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E. Weisshaar 《地球物理与天体物理流体动力学》2013,107(1-2):141-170
Abstract The two dimensional incompressible MHD equations describing the decay of a random initial velocity field in the presence of a uniform magnetic background field are solved numerically by a Chebyshev spectral method. The nonlinear interactions of standing Alfvén-waves of a given energy are studied for various Reynolds numbers and field strengths of the magnetic background field. Small scale structures are generated by these interactions, which increase the energy dissipation, however, the uniform background field suppresses the production of arbitrary small scales. Thus energy dissipation is found to be insignificant at sufficiently high Reynolds numbers. Anisotropies of the fluctuating field components are also studied. In the temporal evolution they appear first in the magnetic field. This is explained by the conservation of mean square vector potential in the limit of infinite conductivity. 相似文献
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Küveler G. Wiehr E. Thomas D. Harzer M. Bianda M. Epple A. Sütterlin P. Weisshaar E. 《Solar physics》1998,182(1):247-255
The primary image reflected from the field stop of solar Gregory telescopes is used for automatic guiding. This new system
avoids temporal varying influences from the bending of the telescope tube by the main mirror's gravity and from offsets between
the telescope and a separate guiding refractor. The required stiffness of the guider mechanics and the small areas of the
sensors demand small f numbers for the guider optics, which cause problems with the image quality and with heat. Problems
also arise from the pinhole in the telescope's field stop which is imaged as a dark dot on the sensor. Pointing errors introduced
by the telescope affect shifts of the solar image on the sensor. These are numerically determined by Fourier methods which
are found to be less sensitive to noise than profile centering methods. Several types of guiders are tested, the final equipment,
now installed at the Gregory telescopes at Tenerife and at Locarno, is described. 相似文献
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