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Register a new userA unified model of solar prominence formation
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Several mechanisms have been proposed to account for the formation of solar prominences or filaments, among which direct injection and evaporation-condensation models are the two most popular ones. In the direct injection model, cold plasma is ejected from the chromosphere into the corona along magnetic field lines; In the evaporation-condensation model, the cold chromospheric plasma is heated to over a million degrees and is evaporated into the corona, where the accumulated plasma finally reaches thermal instability or non-equilibrium so as to condensate to cold prominences. In this paper, we try to unify the two mechanisms: The essence of filament formation is the localized heating in the chromosphere. If the heating happens in the lower chromosphere, the enhanced gas pressure pushes the cold plasma in the upper chromosphere to move up to the corona, such a process is manifested as the direct injection model. If the heating happens in the upper chromosphere, the local plasma is heated to million degrees, and is evaporated into the corona. Later, the plasma condensates to form a prominence. Such a process is manifested as the evaporation-condensation model. With radiative hydrodynamic simulations we confirmed that the two widely accepted formation mechanisms of solar prominences can really be unified in such a single framework. A particular case is also found where both injection and evaporation-condensation processes occur together.
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We investigate the process of formation and subsequent evolution of prominence plasma in a filament channel and its overlying arcade. We construct a three-dimensional time-dependent model of an intermediate quiescent prominence. We combine the magnetic field structure with one-dimensional independent simulations of many flux tubes, of a three-dimensional sheared double arcade, in which the thermal nonequilibrium process governs the plasma evolution. We have found that the condensations in the corona can be divided into two populations: threads and blobs. Threads are massive condensations that linger in the field line dips. Blobs are ubiquitous small condensations that are produced throughout the filament and overlying arcade magnetic structure, and rapidly fall to the chromosphere. The threads are the principal contributors to the total mass. The total prominence mass is in agreement with observations, assuming a reasonable filling factor. The motion of the threads is basically horizontal, while blobs move in all directions along the field. The peak velocities for both populations are comparable. We have generated synthetic images of the whole structure in an H$alpha$ proxy and in two EUV channels of the AIA instrument aboard SDO, thus showing the plasma at cool, warm, and hot temperatures. The predicted differential emission measure of our system agrees very well with observations. We conclude that the sheared-arcade magnetic structure and plasma behavior driven by thermal nonequilibrium fit well the abundant observational evidence for typical intermediate prominences.
Prominence plumes are evacuated upflows that emerge from bubbles below prominences, whose formation mechanism is still unclear. Here we present a detailed study of plumes in a quiescent prominence using the high-resolution H-alpha filtergrams at the line center as well as line wing at +/-0.4 angstrom from the New Vacuum Solar Telescope. Enhancements of brightening, blue shifts, and turbulence at the fronts of plumes are found during their formation. Some large plumes split at their heads and finger-shaped structures are formed between them. Blue-shifted flows along the bubble-prominence interface are found before and during the plume formation. Our observations are consistent with the hypothesis that prominence plumes are related to coupled Kelvin-Helmholtz and Rayleigh-Taylor (KH/RT) instabilities. Plume splittings and fingers are evidence of RT instability, and the flows may increase the growth rate of KH/RT instabilities. However, the significant turbulence at plume fronts may suggest that the RT instability is triggered by the plumes penetrating into the prominence. In this scenario, extra mechanisms are necessary to drive the plumes.
(abridged) The star formation rate (SFR) linearly correlates with the amount of dense gas mass (Mdg) involved in the formation of stars both for distant galaxies and clouds in our Galaxy. Similarly, the mass accretion rate (Macc) and the disk mass (Mdisk) of young, Class II stars are also linearly correlated. We plotted the corresponding observational data together, finding a statistically significant correlation that spans ~ 16 orders of magnitude. This probably represents one of the widest ranges of any empirical correlation known, encompassing galaxies that are several kpc in size, pc-size star-forming clouds within our Galaxy, down to young, pre-main sequence stars with au-size protoplanetary disks. We propose a bottom-up hypothesis suggesting that a relation between Macc and the total circumstellar mass surrounding Class 0/I sources (Mcs; disk+envelope) drives the correlation in protostellar-hosting clouds and cloud-hosting galaxies. This is consistent with the fact that the SFRs derived for clouds over a timescale of 2 Myr can be roughly recovered from the sum of instantaneous Macc values of the protostars embedded within them, implying that galactic SFRs averaged over ~ 10-100 Myr should be constant over this period too. Moreover, the sum of the Mcs values directly participating in the formation of the protostellar population in a cloud likely represents a non-negligible fraction of the Mdg within the cloud. If such fraction is ~ 1-35% of the Mdg associated with star-forming clouds and galaxies, then the global correlation for all scales has a near unity slope and an intercept consistent with the (proto-)stellar accretion timescale, Mcs/Macc. Therefore, an additional critical test of our hypothesis is that the Macc-Mdisk correlation for Class II stars should also be observed between Macc and Mcs for Class 0/I sources with similar slope and intercept.
Solar filaments are dark structures on the solar disk, with an elongated spine and several barbs extending out from the spine. When appearing above the solar limb, a filament is called a prominence, with several feet extending down to the solar surface. It was generally thought that filament barbs are simply the prominence feet veering away from the spine and down to the solar surface. However, it was recently noticed that there might be another dynamic type of barbs, which were proposed to be due to filament thread longitudinal oscillation. If this is the case, the dynamic barbs would not extend down to the solar surface. With the quadrature observations of a filament barb on 2011 June 5 from the {it Solar Dynamics Observatory} and the {it STEREO} satellites, we confirm that the filament barb is due to filament thread longitudinal oscillations. Viewed from the side, the filament barb looks like an appendix along the spine of the prominence, and does not extend down to the solar surface as a foot.
We report on observations of a solar prominence obtained on 26 April 2007 using the Extreme Ultraviolet Imaging Spectrometer on Hinode. Several regions within the prominence are identified for further analysis. Selected profiles for lines with formation temperatures between log(T)=4.7-6.3, as well as their integrated intensities, are given. The line profiles are discussed. We pay special attention to the He II line which is blended with coronal lines. Our analysis confirms that depression in EUV lines can be interpreted by two mechanisms: absorption of coronal radiation by the hydrogen and neutral helium resonance continua, and emissivity blocking. We present estimates of the He II line integrated intensity in different parts of the prominence according to different scenarios for the relative contribution of absorption and emissivity blocking on the coronal lines blended with the He II line. We estimate the contribution of the He II 256.32 line in the He II raster image to vary between ~44% and 70% of the rasters total intensity in the prominence according to the different models used to take into account the blending coronal lines. The inferred integrated intensities of the He II line are consistent with theoretical intensities obtained with previous 1D non-LTE radiative transfer calculations, yielding a preliminary estimate for the central temperature of 8700 K, central pressure of 0.33 dyn/cm^2, and column mass of 2.5 10^{-4} g/cm^2. The corresponding theoretical hydrogen column density (10^{20} cm^{-2}) is about two orders of magnitude higher than those inferred from the opacity estimates at 195 {AA}. The non-LTE calculations indicate that the He II 256.32 {AA} line is essentially formed in the prominence-to-corona transition region by resonant scattering of the incident radiation.
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