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 comparative analysis of zebra-pattern structures at frequencies from 20 to 7000 MHz

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.     

Pulsations of microwave flaring emission at low and high frequencies

A wavelet analysis of the flare-intensity variations has been carried out for a solar flare observed with the Nobeyama Radioheliograph at 17 and 34 GHz and the spectrometer of Purple Mountain Observatory at 4.5–7.5 GHz. The signals contain a well-pronounced periodicity with a period of P = 14–17 s and stable, coherent behavior at frequencies higher and lower than the peak frequency. We simulated the modulation of the gyrosynchrotron emission by fast sausage magnetoacoustic oscillations for the cases of low and high plasma density in the radio source. The synchronism of the pulsations at high and low frequencies can be realized only in the case of high plasma density, when the low-frequency turnover of the microwave spectrum is due to the Razin effect, not self-absorption. Published in Russian in Astronomicheskiĭ Zhurnal, 2007, Vol. 84, No. 7, pp. 655–664. The article was translated by the authors.     

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.     

Turbulent interstellar plasma and ultrahigh angular resolution in radio astronomy

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.     

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.     

New measurements of the circular polarization of the radio emission of Active Galactic Nuclei on parsec scales

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.     

Long-period pulsations of the thermal microwave emission of the solar flare of June 2, 2007 from data with high spatial resolution

Data from the Nobeyama Radioheliograph at 17 GHz with high spatial and temporal resolution are used to detect quasi-periodic pulsations with periods from 55 to 250 s in the thermal component of the microwave emission of a solar flare loop observed on June 2, 2007. Observed pulsations with periods of about 110–120 s are co-phased along the entire loop axis. The observed periodicity is most likely due to modulation of the radio emission by slow magnetoacoustic waves trapped in the filamentary flare loop.     

Torsional wave propagation in non‐homogeneous layer between non‐homogeneous half‐spaces

The study of surface wave in a layered medium has their possible application in geophysical prospecting. In the present work, dispersion equation for torsional wave in an inhomogeneous isotropic layer between inhomogeneous isotropic half‐spaces has been derived. Two cases are discussed separately for torsional wave propagation in inhomogeneous layer between homogeneous and non‐homogeneous half‐spaces, respectively. Further, two possible modes for torsional wave propagation are obtained in case of inhomogeneous layer sandwiched between non‐homogeneous half‐spaces. Closed form solutions for displacement in the layer and half‐spaces are obtained in each case. The study reveals that the layer width, layer inhomogeneity, frequency of inhomogeneity, as well as inhomogeneity in the half‐space has significant effect on the propagation of torsional surface waves. Displacement and implicit dispersion equation for torsional wave velocities are expressed in terms of Heun functions and their derivatives. Effects of inhomogeneity on torsional wave velocity are also discussed graphically by plotting the dimensionless phase velocity against dimensionless and scaled wave number for different values of inhomogeneity parameter. Copyright © 2012 John Wiley & Sons, Ltd.     

Study of the physical conditions in active galactic nuclei. Physical conditions in the cores of two nearby radio galaxies

We have carried out a search for compact radio sources in the cores of 16 nearby radio galaxies. We detected compact components in four radio galaxies, and found upper limits for the flux density in compact components in ten radio galaxies. VLBI observations enabled the detection of a turnover in the spectra of the two nearby radio galaxies 3C 111 and 3C 465. Using a method based on an inhomogeneous model for a synchrotron source, we estimate the magnetic-field strength and the energy densities in the magnetic field and relativistic electrons in the cores of these radio galaxies. Strong inhomogeneity in the distribution of the magnetic fields in the cores of 3C 111 and 3C 465 is implied by our analysis. The magnetic-field strengths in the central regions of these galactic nuclei, on scales of ～0.1 pc, exceed the mean strength by four to five orders of magnitude, and lie in the range 10² G < H < 10⁴ G.     

3C 66A: Variability in 2007–2015

The results of photometric (BV RIJHK) and polarimetric (R)monitoring of the blazar 3C 66A performed at the St. Petersburg State University and the Central AstronomicalObservatory of the Russian Academy of Sciences in 2007–2015, radio observations performed by the Boston University team with the Very Long Baseline Array at 43 GHz, and a gamma-ray light curve based on observations with the Fermi SpaceObservatory are presented. Color variations of the object are studied. Changes in the optical spectral energy distribution are observed at some times, indicating the appearance and disappearance of individual variable sources. A variable source with a degree of polarization of 36% is identified, which is responsible for the polarization variations observed during one episode. The correlations between the variations in the different spectral ranges indicate that the optical and gamma-ray radiation originates near the radio core detected at 43 GHz. The presence of five superluminal components emerging from the core is detected.     

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.     

Giant pulses—The main component of the radio emission of the crab pulsar

The paper presents an analysis of dual-polarization observations of the Crab pulsar obtained on the 64-m Kalyazin radio telescope at 600 MHz with a time resolution of 250 ns. A lower limit for the intensities of giant pulses is estimated by assuming that the pulsar radio emission in the main pulse and interpulse consists entirely of giant radio pulses; this yields estimates of 100 and 35 Jy for the peak flux densities of giant pulses arising in the main pulse and interpulse, respectively. This assumes that the normal radio emission of the pulse occurs in the precursor pulse. In this case, the longitudes of the giant radio pulses relative to the profile of the normal radio emission turn out to be the same for the Crab pulsar and the millisecond pulsar B1937+21, namely, the giant pulses arise at the trailing edge of the profile of the normal radio emission. Analysis of the distribution of the degree of circular polarization for the giant pulses suggests that they can consist of a random mixture of nanopulses with 100% circular polarization of either sign, with, on average, hundreds of such nanopulses within a single giant pulse.     

Broadening of electron cyclotron maser emission lines in a nonuniform magnetic field

The formation of cyclotron maser emission lines in a non-uniform (regular or random) magnetic field is studied. In the presence of sufficiently small inhomogeneity, the line shape can be described by a broadened Gaussian profile. In the case of stronger inhomogeneity, the initial Gaussian profile splits into two Gaussian components, which could be observationally perceived as “harmonics.” A relation between the distribution of local magnetic trap sizes and the distribution of the spectral widths of solar radio spikes is derived. Possible applications of the results to the interpretation of solar radio spikes and related problems are discussed.     

Spectral characteristics of components of a model radio-brightness distribution for the radio galaxy 3C 234

Physical processes that can, under cosmic conditions, give rise to emission whose spectrum peaks at some frequency are discussed in the context of the spectrum of the central extended component of a model brightness distribution for the radio galaxy 3C 234. This component is not detected at decameter wavelengths, probably due to the absorption of the radiation in the plasma in the source itself.     

The “X component” of the radio background

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.     

Microstructure of pulsar radio pulses measured with a time resolution of 62.5 ns at 1650 MHz

We present an analysis of pulsar observations carried out on two frequency channels at 1634 MHz and 1650 MHz with a time resolution of 62.5 ns on the 70-m radio telescope of the NASA Deep Space Network in Tidbinbilla. The data were recorded using the S2 system, intended primarily for VLBI observations. Microstructure with characteristic timescales of 270, 80, and 150 µs was detected in pulsars B0833-45, B1749-28, and B1933 + 16, respectively. The distribution of microstructure timescales for the Vela pulsar (B0833-45) is characterized by a gradual growth with decreasing timescale to 200 µs; the distribution has a maximum at 20–200 µs and falls off sharply for timescales below 20 µs. The statistical relation between the microstructure modulation index m and the corresponding timescale τ_µ can be approximated by the power law dependence R∝τ _⊙ ^0.5 ; i.e., the intensity is higher for micropulses with longer durations. This contradicts the predictions of nonlinear models for the formation of micropulses by supercompact soliton wave packets. In all the pulsars studied, the time delays of the micropulses between the two frequency channels deviate from the expected dispersion laws for the interstellar plasma. In particular, the micropulses in the low-frequency channel arrive earlier than predicted by the dispersion measures derived previously from the mean pulse profiles. The deviation from the dispersion delay is determined most accurately for B0833-45, and is 4.9±0.2 µs. Such anomalous delays are probably associated with the effects of propagation of the radio emission within the pulsar magnetosphere.     

Parameters of giant pulses from the Crab pulsar measured with the Mark5A VLBI system

Radio observations of the Crab pulsar were performed on the 100-m radio telescope of the Green Bank Observatory at a frequency of 2100 MHz in a 64-MHz band in two channels with right-and left-circular polarization. The Mark5A recording system was used. During 15 min of observing time, 609 giant pulses were recorded; the brightest had a peak flux density of 670 kJy. The energy distribution has been constructed, polarization properties have been analyzed, and the characteristic temporal and frequency scales in the radio emission of the detected giant pulses have been found. Comparison of these parameters indicates that the properties of giant pulses detected at the main-pulse and interpulse longitudes do not differ, as is clearly observed at frequencies above 4 GHz. Probable origins of the frequency evolution of the properties of giant pulses are discussed.     

Excitation of electromagnetic lower hybrid instability in the region of bouncing electron beams

The region of bouncing electron beams in the earth’s magnetosphere can be unstable against a non-resonant electromagnetic lower hybrid instability. The instability is purely growing in the rest frame of the plasma, and can be excited either by the temperature anisotropy or the drift velocity of the bouncing electron beams. The growth rates of the instability decrease with the increase of cold electron density. Consequently the growth rate is maximum at the equator where the cold electron density is minimum. The intense turbulence generated by this instability could broaden the bouncing electron beams thereby explaining the observed wider cone width of the beams at the equator. The instability could generate magnetic pulsations in the frequency range of orderPc ₁?Pc₃ with typical wavelength ≈ (3–10) km in the, magnetosphere during magnetic storms or substorms.     

Harmonic oscillations of the X-ray emission of a solar flare

An analysis is presented for the class-M9.3 solar flare of November 6, 2004, whose decay phase displayed weakly damped harmonic oscillations of the predominantly thermal X-ray flux detected by the RHESSI spacecraft (at energies ≲25 keV). The period of these oscillations was ≈78 s, and their characteristic decay time ≈100 min. Similar quasi-periodic pulsations were observed in the decimeter-centimeter radio flux (pulsations of a type-IV radio outburst), but were less pronounced in the non-thermal hard X-ray flux (≳25 keV). The area of the quasi-stationary X-ray source, which was located primarily at the apex of a set of flare loops (≲15 keV) that were cooled primarily via thermal conduction, was found to be in anti-phase with the oscillating X-ray flux it emitted. The observed oscillations are interpreted as harmonic modulations of the radiation flux emitted by the heated thermal flare-loop plasma, due to the global, standing, sausage mode of fast magnetoacoustic waves excited in the loop.