Microwave Flare Morphologies

This study has been motivated by the detection of a small number of optically thin microwave bursts with maximum emission near the loop top, which is contrary to the prediction of isotropic gyrosynchrotron models. Using Nobeyama Radioheliograph (NoRH) high-spatial-resolution images at 17 and 34 GHz, we study the morphology at the radio peak of 104 flares that occurred relatively close to the limb. Using data from the Nobeyama Polarimeter we were able to determine whether the 17- and 34-GHz emissions came from optically thin or thick sources. We identified single-loop events, taking into account supplementary information from EUV and soft X-ray (SXR) images. We found optically thin emission from the top of the loop in 36% of single-loop events. In agreement with standard models, in this sample 46% and 18% of the events showed optically thin emission from the footpoints and optically thick emission from the entire loop, respectively. The derived percentage of events with gyrosynchrotron emission from isotropic populations of energetic electrons is possibly an upper limit. This point is illustrated by the analysis of an optically thin event that shows footpoint emission during the rise phase and loop-top emission during the decay phase. A model that takes into account both anisotropies in the distribution function of nonthermal electrons and time evolution can reproduce the observed transition from footpoint to loop-top morphology, if electrons with pitch-angle anisotropy are injected near one of the footpoints.     

Radiation transfer of gyrosynchrotron emission in stellar environments

Non-thermal radio emission has been detected from several kinds of active stars. Polarization and intensity measurements of the quiescent (i.e. non-flaring) emission indicate gyrosynchrotron emission. A three-dimensional magnetic field model for the stellar field is presented and the gyrosynchrotron intensity and polarization emerging from such a model is calculated and compared with observations. Model spectra agree well with observations. Model polarization results indicate that the emission region is more complex than the simple model and may indicate the presence of multipole components in the field, small loops of flux near the stellar surface and significant energy loss in the radiating electron distribution over time.     

Thermal cyclotron radiation from hot coronal loops and peculiarities of the polarization structure of solar microwave emission sources: I. Brightness temperature

We discuss the possible contribution of the thermal cyclotron radiation from hot coronal magnetic loops to the observed characteristics of the microwave emission from solar active regions. Based on the simplest three-dimensional model of a loop in the shape of a hot torus, we have calculated the expected peculiarities of the frequency and polarization structures of microwave emission sources associated with sunspots and containing coronal loops. Our model calculations of the two-dimensional brightness temperature distributions at various wavelengths for the ordinary and extraordinary modes and the wavelength dependences of the brightness temperatures are presented in the first part of the work. The loop size, the electron density, and the source position on the disk have been found to affect these characteristics. Our numerical calculations of the brightness temperature distributions and spectra have confirmed the well-known assumption that under certain conditions the spectrum of a hot filament can contain cyclotron lines and the sense of the polarization can change over the range. The results obtained here refer to the brightness temperature along the line of sight that crosses the photosphere at a point with given coordinates, i.e., these are the emission characteristics at a fixed point of the source. Integrated characteristics (the flux from the entire source and its polarization) and a discussion of the hot loop model will be given in the second part of the work.     

Resonant Transition Radiation and Solar Radio Bursts

This paper presents general relations for the intensity of the resonant transition radiation (RTR) and their detailed analysis. This analysis shows that the spectrum amplitude of the x-mode at some frequencies for high-energy electrons can grow with the magnetic field increase in some interval from zero value; it can even dominate over that for the o-mode. With further magnetic field increase, the intensity of the RTR x-mode decreases in comparison with the intensity of the o-mode and this decrease is higher for higher velocities of energetic electrons. The polarization of the RTR depends on the velocity of energetic electrons, too. For velocities lower than some velocity limit v<v _i the RTR emission is unpolarized in a broad interval of magnetic field intensities in the radio source. For reasonable values of indices of the power-law distribution functions of energetic electrons, the RTR is broadband in frequencies (df/f≈0.2−0.4). Furthermore, we show various dependencies of the RTR and its spectral characteristics. Assuming the same radio flux of the transition radiation and the gyro-synchrotron one at the Razin frequency, we estimate the limit magnetic field in the radio source of the transition radiation. Then, we analyze possible sources of small-scale inhomogeneities (thermal density fluctuations, Langmuir and ion-sound waves), which are necessary for the transition radiation. Although the small-scale inhomogeneities connected with the Langmuir waves lead to the plasma radiation, which is essentially stronger than RTR, the inhomogeneities of the ion-sound waves are suitable for the RTR without any other radiation. We present the relations describing the RTR for anisotropic distribution functions of fast electrons. We consider the distribution functions of fast electrons in the form of the Legendre polynomials which depend on the pitch-angle. We analyze the influence of the degree of the anisotropy (an increase of the number of terms in the Legendre polynomial) on spectral characteristics of the RTR. A comparison with previous studies is made. As an example of the use of the derived formulas for the RTR, the 24 December 1991 event is studied. It is shown that the observed decimetric burst can be generated by the RTR in the plasma with the density inhomogeneities at the level 〈ΔN ²〉/N ²=2.5⋅10⁻⁵.     

Energetics and morphology of powerful impulsive solar flares

We have studied the energetics of two impulsive solar flares of X-ray class X1.7 by assuming the electrons accelerated in several episodes of energy release to be the main source of plasma heating and reached conclusions about their morphology. The time profiles of the flare plasma temperature, emission measure, and their derivatives, and the intensity of nonthermal X-ray emission are compared; images of the X-ray sources and magnetograms of the flare region at key instants of time have been constructed. Based on a spectral analysis of the hard X-ray emission from RHESSI data and GOES observations of the soft X-ray emission, we have estimated the spatially integrated kinetic power of nonthermal electrons and the change in flare-plasma internal energy by taking into account the heat losses through thermal conduction and radiation and determined the parameters needed for thermal balance. We have established that the electrons accelerated at the beginning of the events with a relatively soft spectrum directly heat up the coronal part of the flare loops, with the increase in emission measure and hard X-ray emission from the chromosphere being negligible. The succeeding episodes of electron acceleration with a harder spectrum have virtually no effect on the temperature rise, but they lead to an increase in emission measure and hard X-ray emission from the footpoints of the flare loops.     

On the Microwave Spike Emission of the September 6, 1992 Flare

The main aim of this paper is to estimate, from multispectral observations, the plasma parameters in a microwave burst source which was also the site of spike emission. This information is essential for the determination of the spike emission process. By analyzing one-dimensional source distributions observed with the SSRT at 5.7 GHz and correlating them with Yohkoh X-ray and Nobeyama 17 GHz images, we have concluded that the microwave emitting region was larger than the soft X-ray loop-top source, and that the origin of the burst could be explained by gyrosynchrotron emission of non-thermal electrons in a magnetic field of approximately 100 G. It has been shown that the source of 5.7 GHz spikes observed during the burst was located close to an SXR-emitting loop with high density and temperature and a relatively low magnetic field. Thus, plasma emission is the most favourable radiation mechanism for the generation of the sub-arc-second microwave pulses.     

Flare SOL2012-07-06: On the Origin of the Circular Polarization Reversal Between 17 GHz and 34 GHz

The new generation of multiwavelength radioheliographs with high spatial resolution will employ microwave imaging spectropolarimetry to recover flare topology and plasma parameters in the flare sources and along the wave propagation paths. The recorded polarization depends on the emission mechanism and emission regime (optically thick or thin), the emitting particle properties, and propagation effects. Here, we report an unusual flare, SOL2012-07-06T01:37, whose optically thin gyrosynchrotron emission of the main source displays an apparently ordinary mode sense of polarization in contrast to the classical theory that favors the extraordinary mode. This flare produced copious nonthermal emission in hard X-rays and in high-frequency microwaves up to 80 GHz. It is found that the main flare source corresponds to an interaction site of two loops with greatly different sizes. The flare occurred in the central part of the solar disk, which allows reconstructing the magnetic field in the flare region using vector magnetogram data. We have investigated the three possible known reasons of the circular polarization sense reversal – mode coupling, positron contribution, and the effect of beamed angular distribution. We excluded polarization reversal due to contribution of positrons because there was no relevant response in the X-ray emission. We find that a beam-like electron distribution can produce the observed polarization behavior, but the source thermal density must be much higher than the estimate from to the X-ray data. We conclude that the apparent ordinary wave emission in the optically thin mode is due to radio wave propagation across the quasi-transverse (QT) layer. The abnormally high transition frequency (above 35 GHz) can be achieved reasonably low in the corona where the magnetic field value is high and transverse to the line of sight. This places the microwave source below this QT layer, i.e. very low in the corona.     

Synchrotron X-ray emission from supernova remnants. Exponential cut-off in the electron spectrum

We present a model which describes the evolution of the energy spectrum of relativistic electrons in supernova remnants, with radiation losses of electrons taken into account. The model can be used to calculate the synchrotron X-ray emission from supernova remnants in the uniform interstellar medium and in the uniform interstellar magnetic field. The importance of various factors in the variations of spatial distributions of nonthermal electrons and their synchrotron emissive capacity is demonstrated. We analyze the errors which arise in the magnetic field strength when it is estimated with the use of the models which ignore the detailed pattern of the evolution of the magnetic field and the electron spectrum behind the shock front in the remnant. The evolution of synchrotron emission spectrum and the ratio between the synchrotron radio and X-ray fluxes from supernova remnants are calculated.     

Magnetography of Solar Flaring Loops with Microwave Imaging Spectropolarimetry

We have developed a general framework for modeling gyrosynchrotron and free–free emission from solar flaring loops and used it to test the premise that 2D maps of source parameters, particularly the magnetic field, can be deduced from spatially resolved microwave spectropolarimetry data. We show quantitative results for a flaring loop with a realistic magnetic geometry, derived from a magnetic-field extrapolation, and containing an electron distribution with typical thermal and nonthermal parameters, after folding through the instrumental profile of a realistic interferometric array. We compare the parameters generated from forward-fitting a homogeneous source model to each line of sight through the folded image data cube both with the original parameters used in the model and with parameters generated from forward-fitting a homogeneous source model to the original (unfolded) image data cube. We find excellent agreement in general, but with systematic effects that can be understood as due to the finite resolution in the folded images and the variation of parameters along the line of sight, which are ignored in the homogeneous source model. We discuss the use of such 2D parameter maps within a larger framework of 3D modeling, and the prospects for applying these methods to data from a new generation of multifrequency radio arrays now or soon to be available.     

Nature of the iogenic plasma source in Jupiter's magnetosphere: I. Circumplanetary distribution

Three-dimensional calculations are presented for the circumplanetary nature of the iogenic plasma source (pickup ions produced by electron and charge exchange processes in the plasma torus) created by O and S gases located above Io's exobase in its corona and escaping extended neutral clouds (designated as the “Outer Region”). These calculations are undertaken using neutral cloud models for O and S with realistic incomplete collisional cascade source velocity distributions and rates at Io's exobase and realistic spacetime loss processes in the plasma torus. The resulting spatial distributions for O and S about Jupiter are highly peaked at Io but extend at much lower density levels all about the planet, particularly within Io's orbit where they may play a role in the pitch angle scattering and energy loss of radially inward diffusing energetic electrons for the synchrotron radiation belts of Jupiter, in producing bite-outs in the energy distribution of energetic heavy ions near Io's orbit, and in providing a charge exchange source for energetic neutral atoms (ENAs) detected both near and far from Jupiter. For the iogenic plasma source created by these neutrals, two-dimensional distributions produced by integrating the three-dimensional information along the magnetic field lines are presented for the instantaneous values of the pickup ion rates, the total- and net-mass loading rates, the mass-per-unit-magnetic-flux source rate, the pickup conductivity, the pickup radial current, and the pickup ion power (or energy rate). On the circumplanetary spatial scale, the instantaneous iogenic plasma source is highly peaked about Io's position on its orbit around Jupiter. The degree of orbital asymmetry and its physical origin are discussed, and overall spatially integrated rates are presented. The spatially integrated net-mass loading rate is 154 kg s⁻¹ and the total (electron impact and charge exchange) mass loading rate is 275 kg s⁻¹. Rough minimum estimates are made for the spatially integrated total-mass loading rate created by the “Inner Region” (spatial region below Io's exobase) and are at least ∼1 to 2.5 times larger than that for the Outer Region. Implications of the iogenic plasma source created by the Outer Region and the Inner Region are discussed.     

从微波爆发谱光薄部分辐射估计日冕磁场强度的一种方法

从微波爆发谱光薄部分辐射估计日冕磁场强度的一种方法周爱华（中国科学院紫金山天文台南京２１０００８）关键词回旋同步加速辐射谱，磁场强度１引言目前对太阳活动区物理和耀斑物理过程的真正认识仍是十分困难的，原因之一在于迄今为止人们仍未找到一种完善的，能被大家...     

Evidence that Synchrotron Emission from Nonthermal Electrons Produces the Increasing Submillimeter Spectral Component in Solar Flares

We investigate the origin of the increasing spectra observed at submillimeter wavelengths detected in the flare on 2 November 2003 starting at 17:17 UT. This flare, classified as an X8.3 and 2B event, was simultaneously detected by RHESSI and the Solar Submillimeter Telescope (SST) at 212 and 405 GHz. Comparison of the time profiles at various wavelengths shows that the submillimeter emission resembles that of the high-energy X rays observed by RHESSI whereas the microwaves observed by the Owens Valley Solar Array (OVSA) resemble that of ∼50 keV X rays. Moreover, the centroid position of the submillimeter radiation is seen to originate within the same flaring loops of the ultraviolet and X-ray sources. Nevertheless, the submillimeter spectra are distinct from the usual microwave spectra, appearing to be a distinct spectral component with peak frequency in the THz range. Three possibilities to explain this increasing radio spectra are discussed: (1) gyrosynchrotron radiation from accelerated electrons, (2) bremsstrahlung from thermal electrons, and (3) gyrosynchrotron emission from the positrons produced by pion or radioactive decay after nuclear interactions. The latter possibility is ruled out on the grounds that to explain the submillimeter observations requires 3000 to 2×10⁵ more positrons than what is inferred from X-ray and γ-ray observations. It is possible to model the emission as thermal; however, such sources would produce too much flux in the ultraviolet and soft X-ray wavelengths. Nevertheless we are able to explain both spectral components at microwave and submillimeter wavelengths by gyrosynchrotron emission from the same population of accelerated electrons that emit hard X rays and γ rays. We find that the same 5×10³⁵ electrons inferred from RHESSI observations are responsible for the compact submillimeter source (0.5 arcsec in radius) in a region of 4500 G low in the atmosphere, and for the traditional microwave spectral component by a more extended source (50 arcsec) in a 480 G magnetic field located higher up in the loops. The extreme values in magnetic field and source size required to account for the submillimeter emission can be relaxed if anisotropy and transport of the electrons are taken into account.     

First Images of Impulsive Millimeter Emission and Spectral Analysis of the 1994 August 18 Solar Flare

We present here the first images of impulsive millimeter emission of a flare. The flare on 1994 August 18 was simultaneously observed at millimeter (86 GHz), microwave (1-18 GHz), and soft and hard X-ray wavelengths. Images of millimeter, soft and hard X-ray emission show the same compact ( 8) source. Both the impulsive and the gradual phases are studied in order to determine the emission mechanisms. During the impulsive phase, the radio spectrum was obtained by combining the millimeter with simultaneous microwave emission. Fitting the nonthermal radio spectra as gyrosynchrotron radiation from a homogeneous source model with constant magnetic field yields the physical properties of the flaring source, that is, total number of electrons, power-law index of the electron energy distribution, and the nonthermal source size. These results are compared to those obtained from the hard X-ray spectra. The energy distribution of the energetic electrons inferred from the hard X-ray and radio spectra is found to follow a double power-law with slope 6–8 below 50 keV and 3–4 above those energies. The temporal evolution of the electron energy spectrum and its implication for the acceleration mechanism are discussed. Comparison of millimeter and soft X-ray emissions during the gradual phase implies that the millimeter emission is free-free radiation from the same hot soft X-ray emitting plasma, and further suggests that the flare source contains multiple temperatures.     

‘Time-integrated” synchrotron radiation in high-energy astrophysics

We survey the observational data on infrared, optical and X-radiation sources associated with energetic cosmic events, and note the occurrence of an apparently preferred value of the spectral index,n=1, for the radiation continua. We review the essentials of standard synchrotron radiation theory; the conventional interpretation of the observational data in terms of an energy distribution of electrons injected into a constant, low valued magnetic field; and the somewhat unsatisfactory attempts that can be made to explain this electron energy distribution in terms of the Fermi acceleration mechanism. We examine the evidence for the presence in the radiation sources of high magnetic fields, which cause evolution of the synchrotron radiation power spectrum to occur. We work out the consequences of this evolution, and obtain a new form of synchrotron radiation theory, which we describe astime-integrated synchrotron radiation theory, the particular advantage of which is that it is able to give a unique value (n=1/2 of the spectral index for radiation produced by a single high energy electron, independently of the initial electron energy. We consider the consequences of there being a distribution of magnetic field values in a radiation source; and in particular we consider a uniform distribution (in which all values are equally probable), which is capable of producing the required spectral indexn=1. We show that this uniform distribution can be explained in terms of a model in which there exist condensations of material containing high magnetic fields and within which electrons can be generatedin situ, through the familiar pion production and decay processes. We also consider systems in which electrons in a radiation source have injection patterns that enable the radiation continua to be interpreted in terms of time-integrated synchrotron radiation theory, originally devised for a single electron. We apply these considerations to sources of optical and higher frequency radiation; we also show that they have limited application to certain types of radio source. We suggest in conclusion that the condensations that feature in our model could act as basic units of structure for complex radiation sources associated with different types of energetic cosmic event, and that therein could lie the clue to the evident similarity of their radiation continua.     

Thermal cyclotron radiation from hot coronal loops and peculiarities of the polarization structure of solar microwave emission sources: II. Integrated characteristics

We present our calculations of the expected characteristics of the integrated spectrum of thermal cyclotron radiation from a solar active region with a coronal magnetic loop. A hot torus is considered as a three-dimensional loop model. We show that the hot-loop emission can change appreciably the emission characteristics of the active region at centimeter and decimeter wavelengths. At certain loop parameters, the emission frequency spectrum can have a nonmonotonic and complex pattern with several peaks or contain narrow-band cyclotron lines. The polarization structure of the source with a hot loop is also complex and the polarization is repeatedly reversed over the observed frequency range under certain conditions. The revealed spectral-polarization peculiarities are considered from the standpoint of whether some atypical observed properties of the emission sources associated with solar active regions can be explained.     

Electron precipitation in a non-uniform glow aurora

Electron intensities at 5 keV >18 keV and >45 keV, were measured on a Petrel rocket flown from Kiruna, Sweden, into a non-uniform glow aurora during the recovery phase of a magnetic bay. The intensities depend on pitch-angle in a way that is consistent with the precipitation being caused by pitch-angle diffusion from reservoirs of geomagnetically-trapped electrons. The scattering process that causes pitch-angle diffusion, and leads to three regions of relatively high intensity, appears to have properties different from the scattering process that leads to two intervening regions of low intensity. A spatial structure in electron reservoir intensity, is attributed mainly to variations in the rate of erosion by pitch-angle diffusion of an initially nearly-uniform reservoir intensity. An expression is derived for the minimum lifetime of trapped electrons undergoing strong pitch-angle diffusion.     

Annihilation radiation in cosmic gamma-ray bursts

The emission features observed in the energy spectra of cosmic gamma-ray bursts imply the existence of two radiation components of comparable intensity. The softer component is similar to the continua of featureless bursts. The fast decrease in the intensity of this radiation with increasing photon energy is apparently due to the neutron star's magnetosphere being opaque to hard photons because of the formation of electron-positron pairs in single- (,B) and two-photon (,), processes. The hard component originates from the annihilation of electron-positron pairs, its spectrum representing a broad line with an extended power-law wing. Such a shape of the spectrum is apparently due to either thermal broadening in a source with a spatially inhomogeneous and rapidly time-varying plasma temperature, or nonthermal energy distribution of particles in their motion along the magnetic field lines. It is assumed that the sources of these components are spatially separated, the annihilation radiation escaping from the polar regions of a strongly magnetized neutron star in a collimated beam without appreciable attenuation.