Properties of oscillations in sunspot penumbras

Observations of oscillations in the penumbras of seven sunspots are analyzed. High-sensitivity differential measurements of the line-of-sight velocity (11 time series) and variations of the Ni I 4857 Å and H_β line profiles (four series) have provided new data making it possible to improve estimates of the amplitude and spectral characteristics of the oscillations. In the middle penumbras, oscillations of the line-of-sight velocity with fundamental periods of 5 and 8–10 min predominate at the photospheric level; their amplitude does not exceed 40–50 m/s, and the spatial coherence scale in the radial direction is no greater than 5″–10″. At frequencies of 0.5–2.0 mHz, the phase difference between the photosphere and chromosphere (NiI 4857 Å-H_β) is close to 180°. The line-of-sight velocity component due to Evershed motions is responsible for oscillations with periods of 15–35 min, which occur synchronously at both heights.     

Characteristics of oscillatory-wave processes in solar structures with various magnetic field topology

Recognizing that waves play an important role in energy-exchange processes between layers of the solar atmosphere, and that the characteristics of propagating waves are determined by the physical conditions of the medium, and, most importantly, the magnetic-field configuration, we have carried out a comparative analysis of the properties of oscillations in solar structures with various magnetic-field topologies: sunspots, faculae, and coronal holes. Simultaneous measurements of the Doppler velocities and intensities at the photospheric and chromospheric levels were accompanied by episodic measurements of the longitudinal magnetic field. In the chromosphere, spot umbrae dominate the three-minute oscillations, while lower-frequency modes are also observed in the penumbrae and at the outer parts of spots. Clear signs of propagating waves have been observed at the bases of coronal holes and in faculae only at frequencies close to 3 mHz.     

The Motion of Magnetic Elements in and around Sunspot Penumbrae

Structural magnetic elements observed in sunspot penumbrae are employed as indicators of motions occurring in and around penumbrae. The analysis presented here is base on SDO/HMI continuum images and magnetograms of the line-of-sight field obtained for the active region NOAA 11117. In a first approximation, the penumbral magnetic fields can be considered alternating spines and interspine filaments. In the plane of the sky, spines are thin radial elements with higher field strengths and lower magnetic-field inclinations compared with those in surrounding areas. It is confirmed that spines first appear as protrusions of the umbra magnetic fields visible in magnetograms, and then develop simultaneously with the growth of the penumbra. The departure of magnetic elements from penumbrae as a result of the detachment of the ends of spines begin 1–1.5 h after the spine formation. Inmature penumbrae, magnetic elements emerge fairly often, and the departure of groups of field elements sometimes generates structures resembling moving ribbons. The velocities of magnetic elements that have separated from spines are a factor of two to three lower than those of elements that have separated from inter-spine filaments. The results obtained agree well with an “uncombed” model for the penumbral magnetic fields.     

Quasi-periodic line-of-sight velocity variations at the bases of polar coronal holes

Spatial and temporal variations in the line-of-sight velocities and brightnesses measured in the Hα and FeI 6564 Å Hβ and FeI 4864 Å NiI 4857 Å lines at the bases of polar coronal holes are analyzed. Time series with durations of 43–120 min were recorded using a CCD strip (3700 pixels 200×7 µm in size) and a CCD array (256×1024 pixels 24 µm in size). Quasi-stationary upward flows (with radial velocities reaching 3 km/s in the photosphere and 12–15 km/s in the chromosphere) were observed near dark points at the boundaries of the chromospheric network. The acoustic 3-min and 5-min oscillations are amplified in the coronal hole, and reach 1 km/s in the photosphere and 3–4 km/s in the chromosphere. The spectra of fluctuations of the line-of-sight velocity exhibit significant maxima at low frequencies, clustering near 0.4, 0.75, and 1 mHz.     

Periodic processes and plasma motions in solar coronal holes

Episodic observations of coronal holes were carried out simultaneously in several spectral lines during the 2002–2005 observational seasons. An analysis of eighteen time series is used to obtain the amplitude—spectral properties of oscillatory wave motions of the solar plasma at the bases of coronal holes. It is found that the amplitudes of the 5-min and 3-min line-of-sight velocity oscillations increase in coronal holes. Low-frequency (1–2 mHz) oscillations are concentrated at the boundaries of the chromospheric network, while the 3-mHz and 5-mHz oscillations dominate in the network cells. Clear indications of propagating waves have been found at the bases of coronal holes. The 3 mHz phase velocities are 45 ± 5 km/s and 80–100 km/s for the equatorial and polar coronal holes, respectively.     

The properties of long-term sunspot oscillations

It is shown that neglecting the motion of sunspots in the plane of the sky in pixels of SOHO MDI magnetograms obtained for the vertical direction results in false periods of 700–1300 min in the long-term oscillations of the magnetic fields of sunspots observed near the central meridian (the Y artefact). The oscillation mode proposed by Efremov, Parfinenko, and Solov’ev in 2012 to be the lowest-frequency sunspot mode is an artefact. A proposed technique for monitoring this artefact using wavelet transforms can be used to study oscillation periods in the range 15 min < T < 500 min. The observational dependence of the oscillation frequency of the sunspot magnetic field on the field strength is constructed using observations of 45 sunspots. This dependence shows a multimode behavior that is consistent with earlier ground observations. One interpretation of this dependence based on the existence of four geometrical oscillation modes detected earlier is proposed.     

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.     

Internal gravity waves over an oscillating sunspot

Internal gravity waves excited in the overlying atmosphere by the proper vertical oscillations of an entire sunspot are studied. For simplicity, the oscillations of the sunspot are introduced into the model through the lower boundary conditions, by specifying oscillations in the vertical velocity. The characteristic radius of the oscillating region is assumed to equal the spot size and the frequency to correspond to the long-period natural oscillations of the spot. Results of numerical computations are presented. It is shown that internal gravity waves propagate nearly in the horizontal direction. Therefore, immediately above the spot, the wave energy drops exponentially with the height, in good agreement with the available observational data. The level where the amplitude and energy density of the wave are maximum rises slowly with distance from the spot.     

Orbital-period variations in the eclipsing binary Y Cam

We have studied the time behavior of the orbital period and the primary’s pulsation period for the eclipsing binary system Y Cam, whose secondary fills its Roche lobe and whose primary is a δ Scuti star. The times of minima available for this eclipsing binary cover 120 years. δ Scuti pulsations of the primary have been observed over the last 50 years, with the period of these pulsational brightness variations remaining virtually unchanged during the entire observed time interval. The large-amplitude cyclic variations of the orbital period of Y Cam cannot be explained solely by the presence of a third body in the system. It is possible to explain the period variations of Y Cam with magnetic oscillations or a superposition of a stationary matter flow from the lower-mass to the higher-mass component together with magnetic oscillations, similar to the case of AB Cas. A good agreement with observations is provided by a model assuming a stationary matter flow from the secondary filling its Roche lobe to the primary, at the rate of 2.85 × 10^?7 M _⊙/year, superposed with irregular period jumps that can be explained by instabilities in the matter flow. We have detected cyclic variations of the orbital period of Y Cam with an amplitude of 0.011^d, which can be understood if the binary moves in a long-period orbit (with a period of 38.6 years) around a third body with mass M ₃ s> 0.30M _⊙. These cyclic period variations of the eclipsing binary agree with the observed small period variations of the δ Scuti pulsations.     

Pulsating evershed flows and propagating waves in a sunspot

A comparative analysis of oscillatory spectra based on 66 time series for 14 active regions observed in 2001 shows that, although the chromospheric and photospheric oscillations in the Evershed flow zone possess many common features, there is no firm evidence that the direct and inverse flows have the same physical origin. The interactions between the various oscillation modes and stationary flows results in a complex pattern of wave motions in a sunspot. We studied the Doppler-velocity variations in the sunspot NOAA 0051 during its motion over the disk. The spatial-temporal distribution of the line-of-sight velocity in the chromospheric umbra displays a chevron structure, clearly indicating the presence of propagating waves. These waves move from the center of the umbra to outer regions with a phase speed of 45–60 km/s, a period of 2.8 min, and a measured Doppler speed of 2 km/s. The amplitude of these oscillations decreases abruptly at the boundary between the umbra and penumbra, and the observed waves are not directly related to propagating penumbral waves. Furthermore, the observed pattern of the photospheric velocities shows periodic motions (with a period of 5 min) directed from the inner boundary of the penumbra and superpenumbra toward the line of maximum Evershed velocity.     

Wavelet-based variability of Yellow River discharge at 500-, 100-, and 50-year timescales

Water scarcity in the Yellow River, China, has become increasingly severe over the past half century. In this paper, wavelet transform analysis was used to detect the variability of natural, observed, and reconstructed streamflow in the Yellow River at 500-, 100-, and 50-year timescales. The periodicity of the streamflow series and the co-varying relationships between streamflow and atmospheric circulation indices/sunspot number were assessed by means of continuous wavelet transform (CWT) and wavelet transform coherence (WTC) analyses. The CWT results showed intermittent oscillations in streamflow with increasing periodicities of 1–6 years at all timescales. Significant multidecadal and century-scale periodicities were identified in the 500-year streamflow series. The WTC results showed intermittent interannual covariance of streamflow with atmospheric circulation indices and sunspots. At the 50-year timescale, there were significant decadal oscillations between streamflow and the Arctic Oscillation (AO) and the Pacific Decadal Oscillation (PDO), and bidecadal oscillations with the PDO. At the 100-year timescale, there were significant decadal oscillations between streamflow and Niño 3.4, the AO, and sunspots. At the 500-year timescale, streamflow in the middle reaches of the Yellow River showed prominent covariance with the AO with an approximately 32-year periodicity, and with sunspots with an approximately 80-year periodicity. Atmospheric circulation indices modulate streamflow by affecting temperature and precipitation. Sunspots impact streamflow variability by influencing atmospheric circulation, resulting in abundant precipitation. In general, for both the CWT and the WTC results, the periodicities were spatially continuous, with a few gradual changes from upstream to downstream resulting from the varied topography and runoff. At the temporal scale, the periodicities were generally continuous over short timescales and discontinuous over longer timescales.     

Form of the latitude distribution of sunspot activity

The spatial (latitude) distribution of sunspots is studied, including its dependence on solar activity. It is shown that the latitude distributions of sunspots for a given year can be approximately described by the normal law, with its variance being a linear function of the current level of solar activity. Thus, an increase in activity is accompanied by an expansion of the zone of solar activity, in good agreement with earlier results. As the solar activity increases, the width of the zone of sunspot generation and the latitude maximum of the sunspot density grow somewhat more slowly than the number of sunspots, in agreement with observations. The results obtained can be used to reconstruct the spatial distributions of sunspots in the past, interpret the magnetic activity of stars, and address the requirements of the dynamo theory in the form of constraints imposed on models of cyclicity.     

The dynamics of photospheric line-of-sight velocities in emerging active regions

The emergence of photospheric magnetic fields and the dynamics of the associated pattern of vertical motions in a developing active region are studied based on SOHO/MDI data. Objects were selected for which complete time series of data were available, so as to make it possible to determine the onset time of the magnetic-field emergence at the surface and tracing the formation of the first pores. The active regions studied originated near the central meridian. The total area of sunspots in these regions exceeded 100 millionths of the hemisphere at the maximum of active region evolution. A generalized evolutionary scenario is constructed for the magnetic field and vertical motions in the emerging active region. An asymmetry in the Doppler velocities is noted at an early stage of the active-region development, which corresponds to a matter flow from the leading to the trailing end of the emerging Ω-shaped tube. A direct relationship is found between the matter-downflow velocity in the area of the pore development and the growth in the strength of the longitudinal magnetic field.     

Streaming motions of molecular clouds, ionized hydrogen, and OB stars in the Cygnus arm

The radial velocity fields of molecular clouds, OB stars, and ionized hydrogen in the Cygnus arm (l ～ 72°–8°) are analyzed. A gradientΔV _LSR/Δlin the mean line-of-sight velocities of molecular clouds and ionized hydrogen due to differential Galactic rotation is detected, and two groups of physically and genetically associated objects moving with different line-of-sight velocities are identified. One of the two molecular-cloud complexes (l～77.3°–80°) is located within 1 kpc of the Sun, closer to the inner edge of the arm, whereas the other complex (l～78.5°–85°) lies 1–1.5 kpc from the Sun and is farther from the inner edge of the arm. The residual azimuthal velocities of the objects in both groups are analyzed. The residual azimuthal velocities of the first molecular-cloud complex are directed opposite to the Galactic rotation (V _Θ ～ ?7 km/s), while those of the second complex are near zero or in the direction of Galactic rotation, independent of the distance to the complex (V _Θ ≥ 1 km/s). Like the molecular clouds, stars of the Cygnus arm form two kinematic groups with similar azimuthal velocities. On the whole, the mean azimuthal velocities V _Θ for the ionized hydrogen averaged over large areas agree with the velocities of either the first or second molecular-cloud complex. In terms of density-wave theory, the observed differences between the magnitudes and directions of the azimuthal velocities of the kinematic groups considered could be due to their different locations within the arm.     

Mean absolute strength of the solar magnetic field in 1968–2006

Measurements of the mean magnetic field of the Sun as a star (the line-of-sight component of the magnetic field of the visible hemisphere for a given day) carried out at six observatories are used to compile a catalog of the mean magnetic field for 1968–2006 (containing about 18 000 daily values). The cataloged data are compared with direct daily measurements of the absolute line-of-sight field made at the Kitt Peak Observatory in 2003–2006 (original data with a resolution of 1″ averaged over the solar disk). The true absolute mean field strength averaged over the visible solar hemisphere is determined for 1968–2006 to be B ₀ = 7.7 ± 0.2 G. This figure exceeds previous estimates by almost a factor of four. B ₀ exhibits no appreciable slow trend over the entire 39-year interval, but varies substantially with the cycle. The period of this variation is 10.5 ± 0.7 yr, and its harmonic amplitude is 1.7 G. The magnetic flux of spots and active regions makes B ₀ almost twice the field strength in the “normal” photosphere at the solar minimum, i.e., for the “quiet” Sun.     

Cyclostratigraphy of Pontian deposits of the Eastern Paratethys (Zheleznyi Rog section,Taman Region)

The first data on the cyclostratigraphy of Pontian deposits of the Eastern Paratethys were obtained by studying the magnetic susceptibility of Upper Miocene rocks of the Zheleznyi Rog section in the Taman Region. Based on statistical methods, using the Lomb-Scargle and REDFIT periodograms, cycles related to long-period insolation oscillations (precession of the Earth’s orbit and variations in Earth’s axial tilt) were revealed. It is proposed that a hiatus occurred (about 150000–200000 years) at the Novorossian/Portaferian boundary (Lower Pontian/Upper Pontian) due to the onset of the maximum Messinian Salinity Crisis.     

Evolution of Maser Emission in the Region of Active Star Formation G43.8–0.1. I. OH Maser Emission at 18 cm

The results of observations of OH maser emission in the star-forming region G43.8–0.1 are presented. In spite of strong flux-density variations in the main lines at 1665 and 1667 MHz, the radial velocities of the spectral features varied only slightly. The main spectral features are identified with maser spots in previously published maps for epochs 1993 and 2001. It is suggested that the regions of OH maser emission may be elongated, nonuniform structures with weak radial velocity gradients (larger-scale analogs of water-maser filaments). The line-of-sight magnetic fields are determined for two Zeeman pairs, which remained essentially constant over at least 17 years.

     

The subphotospheric structure of a sunspot

Local helioseismology techniques yielding the temperature and flow-velocity distributions under a sunspot indicate an unambiguous sign for the horizontal gas-pressure difference between the spot and ambient medium at depths of 4 Mm and more. In the Parker sunspot model, the transverse equilibrium condition cannot be satisfied in these layers: a cluster of vertical, strongly compressed magnetic flux tubes in a plasma that is hotter than the ambient medium with flows that diverge sidewise cannot be in equilibrium. Equilibrium can be satisfied in the hot zone under the spot only if the magnetic flux tube expands sharply with depth, so that the mean magnetic-field strength decreases dramatically at depths exceeding 4 Mm. This corresponds to the “shallow” sunspot model that has been used to interpret long-period sunspot oscillations.     

A new approach to modeling the surface magnetic fields of chemically peculiar stars

A reconstruction of the line-of-sight component and modulus of the surface-magnetic-field vector is proposed, using a model with artificial point sources of the magnetic field with “virtual” magnetic charges in the body of a star. This approach for the direct calculation of the field from the superposition of the potentials of individual magnetic charges enables the reconstruction of most possible configurations of the surface magnetic field of a star. Proper choice of the axis orientations for the three coordinate systems used makes it possible to obtain a simple representation for the vector components of the surface magnetic field. In a dipole approximation, the expression for the line-of-sight component of the magnetic field averaged over the visible disk of the star reduces to the form obtained in other studies.     

Short-period variations in the proper motions of sunspots from SOHO (MDI) observations

Short-period (1–60 min) variations in the coordinates of the centers of gravity of isolated sunspots are analyzed. The sunspot coordinated were determined using two sets of observational data—magnetograms and intensities—obtained by SOHO (MDI) on December 6, 1998, from 01:00 to 21:57 UT with temporal resolution 60 s and spatial resolution 0.6″/pixel. A slow drift in the sunspot coordinates was removed using a low-frequency filter with a 61-min integration window. The guiding errors (RMS～0.014″) were determined by analyzing correlated motions in pairs of sunspots, and were removed from the time series before determining the sunspot proper motions. Based on the calculated power spectra for the sunspot proper motions, two period intervals containing appreciable power were identified. One coincides with the well-known 5-min acoustic solar oscillations. The concentration of power in this interval is greater for the coordinate variations derived the magnetograms than those derived from the intensities; the harmonic amplitude for some peaks reaches ～±30 km. The other spectral interval corresponds to periods exceeding 30 min. Overall, the rms short-period variations in the sunspot proper motions are 9.9±2.2 and 16.7±7.6 km (0.014″±0.003″ and 0.024″±0.010″) for the magnetogram and intensity data, respectively.