Results 1 - 10 of 3252
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[en] The observed form of the emission measure (EM) is used as a function of temperature to infer the wave energy flux density and pressure throughout the lower transition region (TR). This procedure eliminates the need for specifying how the wave energy flux density is damped and addresses the question of whether there is any form of the mechanical heating associated with the degradation of an upward traveling wave energy flux density which is consistent with the observed EM and other observational constraints for the quiet sun. It is found that the observed form of the EM curve is incompatible with waves traveling vertically at the sound speed, regardless of any filling factor arguments. The same conclusion also applies to waves traveling at the Alfven speed, unless it is assumed that the emission in lower TR lines originates solely from small, spatially unresolved regions of large magnetic field strength (100 G), which cover a small fraction (filling factors of 1 percent) of the solar surface. 22 refs
[en] A study is made of the observational consequences of the hypothesis that there is no steady coronal heating, the solar corona instead being heated episodically, such that each short burst of heating is followed by a long period of radiative cooling. The form of the resulting contribution to the differential emission measure (DEM), and to a convenient related function (the differential energy flux, DEF) is calculated. Observational data for the quiet solar atmosphere indicate that the upper branch of the DEM, corresponding to temperatures above 100,000 K, can be interpreted in terms of episodic energy injection at coronal temperatures. 22 refs
[en] An investigation is conducted of physical heating mechanisms due to the ponderomotive forces exerted by turbulent waves along the solar atmosphere's curved magnetic flux loops. Results indicate that the temperature difference between the inside and outside of the flux loop can be classified into three parts, two of which represent the cooling or heating effect exerted by the ponderomotive force, while the third is the heating effect due to turbulent energy conversion from the localized plasma. This heating mechanism is used to illustrate solar atmospheric heating by means of an example that leads to the formulation of plages. 9 references
[en] A numerical simulation of the effect of a random photospheric flow on the magnetic structure of a coronal loop is presented. An initially uniform field embedded in a perfectly conducting plasma is assumed, extending between two flat parallel plates representing the solar photosphere at both ends of the loop. The field is perturbed by a sequence of randomly phased, sinusoidal flow patterns applied at one of the boundary plates, and the corresponding sequence of force-free fields is determined. It is found that the electric currents generated by these flows develop a fine structure on a scale significantly smaller than the wavelength of the velocity patterns. This suggests that magnetic energy is transferred to smaller scale via a cascade process. 22 references
[en] The coronal magnetic field rotates differently than the photosphere. The field configuration of the corona can be calculated from the observed photosphpere field using a potential field model. Correlation of the field patterns at different latitudes with a lag near one solar rotation shows much less differential rotation than observed in the photospheric field; however, the peak is very broad and determines the rotation rate rather poorly. Consideration of longer lags reveals a more complex rotational structure and indicates different rotation rates in the Northern and Southern Hemispheres. Spectral analysis of the equatorial dipole component of the coronal field reveals an organization into just a few discrete rotation frequencies which are apparently present simultaneously. Spectral analysis of the field at different latitudes shows that the frequencies are present simultaneously. Spectra analysis of the field at different latitudes shows that the frequencies are present simultaneously, but in different hemispheres, and that the Southern Hemisphere fields rotate more slowly than those in the north in solar cycle 21. 32 references
[en] Several theoretical studies have proposed that, in response to photospheric footpoint motions, current sheets can be generated in the solar corona without the presence of a null point in the initial potential magnetic field. A fundamental assumption in these analyses, commonly referred to as the line-tying assumption, is that all coronal field lines are anchored to a boundary surface representing the top of the dense, gas pressure-dominated photosphere. It is shown here that line-typing cannot be applied indiscriminately to dipped coronal fields, and that the conclusions of the line-tied models are incorrect. To support the theoretical arguments, the response of a dipped potential magnetic field in a hydrostatic-equilibrium atmosphere to shearing motions of the footpoints is studied, using a 2.5-dimensional MHD code. The results show that, in the absence of artificial line-tying conditions, a current sheet indeed does not form at the location of the dip. Rather, the dipped magnetic field rises, causing upflows of photospheric and chromospheric plasma. 20 refs
[en] Two popular theories of solar system formation are briefly reviewed, then used as background in an examination of several new developments related to planetary ring dynamics that promise to have great impact on future research. Most important are the incorporation of accretion disk and density wave theories into cosmogonic theory. A successful integration of these mechanisms may significantly constrain evolutionary models of the early solar system and also provide new insight into the mechanisms themselves. 49 references
[en] Equations are found for force-free magnetic equilibria in the 'coronal half-space' z>0, subject to boundary conditions on the normal magnetic field at z=0. The distribution of normal field is assumed to be composed of NS isolated unipolar source regions of arbitrary shape, arranged arbitrarily on the plane. The equilibria are found by minimizing the magnetic energy subject to constraints on the total flux interconnecting pairs of source regions. For NS source regions interconnected in ND ways, there are Nc=ND-NS+1 distinct constraints on the field. Minimization subject to these constraints leads to an Nc-dimensional space of equilibria, for given boundary data. All field-lines connecting source regions are current-free, but the equilibrium contains Nc current-sheets lying at certain interfaces. In a two-dimensional magnetic field current sheets occur at points topologically equivalent to X-type neutral points in the potential field. In a three-dimensional field current sheets occur at points topologically equivalent to separators in the potential field. The free magnetic energy is a function of the Nc fluxes used to constrain the variation
[en] This work includes a study of some properties such as speed, apparent width, acceleration and latitudes, etc. of all types of Prominence Eruptions (PEs) and the associated Coronal Mass Ejections (CMEs) observed during the period of 1997–2006 by Nobeyama Radioheliograph (NORH) and SOHO/LASCO covering the solar cycle 23. The average speed of prominences and associated CMEs are 51 km/sec and 559 km/sec, respectively. The average angular width is 32∘ and 74∘, respectively. As expected the associated CMEs are relatively faster and wider than the prominences.
[en] Our limited knowledge of the magnetic fields structuring in the solar corona represents today the main hurdle in our understanding of its structure and dynamic. Over the last decades significant efforts have been dedicated to measure these fields, by approaching the problem on many different sides and in particular: (i) by improving our theoretical understanding of the modification (via Zeeman and Hanle effects) induced by these fields on the polarization of coronal emission lines, (ii) by developing new instrumentation to measure directly with spectro-polarimeters these modifications, (iii) by improving the reliability of the extrapolated coronal fields starting from photospheric measurements, (iv) by developing new techniques to analyse existing remote sensing data and infer properties of these fields, or by combining all these different approaches (e.g., Chifu et al.,).