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[en] In the stellar chromospheres, radiative energy transport is dominated by only the strongest spectral lines. For these lines, the approximation of local thermodynamic equilibrium (LTE) is known to be very inaccurate, and a state of equilibrium cannot be assumed in general. To calculate the radiative energy transport under these conditions, the population evolution equation must be evaluated explicitly, including all time-dependent terms. We develop a numerical method to solve the evolution equation for the atomic-level populations in a time-implicit way, keeping all time-dependent terms to first order. We show that the linear approximation of the time dependence of the populations can handle very large time steps without losing accuracy. We reproduce the benchmark solutions from earlier, well-established works in terms of non-LTE kinetic equilibrium solutions and typical ionization/recombination timescales in the solar chromosphere.
[en] The paper deals with the review of works on solar observations at Abastumani Astrophysical Observatory during the past 12 years. The works accomplished on the basis of patrol observations, with the horizontal solar telescope, small and large coronagraphs, and those based on the data obtained during eclipse observations are discussed separately. (orig.)
[en] This essay on the chromosphere and transition region gegins with a general discussion of mechanically heated atmospheres and goes on to discuss both descriptive and physical models. The descriptive models include the thermodynamic and fluid dynamic properties of the atmosphere as deduced from observations. Particular features of the models are identified with the properties of the radiation loss associated with the ionization of hydrogen and with the properties of thermal conduction. The role of spicules in chromosphere and transition region properties is emphasized. Physical models that attempt to predict the basic features of the descriptive models are reviewed. In summary, it seems likely that no one mechanism will satisfy the needs of the lower transition region. The region is observed to be highly dynamic and highly structured thermally. Certainly there are magnetic loops, and certainly there are non-LTE effects in electron distribution functions. Just as certainly, each will play a role in the structure and in the energy balance. The challenge ahead is to discover the relative magnitudes of these and other important effects
[en] The program was based on simultaneous observations of the same solar region with the Universal Birefringent Filter (UBF) at the Vacuum Tower Telescope (VTT) and with the Big Dome BD Universal Spectrograph (USG). For the UBF images analysis it is referred to the June 13, 1980 RG 2502/2511 (N12-E11) observations. Selected pictures present the line profiles for H-alpha, H-beta, Mg-b1 and Na-D2 at some interesting pixels of the analyzed AR. The detailed photometric morphology of the observed active area and the longitudinal velocity field pattern are presented. A comparison with the corresponding UVSP data is given
[en] Fine prominence features have been predicted to explain the presence of faint luminous chromospheric emissions in the solar corona. This paper describes very fine structures of the order of 200 km in diameter seen at Hα and extending from the center of a sunspot into the corona
[en] High-resolution spectropolarimetric observations of three sunspots taken with Hinode demonstrate the existence of supersonic downflows at or close to the umbra-penumbra boundary which have not been reported before. These downflows are confined to large patches, usually encompassing bright penumbral filaments, and have lifetimes of more than 14 hr. The presence of strong downflows in the center-side penumbra near the umbra rules out an association with the Evershed flow. Chromospheric filtergrams acquired close to the time of the spectropolarimetric measurements show large, strong, and long-lived brightenings in the neighborhood of the downflows. The photospheric intensity also exhibits persistent brightenings comparable to the quiet Sun. Interestingly, the orientation of the penumbral filaments at the site of the downflows is similar to that resulting from the reconnection process described by Ryutova et al. The existence of such downflows in the inner penumbra represents a challenge for numerical models of sunspots because they have to explain them in terms of physical processes likely affecting the chromosphere.
[en] Solar flares are rapid energy release phenomena that appear as bright ribbons in the chromosphere and high temperature loops in the corona, respectively. Supra-arcade Downflows (SADs) are plasma voids that first come out above the flare loops and then move quickly toward the flare loop top during the decay phase of the flare. In our work, we study 20 SADs appearing in three flares. By differential emission measure (DEM) analysis, we calculate the DEM weighted average temperature and emission measure of the front region and the main body of SADs. It is found that the temperatures of the SAD front and body tend to increase during the course of SADs flowing downwards. The relationship between the pressure and temperature fits well with the adiabatic equation for both the SAD front and body, suggesting that the heating of SADs is mainly caused by adiabatic compression. Moreover, we also estimate the velocities of SADs via the Fourier Local Correlation Tracking method and find that increase of the temperature of the SAD front presents a correlation with the decrease of the SAD kinetic energy, while the SAD body does not, implying that the viscous process may also heat the SAD front in spite of a limited role.
[en] The plasma contributing to emission from the Sun between the cool chromosphere (≤104 K) and hot corona (≥106 K) has been subjected to many different interpretations. Here we look at the magnetic structure of this transition region (TR) plasma, based upon the implications of CLASP2 data of an active region recently published by Ishikawa et al., and earlier Interface Region Imaging Spectrograph (IRIS) and Solar Dynamics Observatory (SDO) data of quiet regions. Ishikawa et al. found that large areas of sunspot plages are magnetically unipolar as measured in the cores of Mg ii resonance lines, formed in the lower TR under low plasma-β conditions. Here we show that IRIS images in the line cores have fibrils that are well aligned with the overlying coronal loop segments seen in the 171 Å channel of SDO. When the TR emission in active regions arises from plasma magnetically and thermally connected to the corona, then the line cores can provide the first credible magnetic boundary conditions for force-free calculations extended to the corona. We also re-examine IRIS images of dynamic TR cool loops previously reported as a major contributor to TR emission from the quiet Sun. Dynamic cool loops contribute only a small fraction of the total TR emission from the quiet Sun.