A. A. Solov’ev

Sunspot as an isolated magnetic structure: Stability and oscillations

A simple energy model of a sunspot as a compact magnetic feature is described where the main energy contribution is provided by the coolest and most compressed part of the magnetic force tube of the spot at depths ranging from Wilson’s depression level (300–500 km) down to 2–3 thousand km. The equilibrium and stability conditions for such a system are analyzed using the variation principle, and oscillations of the system as a whole about the inferred equilibrium position are studied. The sunspot is shown to be stable in the magnetic field strength interval from 0.8–1 to 4–5 kG. The dependence of the eigenfrequency on magnetic field strength ω(B) is computed for the main oscillatory mode, where only the umbra of the sunspot takes part in oscillations, ω = ω1 (B). Lower subharmonics may appear in the case where penumbra too becomes involved in the oscillatory process: ω2 = ω1/2, ω3 = ω1/3. Theoretical curves agree well with the observational data obtained in Pulkovo using various independent methods: from temporal variations of sunspot magnetic field and from line-of-sight-velocity measurements. The periods of oscillations found range from 40 to 200 minutes.

Long-period oscillations of the line-of-sight velocities in and near sunspots at various levels in the photosphere

New observational data on long-period oscillations of the line-of-sight velocities detected via the Doppler shifts of spectral lines observed at various heights in and near sunspots are presented. The sunspots and nearby magnetic elements oscillate with periods ranging from 40 to 80 min. The oscillations in the line-of-sight velocities persist over the entire observation session (up to four hours). These results support theoretical models in which this phenomenon represents natural long-period oscillations (vertical-radial displacements) of entire magnetic elements (sunspots, pores, and magnetic knots) about some stable equilibrium positions.

The Structure of Solar Filaments. Prominences in the Corona Free from External Magnetic Field

The new approach to the modeling of quiescent solar prominences is proposed. We solve the inverse magnetohydrostatic problem, when the pressure, density and temperature of plasma in the filament are calculated from the equilibrium equations using the given magnetic structure (magnetic flux function is proposed to be known). The new exact nonlinear solutions for dense (n ≈ (2−3) × 1011 cm−3) and cold (T ≈ (5−10) × 103 K) filaments, embedded in the plan, vertically stratified atmosphere (hot solar corona) free of magnetic field, are derived. The filaments are stretched along the horizontal axisy(the translational symmetry is assumed: ∂/∂y = 0) and located parallel to and above a photospheric, magnetic polarity reversal line. The magnetic field lines have a structure of magnetic flux rope with helical field lines in three-dimensional space; the strength of magnetic field falls rapidly with distance from a rope axis. No external longitudinal magnetic field is needed to equilibrate the prominence. The net electric current along the filament is equal to zero. The model of magnetic arcade with the deflection (sag) on the top, proposed by Pikelner (1971) as a basic form of normal prominence, is calculated also using the method proposed. It is shown that such magnetic arcade, having the magnetic field strength of few gauss only, can effectively maintain the equilibrium of cool dense filament at the heights about 50–60 Mm.

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.

Artifacts of SDO/HMI data and long-period oscillations of sunspots

Aims. The artifacts of SDO/HMI magnetograms that may affect the low-frequency power spectrum of sunspot oscillations are analyzed. Methods. Several examples are given that present false (artificial) harmonics, which are produced by Doppler shifts in the power spectra of long-period oscillations of sunspots. This arises from peculiarities in the orbital movements of SDO. Results. It was found that those artifacts with periods of 12 and 24 h, as revealed even in variations of weak background magnetic fields, are actually present in SDO/HMI magnetograms. However, the quantitative impact of artifacts remains quite weak and does not change the picture of sunspot oscillations dramatically for as long as the magnetic field in the spot is less than about of 2000 Gauss. When the magnetic field strength is greater than 2000 G, the influence of these artifacts increases sharply to become the dominant factor. One can suggest that the amplification of noise components of these artifacts has a highly nonlinear character with the growth of the magnetic field, and the field strength of about 2000 G then takes on meaning of a threshold value.

Mid-temperature solid oxide fuel cells with thin film ZrO2: Y2O3 electrolyte

Data on the mid-temperature solid-oxide fuel cells (SOFC) with thin-film ZrO2-Y2O3 (YSZ) electrolyte are shown. Such a fuel cell comprises a carrying Ni-YSZ anode, a YSZ electrolyte 3–5 μm thick formed by vacuum ion-plasma methods, and a LaSrMnO3 cathode. It is shown that the use of a combined method of YSZ electrolyte deposition, which involves the magnetron deposition of a 0.5–1.5-μm thick sublayer and its pulse electron-beam processing allows a dense nanostructured electrolyte film to be formed and the SOFC working temperature to be lowered down as the result of a decrease in both the solid electrolyte Ohmic resistance and the Faradaic resistance to charge transfer. SOFC are studied by the methods of voltammentry and impedance spectroscopy. The maximum power density of the SOFC under study is 250 and 600 mW/cm−2 at temperatures of 650 and 800°C, respectively.

Sunspot oscillations as derived from the SOHO/MDI magnetograms

As a result of processing long (up to 144 h) series of sunspot magnetograms obtained on the SOHO (Solar and Heliospheric Observatory) spacecraft with the MDI (Michelson Doppler Imager) instrument it is shown that the mode with a period of 800–1300 min is a limiting low-frequency oscillation mode of the magnetic field of a sunspot as a whole. Its period is essentially and nonlinearly depends on the sunspot magnetic field strength. In addition to this mode, higher harmonics are also revealed in the sunspot oscillation spectra in the bands 40–45, 60–80, 135–170, 220–250, and 480–520 min. The oscillation power in these bands monotonically and rapidly decreases with increasing frequency, which is characteristic for overtones arising due to the nonlinear nature of oscillations. The limiting oscillation mode stably exists in sunspots for 1.5–2 days, which coincides with the average lifetime of a supergranular cell. The mode with the period of 35–48 h observed in the power spectrum is not an eigen mode of sunspots, because its period is independent of its magnetic field strength. Probably, it occurs as a quasiperiod of an external exciting force caused by disturbances from supergranular cells surrounding the sunspot.

Investigation of Plasma Characteristics in an Unbalanced Magnetron Sputtering System

Results are presented from experimental studies of a magnetron sputtering system for different configurations of the magnetic field above the cathode surface. The current-voltage characteristics of a magnetron discharge at different working gas pressures (0.08–0.3 Pa) and currents in the unbalancing coil were studied. The production and transport of charge carriers in a magnetron discharge with an unbalanced magnetic field was investigated by means of probe measurements of plasma characteristics and ion energies in the region between the substrate and the magnetic trap at the cathode surface. The radial distributions of the ion current density, plasma potential, and floating potential in the unbalanced operating mode are found to have pronounced extrema at the magnetron axis. It is shown that the plasma density near the substrate can be increased considerably when the axial magnetic field is high enough to efficiently confine plasma electrons and prevent their escape to the chamber wall.

Monitoring of seismoacoustic signals and anthropogenic noise on the shelf of Sakhalin Island

We present a description of the technique and results of many days’ measurements of acoustic noise and the parameters of seismoacoustic signals on the northeast shelf of Sakhalin Island generated in the course of seismoacoustic research in the Lebedinsky licensed sector. The goal of measurements was acoustic control of the water territory around the emitting vessel. We present the results of studying the propagation of seismoacoustic signals on land and in the sea.

Magnetohydrostatics of a Vertical Flux Tube in the Solar Atmosphere: Coronal Loops, a Model of a Ring Flare Filament

The magnetohydrostatic theory of a twisted magnetic flux tube (rope) immersed in a realistic solar atmosphere is presented in a closed analytical form for the first time. General formulas that allow the equilibrium plasma density, pressure, and temperature distributions inside an axisymmetric vertical flux tube to be calculated from its magnetic structure, which is assumed to be known (fixed), have been derived. An analytical model of the external hydrostatic medium free of a magnetic field, the solar atmosphere, where the temperature profile of the semi-empirical tabulated Avrett-Loeser model is used, has been constructed. The distribution of plasma parameters in a twisted magnetic flux tube at small deviations of its internal magnetic structure from the force-free one has been calculated as an example of applying the general theoretical formulas. Since the tube cross section does not change with height, the constructed model can be applied to describe the vertical parts of coronal loops. It has been found that the plasma density in the magnetic flux tube rises when the field twisting exceeds the force-free level and falls with decreasing field twisting compared to the force-free level. This property of a twisted magnetic flux tube is of fundamental importance for justifying the mechanism of flare energy release in magnetic flux ropes. A model of a flare in a ring chromospheric configuration is considered.