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  • 1
    Electronic Resource
    Electronic Resource
    Is there a weak mixed polarity background field? Theoretical arguments (1987)
    Springer
    Solar physics 110 (1987), S. 115-128 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A number of processes associated with the formation of active regions produce ‘U-loops’: fluxtubes having two ends at the photosphere but otherwise still embedded in the convection zone. The mass trapped on the field lines of such loops makes them behave in a qualitative different way from the ‘omega-loops’ that form active regions. It is shown that U-loops will disperse though the convection zone and form a weak (down to a few gauss) field that covers a significant fraction of the solar surface. This field is tentatively identified with the inner-network fields observed at Kitt Peak and Big Bear. The process by which these fields escape through the surface is described; a remarkable property is that it can make active regions fields apparently disappear in situ. The mixed polarity moving magnetic features near sunspots are interpreted as a locally intense form of this disappearance by escape of U-loops.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00148207
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    The Ultraviolet Coronagraph Spectrometer for the solar and heliospheric observatory (1995)
    Springer
    Solar physics 162 (1995), S. 313-356 
    Details
    ISSN: 1573-093X
    Keywords: Sun ; Solar Corona ; Solar Wind ; UV Spectroscopy ; UV Coronagraph
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) is composed of three reflecting telescopes with external and internal occultation and a spectrometer assembly consisting of two toric grating spectrometers and a visible light polarimeter. The purpose of the UVCS instrument is to provide a body of data that can be used to address a broad range of scientific questions regarding the nature of the solar corona and the generation of the solar wind. The primary scientific goals are the following: to locate and characterize the coronal source regions of the solar wind, to identify and understand the dominant physical processes that accelerate the solar wind, to understand how the coronal plasma is heated in solar wind acceleration regions, and to increase the knowledge of coronal phenomena that control the physical properties of the solar wind as determined byin situ measurements. To progress toward these goals, the UVCS will perform ultraviolet spectroscopy and visible polarimetry to be combined with plasma diagnostic analysis techniques to provide detailed empirical descriptions of the extended solar corona from the coronal base to a heliocentric height of 12 solar radii.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00733433
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    UVCS Observations and Modeling of Streamers (1999)
    Springer
    Space science reviews 87 (1999), S. 335-338 
    Details
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present results derived from the analysis of an equatorial streamer structure as observed by the UVCS instrument aboard SOHO. From observations of the H I Lyα and Lyβ lines we infer the density and temperature of the plasma. We develop a preliminary axisymmetric, magnetostatic model of the corona which includes the effects of gas pressure gradients on the magnetic structure. We infer a coronal plasma β 〉 1 in the closed field regions and near the cusp of the streamer. We add to the model a parallel velocity field assuming mass flux conservation along magnetic flux tubes. We then compute the Lyα emissivity and the line-of-sight integrals to obtain images of Lyα intensity, taking into account projection effects and Doppler dimming. The images we obtain from this preliminary model are in good general agreement with the UVCS observations, both qualitatively and quantitatively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005106131009
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Role of Helicity in the Formation of Intermediate Filaments (1998)
    Springer
    Solar physics 180 (1998), S. 299-312 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract In the last few years new observations have shown that solar filaments and filament channels have a surprising hemispheric pattern. To explain this pattern, a new theory for filament channel and filament formation is put forward. The theory describes the formation of a specific type of filament, namely the ‘intermediate filament’ which forms either between active regions or at the boundary of an active region. It describes the formation in terms of the emergence of a sheared activity complex. The complex then interacts with remnant flux and, after convergence and flux cancellation, the filament forms in the channel. A key feature of the model is the net magnetic helicity of the complex. With the correct sign a filament channel can form, but with the opposite sign no filament channel forms after convergence. It is shown how the hemispheric pattern of helicity in emerging flux regions produces the observed hemispheric pattern for filaments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005028930121
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    On the temperature structure of sunspot umbrae (1984)
    Springer
    Solar physics 91 (1984), S. 195-217 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract From a high-resolution spectrum of a sunspot umbra (1.1 〈 λ 〈 2.3 μm) we derive models of the temperature stratification in the deep layers of the umbra. The observed spectrum is corrected for straylight using the Hi Paschen line at gl = 1.282 μm. A method is described for the iterative fitting of empirical temperature models to spectral information, and the method is applied to the present data. We find that the observed profiles of 3 high-excitation lines of Sii and the observed continuum contrast between umbra and photosphere cannot be reproduced with a single one-component model of the umbral atmosphere: the Si i lines require a model that is 460 K hotter at gt 0.5 = 3 than the continuum model. This indicates that hot and cool components coexist within the umbra. A temperature model derived from the relative intensity in the wings of 3 low-excitation lines of Mgi, Ali, and Sii is not significantly different from the continuum model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00146294
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    Paper (German National Licenses)
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  • 6
    Electronic Resource
    Electronic Resource
    Magnetic fine structures in coronal loops (1994)
    Springer
    Space science reviews 70 (1994), S. 31-35 
    Details
    ISSN: 1572-9672
    Keywords: Sun ; Corona
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The formation of magnetic fine structures and associated electric currents is considered in the context of the coronal heating problem. The penetration of field-aligned electric currents into the lower atmosphere is discussed. It is argued that currents strong enough to heat the corona can persist only for short periods of time. The formation of thin current sheets is discussed. It is argued that photospheric magnetic structures (flux tubes) play an important role in the generation of coronal currents.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00777838
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    Paper (German National Licenses)
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  • 7
    Electronic Resource
    Electronic Resource
    Energy release in stellar magnetospheres (1994)
    Springer
    Space science reviews 68 (1994), S. 299-307 
    Details
    ISSN: 1572-9672
    Keywords: accretion disks ; magnetic field
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The interaction of a stellar magnetosphere with a thin accretion disk is considered. Specifically, I consider a model in which (1) the accretion disk is magnetically linked to the star over a large range of radii and (2) the magnetic diffusivity of the disk is sufficiently small that there is little slippage of field lines within the disk on the rotation time scale. In this case the magnetic energy built up as a result of differential rotation between the star and the disk is released in quasi-periodic reconnection events occuring in the magnetosphere (Aly and Kuijpers 1990). The radial transport of magnetic flux in such an accretion disk is considered. It is show that the magnetic flux distribution is stationary on the accretion time scale, provided the time average of the radial component of the field just above the disk vanishes. A simple model of the time-dependent structure of the magnetosphere is presented. It is shown that energy release in the magnetosphere must take place for (differential) rotation angles less than about 3 radians. The magnetic flux distribution in the disk depends on the precise value of the rotation angle.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00749156
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    Paper (German National Licenses)
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