[en] The so-called 'plasma horizon' of Ruffini is examined critically to see if it has any physical significance: very little is found. (author)

[en] An analysis of solar velocity data obtained at the Stanford Solar Observatory shows the existence of solar global oscillations in the range 45-105 μHz. The oscillations are interpreted as internal gravity modes of degree 1=1 and 1=2. A good estimate of the order of the modes has also been made. (author)

[en] The solar chromosphere is identified with the atmosphere inside magnetic flux tubes. Between the temperature minimum and the 7000 K level, the chromosphere in the bright points of the quiet sun is heated by large-amplitude, long-period, compressive waves with periods mainly between 2 and 4 minutes. These waves do not observe the cutoff condition according to which acoustic waves with periods longer than 3 minutes do not propagate vertically in the upper solar photosphere. It is concluded that the long-period waves probably supply all the energy required for the heating of the bright points in the quiet solar chromosphere. 42 refs

[en] The impulsive heating events and their corresponding nonlinear dynamics remain one of the most obscure physical processes in solar atmospheric physics. The complicacy of these processes together with limited observations have greatly hampered our understanding of them. Here, we present, for the first time, an unambiguous example of a nonlinear acoustic wave in a closed coronal loop or loop segment, which appeared as a fast propagating ultra-hot disturbance cohesively in an indistinguishable corona loop with a highly evolving emission intensity profile. Based on the theory of propagating nonlinear waves, we argue that this type of observation can provide further information for the disturbance during its propagation. With this information, we conclude that the propagating nonlinear disturbance can quickly heat the corona through the rarefaction wave, and the disturbance-induced magnetic reconnection should not happen in our observation. Besides, a convenient criterion has also been deduced for the existence of the disturbance-induced reconnection mechanism. All of this provides us with a new insight into the accompanying nonlinear dynamics of solar impulsive heating events, which can not only shed light on problems including coronal heating and the fast formation of hot coronal loops, but also show us a very novel and prospective seismology scheme for the diagnosis of coronal plasma properties.

[en] Resonant absorption is considered to be a crucial mechanism for the damping of the coronal loop oscillations and plasma heating. We study resonant absorption of the coronal loop kink oscillations excited by such external drivers as flares, assuming that there is an intermediate shear flow region surrounding the loop. We find that for long coronal loops resonant absorption can be highly enhanced or reduced depending sensitively on the magnitude and direction of the flow and the spatial extent of the flow region when the transitional layer is thin. For short coronal loops, high flow speed and a thick transitional layer are needed to have a substantial resonant absorption. We provide a potential picture to explain the results where the external Alfvén speed and phase speed of the wave are important parameters. These results imply that the transport of the external wave energy into the loop is significantly changed by the shear flow region, which may cause the selective excitation of the coronal loop oscillations.

[en] Chromospheric activity is found in ten types of stars, both single and binary. Variability at many time scales is reviewed, from minutes to centuries. The emphasis is on what is not understood, incorrectly understood, and understood but not appreciated. Observational results in hand include photometry, spectroscopy, spectrophotometry, speckle interferometry, magnetic fields, and orbital period variations. Certain physical mechanisms responsible for variability are discussed, making the distinction between the well-established, the suspected, and the possible. Specific topics include flares, rotation, differential rotation, synchronization, circularization, starspots, spot models, migration periods, spot cycles, magnetic activity cycles, sector structure, Maunder minima, pulsation, light curve solutions, convective envelope changes, mass transfer, mass loss, and orbital period variations. Specific directions for future research are identified. (author). 2 tabs., 97 refs

[en] The standard mathematical procedures in Helioseismology field are based on normal mode approach for various models of solar interior and atmosphere. We consider a two region model of a system solar interior and solar atmosphere. For simplicity, the two different regions are assumed quasi-isothermal, semi-infinite, without magnetic fields and separated by a boundary z=0. It was found useful to phrase of stability as initial value problem (IPV) in order to ensure the inclusion of certain continuum modes otherwise neglected. In addition to discrete mode (f-mode), sets of continuum modes due to a branch cuts in the complex plane, not treated explicitly in the literature, appears. It will be seen that an ambiguity of the usual normal mode method is avoided.

[en] The nonlinear properties of MHD surface waves in the solar atmosphere are investigated analytically, assuming that the fluid is incompressible and that the waves are confined to a single surface, with semiinfinite regions on both sides. The governing equations are derived in detail, and qualitative results are presented in a graph. For propagating waves, second-order terms in the wave amplitude are found to lead to wave steepening at leading or trailing edges, the steepening rate becoming very large as the threshold for the linear Kelvin-Helmholtz instability is approached. Second-order effects on standing waves include crest and trough sharpening (increasing with time), a current independent of distance on the surface but decreasing exponentially with distance from the surface, and pressure-field fluctuations of infinite extent. It is suggested that these effects could account for a large fraction of solar-atmosphere heating. 23 references

[en] We investigate if the super-saturation phenomenon observed at X-ray wavelengths for the corona exists in the chromosphere for rapidly rotating late-type stars. Moderate resolution optical spectra of fast-rotating EUV- and X-ray-selected late-type stars were obtained. Stars in α Per were observed in the northern hemisphere with the Isaac Newton 2.5 m telescope and Intermediate Dispersion Spectrograph. Selected objects from IC 2391 and IC 2602 were observed in the southern hemisphere with the Blanco 4 m telescope and R-C spectrograph at CTIO. Ca II H and K fluxes were measured for all stars in our sample. We find the saturation level for Ca II K at log (L_CaK/L_bol) = -4.08. The Ca II K flux does not show a decrease as a function of increased rotational velocity or smaller Rossby number as observed in the X-ray. This lack of 'super-saturation' supports the idea of coronal stripping as the cause of saturation and super-saturation in stellar chromospheres and coronae, but the detailed underlying mechanism is still under investigation.

[en] Obstacles encountered in the fields of atmospheric modeling and line synthesis are discussed with attention focused on the following: (1) velocity fields, (2) thermal inhomogeneities, (3) geometry, and (4) finite computing resources. It is noted that in some cases, such as global P-mode oscillations, the velocity fields are indicative of fundamental solar properties; in others, such as turbulence in the upper chromosphere and lower transition zone, these fields signify the operation of energy transport and dissipation mechanisms. With regard to geometry, particular emphasis is placed on the vertical spreading of magnetic flux tubes. It is concluded that in order to solve the current problems in this field, high-quality ultraviolet, optical and infrared observations of the sun from space must be obtained