The magnetic fields and angular sizes of radio sources displaying synchrotron self-absorption

A method for estimating the magnetic-field strengths and angular sizes of radio sources displaying synchrotron self-absorption based on their observed radio spectra is considered. The method is used to derive the angular sizes of compact radio sources (components) and the magnetic fields in these regions, as well as the energy and number of relativistic electrons and the radiative power of a number of quasars and radio galaxies.     

Long-term variations in the rotation of the solar corona

The characteristic time scales for variations in the differential rotation of the solar corona are determined using measurements of the intensity of the FeXIV 5303 Å coronal line made from 1939–2004. Drift waves of the variations in the rotational speed with an 11-year periodicity can be distinguished. Moving averages with time intervals from two to five years are used to identify torsional waves. In addition, longer-period variations in the rotational speed can be distinguished when longer averaging intervals are used. When the interval used for the moving average is increased to 8–12 years, a quasi-22-year rotational period appears. The low-latitude corona rotates more slowly in odd cycles than in even cycles. Increasing the duration of the averaging interval further shows that rapid rotation at low latitudes was observed in 1940–1950 and 1990–2000, while slow rotation was observed in 1960–1980, possibly suggesting the presence of a 55-year period in the rotational variations. Long-term variations are found in the rotation of polar regions. The rotational variations for high-latitude corona are in antiphase with those for the low-latitude corona. The origins of zones of anomalous coronal rotation and their dynamics in the global activity cycle are discussed.     

The three-dimensional structure of the solar corona

The surface where the radial component of the solar magnetic field changes sign is computed for a minimum corona. It is shown that (1) the projection of the folds of this surface onto the plane of the sky is consistent with the helmet structures observed during the eclipse of June 30, 1954; (2) there are type 1 and type 2 helmets, according to the well-known classification of coronal structures; (3) some elements of this sign-change surface of the radial field can be classified as so-called envelopes. The results obtained suggest that more complex coronal structures can be described in a similar way. An MHD model of polar plumes is considered.     

Geometry of solar prominences and magnetic fields in the corona

The spatial location of the surface at which most of the prominence mass is concentrated is compared with the location of the “neutral surface” where B _r = 0 (B _r is the magnetic field) calculated in a potential approximation using photospheric data. More than fifty prominences (filaments) observed in 1999–2003 are studied. The vertical deviations of the prominences (predominantly toward the west) correspond well to the inclination of the neutral surface. The results provide evidence for the magnetic support of filaments of opposite polarities (the magnetic-rope model).     

Motion of an eruptive prominence in the solar corona

A model for the nonradial motion of an eruptive prominence in the solar corona is proposed. Such motions, which can sometimes be inaccessible to observation, result in an apparent break in the causal link between eruptive prominences and coronal mass ejections. The global magnetic field of the Sun governs coronal plasma motions. The complex structure of this field can form prominence trajectories that differ considerably from a simple vertical rise (i.e., radial motion). A solar filament is modeled as a current-carrying ring or twisted toroidal magnetic rope in equilibrium with the coronal magnetic field. The global field is described using two spherical harmonics. A catastrophic violation of the filament equilibrium followed by its rapid acceleration—eruption—is possible in this nonlinear system. The numerical solution of the equations of motion corresponds well to the eruption pattern observed on December 14, 1997.     

Cyclic variations in the differential rotation of the solar corona

The rotation of the solar corona is analyzed using the original database on the brightness of the FeXIV 530.3 nm coronal green line covering six recent activity cycles. The rate of the differential rotation of the corona depends on the cycle phase. In decay phases, there are only small differences in the rotation, which are similar to that of a rigid body. The differences are more significant (though less pronounced than in the photosphere) during rise phases, just before maxima, and sometimes at maxima. The total rate of the coronal rotation is represented as a superposition of two, i.e., fast and slow modes. The synodic period of the fast mode is approximately 27 days at the equator and varies slightly with time. This mode displays weak differences in rotation and is most pronounced in the middle of decay phases. The slow mode is manifested only at high latitudes during the rise phases of activity, and displays a mean period of 31 days. The relative contribution of each mode to the total rotational rate is determined as a function of time and heliographic latitude. These results indicate that the structure of the velocity field in the convective zone must also vary with time. This conclusion can be verified by helioseismology measurements in the near future.     

Determining source angular sizes from interplanetary-scintillation observations in the saturated regime

Interplanetary-scintillation observations of the radio source B0531+194 (J0534+1927) obtained over a wide range of elongations at 111 MHz using the Big Scanning Antenna of the Lebedev Physical Institute are presented. Near the Sun, the temporal spectra of the scintillations have a two-component form, corresponding to the superposition of refractive and diffractive scintillations that is characteristic of the saturated regime. A method for estimating the angular size of the scintillating component based on measurement of the break frequency in the diffractive part of the scintillation spectrum is presented. The scintillating component as a fraction of the total flux can be determined using the maximum scintillation index. The angular size of the scintillating component in B0531+194 is found to be 0.24″ ± 0.05″, and the ratio of the fluxes in the core and halo to be roughly one-third. The flux density in the compact radio component is 5 Jy. The estimated parameters of the angular structure of the source are compared with observations at other frequencies.     

Two modes of the differential rotation of the solar corona

The differential rotation of the solar corona is studied using the brightness of the Fe XIV 530.3 nm green coronal line collected over 5.5 solar-activity cycles. The total observed velocity of the coronal rotation is analyzed as a superposition of two modes—fast and slow. A technique for separating two data series composing the initial data set and corresponding to the two differential-rotation modes of the solar corona is proposed. The first series is obtained by averaging the initial data set over six successive Carrington rotations; this series corresponds to long-lived, large-scale coronal regions. The second series is the difference between the initial data and the averaged series, and corresponds to relatively quickly varying coronal component. The coronal rotation derived from the first series coincides with the fast mode detected earlier using the initial data set; i.e., the synodic period of this mode is 27 days at the equator, then weakly increases with latitude, slightly exceeding 28 days at high latitudes. The second series describes a slow rotation displaying a synodic period of about 34 days. This coincides with the period of rotation of the high-latitude corona derived by M. Waldmeier for polar faculae. We expect that coronal objects corresponding to the fast mode are associated with magnetic fields on the scales typical for large activity complexes. The slow mode may be associated with weak fields on small scales.     

The magnetic-field geometry in polar ray structures of the solar corona

Variations in the positions of the intersection points of tangents to ray structures in the polar corona of the Sun during the solar cycle are considered. At first glance, the decrease in the distance q between the tangent intersection point and the center of the solar disk during activity maximum contradicts harmonic analyses that indicate that the relative weight of higher harmonics in the global field increases during this period. Indeed, the higher the harmonic number in an axisymmetric field, the closer the intersection point of the field-line tangents (the magnetic focus) to the solar surface. It is shown that q for a field composed of two harmonics with opposite polarities at the poles can be smaller than q for either of them taken alone. A simple model representing the global field using the third and seventh harmonics is analyzed; this model can reproduce quite satisfactorily the observed dynamics of magnetic foci of the polar field.     

The total number of spicules on the solar surface and their role in heating and mass balanace in the solar corona

A critical review of determinations of the number of spicules is presented, and the role of both classical and Type 2 spicules in heating and mass balance in the corona is considered. The total number of Type 2 spicules is determined, together with the upward fluxes of energy and mass to which they give rise. The total number of Type 2 spicules on the solar surface is found to be ~10⁵, close to values obtained in other studies. The associated particle flux toward the corona is 2.5 × 10¹⁴ cm^?2 s^?1, an order of magnitude lower than the corresponding flux for classical spicules. The associated energy flux is 10⁴ erg cm^?2 s^?1, an order of magnaitude lower than estimates obtained in other studies. The results indicate that Type 2 spicules can supply the mass lost from the corona, but are not able to fully explain coronal energy losses.     

Long-period oscillations of the solar microwave emission

Modulations of the microwave emission of the Sun at 11.7 GHz have been studied using more than 40 events observed in 2001 at the Mets?hovi Radio Observatory. In nearly all the observed events, low-frequency modulations with periods of 3–90 min were detected. As a rule, simultaneous modulation of the emission at several frequencies was observed. One possible origin of such modulations with periods 5–10 min is parametric resonance arising in coronal magnetic loops as a result of interactions with the 5-min photospheric oscillations, while the long-period modulations could be a manifestation of sunspot oscillations. Torsional (ϑ-mode) and radial (r-mode) oscillations have such periods. The frequency of occurrence of oscillations with the determined periods is considered, and a lower limit for the brightness temperature of the oscillations is estimated.     

Polarization of the solar corona in the green line: Contradiction between theory and observations

The main results of polarization observations in the 530.3-nm line and their role in studying the physical conditions, structure, and magnetic field in the solar corona are discussed. A serious discrepancy between the observations and widely-accepted theoretical concepts was revealed: the theory predicts that the orientation of the polarization electric vector should be nearly radial, in contradiction with the observational results. In particular, the polarization vectors for both the green line and white-light corona in high-latitude streamers were tangential during the eclipse of July 11, 1991. The dependence of the degree of polarization on the angle between the radial direction and the magnetic-field vector was calculated without any a priori assumptions about the configuration of coronal fields. This theoretical analysis of the polarization-vector orientation for magnetic-dipole emission in the green line are in agreement with results obtained previously in other studies. Some ways to resolve the observed discrepancies are discussed.     

The polarization of the solar corona on March 29, 2006

The effect of the auroral ring on the polarization of the solar corona during the solar eclipse of March 29, 2006 is studied. The angle and degree of polarization for emission arising from the combination of two partially polarized components is calculated. The emission of each component is described in terms of the Stokes parameters, and the corresponding parameters added. The position angle and degree of polarization are found for all cases realized in observations of the polarized corona with polarizing filter positions of 0°, 60°, 120° and 0°, 45°, and 90°. These calculations indicate that singular polarization points (saddles with index ?1/2) arise at a distance of about 1R_⊙ from the limb. A model for the total emission of the corona is constructed, which is used to calculate deviations of the polarization plane from the tangential direction (with respect to the limb).     

Evidence for shock generation in the solar corona in the absence of coronal mass ejections

The solar event SOL2012–10–23T03:13, which was associated with a X1.8 flare without an accompanying coronal mass ejection (CME) and with a Type II radio burst, is analyzed. A method for constructing the spatial and temporal profiles of the difference brightness detected in the AIA/SDOUVand EUV channels is used together with the analysis of the Type II radio burst. The formation and propagation of a region of compression preceded by a collisional shock detected at distances R < 1.3R _⊙ from the center of the Sun is observed in this event (R _⊙ is the solar radius). Comparison with a similar event studied earlier, SOL2011–02–28T07:34 [1], suggests that the region of compression and shock could be due to a transient (impulsive) action exerted on the surrounding plasma by an eruptive, high-temperature magnetic rope. The initial instability and eruption of this rope could be initiated by emerging magnetic flux, and its heating from magnetic reconnection. The cessation of the eruption of the rope could result from its interaction with surrounding magnetic structures (coronal loops).     

Observational constraints on the angular and spectral distributions of photons in gamma-ray burst sources

The typical spectra of gamma-ray bursts (GRBs) are discussed in the context of the compactness problem for GRB sources and how it is resolved in the popular fireball model. In particular, observational (model-independent) constraints on the collimation of the gamma-rays and the dependence of the collimation angle on the photon energy are considered. The fact that the threshold for the creation of e ^? e ⁺ pairs depends on the angle between the momenta of the annihilating photons in the GRB source provides an alternative solution to the compactness problem. A new approach to explaining GRBs, taking into account the angular dependence for pair creation, is proposed, and the main features of a scenario describing a GRB source with a total (photon) energy smaller or of the order of 10⁴⁹ erg are laid out. Thus, we are dealing with an alternative to an ultra-relativistic fireball, if it turns out (as follows from observations) that all “long” GRBs are associated with normal (not peculiar) core-collapse supernovae. The effects of radiation pressure and the formation of jets as a consequence of even a small amount of anisotropy in the total radiation field in a (compact) GRB source are examined in this alternative model. Possible energy-release mechanisms acting in regions smaller or of the order of 10⁸ cm in size (a compact model for a GRB) are discussed. New observational evidence for such compact energy release in the burst source is considered.     

The microwave radiation of the corona above a large single sunspot in right- and left-circular polarization

Results of a study of the corona above a large sunspot in the active region NOAA 10105 with a penumbra size of ～70″ observed in September 2002 are reported. Maps of the active region and emission spectra were constructed using observational data from the NoRH, SSRT, and RATAN-600 telescopes. The sizes and brightness temperatures of the microwave emission above the sunspot are determined. SOHO/MDI and Kitt Peak magnetograms, as well as CaII K line images obtained at the Meudon Observatory, are compared. The derived characteristics are interpreted as cyclotron emission of thermal plasma, assuming a dipole structure for themagnetic field. A stable darkening at the sunspot center observed at short wavelengths and only in the ordinary emission mode was detected. A jump-like change was observed in the structure of the sunspot source in the ordinary emission mode, due to an increase in the size and spectral flux density. These results demand a fundamental correction of model concepts about cyclotron emission sources above sunspots, since they are at variance with the initial assumptions. It is suggested that, at the top of the transition region, the cyclotron emission source may be represented only by the third gyrolevel, but is observed in the extraordinary and ordinary emission modes (in contrast to the generally accepted model, which has a combination of the second and third gyrolevels). Taking into account the new observational data may allow us to refine model distributions of the main parameters of the coronal plasma above sunspots (the electron temperature and density) and information about the character of the magnetic field.     

Some features of the streamer belt in the solar corona and at the Earth’s orbit

The rays of enhanced brightness making up the structure of the coronal-streamer belt can be traced to the lowest atmospheric layers in the Sun, with the angular size remaining nearly constant, d ≈ 2.5° ± 0.5°. This suggests that the physical mechanism generating the slow solar wind in the rays of the streamer belt differs from the mechanism giving rise to the fast solar wind from coronal holes. At distances of R < (4–5) R _⊙, the rays of the streamer belt are not radial in the plane of the sky and show deviations toward the corresponding pole. They then become essentially radial at R > (4–5) R _⊙. A transverse cross section of streamers in the corona and its continuation into the heliosphere—a plasma sheet—can be represented as two radially oriented, closely spaced rays (d ≈ 2.0°–2.5°) with enhanced density and an angular size of d. We also show that the ray structure of the streamer belt is involved in the development of coronal mass ejections (CMEs). The motion of a small-scale CME occurs within a magnetic flux tube (ray of enhanced brightness) and leads to an explosive increase in its angular size (rapid expansion of the tube). It seems likely that large-scale CMEs are the result of the simultaneous expansion of several magnetic tubes. We suggest that a small-scale CME corresponds to a “plasmoid” (clump of plasma of limited size with its own magnetic field) ejected into the base of a magnetic tube, which subsequently moves away from the Sun along the tube.     

A charge-consistent model for the acceleration of iron in the solar corona: Nonisothermal injection

A charge-consistent numerical model for the joint (regular and stochastic) acceleration of iron by a spherical shock wave propagating in the solar corona is proposed. Large-scale irregularities of the plasma density and the nonisothermal injection of ions are taken into account. For the case of iron, the energy dependence of the mean charge q_Fe(E) is determined by the relationships between the characteristic acceleration time, the charge-variation time for the accelerated ions, and the time for their trapping in regions of high plasma density. Due to the global inhomogeneity of the medium, these relationships depend on the shock speed. Our calculations indicate that photoionization by soft X-rays from flare regions can substantially change the charge states of heavy ions only in the most powerful solar events (both impulsive and gradual).     

Search for and study of weak radio galaxies with large angular sizes using the NVSS data

A new method for distinguishing candidate giant radio galaxies is proposed and applied. The method is based on comparing the axes of the extended components of NVSS radio sources with separations exceeding 4′, described in a catalog of presumably independent objects. Objects detected using the proposed algorithm include 16 new weak giant-radio-galaxy candidates, for which optical and radio identifications have been obtained using the CATS, NED, SDSS, and SkyView databases.     

Stellar sources of the short-lived radionuclides in the early solar system

We discuss the possible stellar sources of short-lived radionuclides (SLRs) known to have been present in the early solar system (²⁶Al, ³⁶Cl, ⁴¹Ca, ⁵³Mn, ⁶⁰Fe, ¹⁰⁷Pd, ¹²⁹I, ¹⁸²Hf, ²⁴⁴Pu). SLRs produced primarily by irradiation (⁷Be, ¹⁰Be) are not discussed in this paper. We evaluate the role of the galactic background in explaining the inventory of SLRs in the early solar system. We review the nucleosynthetic processes that produce the different SLRs and place the processes in the context of stellar evolution of stars from 1 to 120 M_⊙. The ejection of newly synthesized SLRs from these stars is also discussed. We then examine the extent to which each stellar source can, by itself, explain the relative abundances of the different SLRs in the early solar system, and the probability that each source would have been in the right place at the right time to provide the SLRs. We conclude that intermediate-mass AGB stars and massive stars in the range from ∼20 to ∼60 M_⊙ are the most plausible sources. Low-mass AGB stars fail to produce enough ⁶⁰Fe. Core-collapse Type II supernovae from stars with initial masses of <20 M_⊙ produce too much ⁶⁰Fe and ⁵³Mn. Sources such as novae, Type Ia supernovae, and core-collapse supernovae of O-Ne-Mg white dwarfs do not appear to provide the SLRs in the correct proportions. However, intermediate-mass AGB stars cannot provide ⁵³Mn or the r-process elements, so if an AGB star provided the ⁴¹Ca, ³⁶Cl, ²⁶Al, ⁶⁰Fe, and ¹⁰⁷Pd, and if a late stellar source is required for ⁵³Mn and the r-process elements, then two types of sources would be required. A separate discussion of the production of r-process elements highlights the difficulties in modeling their production. There appear to be two sources of r-process elements, one that produces the heavy r-process elements, including the actinides, and one that produces the elements from N to Ge and the elements ∼110 < A < ∼130. These can be assigned to SNII explosions of stars of ?11 M_⊙ and stars of 12-25 M_⊙, respectively. More-massive stars, which leave black holes as supernova remnants, apparently do not produce r-process elements.