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  • 1
    Electronic Resource
    Electronic Resource
    X-ray flare sparks quake inside Sun (1998)
    [s.l.] : Macmillan Magazines Ltd.
    Nature 393 (1998), S. 317-318 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Solar flares involve a release of the Sun's magnetic energy as X-radiation, particle beams and high-speed plasma flows. But we have discovered, using data from the Solar and Heliospheric Observatory (SOHO), that these flares also affect the Sun's interior, generating seismic waves similar to ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/30629
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  • 2
    Electronic Resource
    Electronic Resource
    Numerical simulations of spicule driving mechanisms (1994)
    Springer
    Space science reviews 70 (1994), S. 53-56 
    Details
    ISSN: 1572-9672
    Keywords: Sun: atmosphere ; Sun: corona
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Spicules are known as one of the most prevalent small-scale dynamic phenomena on the sun, which are likely to give considerable contribution to coronal heating and mass supply. We discuss a model of the spicules driven by a train of slow MHD shock waves propagating along a vertical expanding magnetic flux tube. The shocks are initiated due to compression of the tube by the increasing external pressure in the lower chromosphere. Downflow of spicular material depends on radiative cooling and other dissipative processes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00777841
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  • 3
    Electronic Resource
    Electronic Resource
    Modeling of dynamic evolution of coronal loops (1994)
    Springer
    Space science reviews 70 (1994), S. 171-174 
    Details
    ISSN: 1572-9672
    Keywords: Sun: corona ; Sun: activity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Parameters of expanding magnetic loops and arches and of mass flows generated by them in the corona have been computed in a 1D two-fluid approximation. Two possible trigger mechanisms of the coronal transients have been considered: (i) sudden increase of the background magnetic field strength, and (ii) heating and compression plasma inside these magnetic structures. We discuss the formation of shock waves and their dependence on dynamics and geometry of the magnetic structures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00777862
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  • 4
    Electronic Resource
    Electronic Resource
    STRUCTURE AND ROTATION OF THE SOLAR INTERIOR: INITIAL RESULTS FROM THE MDI MEDIUM-L PROGRAM (1997)
    Springer
    Solar physics 170 (1997), S. 43-61 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The medium-l program of the Michelson Doppler Imager instrument on board SOHO provides continuous observations of oscillation modes of angular degree, l, from 0 to ∽ 300. The data for the program are partly processed on board because only about 3% of MDI observations can be transmitted continuously to the ground. The on-board data processing, the main component of which is Gaussian-weighted binning, has been optimized to reduce the negative influence of spatial aliasing of the high-degree oscillation modes. The data processing is completed in a data analysis pipeline at the SOI Stanford Support Center to determine the mean multiplet frequencies and splitting coefficients. The initial results show that the noise in the medium-l oscillation power spectrum is substantially lower than in ground-based measurements. This enables us to detect lower amplitude modes and, thus, to extend the range of measured mode frequencies. This is important for inferring the Sun's internal structure and rotation. The MDI observations also reveal the asymmetry of oscillation spectral lines. The line asymmetries agree with the theory of mode excitation by acoustic sources localized in the upper convective boundary layer. The sound-speed profile inferred from the mean frequencies gives evidence for a sharp variation at the edge of the energy-generating core. The results also confirm the previous finding by the GONG (Gough et al., 1996) that, in a thin layer just beneath the convection zone, helium appears to be less abundant than predicted by theory. Inverting the multiplet frequency splittings from MDI, we detect significant rotational shear in this thin layer. This layer is likely to be the place where the solar dynamo operates. In order to understand how the Sun works, it is extremely important to observe the evolution of this transition layer throughout the 11-year activity cycle.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1004949311268
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  • 5
    Electronic Resource
    Electronic Resource
    The optical continuum of solar and stellar flares (1981)
    Springer
    Solar physics 73 (1981), S. 269-288 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A further development of the Kostyuk-Pikelner's model is presented. The response of the chromosphere heated by non-thermal electrons of the power-law energy spectrum has been studied on the basis of the numerical solution of the one-dimensional time-dependent equations of gravitational gas dynamics. The ionization and energy loss for the emissions in the Lyman and Balmer lines have been determined separately for the optically thin and thick Lα-line layers. Due to the initial heating, a higher-pressure region is formed. From this region, disturbances propagate upwards (a shock wave with a velocity of more than 1000 km s-1) and downwards. A temperature jump propagates downwards, and a shock is formed in front of the thermal wave. During a period of several seconds after the beginning of this process, the temperature jump intensifies the downward shock wave and the large radiative loss gives rise to the high density jump (ϱ 2/ϱ 1 ∼ 100). The numerical solution has been analyzed in detail for the case heating of the ionized and neutral plasma, and a value of this heating is close to the upper limit of the admissible values. In this case, the condensation located between the temperature jump and the shock wave front, may emit in the observed optical continuum. In their essential features, the gas dynamic processes during the flares in red dwarf atmospheres are the same as those in the solar atmosphere. However, the high atmospheric densities, smaller height scale in red dwarf atmospheres, and greater energy of this processes in stellar flares, give rise, in practice, to the regular generation of optical continuum. The photometric parameters of a source with n ∼ 015 cm-3, T ∼ 9000 K, and Δ z ∼ 10 km are in a good agreement with observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00151682
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  • 6
    Electronic Resource
    Electronic Resource
    On the excitation of oscillations of the Sun (numerical models) (1983)
    Springer
    Solar physics 82 (1983), S. 323-329 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Numerical solutions of the general time-dependent gas-dynamical equations in linear adiabatic approximation are given for initial conditions imitating: (a) a central perturbation, (b) a boundary perturbation (in the convective envelope), and (c) a ‘shrinking’ of the Sun as a whole. For a variety of models of the Sun it is found that at the surface the radial component v r of velocity is much greater than the tangential component v t , and that the period T of stationary oscillations does not exceed 131m. The appearance at the surface of a g mode with period 160m is found to be improbable. With the initial conditions adopted, a propagating wave is produced which is reflected successively from the centre to the periphery and back, producing 5-min oscillations at the surface of the Sun. Expansion of this wave into separate modes leads to a power spectrum qualitatively similar to that observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00145571
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  • 7
    Electronic Resource
    Electronic Resource
    The Solar Oscillations Investigation - Michelson Doppler Imager (1995)
    Springer
    Solar physics 162 (1995), S. 129-188 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The Solar Oscillations Investigation (SOI) uses the Michelson Doppler Imager (MDI) instrument to probe the interior of the Sun by measuring the photospheric manifestations of solar oscillations. Characteristics of the modes reveal the static and dynamic properties of the convection zone and core. Knowledge of these properties will improve our understanding of the solar cycle and of stellar evolution. Other photospheric observations will contribute to our knowledge of the solar magnetic field and surface motions. The investigation consists of coordinated efforts by several teams pursuing specific scientific objectives. The instrument images the Sun on a 10242 CCD camera through a series of increasingly narrow spectral filters. The final elements, a pair of tunable Michelson interferometers, enable MDI to record filtergrams with a FWHM bandwidth of 94 mÅ. Normally 20 images centered at 5 wavelengths near the Ni I 6768 spectral line are recorded each minute. MDI calculates velocity and continuum intensity from the filtergrams with a resolution of 4″ over the whole disk. An extensive calibration program has verified the end-to-end performance of the instrument. To provide continuous observations of the longest-lived modes that reveal the internal structure of the Sun, a carefully-selected set of spatial averages are computed and downlinked at all times. About half the time MDI will also be able to downlink complete velocity and intensity images each minute. This high rate telemetry (HRT) coverage is available for at least a continuous 60-day interval each year and for 8 hours each day during the rest of the year. During the 8-hour HRT intervals, 10 of the exposures each minute can be programmed for other observations, such as measurements in MDI's higher resolution (1.25″) field centered about 160″ north of the equator; meanwhile, the continuous structure program proceeds during the other half minute. Several times each day, polarizers will be inserted to measure the line-of-sight magnetic field. MDI operations will be scheduled well in advance and will vary only during the daily 8-hour campaigns. Quick-look and summary data, including magnetograms, will be processed immediately. Most high-rate data will be delivered only by mail to the SOI Science Support Center (SSSC) at Stanford, where a processing pipeline will produce 3 Terabytes of calibrated data products each year. These data products will be analyzed using the SSSC and the distributed resources of the co-investigators. The data will be available for collaborative investigations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00733429
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  • 8
    Electronic Resource
    Electronic Resource
    Constrained estimates of low-degree mode frequencies and the determination of the interior structure of the Sun (1995)
    Springer
    Solar physics 157 (1995), S. 1-15 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Low-degreep-modes penetrate to the solar centre and provide direct information about the core. However, the high observational accuracy that is required to resolve the details of the structure of the core is difficult to achieve because the oscillation power spectrum is significantly distorted by stochastic forcing of the oscillations, which appears as multiplicative noise. Here, an attempt is reported to reduce uncertainties of spectral parameter estimation by incorporating constraints imposed by smooth behaviour of some of the parameters (e.g., linewidths, background noise, rotational splitting) over a group of lines. Instead of estimating these parameters independently for each line, we determine them as smooth functions of frequency. It is expected that this procedure gives more accurate estimates of the average frequencies of any multiplet in the power spectrum, to which we have applied no constraints. We give some examples of the procedure for whole-disk measurements by the IPHIR space experiment. It is shown that the additional constraints do not result in significant changes in the frequency estimates, except for one mode whose peak in the power spectrum has the lowest signal-to-noise ratio. However, the uncertainty in the frequency of that mode does not influence substantially the results of the structure inversion in the core. Inversions of the IPHIR datasets are compared with corresponding inversions of data from the Birmingham Solar Oscillation Network (BISON). The IPHIR data indicate a sharp increase towards the centre of the deviation of the squared sound speed of the sun from that of a standard solar model, whereas the BISON data show a decrease. The difference between the IPHIR and BISON inversions is significant, preventing any definite conclusion about the deviation of the structure of the solar core from that of the model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00680605
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  • 9
    Electronic Resource
    Electronic Resource
    Skinning process stability of the magnetic field in the solar active regions (1977)
    Springer
    Solar physics 51 (1977), S. 293-306 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Skinning process stability of the magnetic field in homogeneous plasma is studied. A set of magnetohydrodynamic equations is used. Dependence of electrical conductivity on the plasma parameters and radiation intensity in grey-body approximation are taken into account. The investigation is carried out on the model problems in linear approximation and by means of numerical solution of MHD equations. Threshold of stability and critical gradient of magnetic field in skin-layer are obtained. The model of the phenomenon proposed in the paper indicates on overheating instability of plasma with electric current in large gradient magnetic field zones as a possible trigger mechanism of solar flare origin.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00216367
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  • 10
    Electronic Resource
    Electronic Resource
    Nonlinear stage of instability due to local Joule-overheating in the solar active regions (1978)
    Springer
    Solar physics 57 (1978), S. 73-79 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The numerical solution by a computer of the system of magnetohydrodynamics equations in the one-dimensional approximation serves as the basis for studying the non-linear stage of the instability due to local Joule-overheating of zones with large values of magnetic field gradients in the active regions of the Sun. We have demonstrated the formation of a system of current layers responsible for efficient transformation of magnetic energy into Joule heat and kinetic energy of the macroscopic motion. The specific features of quasi-stationary skinning of magnetic field with gravitation have been noted.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00152045
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