Quiescent Prominences Research Articles

CLUSTER 77 IN NGC 4449: THE NUCLEUS OF A SATELLITE GALAXY BEING TRANSFORMED INTO A GLOBULAR CLUSTER?

Imaging solar coronal condensation forming prominences was difficult in the past, a situation recently changed by Hinode and SDO. We present the first example observed with SDO/AIA, in which material gradually cools through multiple EUV channels in a transequatorial loop system that confines an earlier eruption. Nine hours later, this leads to eventual condensation at the dip of these loops, forming a moderate-size prominence of ~$10^{14}$ gram, to be compared to the characteristic $10^{15}$ gram mass of a CME. The prominence mass is not static but maintained by condensation at a high estimated rate of $10^{10}$ gram/sec against a comparable, sustained drainage through numerous vertical downflow threads, such that 96% of the total condensation (~$10^{15}$ gram) is drained in approximately one day. The mass condensation and drainage rates temporally correlate with the total prominence mass. The downflow velocity has a narrow Gaussian distribution with a mean of 30 km/s, while the downward acceleration distribution has an exponential drop with a mean of ~1/6 $g_{Sun}$, indicating a significant canceling of gravity, possibly by the Lorentz force. Our observations show that a macroscopic quiescent prominence is microscopically dynamic, involving the passage of a significant mass through it, maintained by a continual mass supply against a comparable mass drainage, which bears important implications for CME initiation mechanisms in which mass unloading is important.

Observations of Plasma Blob Ejection from a Quiescent Prominence by Hinode Solar Optical Telescope

Abstract We report findings from 0$''\!\!\!.$2 resolution observations of the 2007 October 03 quiescent prominence observed with the Solar Optical Telescope on the Hinode satellite. The observations show clear ejections from the top of the quiescent prominence of plasma blobs. The ejections, originating from the top of prominence threads, are impulsively accelerated to approximately Alfvén velocities and then undergo ballistic motion. The ejections have a characteristic size between $\sim$ 1000–2000 km. These characteristics are similar to downwardly propagating knots (typical size $\sim$ 700 km) that have been observed in prominence threads, we suggest that the plasma blob ejections could be the upward moving counterpart to the downwardly propagating knots. We discuss the tearing instability as a possible mechanism to explain the ejections.

2D radiative-magnetohydrostatic model of a prominence observed by Hinode, SoHO/SUMER and Meudon/MSDP

Aims. Prominences observed by Hinode show very dynamical and intriguing structures. To understand the mechanisms that are responsible for these moving structures, it is important to know the physical conditions that prevail in fine-structure threads. In the present work we analyse a quiescent prominence with fine structures, which exhibits dynamic behaviour, which was observed in the hydrogen Hα line with Hinode/SOT, Meudon/MSDP and Ondřejov/HSFA2, and simultaneously in hydrogen Lyman lines with SoHO/SUMER during a coordinated campaign. We derive the fine-structure physical parameters of this prominence and also address the questions of the role of the magnetic dips and of the interpretation of the flows.

A DROPLET MODEL OF QUIESCENT PROMINENCE DOWNFLOWS

Observations of quiescent prominences with the Solar Optical Telescope on the Hinode satellite have revealed the ubiquitous existence of downflows forming coherent thin and highly structured vertically oriented threads with velocities between 10 and 20 km s{sup -1}. Their widths range between 300 and 500 km. They are often initiated at the top of the visible prominence, but sometimes also at intermediate level. We propose that the downflows are made of plasma packets that squeeze themselves through the dominantly horizontal field under the action of gravity. Their origin is assumed to be hot plasma supplied from either inside or the immediate vicinity of the prominence and condensing at its top. Under compression and further cooling, the matter overflows to the flanks of the prominence dragging its magnetic field with it. Under the increasing action of gravity, vertical structures are forming which eventually disconnect from the field of the inflow channel thus forming finite plasma packets. This process is reminiscent of water flowing over a mountain ridge and breaking up into a multitude of droplets. Like water droplets being subject to air drag, the falling plasma droplets experience a drag force by the horizontal prominence field and assume a steady verticalmore » velocity. This happens via the excitation of Alfven waves. Lateral confinement by the prominence field determines their spatial extent. The small scales of the droplets and the directional balance of their internal tangled magnetic fields can explain the absence of appreciable vertical components in magnetic field measurements. On the basis of the observed width and vertical speed of the downflows and by adopting a prominence field of about 8 G, we derive central density and temperature of the droplets, which turn out to be quite consistent with known prominence characteristics. In the formulation of the drag force a dimensionless 'magnetic drag coefficient' has been introduced with a value well below unity.« less

Observation of Plasma Instabilities in Quiescent Prominences

We study dynamics of quiescent prominences using several data sets taken with the Solar Optical Telescope (SOT) on Hinode. We find a number of processes occurring at different stages of prominence evolution that are common for all of our chosen cases and, having universal character, can be related to fundamental plasma instabilities. We combine the observational evidence and theory to identify these instabilities. Here we discuss three examples: i) prominence cavity formation and its evolution, associated with a screw-pinch instability; ii) development of a regular series of plumes and spikes typical to the Rayleigh – Taylor (RT) instability; and iii) the appearance of growing ripples at the prominence/corona interface, often followed by a sudden collimated mass upflow, attributed to the Kelvin – Helmholtz (KH) instability. The conditions for transition from a linear (rippling mode) to nonlinear stage of the KH instability, known to have an explosive character, are specified. Given excellent Hinode data, all three aspects of prominence dynamics allow quantitative analysis.

Observations of density fluctuations in a quiescent prominence

AbstractDensity fluctuations in a torsional quiescent prominence were studied with Solar Optical Telescope on board of Hinode satellite. Continuous observations were made in Ca II H line from ~15:00 UT May 03, 2008; the observational duration was ~1 hr with a cadence time ~ 30s. The emission intensity along the prominence axis as functions of altitudes and time shows fluctuations in brightness with local peaks in Fourier power spectra, indicating the presence of periods and intervallic displacements to be 3 ~ 11 min and 2 ~ 7 Mm respectively and statistically significant peaks at 368 ± 63s & 620 ± 41 s, and 3.3 ± 0.6 Mm regime. The distance is the line of sight projection. These intensity disturbances may perhaps be caused by density fluctuations originated from compressional waves in which the possible origin could be the combinations of p-mode oscillations induced at the surface and twisting of the flux tube.

A RISING COOL COLUMN AS A SIGNATURE OF HELICAL FLUX EMERGENCE AND FORMATION OF PROMINENCE AND CORONAL CAVITY

Continuous observations were performed of a quiescent prominence with the Solar Optical Telescope (SOT) on board the /emph{Hinode} satellite on 2006 December 23--24. A peculiar slowly-rising column of $/sim10^{4}$ K plasma develops from the lower atmosphere during the observations. The apparent ascent speed of the column is 2 km s$^{-1}$, while the fine structures of the column exhibit much faster motion of up to 20 km s$^{-1}$. The column eventually becomes a faint low-lying prominence. Associated with the appearance of the column, an overlying coronal cavity seen in the X-ray and EUV moves upward at $/sim$5 km s$^{-1}$. We discuss the relationship between these episodes, and suggest that they are due to the emergence of a helical flux rope that undergoes reconnection with lower coronal fields, possibly carrying material into the coronal cavity. Under the assumption of the emerging flux scenario, the lower velocity of 2 km s$^{-1}$ and the higher one of 20 km s$^{-1}$ in the column are attributed to the rising motion of the emerging flux and to the outflow driven by magnetic reconnection between the emerging flux and the pre-existing coronal field, respectively. The present paper gives a coherent explanation of the enigmatic phenomenon of the rising column with the emergence of the helical rope, and its effect on the corona. We discuss the implications that the emergence of such a helical rope has on the dynamo process in the convection zone.

Strong rotation of an erupting quiescent polar crown prominence

On 5–6 June 2007, a large quiescent polar crown prominence was observed to erupt by the two Solar Terrestrial Relations Observatory (STEREO) spacecraft. This eruption was particularly visible in the 304 Å channel of the Extreme Ultraviolet Imager (EUVI) telescopes. A detailed analysis of the fine structures in the images allows the three-dimensional structure of the erupting prominence to be derived. The prominence is seen to undergo substantial rotation of at least 90 ∘ along the radial axis as it rises, with indications that additional rotation occurred before the prominence rose into the STEREO fields of view. Two temporary structures (“spurs”) are seen to form at an angle to the main spine of the prominence, and are interpreted as signs of reconnection. These reconnection events contribute to the overall rotation of the prominence. A significant fraction of the prominence material is drained through new field lines caused by one of the reconnection events, resulting in only a weak coronal mass ejection event observed by the STEREO and SOHO coronagraphs. The eruption is interpreted as being initiated by the helical kink instability, with subsequent modification by the reconnection events.

QUIESCENT PROMINENCE DYNAMICS OBSERVED WITH THE HINODE SOLAR OPTICAL TELESCOPE. I. TURBULENT UPFLOW PLUMES

ABSTRACT Hinode/Solar Optical Telescope (SOT) observations reveal two new dynamic modes in quiescent solar prominences: large-scale (20–50 Mm) “arches” or “bubbles” that “inflate” from below into prominences, and smaller-scale (2–6 Mm) dark turbulent upflows. These novel dynamics are related in that they are always dark in visible-light spectral bands, they rise through the bright prominence emission with approximately constant speeds, and the small-scale upflows are sometimes observed to emanate from the top of the larger bubbles. Here we present detailed kinematic measurements of the small-scale turbulent upflows seen in several prominences in the SOT database. The dark upflows typically initiate vertically from 5 to 10 Mm wide dark cavities between the bottom of the prominence and the top of the chromospheric spicule layer. Small perturbations on the order of 1 Mm or less in size grow on the upper boundaries of cavities to generate plumes up to 4–6 Mm across at their largest widths. All plumes develop highly turbulent profiles, including occasional Kelvin–Helmholtz vortex “roll-up” of the leading edge. The flows typically rise 10–15 Mm before decelerating to equilibrium. We measure the flowfield characteristics with a manual tracing method and with the Nonlinear Affine Velocity Estimator (NAVE) “optical flow” code to derive velocity, acceleration, lifetime, and height data for several representative plumes. Maximum initial speeds are in the range of 20–30 km s−1, which is supersonic for a ∼10,000 K plasma. The plumes decelerate in the final few Mm of their trajectories resulting in mean ascent speeds of 13–17 km s−1. Typical lifetimes range from 300 to 1000 s (∼5–15 minutes). The area growth rate of the plumes (observed as two-dimensional objects in the plane of the sky) is initially linear and ranges from 20,000 to 30,000 km2 s−1 reaching maximum projected areas from 2 to 15 Mm2. Maximum contrast of the dark flows relative to the bright prominence plasma in SOT images is negative and ranges from −10% for smaller flows to −50% for larger flows. Passive scalar “cork movies” derived from NAVE measurements show that prominence plasma is entrained by the upflows, helping to counter the ubiquitous downflow streams in the prominence. Plume formation shows no clear temporal periodicity. However, it is common to find “active cavities” beneath prominences that can spawn many upflows in succession before going dormant. The mean flow recurrence time in these active locations is roughly 300–500 s (5–8 minutes). Locations remain active on timescales of tens of minutes up to several hours. Using a column density ratio measurement and reasonable assumptions on plume and prominence geometries, we estimate that the mass density in the dark cavities is at most 20% of the visible prominence density, implying that a single large plume could supply up to 1% of the mass of a typical quiescent prominence. We hypothesize that the plumes are generated from a Rayleigh–Taylor instability taking place on the boundary between the buoyant cavities and the overlying prominence. Characteristics, such as plume size and frequency, may be modulated by the strength and direction of the cavity magnetic field relative to the prominence magnetic field. We conclude that buoyant plumes are a source of quiescent prominence mass as well as a mechanism by which prominence plasma is advected upward, countering constant gravitational drainage.

Velocity vectors of a quiescent prominence observed byHinode/SOT and the MSDP (Meudon)

The dynamics of prominence fine structures is a challenge to understand the formation of cool plasma prominence embedded in the hot corona. Recent observations from the high resolution Hinode/SOT telescope allow us to compute velocities perpendicularly to the line-of-sight or transverse velocities. Combining simultaneous observations obtained in H-alpha with Hinode/SOT and the MSDP spectrograph operating in the Meudon solar tower we derive the velocity vectors of a quiescent prominence. The velocities perpendicular to the line-of-sight are measured by time slice technique, the Dopplershifts by the bisector method. The Dopplershifts of bright threads derived from the MSDP reach 15 km/s at the edges of the prominence and are between +/- 5 km/s in the center of the prominence. Even though they are minimum values due to seeing effect, they are of the same order as the transverse velocities. These measurements are very important because they suggest that the verticalstructures shown in SOT may not be real vertical magnetic structures in the sky plane. The vertical structures could be a pile up of dips in more or less horizontal magnetic field lines in a 3D perspective, as it was proposed by many MHD modelers. In our analysis we also calibrate the Hinode H-alpha data using MSDP observations obtained simultaneously.

DYNAMICS OF VERTICAL THREADS AND DESCENDING KNOTS IN A HEDGEROW PROMINENCE

The existence and behavior of vertical fine structures of plasma—threads and knots—are a significant observational clue to understanding the magnetic structure and dynamics of quiescent prominences on the quiet Sun. Based on the equation of motion in ideal MHD, we reason that the non-hydrostatic support of plasma against gravity in general requires either the motion of plasma with a high value of downward acceleration (dynamical support) or the role of horizontal magnetic fields (magnetic support). By carefully tracking the motion of several bright threads seen in a hedgerow prominence observed by the Solar Optical Telescope aboard Hinode, we confirm that these threads are essentially static and stable, which negates the dynamic support. The application of the Kippenhahn-Schluter solution suggests that they may be supported by sagged magnetic field lines with a sag angle of about 43°. We also track several bright descending knots and find that their descending speeds range from 10 to 30 km s–1, with a mean value of 16 km s–1, and their vertical accelerations from –0.10 to 0.10 km s–2, with a mean of practically zero. This finding suggests that these knots are basically supported by horizontal magnetic fields against gravity even when they descend, and the complex variations of their descending speeds should be attributed to small imbalances between gravity and the force of magnetic tension. Furthermore, some knots are observed to impulsively get accelerated downward from time to time. We conjecture that these impulsive accelerations are a result of magnetic reconnection and the subsequent interchange of magnetic configuration between a knot and its surrounding structure. It is proposed that this process of reconnection and interchange not only initiates the descending motion of the knots, but also allows knots to keep falling long distance through the medium permeated by horizontal magnetic fields.

The SUMER Ly-α line profile in quiescent prominences

Aims: Out of a novel observing technique, we publish for the first time, SoHO-SUMER observations of the true spectral line profile of hydrogen Lyman-alpha in quiescent prominences. With SoHO not being in Earth orbit, our high-quality data set is free from geocoronal absorption. We study the line profile and compare it with earlier observations of the higher Lyman lines and recent model predictions. Methods: We applied the reduced-aperture observing mode to two prominence targets and started a statistical analysis of the line profiles in both data sets. In particular, we investigated the shape of the profile, the radiance distribution and the line shape-to-radiance interrelation. We also compare Ly-a data to co-temporal 1206 Si III data. Results: We find that the average profile of Ly-a has a blue-peak dominance and is more reversed, if the line-of-sight is perpendicular to the field lines. The contrast of Ly-a prominence emission rasters is very low and the radiance distribution differs from the log-normal distribution of the disk. Features seen in the Si III line are not always co-spatial with Ly-a emission. Conclusions: Our empirical results support recent multi-thread models, which predict that asymmetries and depths of the self-reversal depend on the orientation of the prominence axis relative to the line-of-sight.

The structure of solar filaments. Prominences in the corona free from external magnetic field

The new approach to the modeling of quiescent solar prominences is proposed. We solve the inverse magnetohydrostatic problem, when the pressure, density and temperature of plasma in the filament are calculated from the equilibrium equations using the given magnetic structure (magnetic flux function is proposed to be known). The new exact nonlinear solutions for dense (n ≈ (2−3) × 1011 cm−3) and cold (T ≈ (5−10) × 103 K) filaments, embedded in the plan, vertically stratified atmosphere (hot solar corona) free of magnetic field, are derived. The filaments are stretched along the horizontal axisy(the translational symmetry is assumed: ∂/∂y = 0) and located parallel to and above a photospheric, magnetic polarity reversal line. The magnetic field lines have a structure of magnetic flux rope with helical field lines in three-dimensional space; the strength of magnetic field falls rapidly with distance from a rope axis. No external longitudinal magnetic field is needed to equilibrate the prominence. The net electric current along the filament is equal to zero. The model of magnetic arcade with the deflection (sag) on the top, proposed by Pikelner (1971) as a basic form of normal prominence, is calculated also using the method proposed. It is shown that such magnetic arcade, having the magnetic field strength of few gauss only, can effectively maintain the equilibrium of cool dense filament at the heights about 50–60 Mm.

BEHAVIOR OF THE SPINES IN A QUIESCENT PROMINENCE OBSERVED BYHINODE/SOT

We report the behaviors of the spines in a quiescent prominence from the observations on 2008 January 15 made with Hinode/SOT in H alpha + 0.076 angstrom, H alpha-0.34 angstrom, and Ca II H line filters. Two spines (1 and 2) are visible in this event. In the spacetime plots of the H alpha and CaII intensities, the two spines seem to gradually move closer together, and finally merge, then separate again. Their behaviors are separated into two kinds of typical motions. On the Doppler diagrams, the spine 1 has a dominant redshift, and spine 2 favors a blueshift, which reveals that the spines 1 and 2 firstly display the drifting motions in opposite directions. The former is drifting northward, while the latter drifts southward. Second, both spines display large-scale oscillating motions. Their oscillating velocities, amplitudes, and periods have average values of 3 km s(-1), +/- 5 Mm, and 98 minutes, respectively, indicating a small-amplitude oscillation with a long period. After the sinusoidal fitting, both spines almost exhibit an antiphase oscillating motions. The spine 2 oscillates 135 degrees ahead of the spine 1. Such antiphase oscillations would reflect the coupling of the transverse oscillations of the spines in this prominence.

MULTI-LINE STOKES INVERSION FOR PROMINENCE MAGNETIC-FIELD DIAGNOSTICS

We present test results on the simultaneous inversion of the Stokes profiles of the He I lines at 587.6 nm (D_3) and 1083.0 nm in prominences (90-deg scattering). We created datasets of synthetic Stokes profiles for the case of quiescent prominences (B<200 G), assuming a conservative value of 10^-3 of the peak intensity for the polarimetric sensitivity of the simulated observations. In this work, we focus on the error analysis for the inference of the magnetic field vector, under the usual assumption that the prominence can be assimilated to a slab of finite optical thickness with uniform magnetic and thermodynamic properties. We find that the simultaneous inversion of the two lines significantly reduces the errors on the inference of the magnetic field vector, with respect to the case of single-line inversion. These results provide a solid justification for current and future instrumental efforts with multi-line capabilities for the observations of solar prominences and filaments.

LARGE-SCALE FLOWS IN PROMINENCE CAVITIES

Regions of rarefied density often form cavities above quiescent prominences. We observed two different cavities with the Coronal Multichannel Polarimeter on 2005 April 21 and with Hinode/EIS on 2008 November 8. Inside both of these cavities, we find coherent velocity structures based on spectral Doppler shifts. These flows have speeds of 5-10 km s–1, occur over length scales of tens of megameters, and persist for at least 1 hr. Flows in cavities are an example of the nonstatic nature of quiescent structures in the solar atmosphere.

Prominence Seismology Using Small Amplitude Oscillations

Quiescent prominences are thin slabs of cold, dense plasma embedded in the much hotter and rarer solar corona. Although their global shape is rather irregular, they are often characterised by an internal structure consisting of a large number of thin, parallel threads piled together. Prominences often display periodic disturbances mostly observed in the Doppler displacement of spectral lines and with an amplitude typically of the order of or smaller than 2--3 km s$^{-1}$, a value which seems to be much smaller than the characteristic speeds of the prominence plasma (namely the Alfv\'en and sound velocities). Two particular features of these small amplitude prominence oscillations is that they seem to damp in a few periods and that they seem not to affect the whole prominence structure. In addition, in high spatial resolution observations, in which threads can be discerned, small amplitude oscillations appear to be clearly associated to these fine structure constituents. Prominence seismology tries to bring together the results from these observations (e.g. periods, wavelengths, damping times) and their theoretical modeling (by means of the magnetohydrodynamic theory) to gain insight into physical properties of prominences that cannot be derived from direct observation. In this paper we discuss works that have not been described in previous reviews, namely the first seismological application to solar prominences and theoretical advances on the attenuation of prominence oscillations.

Small-scale oscillations in a quiescent prominence observed by HINODE/SOT

<i>Context. <i/>Investigations of the behavior of small-scale threads can provide an alternative approach to studying prominence dynamics and understanding its origin and nature.<i>Aims. <i/>The behavior of threads are analyzed in a quiescent prominence, including drifting and both the horizontally and vertically oscillating motions. These indicate waves in the solar prominence.<i>Methods. <i/>We used the H<i>α<i/> images at a setting wavelength of +0.076 Å. A quiescent prominence was observed by HINODE/SOT on 2008 January 15 for about 3 h in total.<i>Results. <i/>Consistent with previous findings, prominences show numerous thread-like structures. Some threads clearly exhibit both vertically and horizontally oscillatory motions, while others are only drifting. Complicated cases show both drifting and oscillatory motions simultaneously. In the upper part of the prominence, the threads are oscillating independently of each other. We find that three threads oscillate with the same phase for at least two periods. The oscillations seem to be strongly damped since they disappear after a few periods. The maximum number of observed periods is 8 in our observations. In the lower part of the prominence, however, the different threads have a mixed character with the individual oscillatory motions unstable for one entire period. Most oscillatory motions will disappear after a half period or less, while the new oscillatory motions are excited nearby. A 5-min period is predominant, and the oscillating amplitudes show an average value of <i>±<i/>3.5 km s<sup>-1<sup/>. We find some upflows in the spicule layer, and they appear to transport the mass from photosphere (or spicules themselves) to the prominence. These upflows have an average velocity amplitude of 0.8 km s<sup>-1<sup/>.<i>Conclusions. <i/>The threads exhibit three distinct behaviors. The first is only drifting, the second is typically oscillating, and the third shows both characteristics. There are no substantial differences between the periods of horizontally and vertically oscillating threads in this prominence.

A NARROW STREAMER-PUFF CORONAL MASS EJECTION FROM THE NONRADIAL ERUPTION OF AN ACTIVE-REGION FILAMENT

So-called streamer-puff coronal mass ejections (CMEs) were recently identified by Bemporad et al. as a new variety of CME. They originate from nonradial ejecta from compact ejective flares occurring in the outskirts of the base of a streamer and travel out along it, during which the streamer is transiently inflated by the puff but is not disrupted. Moore & Sterling further indicated that streamer-puff CMEs are a particular subclass of a broad class of CMEs, called "over-and-out" CMEs, which come from flare-producing magnetic explosions of various sizes and are laterally far offset from the flare. In this paper, we present observations of a streamer-puff CME that resulted from the spectacular eruption of an active-region filament inside a single helmet streamer consisting of a double-arcade system, occurring in AR 10792 on 2005 July 29. The filament was covered by a smaller arcade in the outer flank of the streamer base, with a nearby quiescent prominence below another larger arcade. In the inner corona the filament eruption underwent nonradial motion, while in the outer corona the CME had a radial path along the streamer that was laterally offset from the original site of the filament. The CME was narrow with an angular width of only 15° and clearly showed a typical three-part structure below 1.38 R☉. In this eruption, the prominence remained nearly unchanged, but in other stronger eruptions of a homologous sequence from the same AR in the following days, it exhibited obvious oscillations. These observations can be explained by the magnetic-arch-blowout scenario for streamer-puff and over-and-out CMEs, in which the streamer arcade can act on the erupting filament, laterally deflecting and channeling its motion. The prominence oscillations probably indicate an interplay or interaction between the twin arcades that could affect but not destroy the overall field configuration supporting the prominence.

Global prominence oscillations

Aims. The question of the different oscillation periods for global modes of quiescent prominences is discussed. Methods. Simple 1D prominence configurations are used to describe the magnetohydrostatic equilibrium and their oscillations for small amplitudes. Results. Three basic modes of oscillations were found and their periods as a function of the magnetic field configuration and the assumed geometry are given.