Volume 35, Nº 2 (2019)

Examination results for specific features of variation of line-of-sight plasma velocities in different photosphere layers of the NOAA 11024 active region (AR) under the influence of two evolving Ellerman bombs (EB-1 and EB-2) are presented. Spectroscopic data with high spatial and temporal resolution were obtained with the French–Italian THEMIS solar telescope on July 4, 2009. The observation time was 20 min. The AR was undergoing a sharp increase in activity on the day of observations, and the EBs evolved in the region of one of the three magnetic fluxes that were emerging at the time. The brightness of EB-1 decreased during observations, while EB-2 got brighter. The λ ≈ 630 nm spectral region, which includes Fraunhofer lines forming within a wide range of photospheric heights (neutral iron lines Fe I λ 630.15, 630.25, and 630.35 nm and titanium line Ti I λ 630.38 nm), was used. Changes in the velocity and direction of motion of matter in the region of Ellerman bombs and in their immediate vicinity at different photosphere levels and at different stages of EB evolution were determined and analyzed. It was found that upflows were predominant at all levels of the AR photosphere. At the same time, line-of-sight velocities V_los and the amplitude of their oscillations decreased considerably in the region of EBs. Apparently, downward matter flows reduced the velocity of upward plasma motion. This suggests that small-scale downward flows induced by magnetic reconnections were superimposed onto the large-scale upward motion of plasma of the new magnetic flux. The profile shape of photospheric lines reinforces this conclusion. The profiles of strong lines had a red asymmetry. The matter velocity determined based on the shift of a prominent component in the red wing of the Fe I λ 630.35 nm line profiles was as high as 2 km/s. Such a V_los distribution in the EB regions indicates that they consisted of several jets moving with different velocities in different directions. The line-of-sight velocity in the central part of EB-1 and EB-2 varied from –1 to 0 km/s and from –1 to 0.2 km/s in the upper photospheric layer and from –1.6 to –0.2 km/s and from –1.1 to 0.25 km/s in the lower layer of the photosphere, respectively. V_los variations in the vicinity of these Ellerman bombs were oscillatory, and the interval between oscillations was ~5 min. The pattern of quasiperiodic V_los oscillations was disturbed in the EB regions: they often occurred in antiphase. It may be concluded that the excitation caused by a pulsed energy release as a result of successive magnetic reconnections associated with the emergence of the new magnetic flux propagated from the EB-1 site along a magnetic loop and triggered the formation of EB-2, and the Ellerman bombs then evolved as a physically connected pair. The studied features of the line-of-sight velocity variation of chromospheric and photospheric matter indicate that during the evolved of EBs differently directed motions of matter there was: it moved upward in the lower chromospheric layer, while downward flows reduced the velocity of ascending plasma at the photospheric level. Magnetic reconnections occurring in the layer between the upper photosphere and the lower chromosphere, where the core of the H_α line was formed, could induce such a velocity distribution.

Two-dimensional spectra of the Sun (from the center to the limb of the disk) in wavelength ranges of λ = 532.0–532.8 and 539.1–539.9 nm were registered with the ATsU-5 telescope (the Russian abbreviation ATsU corresponds to the Astronomical Coelostat Mounting) of the Main Astronomical Observatory of the National Academy of Sciences of Ukraine (MAO NASU) after replacing the regular main mirror by a short-focus one (with a focal length of 1 m). An advantage of the applied method is that the spectrum is registered simultaneously for different heliocentric positions on the solar disk. The observational data were reduced for the influence of light scattered in the spectrograph, the atmospheric-stray light, the spectrograph instrumental profile, and some aberrations. The center-to-limb ratios of the spectra are compared to the data available in the literature. The data on variations in the profiles of eleven spectral lines in transition from the center to the limb of the solar disk were obtained. The revealed nonmonotonicity of these changes is explained by inhomogeneities in physical conditions on the surface of the Sun. In general, the depths of the studied Fe I lines demonstrate the decreasing tendency in the strength of lines when passing toward the limb. For most lines, the full widths at half maximum increase toward the disk edge. The equivalent widths show differently directed variations. The parameters of the line Mn I λ 539.4 nm behave in a different way: all three analyzed parameters grow toward the disk edge; however, at the extreme limb, the depth and the equivalent width also start to decrease. The limb-effect of lines was measured; it exhibits the highest value when the core positions in weak lines are compared. For strong lines, the limb-effect is strongest while comparing the middle parts of the line bisectors.

Ap stars are chemically peculiar, main-sequence stars with atmospheres having an anomalous chemical composition (compared to the solar one). This abnormal chemical composition is thought to result from diffusion of chemical elements under the influence of radiation pressure and gravitational settling. Depending on the prevailing process, a chemical element either sinks and is accumulated in deeper layers or floats up to the upper layers of the stellar atmosphere. In addition to nonuniform depth distributions of elements, peculiar stars feature spots on their surface enriched or depleted with certain chemical elements. Ap stars have strong global magnetic fields with intensities ranging from several hundred millitesla to several tesla. These fields, as a rule, have a simple dipole configuration that remains stable over time intervals of several decades at the least. One of the unexplored effects observed in chemically peculiar stars is the significant difference in magnetic fields measured using different spectral lines. The present study is focused on measurements of the longitudinal magnetic field of the chemically peculiar Ap star 33 Lib in N = 180 spectral lines. High-resolution circularly polarized spectra collected over four nights in 2006 with the 3.6 m CFHT ESPaDOnS spectrograph were taken from the CADC open database. The magnetic field was calculated based on the Zeeman effect using the single-line method. It was found that the longitudinal magnetic field of 33 Lib averaged over all nights is 〈B_e〉 = 274.9 ± 2.7 mT. Statistically significant differences between the magnetic-field magnitudes determined for different spectral lines (including those of the same chemical element) were also found. The weakest magnetic field was measured in the cores of hydrogen lines H_α and H_β and in Y and Pr lines. The dependence of the magnetic-field magnitude on parameters of spectral lines revealed that weak lines with small Lande factors had the strongest magnetic field. This may be attributed to the nonuniformity of the magnetic field in the stellar atmosphere and the distribution of chemical elements over the surface (and/or with depth) or to both these factors.