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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

FORMATION AND PLASMA CIRCULATION OF SOLAR PROMINENCES

Solar prominences are long-lived cool and dense plasma curtains in the hot and rarefied outer solar atmosphere or corona. The physical mechanism responsible for their formation and especially for their internal plasma circulation has been uncertain for decades. The observed ubiquitous downflows in quiescent prominences are difficult to interpret because plasma with high conductivity seems to move across horizontal magnetic field lines. Here we present three-dimensional numerical simulations of prominence formation and evolution in an elongated magnetic flux rope as a result of in situ plasma condensations fueled by continuous plasma evaporation from the solar chromosphere. The prominence is born and maintained in a fragmented, highly dynamic state with continuous reappearance of multiple blobs and thread structures that move mainly downward, dragging along mass-loaded field lines. The circulation of prominence plasma is characterized by the dynamic balance between the drainage of prominence plasma back to the chromosphere and the formation of prominence plasma via continuous condensation. Plasma evaporates from the chromosphere, condenses into the prominence in the corona, and drains back to the chromosphere, establishing a stable chromosphere–corona plasma cycle. Synthetic images of the modeled prominence with the Solar Dynamics Observatory Atmospheric Imaging Assembly closely resemble actual observations, with many dynamical threads underlying an elliptical coronal cavity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

SPECTRO-POLARIMETRIC IMAGING REVEALS HELICAL MAGNETIC FIELDS IN SOLAR PROMINENCE FEET

Solar prominences are clouds of cool plasma levitating above the solar surface and insulated from the million-degree corona by magnetic fields. They form in regions of complex magnetic topology, characterized by non-potential fields, which can evolve abruptly, disintegrating the prominence and ejecting magnetized material into the heliosphere. However, their physics is not yet fully understood because mapping such complex magnetic configurations and their evolution is extremely challenging, and must often be guessed by proxy from photometric observations. Using state-of-the-art spectro-polarimetric data, we reconstruct the structure of the magnetic field in a prominence. We find that prominence feet harbor helical magnetic fields connecting the prominence to the solar surface below.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1968

Formation of solar prominences.

Thermal instability mechanism for quiescent solar prominence formation by condensation from solar corona, noting condensation inhibition by free electrons heat conduction

Raju, P. K.↗
NASA NTRS1986

On the thermal durability of solar prominences, or how to evaporate a prominence

The thermal disappearance of solar prominences under strong perturbations due to wave heating, Ohmic heating, viscous heating or conduction was investigated. Specifically, how large a thermal perturbation is needed to destroy a stable thermal equilibrium was calculated. It was found that the prominence plasma appears to be thermally very rugged. Its cold equilibrium may most likely be destroyed by either strong magnetic heating or conduction in a range of parameters which is relevant to flares.

Malherbe, J. M.↗
DOE OSTI2016

INTERNAL DYNAMICS OF A TWIN-LAYER SOLAR PROMINENCE

Modern observations revealed rich dynamics within solar prominences. The globally stable quiescent prominences, characterized by the presence of thin vertical threads and falling knobs, are frequently invaded by small rising dark plumes. These dynamic phenomena are related to magnetic Rayleigh–Taylor instability, since prominence matter, 100 times denser than surrounding coronal plasma, is lifted against gravity by weak magnetic field. To get a deeper understanding of the physics behind these phenomena, we use three-dimensional magnetohydrodynamic simulations to investigate the nonlinear magnetoconvective motions in a twin-layer prominence in a macroscopic model from chromospheric layers up to 30 Mm height. The properties of simulated falling “fingers” and uprising bubbles are consistent with those in observed vertical threads and rising plumes in quiescent prominences. Both sheets of the twin-layer prominence show a strongly coherent evolution due to their magnetic connectivity, and demonstrate collective kink deformation. Our model suggests that the vertical threads of the prominence as seen in an edge-on view, and the apparent horizontal threads of the filament when seen top-down are different appearances of the same structures. Synthetic images of the modeled twin-layer prominence reflect the strong degree of mixing established over the entire prominence structure, in agreement with the observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

SOLAR PROMINENCES EMBEDDED IN FLUX ROPES: MORPHOLOGICAL FEATURES AND DYNAMICS FROM 3D MHD SIMULATIONS

The temporal evolution of a solar prominence inserted in a three-dimensional magnetic flux rope is investigated numerically. Using the model of Titov and Démoulin under the regime of weak twist, the cold and dense prominence counteracts gravity by modifying the initially force-free magnetic configuration. In some cases a quasi-stationary situation is achieved after the relaxation phase, characterized by the excitation of standing vertical oscillations. These oscillations show a strong attenuation with time produced by the mechanism of continuum damping due to the inhomogeneous transition between the prominence and solar corona. The characteristic period of the vertical oscillations does not depend strongly on the twist of the flux rope. Nonlinearity is responsible for triggering the Kelvin–Helmholtz instability associated with the vertical oscillations and that eventually produces horizontal structures. Contrary to other configurations in which the longitudinal axis of the prominence is permeated by a perpendicular magnetic field, like in unsheared arcades, the orientation of the prominence along the flux rope axis prevents the development of Rayleigh–Taylor instabilities and therefore the appearance of vertical structuring along this axis.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1986

Measurements of magnetic fields in solar prominences

Magnetic fields can be measured, in solar prominences, by means of two different basic mechanisms that are responsible for the introduction (or the reduction) of a given amount of polarization in spectral lines: these are the Zeeman effect and the Hanle effect. Through the splitting of the magnetic components of a spectral line, the Zeeman effect is capable of introducing a certain amount of circular polarization across the line profile. The Hanle effect consist of a modification of the linear polarization that is induced in spectral lines by the anisotropic illumination of the prominence plasma by the photospheric radiation field. These two effects are briefly discussed.

Deglinnocenti, Egidio Landi↗
DOE OSTI2013

PROPAGATING WAVES TRANSVERSE TO THE MAGNETIC FIELD IN A SOLAR PROMINENCE

We report an unusual set of observations of waves in a large prominence pillar that consist of pulses propagating perpendicular to the prominence magnetic field. We observe a huge quiescent prominence with the Solar Dynamics Observatory Atmospheric Imaging Assembly in EUV on 2012 October 10 and only a part of it, the pillar, which is a foot or barb of the prominence, with the Hinode Solar Optical Telescope (SOT; in Ca II and Hα lines), Sac Peak (in Hα, Hβ, and Na-D lines), and THEMIS ({sup T}élescope Héliographique pour l' Etude du Magnétisme et des Instabilités Solaires{sup )} with the MTR (MulTi-Raies) spectropolarimeter (in He D{sub 3} line). The THEMIS/MTR data indicates that the magnetic field in the pillar is essentially horizontal and the observations in the optical domain show a large number of horizontally aligned features on a much smaller scale than the pillar as a whole. The data are consistent with a model of cool prominence plasma trapped in the dips of horizontal field lines. The SOT and Sac Peak data over the four hour observing period show vertical oscillations appearing as wave pulses. These pulses, which include a Doppler signature, move vertically, perpendicular to the field direction, along thin quasi-vertical columns in the much broader pillar. The pulses have a velocity of propagation of about 10 km s{sup –1}, a period of about 300 s, and a wavelength around 2000 km. We interpret these waves in terms of fast magnetosonic waves and discuss possible wave drivers.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

FINE MAGNETIC STRUCTURE AND ORIGIN OF COUNTER-STREAMING MASS FLOWS IN A QUIESCENT SOLAR PROMINENCE

We present high-resolution observations of a quiescent solar prominence that consists of a vertical and a horizontal foot encircled by an overlying spine and has ubiquitous counter-streaming mass flows. While the horizontal foot and the spine were connected to the solar surface, the vertical foot was suspended above the solar surface and was supported by a semicircular bubble structure. The bubble first collapsed, then reformed at a similar height, and finally started to oscillate for a long time. We find that the collapse and oscillation of the bubble boundary were tightly associated with a flare-like feature located at the bottom of the bubble. Based on the observational results, we propose that the prominence should be composed of an overlying horizontal spine encircling a low-lying horizontal and vertical foot, in which the horizontal foot consists of shorter field lines running partially along the spine and has ends connected to the solar surface, while the vertical foot consists of piling-up dips due to the sagging of the spine fields and is supported by a bipolar magnetic system formed by parasitic polarities (i.e., the bubble). The upflows in the vertical foot were possibly caused by the magnetic reconnection at the separator between the bubble and the overlying dips, which intruded into the persistent downflow field and formed the picture of counter-streaming mass flows. In addition, the counter-streaming flows in the horizontal foot were possibly caused by the imbalanced pressure at the both ends.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

SIMULATING THE IN SITU CONDENSATION PROCESS OF SOLAR PROMINENCES

Prominences in the solar corona are a hundredfold cooler and denser than their surroundings, with a total mass of 10{sup 13} up to 10{sup 15} g. Here, we report on the first comprehensive simulations of three-dimensional, thermally and gravitationally stratified magnetic flux ropes where in situ condensation to a prominence occurs due to radiative losses. After a gradual thermodynamic adjustment, we witness a phase where runaway cooling occurs while counter-streaming shearing flows drain off mass along helical field lines. After this drainage, a prominence-like condensation resides in concave upward field regions, and this prominence retains its overall characteristics for more than two hours. While condensing, the prominence establishes a prominence-corona transition region where magnetic field-aligned thermal conduction is operative during the runaway cooling. The prominence structure represents a force-balanced state in a helical flux rope. The simulated condensation demonstrates a right-bearing barb, as a remnant of the drainage. Synthetic images at extreme ultraviolet wavelengths follow the onset of the condensation, and confirm the appearance of horns and a three-part structure for the stable prominence state, as often seen in erupting prominences. This naturally explains recent Solar Dynamics Observatory views with the Atmospheric Imaging Assembly on prominences in coronal cavities demonstrating horns.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

EVIDENCE FOR ROTATIONAL MOTIONS IN THE FEET OF A QUIESCENT SOLAR PROMINENCE

We present observational evidence of apparent plasma rotational motions in the feet of a solar prominence. Our study is based on spectroscopic observations taken in the He I 1083.0 nm multiplet with the Tenerife Infrared Polarimeter attached to the German Vacuum Tower Telescope. We recorded a time sequence of spectra with 34 s cadence placing the slit of the spectrograph almost parallel to the solar limb and crossing two feet of an intermediate size, quiescent hedgerow prominence. The data show opposite Doppler shifts, {+-}6 km s{sup -1}, at the edges of the prominence feet. We argue that these shifts may be interpreted as prominence plasma rotating counterclockwise around the vertical axis to the solar surface as viewed from above. The evolution of the prominence seen in EUV images taken with the Solar Dynamics Observatory provided us with clues to interpret the results as swirling motions. Moreover, time-distance images taken far from the central wavelength show plasma structures moving parallel to the solar limb with velocities of about 10-15 km s{sup -1}. Finally, the shapes of the observed intensity profiles suggest the presence of, at least, two components at some locations at the edges of the prominence feet. One of them is typically Doppler shifted (up to {approx}20 km s{sup -1}) with respect to the other, thus suggesting the existence of supersonic counter-streaming flows along the line of sight.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1989

Formation and eruption of solar prominences

A model for the magnetic field associated with solar prominences is considered. It is shown that flux cancellation at the neutral line of a sheared magnetic arcade leads to the formation of helical field lines which are capable, in principle, of supporting prominence plasma. A numerical method for the computation of force-free, canceling magnetic structures is presented. Starting from an initial potential field we prescribe the motions of magnetic footpoints at the photosphere, with reconnection occurring only at the neutral line. As more and more flux cancels, magnetic flux is transferred from the arcade field to the helical field. Results for a particular model of the photospheric motions are presented. The magnetic structure is found to be stable: the arcade field keeps the helical field tied down at the photosphere. The axis of the helical field moves to larger and larger height, suggestive of prominence eruption. These results suggest that prominence eruptions may be trigered by flux cancellation.

Van Ballegooijen, A. A.↗
NASA NTRS2016

The Effects of Magnetic-Field Geometry on Longitudinal Oscillations of Solar Prominences: Cross-Sectional Area Variation for Thin Tubes

Solar prominences are subject to both field-aligned (longitudinal) and transverse oscillatory motions, as evidenced by an increasing number of observations. Large-amplitude longitudinal motions provide valuable information on the geometry of the filament channel magnetic structure that supports the cool prominence plasma against gravity. Our pendulum model, in which the restoring force is the gravity projected along the dipped field lines of the magnetic structure, best explains these oscillations. However, several factors can influence the longitudinal oscillations, potentially invalidating the pendulum model. Aims. The aim of this work is to study the influence of large-scale variations in the magnetic field strength along the field lines, i.e., variations of the cross-sectional area along the flux tubes supporting prominence threads. Methods. We studied the normal modes of several flux tube configurations, using linear perturbation analysis, to assess the influence of different geometrical parameters on the oscillation properties. Results. We found that the influence of the symmetric and asymmetric expansion factors on longitudinal oscillations is small.Conclusions. We conclude that the longitudinal oscillations are not significantly influenced by variations of the cross-section of the flux tubes, validating the pendulum model in this context.

Luna, M.↗
DOE OSTI2015

ON THE SUPPORT OF NEUTRALS AGAINST GRAVITY IN SOLAR PROMINENCES

Cool and dense prominences found in the solar atmosphere are known to be partially ionized because of their relatively low temperature. In this Letter, we address the long-standing problem of how the neutral component of the plasma in prominences is supported against gravity. Using the multiple-fluid approach, we solve the time-dependent equations in two dimensions considering the frictional coupling between the neutral and ionized components of the magnetized plasma representative of a solar prominence embedded in a hot coronal environment. We demonstrate that given an initial density enhancement in the two fluids, representing the body of the prominence, the system is able to relax in the vicinity of magnetic dips to a stationary state in which both neutrals and ionized species are dynamically suspended above the photosphere. Two different coupling processes are considered in this study: collisions between ions and neutrals and charge exchange interactions. We find that for realistic conditions, ions are essentially static, while neutrals have a very small downflow velocity. The coupling between ions and neutrals is so strong at the prominence body that the behavior is similar to that of a single fluid with an effective density equal to the sum of the ion and neutral species. We also find that the charge exchange mechanism is about three times more efficient at sustaining neutrals than elastic scattering of ions with neutrals.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

RESONANT ABSORPTION OF TRANSVERSE OSCILLATIONS AND ASSOCIATED HEATING IN A SOLAR PROMINENCE. I. OBSERVATIONAL ASPECTS

Transverse magnetohydrodynamic waves have been shown to be ubiquitous in the solar atmosphere and can, in principle, carry sufficient energy to generate and maintain the Sun’s million-degree outer atmosphere or corona. However, direct evidence of the dissipation process of these waves and subsequent heating has not yet been directly observed. Here we report on high spatial, temporal, and spectral resolution observations of a solar prominence that show a compelling signature of so-called resonant absorption, a long hypothesized mechanism to efficiently convert and dissipate transverse wave energy into heat. Aside from coherence in the transverse direction, our observations show telltale phase differences around 180° between transverse motions in the plane-of-sky and line-of-sight velocities of the oscillating fine structures or threads, and also suggest significant heating from chromospheric to higher temperatures. Comparison with advanced numerical simulations support a scenario in which transverse oscillations trigger a Kelvin–Helmholtz instability (KHI) at the boundaries of oscillating threads via resonant absorption. This instability leads to numerous thin current sheets in which wave energy is dissipated and plasma is heated. Our results provide direct evidence for wave-related heating in action, one of the candidate coronal heating mechanisms.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

DYNAMICS OF A SOLAR PROMINENCE TORNADO OBSERVED BY SDO/AIA ON 2012 NOVEMBER 7–8

We study the detailed dynamics of a solar prominence tornado using time series of 171, 304, 193, and 211 Å spectral lines obtained by the Solar Dynamics Observatory/Atmospheric Imaging Assembly during 2012 November 7–8. The tornado first appeared at 08:00 UT, November 07, near the surface, gradually rose upwards with the mean speed of ∼1.5 km s{sup −1} and persisted over 30 hr. Time–distance plots show two patterns of quasi-periodic transverse displacements of the tornado axis with periods of 40 and 50 minutes at different phases of the tornado evolution. The first pattern occurred during the rising phase and can be explained by the upward motion of the twisted tornado. The second pattern occurred during the later stage of evolution when the tornado already stopped rising and could be caused either by MHD kink waves in the tornado or by the rotation of two tornado threads around a common axis. The later hypothesis is supported by the fact that the tornado sometimes showed a double structure during the quasi-periodic phase. 211 and 193 Å spectral lines show a coronal cavity above the prominence/tornado, which started expansion at ∼13:00 UT and continuously rose above the solar limb. The tornado finally became unstable and erupted together with the corresponding prominence as coronal mass ejection (CME) at 15:00 UT, November 08. The final stage of the evolution of the cavity and the tornado-related prominence resembles the magnetic breakout model. On the other hand, the kink instability may destabilize the twisted tornado, and consequently prominence tornadoes can be used as precursors for CMEs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

MORPHOLOGY AND DYNAMICS OF SOLAR PROMINENCES FROM 3D MHD SIMULATIONS

In this paper we present a numerical study of the time evolution of solar prominences embedded in sheared magnetic arcades. The prominence is represented by a density enhancement in a background-stratified atmosphere and is connected to the photosphere through the magnetic field. By solving the ideal magnetohydrodynamic equations in three dimensions, we study the dynamics for a range of parameters representative of real prominences. Depending on the parameters considered, we find prominences that are suspended above the photosphere, i.e., detached prominences, but also configurations resembling curtain or hedgerow prominences whose material continuously connects to the photosphere. The plasma-β is an important parameter that determines the shape of the structure. In many cases magnetic Rayleigh-Taylor instabilities and oscillatory phenomena develop. Fingers and plumes are generated, affecting the whole prominence body and producing vertical structures in an essentially horizontal magnetic field. However, magnetic shear is able to reduce or even to suppress this instability.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1992

Formation of solar prominences by photospheric shearing motions

The formation of the normal polarity solar prominences in a magnetic arcade by photospheric shearing motions was studied using a two-and-a-half-dimensional MHD code, which includes the gravity, radiative cooling, simplified coronal heating, and thermal conduction along the field lines. It was found that a footpoint shear induces an expansion of the magnetic arcade and cooling of the plasma in it; simultaneously, the denser material from the lower part of the arcade is pulled up by the expanding field lines. Thus, a local enhancement of radiative cooling is effected, leading to the onset of thermal instability and the condensation of coronal plasma. The condensed material grows vertically to form a sheetlike structure leading eventually to the formation of prominence.

Choe, G. S.↗