共查询到20条相似文献，搜索用时 31 毫秒
1.
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. 相似文献
2.
Spectral and polarization characteristics of radio emission from hot X-ray kernels are considered. It is shown that the frequency spectrum of thermal cyclotron radio emission and bremsstrahlung from these regions may contain a set of lines at cyclotron harmonics and the maximum at plasma frequency. This makes possible the diagnostics of the X-ray kernel plasma and magnetic fields according to spectral and polarization observations of microwave radio emission in solar flares. 相似文献
3.
A. A. Kuznetsov 《Solar physics》2008,253(1-2):103-116
Zebra pattern is observed as a number of almost parallel bright and dark stripes in the dynamic spectrum of solar radio emission. Recent observations show that zebra patterns in the microwave range often have superfine temporal structure, when the zebra stripes consist of individual short pulses similar to millisecond spikes. In this article, the burst on 21 April 2002 is investigated. The burst with a distinct superfine structure was detected at the Huairou Station (China) in 2.6?–?3.8 GHz frequency range. It is found that the emission pulses are quasi-periodic, the pulse period is about 25?–?40 ms and decreases with an increase of the emission frequency. The degree of circular polarization of zebra pattern increases with an increase of the emission frequency, it varies from moderate (about 20%) to relatively high (>60%) values. The temporal delay between the signals with left- and right-handed polarization is not found. The conclusion is made that the emission is generated by plasma mechanism at the fundamental plasma frequency in a relatively weak magnetic field. The observed polarization of the emission is formed during its propagation due to depolarization effects. A model is proposed in which the superfine temporal structure is formed due to modulation of the emission mechanism by downward propagating MHD oscillations; this model allows us to explain the observed variation of the pulse period with the emission frequency. 相似文献
4.
We investigated the angular direction and polarization of the solar radio millisecond spike emission in the model in which the spike emission is due to the second harmonic instability modes driven by electron cyclotron maser of loss cone distributed electrons during the propagation of a nonlinear plasma density wave near the magnetic mirror. We found that, when the angle θ between the wave vector and the magnetic field is > 60 °, the emission is in 100% X-mode polarization; when 40 ° < θ<60 °, the emission is in 100% O-mode polarization provided the amplitude of the density wave is below a certain limit; above that limit, the polarization will fall from 100% O-mode to even the X-mode. We also found that only 0.1% of the free energy of energy carrying electrons in the source region is converted into radiation wave energy. 相似文献
5.
6.
Leonid V. Yasnov 《Solar physics》2014,289(4):1215-1225
A number of authors claimed that radio sources above the neutral line of the magnetic field in solar active regions are due to non-thermal emission. This study shows that the thermal mechanism explains the radio emission from such sources. Models similar to those used for interpreting cyclotron lines were used in this study. Such models account for a steep decline in the spectrum at high frequencies and a low degree of polarization. The magnetic field between the two sunspots with an anti-parallel magnetic field has a lower gradient than the field above the sunspots. This, combined with the possibly high temperature in coronal loops connecting the sunspots, leads to the following conclusions. The optical thickness of the gyroresonance layers is increased and leads to more effective radiation at a harmonic number of 4 or 5. The lower gradient of the field between the sunspots also results in more rapid growth of emission intensity with increasing wavelength in this region than in the regions immediately above the sunspots. Additionally, the spatial averaging of the source structure due to the antenna beam pattern leads to a decrease in the degree of polarization in the region between the sunspots. 相似文献
7.
It is believed that magnetic field lines are twisted and bend by turbulent motions in the Galaxy. Therefore, both Galactic synchrotron emission and thermal emission from dust reflects statistics of Galactic turbulence. Our simple model of Galactic turbulence, motivated by results of our simulations, predicts that Galactic disk and halo exhibit different angular power spectra. We show that observed angular spectra of synchrotron emission are compatible with our model. We also show that our model is compatible with the angular spectra of star-light polarization for the Galactic disk. Finally, we discuss how one can estimate polarized microwave emission from dust in the Galactic halo using star-light polarimetry. 相似文献
8.
T. M. Gledhill A. N. Witt U. P. Vijh C. J. Davis 《Monthly notices of the Royal Astronomical Society》2009,392(3):1217-1224
Imaging polarimetry through J and H broad-band filters and a 3.4 μm narrow-band filter is used to highlight the regions of scattered light in the Red Rectangle. We find that the scattered light identifies the circumbinary dust component of the molecular disc seen in CO emission. This region also appears to be the origin of the recently discovered Blue Luminescence. We find that the degrees of polarization are consistent with the amorphous carbon dust model invoked by Men'shchikov. Spectropolarimetry from 1.4 to 2.5 μm confirms that the degree of polarization in the central arcsecond region is very low. This suggests that the central bicone seen in the near-infrared is predominantly due to emission from hot dust and/or from stochastically heated nanoparticles, rather than due to scattering by large grains. 相似文献
9.
Cyclotron radiation (its frequency spectrum and polarization) of thermal electrons in a neutral current sheet is considered. It is shown that cyclotron radiation is able to escape only from a thin edge of the sheet where the magnetic field is practically homogeneous. Due to this, the frequency spectrum has the form of comparatively narrow lines with integer ratio of frequencies. This fact enables one to recognize neutral current sheets in the solar corona by their radio emission. 相似文献
10.
It has been suggested by several authors that the infra-red emission from galactic nuclei is due to synchrotron process from compact objects or due to thermal processes. We have examined here the coherent and incoherent synchrotron mechanisms from compact objects and find that these are unlikely to explain the observed emission. 相似文献
11.
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. 相似文献
12.
M. J. Aschwanden 《Solar physics》1986,104(1):57-65
A sample of 10 decimetric broadband pulsations were observed in 1980–1983 and analyzed in polarization. Half of the data set was 85–100% circularly polarized, the other half showed a mild polarization of 15–55%. The polarization is constant in time and frequency for the strongly polarized group. All the mildly polarized bursts originate from near the limb; the lower degree of circular polarization is likely to be caused by depolarization due to propagation effects. The degree of polarization is constant throughout the event, but varies in frequency for the mild polarized group. Following the leading spot hypothesis, the magneto-ionic mode of the emission was found to be extraordinary. The high circular polarization of the pulsations was interpreted to be determined by the emission mechanism itself, not by propagation effects or cut-offs (contrary to the metric type I noise storms). Implications for pulsation models are discussed.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985. 相似文献
13.
The IRAS and 2MASS associations for 193 T Tauri stars are identified in this paper. From the color–color diagrams and spectral index, it is found that the IR excesses for most samples are due to thermal emission from the circumstellar material, as suggested previously. It is also found that the IR excesses at IRAS region for few T Tauri stars and the near-IR excesses for some T Tauri stars are likely attributed to free-free emission or free-bound emission from the circumstellar ionized gas. Moreover, It is found in deredened J–H versus H–K color–color diagram that there is a slight separation in different spectral groups. The T Tauri stars locus equation in J–H versus H–K color–color diagram for our sample is also presented. 相似文献
14.
This work demonstrates the possibility of magnetic-field topology investigations using microwave polarimetric observations. We study a solar flare of GOES M1.7 class that occurred on 11 February, 2014. This flare revealed a clear signature of spatial inversion of the radio-emission polarization sign. We show that the observed polarization pattern can be explained by nonthermal gyrosynchrotron emission from the twisted magnetic structure. Using observations of the Reuven Ramaty High Energy Solar Spectroscopic Imager, Nobeyama Radio Observatory, Radio Solar Telescope Network, and Solar Dynamics Observatory, we have determined the parameters of nonthermal electrons and thermal plasma and identified the magnetic structure where the flare energy release occurred. To reconstruct the coronal magnetic field, we use nonlinear force-free field (NLFFF) and potential magnetic-field approaches. Radio emission of nonthermal electrons is simulated by the GX Simulator code using the extrapolated magnetic field and the parameters of nonthermal electrons and thermal plasma inferred from the observations; the model radio maps and spectra are compared with observations. We have found that the potential-magnetic-field approach fails to explain the observed circular polarization pattern; on the other hand, the Stokes-\(V\) map is successfully explained by assuming nonthermal electrons to be distributed along the twisted magnetic structure determined by the NLFFF extrapolation approach. Thus, we show that the radio-polarization maps can be used for diagnosing the topology of the flare magnetic structures where nonthermal electrons are injected. 相似文献
15.
The problem of strong polarization of the zebra-type fine structure in solar radio emission is discussed. In the framework of the plasma mechanism of radiation at the levels of the double plasma resonance, the polarization of the observed radio emission may be due to a difference in rates of plasma wave conversion into ordinary and extraordinary waves or different conditions of escaping of these waves from the source. In a weakly anisotropic plasma which is a source of the zebra-pattern with rather large harmonic numbers, the degree of polarization of the radio emission at twice the plasma frequency originating from the coalescence of two plasma waves is proportional to the ratio of the electron gyrofrequency to the plasma frequency, which is a small number and is negligible. Noticeable polarization can therefore arise only if the observed radio emission is a result of plasma wave scattering by ions (including induced scattering) or their coalescence with low-frequency waves. In this case, the ordinary mode freely leaves the source, but the extraordinary mode gets into the decay zone and does not exit from the source. As a result, the outgoing radio emission can be strongly polarized as the ordinary mode. Possible reasons for the polarization of the zebra pattern in the microwave region are discussed. 相似文献
16.
A.D. Kuzmin 《Astronomische Nachrichten》2007,328(5):434-435
The radio emission that was observed in earliest times could not be interpreted in terms of thermal emission process. The suggestion that synchrotron radiation was observed required the confirmation by the detection of linear polarization. We have made such a detection in 1957 which we describe in the present article. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
17.
钱善瑎 《中国天文和天体物理学报》1992,(4)
在许多激变射电源(Blazar天体)中,观测到一种非常特别的现象,即偏振角的快速旋转.在相对论喷流的框架下,这种偏振角的旋转可以用双成分模型来解释.在这个模型中,一个成分是作为背景喷流本身的辐射,是恒定不变的成分;另一个成分被认为是沿喷流传播的相对论性激波,它产生变化的流量和偏振.这两个成分的偏系辐射的叠加可以产生观测到的偏振角旋转。本文讨论了三个激变射电源(BL Lac,AO0235+164,0727-115)中发生的偏振角旋转.结果表明,用相对论喷流-激波双成分模型可以很好地拟合观测到的偏振角旋转、偏振度和流量变化。说明射电激变源中出现的快速偏振和流量变化可能是由于相对论激波沿喷流传播时,激波辐射区中磁场取向和有序性以及强度和电子密度的变化所引起的。 相似文献
18.
H. U. Nørgaard-Nielsen 《Astrophysics and Space Science》2012,340(1):161-173
One of the fundamental problems in extracting the cosmic microwave background signal (CMB) from millimeter/submillimeter observations
is the pollution by emission from the Milky Way: synchrotron, free-free, and thermal dust emission. To extract the fundamental
cosmological parameters from CMB signal, it is mandatory to minimize this pollution since it will create systematic errors
in the CMB power spectra. In previous investigations, it has been demonstrated that the neural network method provide high
quality CMB maps from temperature data. Here the analysis is extended to polarization maps. As a concrete example, the WMAP
7-year polarization data, the most reliable determination of the polarization properties of the CMB, has been analyzed. The
analysis has adopted the frequency maps, noise models, window functions and the foreground models as provided by the WMAP
Team, and no auxiliary data is included. Within this framework it is demonstrated that the network can extract the CMB polarization
signal with no sign of pollution by the polarized foregrounds. The errors in the derived polarization power spectra are improved
compared to the errors derived by the WMAP Team. 相似文献
19.
We have measured polarization of the 1.1 mm and 0.8 mm continuum emission for 3 pre-T Tauri stars and 2 T Tauri stars. Positive detections were made for NGC 1333 IRAS 4 and IRAS 16293-2422, while L1551 IRS 5 and HL Tau were only marginally detected. For GG Tau we measured a 2 upper limit of 3%. The polarization is interpreted in terms of thermal emission by magnetically aligned dust grains in circumstellar disks or envelopes. We have found a definite geometrical relation between the polarization and other circumstellar structure. 相似文献
20.
Lara A. Gopalswamy N. Kundu M. R. Pérez-EnrÍquez R. Koshiishi H. Enome S. 《Solar physics》1998,178(2):353-378
We have studied the properties and evolution of several active regions observed at multiple wavelengths over a period of about 10 days. We have used simultaneous microwave (1.5 and 17 GHz) and soft X-ray measurements made with the Very Large Array (VLA), the Nobeyama Radio Heliograph (NRH) and the Soft X-ray Telescope (SXT) on board the Yohkoh spacecraft, as well as photospheric magnetograms from KPNO. This is the first detailed comparison between observations at radio wavelengths differing by one order of magnitude. We have performed morphological and quantitative studies of active region properties by making inter-comparison between observations at different wavelengths and tracking the day-to-day variations. We have found good general agreement between the 1.5 and 17 GHz radio maps and the soft X-rays images. The 17 GHz emission is consistent with thermal bremsstrahlung (free-free) emission from electrons at coronal temperatures plus a small component coming from plasma at lower temperatures. We did not find any systematic limb darkening of the microwave emission from active regions. We discuss the difference between the observed microwave brightness temperature and the one expected from X-ray data and in terms of emission of a low temperature plasma at the transition region level. We found a coronal optical thickness of 10-3 and 1 for radiation at 17 and 1.5 GHz, respectively. We have also estimated the typical coronal values of emission measure ( 5 × 1028 cm-5), electron temperature ( 4.5 × 1066 K) and density ( 1.2 × 109 cm3). Assuming that the emission mechanism at 17 GHz is due to thermal free-free emission, we calculated the magnetic field in the source region using the observed degree of polarization. From the degree of polarization, we infer that the 17 GHz radiation is confined to the low-lying inner loop system of the active region. We also extrapolated the photospheric magnetic field distribution to the coronal level and found it to be in good agreement with the coronal magnetic field distribution obtained from microwave observations. 相似文献