The Astrophysical Journal, 721:74–79, 2010 September 20 C 2010.
doi:10.1088/0004-637X/721/1/74
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
PATTERNS OF FLOWS IN AN INTERMEDIATE PROMINENCE OBSERVED BY HINODE Kwangsu Ahn1,2 , Jongchul Chae1,2 , Wenda Cao2 , and Philip R. Goode2 1
Department of Physics and Astronomy, Seoul National University, Seoul, 151-747, Republic of Korea;
[email protected] 2 Big Bear Solar Observatory, Big Bear City, CA 92314, USA Received 2009 November 3; accepted 2010 July 20; published 2010 August 26
ABSTRACT The investigation of plasma flows in filaments/prominences gives us clues to understanding their magnetic structures. We studied the patterns of flows in an intermediate prominence observed by Hinode/SOT. By examining a time series of Hα images and Ca ii H images, we have found horizontal flows in the spine and vertical flows in the barb. Both of these flows have a characteristic speed of 10–20 km s−1 . The horizontal flows displayed counterstreaming. Our detailed investigation revealed that most of the moving fragments in fact reversed direction at the end point of the spine near a footpoint close to the associated active region. These returning flows may be one possible explanation of the well-known counterstreaming flows in prominences. In contrast, we have found vertical flows—downward and upward—in the barb. Most of the horizontal flows in the spine seem to switch into vertical flows when they approach the barb, and vice versa. We propose that the net force resulting from a small deviation from magnetohydrostatic equilibrium, where magnetic fields are predominantly horizontal, may drive these patterns of flow. In the prominence studied here, the supposed magnetohydrostatic configuration is characterized by magnetic field lines sagging with angles of 13◦ and 39◦ in the spine and the barb, respectively. Key words: magnetic fields – Sun: filaments, prominences Online-only material: color figure, animations
that claimed the dominance of vertical magnetic structures were based on the vertical thread structures seen in quiescent filaments/prominences (Malville 1976; Engvold 1998). In contrast, those who performed polarimetric observations of some prominences reported dominance of horizontal magnetic structures (Leroy 1978; Casini et al. 2003) even though there is also a study that reported opposing results (Merenda et al. 2006). Is there any correlation between the direction of the magnetic fields and the elongated direction of plasma structures or the direction of plasma flows? Some investigators (Zirker et al. 1998; Lin et al. 2003) thought that the magnetic fields are along the threads and the flows. If this is the case, magnetic fields should be predominantly horizontal in active region prominences, and vertical in the quiet Sun prominences like hedgerow prominences. The same argument was used by Martin (1998) to propose that barbs represent elongated plasma structures that extend downward from the main body of the corresponding prominence with the field aligned with them. Note that field-aligned flows are passive to magnetic fields in that they cannot change the fields. In principle, however, flows in the direction perpendicular to the fields can occur too. If flows are strong enough, then they can change magnetic fields unlike field-aligned flows. Low & Petrie (2005) provided a theory for such plasma flows that can change magnetic field line structures. According to this theory, it is not always required that elongated plasma structures be directed along the direction of the magnetic fields. When one attempts to infer the direction of the magnetic field from the pattern of plasma flows or fine threads seen in a prominence, one should carefully examine whether the plasma flows or the threads are really field aligned as is often believed. In this paper, we report the patterns of flows we found in a prominence of intermediate type observed by Hinode/SOT (Kosugi et al. 2007; Tsuneta et al. 2008) that may shed light on the inference of magnetic structures in prominences. This kind of prominence is useful in that it represents the intermediate regime between active region prominences and the quiet Sun
1. INTRODUCTION Filaments/prominences are relatively cool plasma structures that extend from the chromosphere to the corona. They generally consist of fine thread-like components. The structures of the threads seem to depend on the magnetic environment of the filaments/prominences. In the quiet Sun prominences, threads usually appear vertically aligned, often forming hedgerows (e.g., Chae 2010). In active region prominences, on the other hand, horizontal thread structures dominate (Okamoto et al. 2007; Lin 2004). Intermediate prominences, which generally lie between active regions and unipolar quiescent regions (Engvold 1998; Mackay et al. 1998), could show both horizontal and vertical thread structures. Each filament/prominence is known to consist of one spine and several barbs (Martin 1998). The spine is a long and horizontal structure located in the higher part of the filament, and the barbs are short extrusions from the spine. Barbs are most clearly discernible in filaments of intermediate type while they are hardly visible in active region filaments, and look complicated in the quiet Sun filaments. When seen off the limb, the barbs appear to connect the spine with the underlying chromosphere, showing vertical structures. It has been well known that a variety of flows occur in prominences. The flows reported so far were either vertical (Berger et al. 2008) or horizontal (Chae et al. 2008; Jing et al. 2006), steady (Zirker et al. 1998; Berger et al. 2008; Chae et al. 2008) or oscillatory (Molowny-Horas et al. 1997; Schrijver et al. 1999; Lin et al. 2003; Jing et al. 2006; Chen et al. 2008), and unidirectional (Berger et al. 2008; Chae et al. 2008) or bi-directional counterstreaming (Zirker et al. 1998; Lin et al. 2003). Magnetic configurations of prominences are still poorly known even though they are crucial for understanding the formation, plasma structure, and evolution of prominences. In particular, a controversy exists about which is the dominant direction of a magnetic field: vertical or horizontal. The studies 74
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prominences. Specifically, it displays both spines and barbs clearly so it provides a nice opportunity to study flows in spines and barbs at the same time. As a matter of fact, we found horizontal flows in the spine of the prominence and vertical flows in its barb. It is particularly interesting that the horizontal flows in the spine appear to be not only counterstreaming, but also returning at some points.
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2. DATA AND ANALYSIS We have analyzed data obtained from Hinode/SOT in Hα and in Ca ii H on 2008 January 16. The Hα data set consists of images that were taken from two different wavelength offsets from line center, +76 m Å and −340 m Å, each having a bandwidth of 90 m Å. The bandwidth of the Ca ii filter was around 3 Å. Both data sets had a time cadence of 30 s. Hα images have pixels of 0. 16 and Ca ii H images of 0. 108. Both data sets had a field of view of about 112 × 112 . The time interval between the two wavelength offsets in Hα was 8 s. We also referred to synoptic full-disk Hα images taken on the 12th from the ParisMeudon Observatory and a Hα movie taken on the 10th from Yunnan National Solar Observatory, and He ii 304 Å images from STEREO/EUVI. The Hα images were aligned with the limb profiles as the reference. To reduce the residual errors, we smoothed the displacement values over each run of five images. In addition, we rotated all the images so that the limb appears to be horizontal in the display. A similar process was done for Ca ii H images. We produced pseudo-Hα Dopplergrams by subtracting the red wing images from the blue wing images. In these Dopplergrams, positive values correspond to blueshifts, and negative values to redshifts. The displacements of the moving plasma fragments between two successive images were determined by applying an optical flow technique called the Non-linear Affine Velocity Estimator (NAVE; Chae & Sakurai (2008)) to Hα images. This method tracks a feature by assuming that the local velocity field is affine to the position (x, y), ux = U0 + Ux (x − x0 ) + Uy (y − y0 ),
(1)
uy = V0 + Vx (x − x0 ) + Vy (y − y0 ),
(2)
which is specified by six free parameters: U0 , Ux , Uy , V0 , Vx , and Vy . This velocity profile is a generalization of the constant velocity field conventionally used in the local correlation tracking (LCT). This affine velocity field allows the shape of a feature to change, so that NAVE is superior to LCT in tracking shape-changing features. Due to the complexity of dynamical patterns such as counterstreaming, we performed careful analyses using NAVE. We picked several fragments on the spine that were moving away from us and hence were prominent in the red wing images, and tracked their motion using red wing images. When they arrived near the edge of the spine, the fragments became faint in the red wing images, and instead, they became prominent in blue wing images on the same spot. As we shall see, this change occurred as the fragments turned around. As soon as the intensity of the blue wing images became brighter than that of the red wing images, we switched to the blue wing images to continue to track the motion of the fragments. We have chosen a window for the NAVE technique as large as 50 pixels. If a window that is too small is used, the technique sometimes fails to track a moving feature since it often splits into two or more, or its intensity decreases, for example, by
Figure 1. Snapshot of the prominence taken through the Ca ii H filter on board Hinode. The horizontal structure in the middle is a spine, and the vertical pillarlike structure is a barb. A white bar in the field of view represents the projected horizontal direction of the prominence. (An animation of this figure is available in the online journal.)
expansion. In addition, we found that the red wing images showed not only bright fragments moving in the dominant direction, but also some faint fragments moving in the opposite direction, which made it difficult to track the prominent flows if a small window is used. As far as well-isolated and bright features are concerned, we found that the smaller window size yielded the same results as the window of 50 pixels. This means that our use of the large window does not cause a problem of accuracy in our study. As a matter of fact, we have independently tracked some prominent features by eye and then determined their average speeds by simply dividing the travel distance by the duration; these speeds were close to the average speed determined using the NAVE method. We performed eye inspection and the time-slice technique to determine the velocity component in the barb. The barb in Hα images was too thick for NAVE to identify fragments, and as in Ca ii H images, the segregation of uni-directional flows from a mixture of upflows and downflows was difficult. The timeslice technique makes use of a time sequence of narrow stripes of images (slit images) put together. Such a stack of images is useful to determine the component of velocity along the slit direction. 3. RESULTS 3.1. General Characteristics Figure 1 shows a Ca ii H image of the prominence we studied, and the movie accompanying Figure 1 in the online version of the journal is a set of 1050 successive images taken in Ca ii H with a duration of about 9 hrs. We see from the movie that intensity fragments ceaselessly move. We regard the optically thin, horizontal structures as a spine, and the thick, pillar-like structure as a barb. This prominence may be categorized as an intermediate prominence. The synoptic full-disk Hα images taken a couple of days earlier show that it was located near an active region, AR10980. The active region is located on the left side of the figure. This prominence had a linear shape, extending from the active region to the southeast. Note that the horizontal direction indicated as the line in Figure 1 is oblique to the limb because of the projection on the plane of sky. The He ii 304 Å images taken by STEREO/EUVI revealed that several surges occurred in this active region. They perturbed the prominence so that they could generate oscillatory horizontal motions in the prominence, which lasted for a few oscillatory cycles. A similar kind of oscillatory motion associated with surges were reported by Chen et al. (2008). The prominence eventually erupted on the 18th.
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Figure 3. Right: a Ca ii snapshot image showing the returning points of fragments (cross symbols). Left: the time-slice Ca ii images of the narrow rectangle shown in the right panel. These clearly show the returning motion of the fragment at a cadence of about 30 s.
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Figure 2. Top: Hα red wing image and velocity vectors (red) determined from a pair of such images. Middle: Hα blue wing image and velocity vectors (blue) determined from a pair of such images. Bottom: psuedo-Dopplergram in Hα and all the velocity vectors (red or blue) determined from either red wing images or blue wing images. The field of view is 96 × 49 . (A color version and an animation of this figure are available in the online journal.)
Below, we describe in detail two patterns of flows: the pattern of bi-directional, horizontal counterstreaming flows in the spine and the pattern of upflows and downflows flows in the barb. 3.2. Returning Horizontal Flows In the spine, a pattern of bi-directional, horizontal counterstreaming motions is dominant. A careful examination of the left edge of the spine revealed that most of the leftward-moving fragments shown in this figure reverse their directions to move rightward, except some fragments that flew toward the active region. This behavior results in the mixed existence of both leftward and rightward moving fragments. Thus, it appears that the pattern of horizontal counterstreaming motions in this case is due to the return of moving plasma fragments. Figure 2 shows velocity vectors obtained using NAVE from two successive images in each wavelength offset—the red wing and the blue wing. The velocity vectors were determined on the local intensity maxima, which correspond to the centers of identifiable plasma fragments. The plane of sky speeds of prominent moving fragments is typically about 10 km s−1 , often reaching up to 15 km s−1 . It is obvious that most velocity
vectors determined from the red wing images as represented by red arrows are directed leftward, showing the leftwardmoving pattern, while most velocity vectors from the blue wing images (blue arrows) are directed rightward. This mixed pattern is a clear signature of counterstreaming. It appears that there is a rough balance in number between leftward-moving fragments and rightward-moving ones: the number of leftdirected fragments in the velocity map is 60, and the number of rightward-moving fragments is 68. The leftward-moving motion extends to the left edge of the spine and the rightward-moving velocity components begin to show up, implying the existence of return flows. This characteristic of the returning flows is obvious in the movie associated with Figure 2. This movie shows the time sequence of Hα Dopplergrams with a duration of about 3 hrs. Note that each leftward-moving plasma fragment seen in black (redshift) suddenly disappears at the turning point, and then shows up as a rightward-moving fragment seen in white (blueshift), confirming the returning nature of the motion. The time-slice images of a moving fragment in Figure 3 clearly show that such a return flow is real. We find from these that it took several minutes for a fragment to complete the turnaround. The turning points are located near the edge of the spine, which is close to the associated active region. Note, however, that their locations are not well ordered; they vary both in height and in time, and it seems that each plasma fragment has its own turning point. This suggests that moving fragments behave more or less independently of others. We present two trajectories determined using NAVE in Figure 4. Note that even though these fragments were shown to flow from the barb, we started the tracking of the fragments from the spine, not from the barb because of difficulty in tracking due to the heavy overlapping of fragments in the barb. These trajectories clearly illustrate the returning flows. Moreover, they indicate that the height of fragments changed while they turned around. We see that the fragment that was initially located at low altitudes became even lower when it returned, while the high-lying fragment became even higher when it returned. This characteristic generally holds for other fragments as well. Figure 5 presents the time variation of the velocity of the lower fragment during the returning motion. The velocity changed with a constant acceleration of 0.011 km s−2 , and the acceleration remained almost constant for about 40 minutes. 3.3. Vertical Flows Near the barb, we saw patterns of upflows and downflows. Figure 6 shows the details of the upflow pattern in two examples
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Figure 4. Snapshot of pseudo-Dopplergram in Hα and two selected trajectories of plasma fragments. Bright fragments correspond to blueshifts, and dark fragments to redshifts.
20 Vy Vz
velocity(km/s)
10
0
-10 -20 0
20
40
60
time(min) Figure 5. Velocity of a plasma fragment motion projected on the image plane near the returning point. Acceleration occurred for about 40 minutes. Solid line represents horizontal motion (v y ), dotted line shows vertical motion (vz ).
of fragments. The first fragment shown in the top panel was located on the barb. It moved upward with a constant speed of 13 km s−1 . The second one shown in the bottom panel was not on the barb, but close to it. It accelerated up to a speed of 16 km s−1 , and then decelerated to have zero vertical speed. It eventually began to move horizontally and then merged into the spine. The continuous transitions of horizontal movement to vertical movement or vice versa took place midway between the spine and the barb as identified from the Ca ii movie. Most of the plasma fragments that moved toward the barb from the spine changed their direction to downward when they merged into the barb. Actually, we could not identify any fragment that was created in the spine. On the other hand, the upflowing fragments from the barb changed their direction to leftward horizontal motion in the spine when they arrived near the top of the barb. Almost all the fragments in the spine seemed to originate from the barb. We found that the horizontally fast-moving fragments in the spine have a speed of 14 km s−1 in the plane of sky, and the vertically fast-moving fragments in the barb kept an averaged speed of 17 km s−1 . The barb was vertically extended up to 37,000 km, corresponding to a free-fall speed of 140 km s−1 . Supposing a tilt of 45◦ for the prominence with respect to the line of sight, we obtain the horizontal speed of 20 km s−1 , which is comparable to the measured vertical speed in the barb.
Figure 6. Two examples of fragments moving vertically. The locations of the fragments are shown in snapshot Ca ii images, marked with black rectangles. The time-lapse Ca ii images were taken from the rectangles at a cadence of about 30 s.
4. DISCUSSION We observed returning flows in an intermediate prominence. Because of these returning flows, the prominence appeared to display a counterstreaming pattern of flows. We do not know whether this returning feature is a general source of counterstreaming, but this can be one possible source that can result in the apparent bi-directional motion in prominences. To our knowledge, flows of this kind were never reported in previous observations, even though a variety of oscillatory motions were observed (Molowny-Horas et al. 1997; Schrijver et al. 1999; Jing et al. 2006; Chen et al. 2008; Lin et al. 2003). In particular, the return flows we found appear to be distinct from the oscillation of plasma fragments reported by Schrijver et al. (1999), Lin et al. (2003), and Jing et al. (2006) in that the fragments studied in these studies oscillated with the same phase, while all the fragments in our study did not move in the phase. Our observations suggest that the motion of the plasma fragments may not be fully field aligned. The change of the height of the fragments after return—either rising or falling—indicates that the magnetic field is not strong enough to hold the fragments with a fixed trajectory. Moreover, the upflow speed of 17 km s−1 in the barb is far lower than the speed required to elevate fragments up to the height of the barb against gravity. Thus, this motion is far from the ballistic one expected under gravity in a vertical magnetic field. In other words, the existence of horizontal magnetic field component is necessary, suggesting that the observed vertical motion may not be field aligned. This conjecture is in line with Chae’s (2010) argument against vertical magnetic field in a hedgerow prominence he studied.
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The existence of returning flows implies there is a force that keeps the plasma fragments from moving further and pushes them back. The relatively long duration of the acceleration means that the force is not impulsive. Moreover, the observed acceleration caused by this force is much lower than gravity. This means that the system is close to a magnetohydrostatic equilibrium, and the force is a result of a slight deviation from it. Low & Petrie (2005) and Chae et al. (2008) proposed that slow and steady motion, in either horizontal or vertical, can be generated when the magnetohydrostatic equilibrium is perturbed. The restoring force may be magnetic in nature arising from either magnetic tension or magnetic pressure gradient. Even though gas pressure gradient should be considered as in Karpen et al. (2003), it may play little role in our case considering that there is no dense plasma material near the identified plasma fragments. The He ii 304 Å image data also show that there is no dense material around the prominence. We think that both the vertical motion in the barb and the horizontal motion in the spine we observed may be explained in similar ways. In a magnetohydrostatic equilibrium, each of the force components Bz2 Bz ∂By ∂ Fy = − p+ + (3) ∂y 8π 4π ∂z By2 By ∂Bz ∂ (4) Fz = − ρg − p+ + ∂z 8π 4π ∂y should be equal to zero, where we set yˆ -axis to the direction along the sheet, and zˆ -axis to the vertical direction. The equilibrium is established as a balance among magnetic pressure force, magnetic tension force, gas pressure force, and the gravitational force. If one of the forces has an excess, the equilibrium breaks down and plasma fragments experience a net force. From these equations, we can understand that horizontal gradient of magnetic pressure or horizontal component of magnetic tension force may drive the horizontal motion in the spine. The competition among the gravitational force, the vertical component of magnetic tension force, and the magnetic pressure gradient along the vertical direction may cause the vertical motions observed in the barb. Suppose that the magnetohydrostatic equilibrium is established between the downward gravitational force and the upward force of magnetic tension, as described by the famous Kippenhahn–Schl¨uter solution (Kippenhahn & Schl¨uter 1957). The resulting magnetic configuration is a dipped field line with a sag angle Φ that is determined by the total amount of mass loaded on the field line and the strength of magnetic field (Low & Petrie 2005). The sag angle is also related to the width of the thread L measured along the direction of horizontal field, and the local scale height of pressure Hp by the equation L = 4Hp (By /Bz0 ) = 4Hp / tan Φ (Low & Petrie 2005), where By is horizontal magnetic field strength and Bz0 is the vertical field strength at y → ∞. From the Ca ii images, we found that the widths of plasma fragments ranged from 3500 km to 6500 km in the spine, and about 1500 km in the barb. By choosing a median value of 5000 km for the spine and by adopting a typical pressure scale height value of 300 km (TandbergHanssen 1995), we obtain 13◦ and 39◦ for the sag angles in the spine and the barb, respectively. It appears that the small sag angle in the spine is physically related to the observed fact that plasma fragments in the spine display horizontal motions. On the
Figure 7. Top: magnetic field and plasma motion in a magnetohydrostatic equilibrium configuration (dotted curves) and a non-equilibrium configuration (solid curves) in the spine. Middle: the same, but in the barb. Bottom: the transition from horizontal plasma motion in the spine (1) to vertical motion in the barb (2).
other hand, the large sag angle in the barb, to the observed more or less vertical motions there, is in line with previous reports of the association of large sag angles (∼45◦ ) and vertical motions in quiescent prominences (Chae et al. 2008; Chae 2010). From the Kippenhahn–Schl¨uter equilibrium (Low & Petrie 2005; Chae 2010), we may estimate the magnetic field strengths near the fragments by using the relation between the gas pressure at the center of√fragments, pc , and the horizontal magnetic field strength By = 8πpc / tan Φ. Since the horizontal field strength is associated with the sag angles, by adopting a typical value of gas pressure (0.51 dyne cm−2 ; Chae 2010), we may estimate the horizontal field strengths near the fragments as 15.4 G and 4.4 G for the spine and the barb, respectively. This is natural in that stronger horizontal magnetic field is expected for the fragments which show horizontal motion. With the finding of the sag angles, we illustrate our interpretation of the observed patterns of flow in Figure 7. The top of the figure is given as an explanation of the pattern of returning horizontal flows in the spine. We suppose that a plasma fragment was initially in equilibrium in the dotted field line. For some reason, it is excited and begins to move horizontally to the left, that is, toward the associated active region. This motion results in the change of magnetic configuration, making it deviate from the equilibrium. Specifically, it increases the strength of the vertical field and the associated magnetic pressure on the left, so that restoring force becomes active to the right. This naturally leads to the returning motion as was observed. The middle of the figure illustrates the pattern of vertical motion in the barb. As mass is added, gravity exceeds the upward
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magnetic tension force, and the result is downward motion. If the magnetic tension overcomes gravity later, the mass will move back upward. The addition of mass in the barb is attributed to the inflow from the spine. As shown in the bottom of the figure, we observed horizontally moving fragments in the spine often go over to the barb, which may increase the mass there. This will increase gravity, and cause the vertical motion in the barb as described above. We greatly appreciate the referee’s critical comments and useful suggestions. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (KRF-2008-220-C00022), NSF grant ATM-0745744, NASA grant NASA-NNX08BA22G, and Hinode grant Solar B Phase E through NNM07AA01C Lockheed subcontract. K. Ahn and W. Cao gratefully acknowledge the support of NSF-CAREER through ATM-0847126. Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in cooperation with ESA and NSC (Norway). REFERENCES Berger, T. E., et al. 2008, ApJ, 676, 89 Casini, R., L´opez Ariste, A., Tomczyk, S., & Lites, B. W. 2003, ApJ, 598, L67
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