共查询到20条相似文献，搜索用时 31 毫秒
1.
I. Yu. Grigor’eva M. A. Livshits G. V. Rudenko I. I. Mysh’yakov 《Astronomy Reports》2013,57(8):611-621
Data on small active regions on the Sun collected over three years (2007–2009) are analyzed. Under very quiescent conditions (a low X-ray background level), the shapes of the coronal loops of some active regions correspond fairly well to the shapes of magnetic-field lines calculated in a potential approximation. This is true of several active regions (e.g., the group AR 10999 in June 2008) in which no flares more powerful than B3 were observed. The radio emission of this active region detected by the RATAN-600 telescope was very weak and virtually no polarization was detected. Subflares were observed in most groups. It is demonstrated using AR 10933 (January 2007) as an example that a growth in the soft X-ray emission by up to factors of ten simultaneous with an increase in the radio flux is characteristic for such active regions. A source with the opposite polarization developed to the Northwest of the main spot in AR 10933. A series of SOHO/MDI (and also Hinode) magnetograms shows the emergence of new magnetic flux before the development of this polarized source, which continued for several hours on January 8, 2007. The current density at surfaces located at various heights is estimated based on observations of the total vector magnetic field (Hinode data) and a non-linear, force-free magnetic-field extrapolation. The height-integrated current becomes appreciably stronger at two nodes above a field neutral line, near the location of the main emerging flux. This supports the idea that the emergence of new magnetic flux is a key factor in the evolution of active regions at all stages of their existence. The development of this picture could help in elucidating the inter-relationship between current enhancements, plasma heating, and particle acceleration, in both weak active regions and strong activity complexes. 相似文献
2.
The recent publication of evidence for a new mechanism producing background radio emission of the Galaxy at centimeter wavelengths (in addition to synchrotron radiation, free—free transitions in ionized gas, and the weak radio emission of standard dust) gave rise to a strong reaction among observers, and requires independent experimental verification. This signal is of special concern in connection with studies of the polarization of the cosmic microwave background (CMB) using new-generation experiments. We have derived independent estimates of the validity of the “spinning-dust” hypothesis (dipole emission of macromolecules) using multi-frequency RATAN-600 observations. Test studies in the Perseus molecular cloud show evidence for anomalous extended emission in the absence of strong radio sources (compact HII regions) that could imitate an anomalous radio spectrum in this region. A statistical analysis at centimeter wavelengths over the Ratan Zenith Field shows that the upper limit for the polarized noise from this new component in the spinning-dust hypothesis is unlikely to exceed 1 µK at wavelengths of 1 cm or shorter on the main scales of the EE mode of Sakharov oscillations. Thus, this emission should not hinder studies of this mode, at least to within several percent of the predicted level of polarization of the CMB emission. 相似文献
3.
New observations of solar microbursts are reported. The spectra of microbursts can contain narrow features (Δf/f ≤ 0.03). Possible mechanisms for the generation of the microburst radio emission are analyzed, focusing on mechanisms that preferentially generate ordinary waves. Together with the well-known generation of radio emission at the fundamental plasma frequency, mechanisms associated with upper-hybrid waves and resonant-transition radiation are considered. The radio emission at the upper-hybrid frequency always corresponds to ordinary waves. Ordinary waves can also dominate in resonant-transition radiation, but the presence of well-developed small-scale turbulence in the emission region is required. Possible mechanisms for the generation of this turbulence include thermal fluctuations, Langmuir turbulence, quasi-stationary structures created by plasmawaves, and ion-acoustic waves. Probable origins of the quasi-periodicity of microbursts are analyzed. 相似文献
4.
Two solar radio bursts exhibiting narrow-band millisecond pulsations in intensity and polarization are analyzed. There were considerable time delays between the left-and right-circularly polarized components of the radio emission. The observed oscillations of the degree of polarization are due to the different group velocities of the ordinary and extraordinary modes in their propagation from the source to the observer; the frequency dependence of the delay is in excellent agreement with the theoretically calculated group delay in a magnetoactive plasma. It unambiguously follows that the pulsed radio emission is generated near the double upper hybrid frequency by the nonlinear plasma mechanism, since the source emission has a low degree of polarization. In addition to dispersion effects, a Fourier analysis also reveals effects associated with the source inhomogeneity. We detected a frequency drift of pulsations (autodelays) with different signs for different polarization components. This drift suggests that, apart from the dispersion effects, there are also the effects related to inhomogeneity of the radio source. It is shown, in particular, that the upper hybrid modes (generating the radio emission) are unstable in regions with enhanced gradients of the plasma density and/or magnetic field. 相似文献
5.
Observations of the active region AR 7962 obtained at 2–32 cm on the RATAN-600 radio telescope on May 10–12, 1996, are presented. The high-resolution measurements detected a narrow feature near 8.5 cm against the background of the smooth spectrum of the local source associated with sunspots. This narrow-band emission is identified with a bright, pointlike, high-frequency source at 1.7 cm recorded on maps made using the Nobeyama radio telescope. The characteristics of the observed line (lifetime 3 days, brightness temperature of the order of several million Kelvin, relative width of about 10%) suggest that it can be explained as thermal cyclotron radiation at the third harmonic of the electron gyrofrequency from a compact source containing a dense, hot plasma; the corresponding higher frequency emission could be due to thermal Bremsstrahlung. Analysis of the RATAN-600 and Nobeyama data can be used to probe the magnetic field, kinetic temperature, and electron density in the radiation source in the corona. 相似文献
6.
Spectra of solar-flare active regions displaying peculiarities in their polarized radio emission observed on the RATAN-600
radio telescope at 2–16 GHz are considered. An appreciable dip of the circularly polarized emission (Stokes parameter V) in the middle of the microwave range (6–12 GHz), sometimes with a reversal of the sign of the polarization, is unusual.
In some cases, the ordinary emission also dominates at long microwave wavelengths. Expected peculiarities of the frequency
structure of microwave sources are calculated in simple models with loops in the form of hot and cool tori. Numerical calculations
of these spectra show that the above features of the polarized emission can be explained by the presence of a hot region in
the solar corona. It is shown that the parameters of the spectrum of the polarized emission can be used to determine the magnetic
field in this hot region and the product of the relative magnetic-field gradient and the loop thickness. 相似文献
7.
Using literature data on approximately 400 compact radio sources detected with the Very Large Array and located in the direction of the Galactic center within 2° of the compact source Sgr A*, 69 sources whose angular sizes are determined by scattering on electron density inhomogeneities were distinguished. Fifty-five of these are extragalactic, two are supercompact HII regions, ten are sources of maser emission, and two are variable Galactic sources. The excess of the apparent angular sizes of maser sources within 2° of the Galactic center above the mean size of objects of this class in other parts of the Galaxy found in many studies cannot be explained purely by the effect of scattering of their radio emission on interstellar plasma inhomogeneities. The angular sizes of these objects are increased due to scattering only within Galactic longitudes of about 0.4° and Galactic latitudes less than 0.1°. The turbulent medium responsible for scattering of radio emission of compact sources in the immediate vicinity of the Galactic center is strongly concentrated toward the compact source Sgr A* at the Galactic center. No extragalactic sources are observed within 0.4° in longitude and 0.2° in latitude of the Galactic center, because of their low brightness due to the superstrong scattering in this region. Data on scatter broadening can be used to study the distribution of turbulent plasma near the Galactic center. 相似文献
8.
We describe and compare methods for reducing radio circular-polarization data for Active Galactic Nuclei on parsec (VLBI) scales, and present results derived by applying these methods to 15 GHz observations of 29 AGN obtained on the Very Long Baseline Array. Circular polarization was detected in six of these objects, with characteristic degrees of several tenths of a percent. According to results obtained in other studies and the new results presented here, the most probable mechanism for the formation of the circular polarization is Faraday conversion of linear polarization when the radio emission passes through a magnetized plasma. For the objects OJ 287, 1334-127, and 3C 279, which were observed at several epochs, we find a constant degree and sign of the circular polarization on time scales of several years, testifying to the presence of stable ordered magnetic fields in the corresponding regions. 相似文献
9.
N. G. Peterova B. V. Agalakov T. P. Borisevich A. N. Korzhavin B. I. Ryabov 《Astronomy Reports》2006,50(8):679-686
We study the active region NOAA 9591 using observations at 1.92–10.17 cm obtained on two large Russian radio instruments: the RATAN-600 radio telescope and the Siberian Solar Radio Telescope. The active region was associated with an isolated spot at the photospheric level, whose magnetic field had a well-defined Delta configuration. The radio observations show that the structure of the coronal source located above the spot cannot be described in a simple (unipolar) cyclotron model. A comparison with X-ray observations indicates that the three-dimensional structure of the corona above the spot can be represented as a strongly elongated loop whose apex resembles a cusp brightening (a qualitative model for the structure is presented). Unexpectedly, radiation with a high degree of polarization (～25%) was detected far (～100 000 km) from the photosphere. The need for a quantitative model for coronal sources above the strong Delta-configuration magnetic fields, which are known to play an important role in active solar processes (flares, phenomena such as coronal-mass ejections) is outlined. Thanks to its simple morphology, which enabled the identification of a pure Delta configuration, the active region NOAA 9591 provides high-quality observational material for the creation of such a model. 相似文献
10.
A comparative analysis of sunspot oscillations and related radio sources in the active regions AR 8949, AR 8951, and AR 8953 is carried out using SOHO MDI data and simultaneous observations with the Nobeyama Radioheliograph, with a one-minute time resolution on scales of tens to hundreds of minutes. The radio sources in the selected active regions are ～40 000–60 000 km away from the corresponding spots, with the periods of long-term oscillations of the radio sources being ～12% longer. 相似文献
11.
An approximate method for calculating the returning positron flux in the polar-cap regions of a radio pulsar is proposed. The pulsar is considered in the Goldreich-Julian model for a regime of free-electron emission from the neutron-star surface in the region of open lines of the dipolar magnetic field. Calculations have been done for the case when the dipolar magnetic moment is aligned with the star's rotational axis. The acceleration of primary electrons is assumed to occur near the neutron-star surface on scales comparable to the transverse radius of the tube of open field lines. The generation of electron-positron pairs by curvature radiation of the primary electrons is taken into account. A considerable contribution to the returning flux is made by the region where the electric field is screened by the electron-positron plasma. 相似文献
12.
V. I. Shishov 《Astronomy Reports》2001,45(3):195-202
The effects of the modulation of radio waves during their passage through turbulent interstellar plasma on measurements of the structure of radio sources made with ultrahigh angular resolution using space radio interferometers are considered. Typical scattering angles?scat, pl for an extragalactic radio source at various wavelengths are estimated from scattering observations for pulsars and extragalactic sources. The (?0λ) plane, where ?0 is the source size and λ is the wavelength of the radio emission, can be divided into five regions, in which different regimes of radio-wave modulation and image reconstruction are realized. Possibilities for image reconstruction in each of these regions are investigated. 相似文献
13.
The main argument against the idea that the intense radio emission observed from active regions on the Sun and flare stars is electron-cyclotron maser (ECM) radiation is that such radiation should be strongly absorbed in higher-lying layers where the condition for the cyclotron resonance at harmonics of the electron gyrofrequency is fulfilled. Cyclotron absorption lowers the efficiency of ECM radiation virtually to zero for a broad range of angles between the direction of propagation of the radiation and the magnetic field. Less severe absorption is possible only in narrow angular “windows” along (for ordinary and extraordinary waves) and perpendicular to (for ordinary waves) the magnetic field. However, the ECM radiation that is generated does not fall into these windows of transparency due to the kinematic conditions corresponding to coronal magnetic traps. We investigate the efficiency of induced scattering of ECM radiation on ions in the equilibrium plasma in the source. Under certain conditions, induced scattering leads to the formation of a condensate of ECM radiation with the direction of its wave vectors approximately along the magnetic field, enabling the escape of the radiation through windows of transparency. The most favorable conditions for this phenomenon are realized for ordinary waves. We estimate the optical depths of the sources of the ECM radiation to the scattering and the angular width of the condensate for ordinary and extraordinary waves for the cases of the flare radio emission of the star AD Leo and the sources of type I noise storms in the solar corona. In both cases, the polarization of the emergent radiation should correspond to the ordinary wave. 相似文献
14.
I. M. Chertok V. A. Slemzin S. V. Kuzin V. V. Grechnev O. I. Bugaenko I. A. Zhitnik A. P. Ignat'ev A. A. Pertsov 《Astronomy Reports》2004,48(5):407-417
We analyze large-scale solar activity following the eruption of a very powerful, geoeffective coronal mass ejection in the 23rd solar cycle, observed at 175, 284, and 304 Å on November 4, 2001, using data from the CORONAS-F/SPIRIT telescope. In particular, we have shown that the restructuring of the magnetic field above the eruption center was accompanied by the formation of a multicomponent post-eruptive arcade, which was observed in all three bands over many hours and had an extent of the order of 0.5R⊙. Two kinds of dimmings were observed, i.e., compact dimmings on either side of this arcade and channeled dimmings along some extended features beyond the active region. The intensity in the dimmings decreased by several tens of percent. The enhanced emission observed at the top of the post-eruptive arcade can be due to energy release in the course of magnetic reconnection high in the corona at the relaxation stage of the perturbed magnetic field to a new equilibrium state with a closed configuration. It can also be due to an enhanced emission measure because of the oblique direction of the line of sight crossing both loop tops and footpoint regions. The spatial coincidence of the main dimmings in lines corresponding to different temperatures indicates that a plasma outflow from the transition region and coronal structures with opened field lines are responsible for these dimmings. Variations in the plasma temperature associated with coronal mass ejections probably play an important role for some dimmings, which appear different in different lines. 相似文献
15.
G. P. Chernov 《Astronomy Reports》2004,48(10):853-870
A joint analysis of several recent solar type IV radio outbursts with zebra structures and fiber bursts in their dynamical radio spectra is carried out using all available ground-based and satellite data (Yohkoh, SOHO, TRACE). Zebra structures and fiber bursts were observed at frequencies from 20 to 6500 MHz. The main relative spectral parameters and degree of circular polarization of the zebra structures and fiber bursts are nearly the same. The relative width of the zebra structures varies only slightly with frequency (≈0.003–0.005); the radio emission is radiated in the ordinary mode. New data on centimeter-wavelength zebra structures and fiber bursts testifies that they are analogous to similar structures observed at meter wavelengths. A double-plasma-resonance model for the zebra structures based on the observational dependences for the electron density and magnetic field yields a frequency dependence for the frequency separation between stripes that does not agree with the observations. Fine structure was observed together with the rise into the corona of new, hot magnetic loops, in which instabilities associated with high-frequency and low-frequency plasma waves develop. The frequency range of the fine structure in the dynamical spectra is probably determined by the extent of these new loops in the corona. The continuous transition of the fiber bursts into zebra structures and vice versa testifies to a single origin for these two structures. All the main properties of the stripes in emission and absorption can be explained if they are associated with interactions between electrostatic plasma waves and whistlers. It is possible to obtain realistic values for the magnetic-field strength of B≈160 G at a plasma level of about 3 GHz in this model. 相似文献
16.
Thermal cyclotron emission features from a sunspot-associated source in a solar active region are considered in the framework of a three-level temperature model with a hot layer. The images of the source have a complex fine structure, with rings with different brightness temperatures and different signs of circular polarization. The proposed model suggests the possibility of a double or multiple reversal of the sign of polarization, as well as a significant increase of the fluxes in the 3–4 cm range, as is typical of active regions associated with proton flares. Energetic particles of the halo cannot provide the required temperatures of the layers. Alternative heating mechanisms are discussed. 相似文献
17.
The magnetic fields of solar active regions are analyzed using a method based on comparing the spatial structures of the reconstructed magnetic field and of the radio emission of the active region. Two approaches are used: comparing the radio size of the active region and the corresponding size calculated using the reconstructed magnetic field, and comparing the radio spectra that are observed and calculated using the reconstructed magnetic field. Overall, the calculated sizes and spectra correspond fairly well to the observational data, making it possible to estimate physical parameters of the emitting region, such as the electron density and temperature. 相似文献
18.
Results of polarization observations of gas-dust condensations obtained on the Nançay radio telescope in the 1665 and 1667 MHz OH lines in all four Stokes parameters are reported. Seven OH maser sources associated with methanol masers were selected for this study. The goal was to estimate the magnetic fields in methanol condensations from the Zeeman splitting of OH maser lines associated with the methanol masers. The Gaussian parameters of features in the OH spectra are presented, and their polarization parameters are estimated: the degree of circular polarization m C , flux density in linear polarization p, and degree of linear polarization m L . The magnetic field intensity B has been estimated from the Zeeman splitting of the OH lines and approximation of the Stokes parameter V from the derivative of Stokes parameter I. B varies from ≤0.5 to 1.4 mG for different sources. The association of OH masers with methanol emission has been analyzed; the magnetic fields of OH masers in interstellar condensations associated with Class I methanol masers can be determined more reliably than the fields in interstellar condensations with OH masers associated with Class II methanol emission, and have higher values. The sizes of the studied regions suggest they may be bound structures such as Bok globules, small IRDC clouds, or protoplanetary disks. 相似文献
19.
O. V. Verkhodanov E. K. Maiorova O. P. Zhelenkova D. I. Solov’ev M. L. Khabibullina 《Astronomy Reports》2016,60(7):630-654
Fields of the Planck cosmic-microwave background maps in the regions of radio sources of the RCR catalog have been studied. Using measurements from the Planck catalog, calibration curves have been plotted in order to determine the objects’ luminosities. The flux densities at millimeter and submillimeter wavelengths of 83 objects with normal radio spectra have been estimated for the first time; their spectra have also been constructed, they have been optically identified, and information available in various databases has been collected. A statistical comparison with a sample of sources with steep radio spectra has been carried out. Faint, difficult to characterize microwave sources make an additional contribution to the secondary anisotropy on angular scales < 7′. An algorithm for selecting candidate objects with the Sunyaev–Zel’dovich effect has been proposed, based on the use of data on the radio spectral indices and the signal in cosmic-microwave background maps. 相似文献
20.
A high-accuracy method for the removal of point sources from maps of the cosmic microwave background
A. T. Bajkova 《Astronomy Reports》2005,49(12):947-957
A new method for removing point radio sources and other non-Gaussian noise is proposed as a means of improving the accuracy of estimates of the angular power spectrum of the cosmic microwave background (CMB). The main idea of the method is to reconstruct fluctuations of the CMB in places contaminated by such emission, while traditional methods simply exclude these regions from consideration, leading to the appearance of “holes” in the resulting maps. The fundamental possibility of reconstructing the CMB signal in such holes follows from the analytical properties of a function with a finite spatial spectrum (the Silk damping frequency). A two-dimensional median filter is used to localize the point radio sources. Results of simulations of the method for maps of modest size are presented. The efficiency of applying the method to reconstruct the CMB from data with limited resolution and contaminated by appreciable pixel noise is investigated. The fundamental possibility of applying the method to reconstruct the CMB distribution in the region of the Galaxy is also demonstrated. 相似文献