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Register a new userDoes a solar filament barb always correspond to a prominence foot?
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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.
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We study recent observations of propagating fluctuations in a prominence foot with Hinode Solar Optical Telescope (SOT) high-resolution observations in Ca~II and H alpha emission which we identify as nonlinear fast magnetosnic waves. Here we analyze further the observations of propagating waves and flows with Interface Region Imaging Spectrograph (IRIS) Mg~II slit jaw images, in addition to Hinode/SOT Ca~II images. We find that the waves have typical periods in the range of 5 - 11 minutes and wavelengths in the plane of the sky (POS) of about 2000 km, while the flows in narrow threads have typical speed in the POS of ~16-46 km/s. We also detect apparent kink oscillations in the threads with flowing material, and apply coronal seismology to estimate the magnetic field strength in the range 5-17 G. Using 2.5D MHD we model the combined effects of nonlinear waves and flows on the observed dynamics of the prominence material, and reproduce the propagating and refracting fast magnetosonic waves, as well as standing kink-mode waves in flowing material along the magnetic field. The modeling results are in good qualitative agreements with the observations of the various waves and flows in the prominence foot, further confirming coronal seismology analysis and improving the understanding of the fine scale dynamics of the prominence material.
We attempt to propose a method for automatically detecting the solar filament chirality and barb bearing. We first introduce the unweighted undirected graph concept and adopt the Dijkstra shortest-path algorithm to recognize the filament spine. Then, we use the polarity inversion line (PIL) shift method for measuring the polarities on both sides of the filament, and employ the connected components labeling method to identify the barbs and calculate the angle between each barb and the spine to determine the bearing of the barbs, i.e., left or right. We test the automatic detection method with H-alpha filtergrams from the Big Bear Solar Observatory (BBSO) H-alpha archive and magnetograms observed with the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Four filaments are automatically detected and illustrated to show the results. The barbs in different parts of a filament may have opposite bearings. The filaments in the southern hemisphere (northern hemisphere) mainly have left-bearing (right-bearing) barbs and positive (negative) magnetic helicity, respectively. The tested results demonstrate that our method is efficient and effective in detecting the bearing of filament barbs. It is demonstrated that the conventionally believed one-to-one correspondence between filament chirality and barb bearing is not valid. The correct detection of the filament axis chirality should be done by combining both imaging morphology and magnetic field observations.
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.
From recent high resolution observations obtained with the Swedish 1 m Solar Telescope in La Palma, we detect swaying motions of individual filament threads in the plane of the sky. The oscillatory character of these motions are comparable with oscillatory Doppler signals obtained from corresponding filament threads. Simultaneous recordings of motions in the line of sight and in the plane of the sky give information about the orientation of the oscillatory plane. These oscillations are interpreted in the context of the magnetohydrodynamic theory. Kink magnetohydrodynamic waves supported by the thread body are proposed as an explanation of the observed thread oscillations. On the basis of this interpretation and by means of seismological arguments, we give an estimation of the thread Alfven speed and magnetic field strength by means of seismological arguments.
We present a statistical study of prominence and filament eruptions observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO). Several properties are recorded for 904 events that were culled from the Heliophysics Event Knowledgebase (HEK) and incorporated into an online catalog for general use. These characteristics include the filament and eruption type, eruption symmetry and direction, apparent twisting and writhing motions, and the presence of vertical threads and coronal cavities. Associated flares and white-light coronal mass ejections (CME) are also recorded. Total rates are given for each property along with how they differ among filament types. We also examine the kinematics of 106 limb events to characterize the distinct slow- and fast-rise phases often exhibited by filament eruptions. The average fast-rise onset height, slow-rise duration, slow-rise velocity, maximum field-of-view (FOV) velocity, and maximum FOV acceleration are 83 Mm, 4.4 hours, 2.1 km/s, 106 km/s, and 111 m/s^2, respectively. All parameters exhibit lognormal probability distributions similar to that of CME speeds. A positive correlation between latitude and fast-rise onset height is found, which we attribute to a corresponding negative correlation in the average vertical magnetic field gradient, or decay index, estimated from potential field source surface (PFSS) extrapolations. We also find the decay index at the fast-rise onset point to be 1.1 on average, consistent with the critical instability threshold theorized for straight current channels. Finally, we explore relationships between the derived kinematics properties and apparent twisting motions. We find that events with evident twist have significantly faster CME speeds and significantly lower fast-rise onset heights, suggesting relationships between these values and flux rope helicity.
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