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RISS 인기검색어
- 검색키워드
- 저자 : Ofman, Leon (검색결과 5 건)
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Wang, Tongjiang,Ofman, Leon,Sun, Xudong,Solanki, Sami K.,Davila, Joseph M. American Astronomical Society 2018 The Astrophysical journal Vol.860 No.2
<P>Standing slow-mode waves have been recently observed in flaring loops by the Atmospheric Imaging Assembly of the Solar Dynamics Observatory. By means of the coronal seismology technique, transport coefficients in hot (similar to 10 MK) plasma were determined by Wang et al., revealing that thermal conductivity is nearly suppressed and compressive viscosity is enhanced by more than an order of magnitude. In this study, we use 1D nonlinear MHD simulations to validate the predicted results from the linear theory and investigate the standing slow-mode wave excitation mechanism. We first explore the wave trigger based on the magnetic field extrapolation and flare emission features. Using a flow pulse driven at one footpoint, we simulate the wave excitation in two types of loop models: Model 1 with the classical transport coefficients and Model 2 with the seismology-determined transport coefficients. We find that Model 2 can form the standing wave pattern (within about one period) from initial propagating disturbances much faster than Model 1, in better agreement with the observations. Simulations of the harmonic waves and the Fourier decomposition analysis show that the scaling law between damping time (tau) and wave period (P) follows tau proportional to P-2 in Model 2, while tau proportional to P in Model 1. This indicates that the largely enhanced viscosity efficiently increases the dissipation of higher harmonic components, favoring the quick formation of the fundamental standing mode. Our study suggests that observational constraints on the transport coefficients are important in understanding both the wave excitation and damping mechanisms.</P>
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Kwon, Ryun-Young,Ofman, Leon,Olmedo, Oscar,Kramar, Maxim,Davila, Joseph M.,Thompson, Barbara J.,Cho, Kyung-Suk IOP Publishing 2013 The Astrophysical journal Vol.766 No.1
<P>We report white-light observations of a fast magnetosonic wave associated with a coronal mass ejection observed by STEREO/SECCHI/COR1 inner coronagraphs on 2011 August 4. The wave front is observed in the form of density compression passing through various coronal regions such as quiet/active corona, coronal holes, and streamers. Together with measured electron densities determined with STEREO COR1 and Extreme UltraViolet Imager (EUVI) data, we use our kinematic measurements of the wave front to calculate coronal magnetic fields and find that the measured speeds are consistent with characteristic fast magnetosonic speeds in the corona. In addition, the wave front turns out to be the upper coronal counterpart of the EIT wave observed by STEREO EUVI traveling against the solar coronal disk; moreover, stationary fronts of the EIT wave are found to be located at the footpoints of deflected streamers and boundaries of coronal holes, after the wave front in the upper solar corona passes through open magnetic field lines in the streamers. Our findings suggest that the observed EIT wave should be in fact a fast magnetosonic shock/wave traveling in the inhomogeneous solar corona, as part of the fast magnetosonic wave propagating in the extended solar corona.</P>
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Selwa, M.,Solanki, S. K.,Ofman, L. IOP Publishing 2011 The Astrophysical journal Vol.728 No.2
<P>We present numerical results of simulations of kink oscillations of coronal loops in an idealized active region (AR) that is initialized as a potential dipole magnetic configuration with gravitationally stratified density. We consider loops, with density higher than the surrounding plasma, embedded into the dipolar AR. We study the excitation of kink oscillations of such loops by velocity pulses at different positions, of a given duration and amplitude. The position of the pulse varies in the parametric studies. For a central (symmetric) loop within the AR, we find that the amplitude of vertical kink oscillations is significantly amplified in comparison to horizontal kink oscillations for exciters located centrally (symmetrically) below the loop. For pulses initiated further from such a symmetric loop a combination of vertical and horizontal oscillations is excited. The scenario changes significantly when we study an inclined loop (non-symmetric within a dipole field). In this case, we do not see vertical kink oscillations of any significant amplitude being excited, while horizontal ones can be easily detected. These results indicate that the reason why vertical kink oscillations are observed so rarely is that their excitation requires a set of conditions to occur simultaneously: the exciting pulse must be located roughly below the loop apex and the loop itself must be located symmetrically within the group of loops. The new findings of the present study show the importance of not only the position of the pulse, but mainly of the location of the loop within the set of field lines having the same magnetic connectivity. We find that the slow propagating wave is excited in all the studied loops and its excitation does not depend either on the geometry of the loop or the pulse. We discuss TRACE observations of coronal loop oscillations in view of our findings and find that our results can be used for identifying the polarization of the kink mode based on the location of the loop within the set of field lines of the same connectivity and the position of the flare.</P>
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