arXiv subjects

A. S. Gadun

Publications and source records attributed to A. S. Gadun.

5 recordsLinked to original sources

arXiv2012

Evolution of solar magnetic tubes and its manifestation in Stokes parameters

Basic scenarios and mechanisms for the formation and decay of small-scale magnetic elements and their manifestation in synthesized Stokes profiles of the Fe I 15648.5 A infrared line are considered in the context of two-dimensional modeling of nonstationary magnetogranulation on the Sun. The stage of convective collapse is characterized by large redshifts in the V profiles accompanied by complete Zeeman splitting of the I profiles. This is due to intense downward flows of material, which facilitates the concentration of longitudinal field with an amplitude of about several kG in the tube. The dissipation of strong magnetic structures is characterized by blueshifts in the Stokes profiles, which result from upward fluxes that decrease the magnetic field in the tube. Typical signatures during key stages in the evolution of compact magnetic elements should be detectable via observations with sufficiently high spatial and temporal resolution.

astro-ph.SR↗

arXiv2010

Absolute shifts of Fe I and Fe II lines in solar active regions (disk center)

We estimated absolute shifts of Fe I and Fe II lines from Fourier-transform spectra observed in solar active regions. Weak Fe I lines and all Fe II lines tend to be red-shifted as compared to their positions in quiet areas, while strong Fe I lines, whose cores are formed above the level $\log τ_5\approx-3$ (about 425 km), are relatively blue-shifted, the shift growing with decreasing lower excitation potential. We interpret the results through two-dimensional MHD models, which adequately reproduce red shifts of the lines formed deep in the photosphere. Blue shifts of the lines formed in higher layer do not gain substance from the models.

astro-ph.SR↗

arXiv2010

Fourier analysis of Fe I lines in the spectra of the Sun, α Centauri A, Procyon, Arcturus, and Canopus

We used spectral observations of Fe I line profiles with a 200 000 resolution to determine micro and macroturbulent velocities in the atmospheres of the Sun as a star, α Cen A, Procyon (α CMi), Arcturus (α Boo), and Canopus (α Car). Isotropic microturturbulent velocities (V_mi) and radial-tangential macroturbulent velocities (V_ma,RT) were found to be a quite suitable approximation to the velocity field in the atmospheres of all stars studied except Canopus. The average velocities V_mi and V_ma,RT are 0.8 +/- 0.1 and 2.6 +/- 0.3 km/s for the Sun as a star, 0.8 +/- 0.2 and 2.9 +/- 0.4 km/s for α Cen A, 0.8 +/- 0.3 and 5.9 +/- 0.2 km/s for Procyon, 1.0 +/- 0.2 and 4.6 +/- 0.3 km/s for Arcturus. The velocity field in the atmosphere of Canopus can be described by an anisotropic radial-tangential distribution of microturbulence with V_mi,RT = 2.1 km/s and anisotropic distribution of macroturbulence with V_ma,rad = 17 +/- 2 km/s and V_ma,tan = 1.3 +/- 1.0 km/s. From Fourier analysis of broadening and shapes of three spectral lines of Fe I, we have derived the rotation velocity V_e sini = 3.5 +/- 0.2 km/s for Canopus.

astro-ph.SR↗

arXiv2010

Convective shifts of iron lines in the spectrum of the solar photosphere

The influence of the convective structure of the solar photosphere on the shifts of spectral lines of iron was studied. Line profiles in the visible and infrared spectrum were synthesized with the use of 2-D time-dependent hydrodynamic solar model atmospheres. The dependence of line shifts on excitation potential, wavelength, and line strength was analyzed, along with the depression contribution functions. The line shifts were found to depend on the location of the line formation region in convective cells and the difference between the line depression contributions from granules and intergranular lanes. In visible spectrum the weak and moderate lines are formed deep in the photosphere. Their effective line formation region is located in the central parts of granules, which make the major contribution to the absorption of spatially unresolved lines. The cores of strong lines are formed in upper photospheric layers where is formed reversed granulation due to convection reversal and physical conditions change drastically there. As a consequence the depression contributions in the strong line from intergranular lanes with downflows substantially increase. This accounts for smaller blue shifts of strong lines. In infrared spectrum the observed decrease in the blue line shifts is explained by the fact that their effective line formation regions lie higher in the photosphere and extend much further into the reversed granulation region due to the line opacity rise with the increase of line wavelength. Additionally the effective line formation depths of the synthesized visible and infrared Fe I lines and their dependence on line parameters is discussed.

astro-ph.SR↗

arXiv2009

Origin of Extremely Asymmetric Stokes V Profiles in an Inhomogeneous Atmosphere

The formation of unusually shaped Stokes V profiles of the Fe I 630.2 nm line in the solar photosphere are investigated. The results of numerical 2-D MHD simulation of solar magnetogranulation are used for this. In their properties, the synthetic unusual profiles with extremely asymmetry are similar to the unusual profiles observed with a spatial resolution better than 1" in the network and internetwork regions. According to our results the unusual profiles mostly appear in clusters along the polarity inversion lines in the regions of weak magnetic fields with mixed polarity. As a rule, they are located at the edges of granules and lanes, and sometimes they are met close to strong magnetic field concentrations with high velocity and magnetic field strength gradients. They turned out to appear as clusters in the regions where large granules disintegrate and new magnetic flux tubes begin to form. The unusual $V$ profiles may have from one to six lobes. The one-lobe and multilobe profiles are of the same origin. The processing causing the extreme asymmetry of the profiles are characterized by one or several polarity reversal along the line of sight as well as by complicated velocity and field strength gradients. The greater the number of profile lobes, the greater is the probability of the field gradient sign change. Hence it follows that the magnetic field should be very complicated in the regions of formation of extremely asymmetric $V$ profiles. This is confirmed by immediate results of MHD granulation simulations, which demonstrate the formation of vortices and turbulence by the velocity shear at down draft edges. These processes add complexity to the magnetic field structure by mixing field polarities, particularly at the edges of granules.

astro-ph.SR↗