Peculiarities of polarized radio emission of solar active regions

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.     

The quiescent X-ray emission of X-ray novae and the coronal emission of late-type stars

The X-ray luminosities and spectra of F-M stars of luminosity classes IV–V are analyzed. In dwarfs with rotational velocities of about 100 km/s, such as the optical components of low-mass X-ray novae with black holes, hot plasma can be confined in coronal loops even in the presence of fairly weak magnetic fields. Thus, the soft X-ray emission of such systems in their quiescent state (to 10³¹ erg/s) could be associated with the coronal emission of the optical component/dwarf. Two systems studied with subgiants (V1033 Sco and V404 Cyg) have X-ray luminosities 2×10³²–2×10³³ erg/s. The X-ray emission of a solar-type corona cannot provide such luminosities. However, a transition to a non-solar corona is possible in rapidly rotating subgiants—a dynamical corona whose X-ray emission can be one to two orders of magnitude higher than observed for more slowly rotating late-type subgiants in the solar neighborhood. This suggests that the quiescent X-ray emission of these two systems is provided by emission from the corona of the subgiant optical component.     

The nature of decimeter-wave microburst emission

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.     

Flare-plasma diagnostics from millisecond pulsations of the solar radio emission

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.     

Detection of a cyclotron line in the radio spectrum of a solar active region and its interpretation

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.     

Spectral-temporal evolution of low-frequency pulsations in the microwave radiation of solar flares

Low-frequency pulsations of 22 and 37 GHz microwave radiation detected during solar flares are analyzed. Several microwave bursts observed at the Metsähovi Radio Observatory are studied with time resolutions of 100 and 50 ms. A fast Fourier transformation with a sliding window and the Wigner-Ville method are used to obtain frequency-time diagrams for the low-frequency pulsations, which are interpreted as natural oscillations of coronal magnetic loops; the dynamical spectra of the pulsations are synthesized for the first time. Three types of low-frequency fluctuations modulating the flare microwave radiation can be distinguished in the observations. First, there are fast and slow magneto-acoustic oscillations with periods of 0.5–0.8 s and 200–280 s, respectively. The fast magneto-acoustic oscillations appear as trains of narrow-band signals with durations of 100–200 s, a positive frequency drift dν/dt=0.25 MHz/min, and frequency splitting δν=0.01–0.05 Hz. Second, there are natural oscillations of the coronal magnetic loops as equivalent electrical circuits. These oscillations have periods of 0.5–10 s and positive or negative frequency drift rates dν/dt=8×10^?3 Hz/min or dν/dt=?1.3×10^?2 Hz/min, depending on the phase of the radio outburst. Third, there are modulations of the microwave radiation by short periodic pulses with a period of 20 s. The dynamical spectra of the low-frequency pulsations supply important information about the parameters of the magnetic loops: the ratio of the loop radius to its length r/L≈0.1, the plasma parameter β≈10^?3, the ratio of the plasma densities outside and inside the loop ρ_e/ρ_i≈10^?2, and the electrical current flowing along the loop I≈10¹² A.     

The possible escape of electron cyclotron maser radiation from hot stellar coronas through a window of transparency

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.     

The relationship between coronal fan structures and oscillations above faculae regions

The power spectra of radial-velocity and intensity oscillations are analyzed using ground-based (the Si I 10 827 Å and He I 10 830 Å lines) and Solar Dynamics Observatory (the Fe I 6173, 1700 Å, He II 304 Å, and Fe IX 171 Å lines) data, with the aim of searching for frequency modes that most efficiently penetrate into the solar corona from the lower layers of solar faculae. Analysis of the spatial distribution of the oscillation power at various heights indicates that fan structures in the corona (at the height of the 171 Å emission) are better reproduced at frequencies of 1–1.5 mHz. This means that oscillations with periods of 10–15 min dominate in coronal loops above faculae regions. The five-minute oscillations that universally dominate in radial-velocity measurements in low layers of faculae are appreciable in coronal loops only in individual compact fragments.     

The altitude structure of the coronal magnetic field of AR 10933

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.     

The generation of hard X-rays and relativistic protons observed during solar flares

The flare source of thermal X-rays above a magnetic arch in the corona arises from the dissipation of the magnetic energy of the current sheet formed at the reconnection of magnetic-field lines. The sources of hard X-rays emitted from the footpoints of the magnetic arch are beams of electrons accelerated in field-aligned currents induced by the Hall electric field generated in the current sheet. Both the hard X-rays detected above the active region and the type III radio emission are radiated by electrons accelerated in the field-aligned currents induced by Alfven waves. The solar cosmic rays are emitted promptly at the instant of the flare. It is important that the Lorentz electric field accelerates protons along the singular magnetic X line. The relativistic protons propagate along the interplanetary magnetic field. These protons have exponential spectra, typical for acceleration occurring in current sheets. A mechanism that is relevant for the generation of delayed cosmic rays, which demonstrate significant anisotropy and a power-law spectrum with γ ∼5, is also discussed.     

Detection of prolonged, extremely faint decimeter bursts on the sun

We report the detection of long-lived sources of radio bursts accompanied by polarized background emission in solar active regions. Both types of radio sources were detected at several decimeter wavelengths in observations on the RATAN-600 radio telescope in one-dimensional scans in intensity and circular polarization with a sensitivity of about 5–10 Jy. The degree of polarization is from 70 to 100%. The microburst and background sources exist for several days and appear at sites of prolonged energy release. The typical duration of an individual microburst is about 1–2 s, and the time interval between microbursts is about 3–5 s. A negative microburst frequency drift of about 100 MHz/s or more is also observed. Some interpretations of the microburst and background sources are discussed. The most probable microburst model involves the generation of radio emission via the plasma mechanism, with the upward propagation of fast electrons above an active region. In this case, the required energy of the Langmuir waves is 2×10^?8 of the heat energy of the background plasma. Microbursts appear in different places in an active region. New methods for determining the magnetic-field intensity in the regions of generation of the decimeter-wavelength emission are proposed.     

A study of eruptive solar events with negative radio bursts

Solar events of June 15/16, 2000, June 1/2, 2002, February 6, 2002, and February 7, 2002, have been studied. These events probably belong to a poorly studied class of explosive eruptions. In such events disintegration of the magnetic structure of an eruptive filament and dispersing of its fragments as a cloud over a considerable part of the solar surface are possible. The analysis of SOHO/EIT extreme ultraviolet images obtained in the 195 Å and 304 Å channels has revealed the appearance of dimmings of various shapes and propagation of a coronal wave for June 1/2, 2002. In all the events the Nobeyama, Learmonth, and Ussuriysk observatories recorded negative radio bursts at several frequencies in the 1–10 GHz range. Most likely, these bursts were due to absorption of solar radio emission in clouds produced by fragments of filaments. Absorption of the solar background radiation can be observed as a depression of the emission in the 304 Å channel. A model has been developed, which permits one to estimate parameters of absorbing plasma such as temperature, optical thickness, area of the absorbing cloud, and its height above the chromosphere from the radio absorption observed at several frequencies. The obtained values of the temperature, 8000–9000 K, demonstrate that the absorber was the material of an erupted cool filament. The model estimate of the masses of the ejecta in the considered events were ～10¹⁵ g, which is comparable to masses of typical filaments and coronal mass ejections.     

Anisotropy of small-scale inhomogeneities in the region of solar-wind acceleration

A model is proposed to explain observational data on the scattering of radio signals, which indicate that small-scale plasma-density inhomogeneities in the region of solar-wind acceleration are strongly elongated in the radial direction, with the degree of elongation sharply decreasing at heliocentric distances of about six solar radii. The evolution of the energy spectra of the fluctuations of the magnetic field and plasma density is studied assuming that the plasma-density fluctuations are generated locally by nonlinear interactions of high-frequency Alfven waves, and that the gradients of the mean plasma parameters are smooth. The growth rates of the main nonlinear processes are estimated. The strong elongation of the inhomogeneities first arises when the Alfven waves travel through the chromosphere-corona transition layer, then survives to considerable distances from the Sun because the associated nonlinear relaxation processes are fairly slow. Estimates of the degree of elongation of the inhomogeneities and the characteristic distance for changes in the angular wave spectra are in good agreement with radio propagation data.     

Plasma heating during the parametric excitation of acoustic waves in coronal magnetic loops

We examine plasma heating due to the dissipation of acoustic waves excited in coronal magnetic loops by parametric resonance with the five-minute oscillations in the velocity of the photospheric convection. The energy of acoustic waves excited in the coronal magnetic loop, rate of dissipation of acoustic waves, and rate of heating of the coronal plasma are determined. The maximum temperature predicted for the apex of the loop is calculated as a function of the velocity of photospheric oscillations, length of the loop, and electric current in the loop. It is shown that the mechanism proposed can explain the origin of quasi-stationary X-ray loops with temperatures of 3–6 MK. The lengths of these loops are resonant for acoustic waves excited by the five-minute photospheric oscillations. The use of the proposed mechanism to explain heating of the X-ray loops expected to be on stars of late spectral types is discussed.     

Progress in low-frequency radio astronomy and I.S. Shklovskii’s contribution to its development

Radio astronomy at decameter wavelengths is currently undergoing very active development. Large-scale, new generation low-frequency radio telescopes are being constructed and already used in many countries around the world. As before, the largest, most sensitive, and most versatile telescope at decameter wavelengths is the Ukrainian UTR-2 radio telescope operating at 8–32 MHz, which has an effective area of more than 10⁵ m² and an angular resolution of about 0.5?, as well as the URAN interferometric system based on the UTF-2. Many studies that have been carried out on these facilities have been based on important results and far-sighted predictions of Shklovskii. These include, in particular, studies of dynamical spectra and the brightness distributions of the sporadic and quiescent decameter radio emission of the hot solar corona, complex, broadband radio spectroscopy of the interstellar medium, and multi-frequency monitoring of secular decreases in the flux densities of supernova remnants. The coordinated use of highly effective existing and newly constructed radio telescopes joined into ground networks, as well as specialized space missions, are opening new prospects for low-frequency radio astronomy.     

Long-term variability of a complete sample of flat-spectrum radio sources at declinations of 4°–6° (B1950)

We present the results of twenty-year observations of a complete sample of 68 flat-spectrum radio sources with flux densities S _{3.9 GHz} > 200 mJy carried out at centimeter wavelengths with the RATAN-600 radio telescope. Since 1995, we have observed simultaneously at six frequencies between 0.97 and 21.7 GHz. Of the 56 sources identified with optical objects, 41 are quasars with redshifts between 0.293 and 3.263. Based on our analysis of the spectral shapes, we divide the sources into four classes. Changes of spectral class for individual sources are fairly rare. Based on the light curves and spectra, in most cases, a flare’s evolution is in accordance with a model in which the variations result from the evolution of a shock in the radio jet. The main result of our study is that there is no redshift dependence for the true linear sizes of the radiating regions, the variability indices derived for all 20 years of data or for individual flares, or the peak frequencies of the spectra of the compact radio emission. We suggest that this testifies to an absence of cosmological evolution of the sample quasars, at least to z ≈ 3.     

Secondary dynamical spectra of pulsars as indicators of inhomogeneities in the interstellar plasma

Observations of ten bright pulsars were obtained on the Giant Meter-wavelength Radio Telescope (GMRT, India) in order to study the effects of scattering of their radio waves by contructing and analyzing secondary dynamical spectra. The observations were conducted at 610 and 1420 MHz using a digital spectral analyzer operating in a real-time regime. The frequency resolution was 32.5 or 65.1 kHz, and the readout time was from 61.44 to 512 μs. Archival data for five pulsars at 327 MHz were also used. Procedures for normalizing the spectra and for constructing the secondary dynamical spectra were developed. Parabolic arcs were found in the secondary spectra of four pulsars (B1642-03, B1556-44, B2154+40, and B2021+51). The curvature of these arcs can be used to determine the distance to the effective scattering screen. In all cases, these screens are located relatively near the pulsars themselves.     

Detection of the new rotating radio transient pulsar PSR J2225+35

We have detected the new pulsar PSR J2225+35, which displays the properties of the new class of radio sources “Rotating Radio Transients” (RRATs). RRATs are distinguished by isolated bursts of radio emission and long quiet periods. Throughout 45 observations with a total duration of about 3 hr, only two bursts of radio emission lasting a total of about 10 min were detected in two observations. The temporal and frequency delay of the pulses corresponds to the dispersion measure DM = 51.8 pc/cm³ and the distance d = 3.05 kpc. The period of the pulses is P = 0.94 s. The emission is polarized, with the rotation measure being RM = 49.8 rad/m².     

一种绿色“斑马石”玉料的矿物组成研究

最近在江苏东海的水晶市场上出现了一种名为“斑马石一的玉石,该玉石总体颜色为墨绿色,其中有白色条纹呈带状分布并呈丝绢光泽。为了查明其组成,利用偏光显微镜、X射线粉末衍射仪、红外光谱仪、拉曼光谱仪以及激光诱导离解光谱仪等测试方法对该样品进行了测试分析。结果表明,该“斑马石”实际就是一种蛇纹石的集合体材料。其绿色部分的主要矿物组成为利蛇纹石,其次为绿泥石,白色部分由纤蛇纹石构成,含少量绿泥石,黑色包裹体为磁铁矿。这种材料不同于一般的蛇纹石玉,在于该玉石中含有一些定向的纤蛇纹石的平行纤维条带,正是这种纤维状平行排列的结构,导致了“斑马石”中呈丝绢光泽的条带。“斑马石”的折射率,相对密度及荧光特征与一般的蛇纹石玉无差异。     

Radio occultation probing of the turbulent circumsolar plasma by pulsar pulses at small elongations

The possibility of using the occultation of a pulsar by the solar corona for diagnostics of the magnitude of coronal electron-density inhomogeneities is analyzed. Coronal probing at small elongations is considered, when a ground-based receiver can be in the vicinity of the caustic surface due to the regular refraction of the pulsar radio emission in the circumsolar plasma.