The Astrophysical Journal, 560:L91–L94, 2001 October 10 䉷 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.
ERUPTION OF A FLUX ROPE ON THE DISK OF THE SUN: EVIDENCE FOR THE CORONAL MASS EJECTION TRIGGER? Carl R. Foley, Louise K. Harra, J. Leonard Culhane, and Keith O. Mason Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, England, UK;
[email protected] Received 2001 June 25; accepted 2001 August 28; published 2001 September 13
ABSTRACT The first evidence of acceleration of a flux rope from the disk of the Sun using the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) is presented. A distinct blueshifted emission component (⫺480 km s⫺1) was observed by the EUV spectrometer on SOHO at the start of the impulsive phase of an X2.3 flare. There is a halo coronal mass ejection associated with this event. Based on a sequence of velocity measurements, we determine the acceleration of the erupting material. These results are supported by simultaneous EUV imaging data from the Transition Region and Coronal Explorer spacecraft, which shows the projected motion of the flux rope. The CDS spectra reveal an initial rapid acceleration phase (3.5 km s⫺2), followed by a transition to a more gradual acceleration (0.68 km s⫺2 ). This may indicate energy input via explosive reconnection. Subject headings: Sun: corona — Sun: flares — Sun: X-rays, gamma rays On-line material: color figures 2001 April 10 in NOAA Active Region 9415, which was located close to the center of the Sun’s disk. The region was observed by the Transition Region and Coronal Explorer (TRACE) and SOHO spacecrafts to eject a portion of the active region in the form of a flux rope. There was an associated halo CME that was observed with the Large Angle Spectroscopic Coronagraph (LASCO; Brueckner et al. 1995). The Yohkoh spacecraft missed the preflare and rise phase because of a transit through the South Atlantic Anomaly. The soft X-ray telescope (SXT) did record some images in the hours preceding. We display a subportion of one of these obtained at 03:38 UT in Figure 2. The active region appears twisted and coiled, in a configuration termed “sigmoidal” and frequently associated with eruptive solar phenomena (Rust & Kumar 1996; Canfield, Hudson, & McKenzie 1999; Glover et al. 2000; Sterling et al. 2000). The eruption was observed by the CDS instrument when the CDS field of view (FOV) was located 200⬙ south of the active region. It recorded the transit of the ejected flux rope as part of its synoptic program in the emission lines of He i, Fe xvi, O v, Mg ix, and Mg x, which cover the temperature regime of 104 to ∼3 # 10 6 K. The CDS FOV is illustrated in Figure 2. The TRACE spacecraft recorded images in a range of wavelengths including white light and passbands centered on 1600, ˚ that were observed at various resolutions. We 1700, and 171 A ˚ (Fe ix/Fe x) passband images since they selected the 171 A offered the largest FOV, thus allowing the progression of the eruption to be tracked. The flux rope is also most clearly visible in the images obtained in this line.
1. INTRODUCTION
One of the primary challenges in studying the Sun and its corona is to understand the initiation and onset of coronal mass ejections (CMEs). Despite considerable observational and theoretical work, their origins and fundamental drivers remain unclear. Previous work has concentrated on investigating the characteristics that exist within the low corona at the time of the CME launch. Recently, signatures such as coronal dimming (e.g., Sterling & Hudson 1997; Zarro et al. 1999), coronal waves (Thompson et al. 1998), and sigmoidal structuring (Rust & Kumar 1996) have been revealed by observations made with Yohkoh and the Solar and Heliospheric Observatory (SOHO). A more long-standing link is established with the eruption of prominence (or, for disk events, filament) material (e.g., Gosling et al. 1974). Prominence/filament material is dense, cold, absorbing material that is suspended at distances of up to 100 Mm above the Sun’s chromosphere. When observed against the solar disk, they appear as threads of dense, dark (absorbing) material. These structures are supported in the corona by strong magnetic fields overlying magnetic neutral lines that separate regions of opposing polarity. Observations and modeling by Rust & Kumar (1996) suggest that prominences are constrained by a helical magnetic field that contains plasma covering a broad range of temperatures from 10,000 K to over 1 MK. Together, the filament and the material associated with this helical field are commonly termed flux ropes. In this Letter, we present the first spectroscopic and imaging observations of the acceleration experienced by an erupting and untwisting flux rope near to the center of the solar disk. As part of these observations the SOHO Coronal Diagnostic Spectrometer (CDS) recorded the largest observed Doppler motions in the EUV regime (⫺480 km s⫺1).
3. ANALYSIS AND RESULTS
3.1. CDS O v Doppler Velocity The CDS has an intrinsic pixel size that corresponds to velocities on the order of 25 km s⫺1. The velocity resolution is related to the statistical quality of the data. This can be enhanced at the cost of either temporal or spatial resolution by summing our counts over time or space. We did this for our absolute
2. OBSERVATIONS
An eruptive two-ribbon flare with a GOES classification of X2.3 (see Fig. 1 for GOES soft X-ray light curve) occurred on L91
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to the bottom left. The first signature of the emission in the CDS FOV was at 05:05 UT. This was a relatively weak redshifted component. The flux rope continued to enter the FOV until 05:07 UT, when its velocity was observed to change from a redshift of 88 km s⫺1 to a strong blueshift of 220 km s⫺1. This material continued to enter the CDS FOV and accelerate as it traveled the full extent of the 240⬙ slit. The evolution of the strong blueshifted emission was extracted by measuring the average blueshift for successive slit positions. This is shown in Figure 4a. We find that the evolution of the blueshifted component proceeded in two phases—initially with an acceleration of 3.5 km s⫺2 that lasted just over a minute, until 05:12 UT. This was followed by a reduced but more sustained acceleration of 0.68 km s⫺2 that was observed until the rope exited the CDS FOV. Similar velocity behavior is seen in all of the other emission lines measured. ˚ 3.2. Image Projected Velocity: TRACE 171 A
˚ passbands Fig. 1.—GOES 10 soft X-ray flux obtained in its 0.5–4 and 1–8 A (upper curve). The time interval of the CDS and TRACE observations is indicated by the hatched region.
wavelength calibration. This was obtained by averaging the quiet-Sun signal from the region surrounding our observations. The strongest emission recorded in the CDS images of the flux rope was that in the O v line that forms at temperatures of around 3 # 10 5 K. The intensity of this line provided velocity resolution of better than 10 km s⫺1 without any summing. We have restricted the analysis to this line for this reason, although we note that there is significant emission recorded in all the available lines. The progression of the flux rope is illustrated in Figure 3. Since the CDS rasters from west to east (right to left; see Harrison et al. 1995 for a description of CDS), its signature in the CDS intensity maps is a streak arching from the top right
˚ images recorded the full progression of The TRACE 171 A the flux rope from its initial motion disturbance until it had entirely exited the TRACE FOV of 8⬘. The eruption of the flux rope was directed to the south and thus into the CDS slit as it scanned at the periphery of the TRACE FOV (see Fig. 2). The TRACE images suggest that the flux rope is untwisting. If we ascribe all of the redshift (88 km s⫺1) observed before the acceleration phase to untwisting, the implied angular velocity is 5.2 # 10⫺3 radians s⫺1. This is of the same order as the rate of untwisting observed with the Ultraviolet Coronagraph Spectrometer for a helical CME by Ciaravella et al. (2000; 9 # 10⫺4 radians s⫺1). At around 05:10 UT, the flux rope appeared to accelerate from a region just above one of its footpoints close to the core of the active region. Additionally, the inner edge of the flux rope was observed to brighten progressively as it moved outward. This was accompanied by a more general expansion from the core of the active region toward the east and west. To evaluate the velocity evolution in the plane of the image, we tracked prominent features of the flux rope as they progressed through the TRACE FOV. As the features progressed out of the FOV, we adopted a bootstrapping method so that
Fig. 2.—AR 9415 as observed by (b) the Yohkoh SXT and (c) TRACE. In (a), we display the position of the partial images. AR 9415 is indicated by the black circle. (c) The TRACE image was recorded at 05:09 UT while the eruption was in progress. On this image, we have overplotted the CDS FOV along with the slit position at this time. The erupting flux rope is arrowed. [See the electronic edition of the Journal for a color version of this figure.]
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we could estimate the progression of the rope that had effectively left the TRACE FOV and entered the CDS FOV. The pixel size of the TRACE images that we used is ∼3 Mm. This is the dominant source of error with this method. However, it is minor in comparison with the cumulative distance traveled by the material during our observations (∼300 Mm). The result of the tracking of the flux rope is illustrated in Figure 4. We found that in the time period up to 05:10 UT, the expansion was relatively uniform and could be ascribed to a constant projected velocity of 155 km s⫺1. The eruption appeared to accelerate in a fashion consistent with that which we found in the latter phases of the Doppler-shift curve of O v. The TRACE data are consistent with a constant projected acceleration of 1.95 km s⫺2 after 05:11 UT. There is a gap in the TRACE data coincident with the high-acceleration phase observed with CDS. 4. DISCUSSION
Fig. 3.—Intensity maps formed by the (a) blue wing and (b) the stationary red wing. The separation of these regions is indicated by the regions at the top of (c), which displays a sample spectrum of the brightest part of the flux rope at 05:15 UT. The rest wavelength is indicated by the dotted line. [See the electronic edition of the Journal for a color version of this figure.]
We have observed the first example of a flux rope lifting off the disk of the Sun, using the CDS instrument on board the SOHO spacecraft and the TRACE EUV imager. We have measured the acceleration of the flux rope as it is ejected. The acceleration was observed to proceed in two phases, seen in both the CDS data and the TRACE data. A high-acceleration phase (∼3.5 km s⫺2) lasted for just over a minute. This was followed by a more gradual acceleration (0.68 km s⫺2) for the next 4 minutes until the flux rope left the CDS FOV. Observations with coronagraphs of prominence eruptions located at the limb of the Sun have demonstrated that they experience constant or no acceleration. Two possible forms of energy input have been postulated to explain the gradual acceleration of CMEs, based on studies of the velocity evolution of coronal flux ropes by Krall, Chen, & Santoro (2000) within the height range of 0.4–5 R,: (1) an increase in the poloidal component of the flux rope’s helical magnetic field and (2) hot plasma injection. Our observations were made early in the impulsive phase and show two phases of acceleration. This may suggest an additional input of energy, possibly due to explosive magnetic reconnection, to explain the initial shortlived higher acceleration phase. This is consistent with the conclusions derived from the observations of flare sprays by Tandberg-Hanssen, Martin, & Hansen (1980). They reported similar morphology and timeheight evolution to us. They attributed the motion observed to an initial acceleration phase that was short-lived and occurred
Fig. 4.—(a) Evolution of the average blueshift that was recorded by CDS for each successive slit position. To this, we have fitted two lines: one representing an initial acceleration of 3.5 km s⫺2 and the other representing the following longer lasting acceleration of 0.68 km s⫺2 . (b) Here we have plotted the expansion ˚ images recorded by TRACE. Initially, during the preflare phase, the expansion is at a constant velocity of the flux rope in the plane of the image from the 171 A of 155 km s⫺1. During the period in which we observe the acceleration in the CDS observations, we also observe one in the TRACE data of 1.95 km s⫺2.
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at low altitudes. It was noted from the work of Gosling et al. (1976) that the CMEs associated with sprays were generally fast, and those from disparitions brusques slow. Observations of CMEs with the LASCOs (Sheeley et al. 1999; Andrews & Howard 2001) have demonstrated that the CMEs originating from nonflaring/flaring regions display a similar anisotropy. The CMEs associated with flares at their onsets often have accompanying Moreton waves (Moreton 1960) and type II bursts (Klassen et al. 2000) and tend to be faster, brighter, and larger. Observations of broadened line profiles above a flare at the Sun’s limb have been reported by Innes et al. (2001) using SUMER data. The line profiles extend out to 650 km s⫺1, but without a resolved high-velocity component as reported here. They attributed the motions they observed to shock-driven acceleration and heating of the active region loops above the flare location and suggest that the shock provides the energy to drive the CME. Our observations support the idea that explosive reconnection is the underlying CME trigger. However, in our case, instead of a shock front, we see one end of a flux rope
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being violently disconnected from the Sun. What we observe may therefore be characteristic of those CMEs that have associated filament eruption. We would like to thank the anonymous referee for useful comments that improved this Letter. L. K. H. acknowledges the support of a PPARC advanced fellowship. We thank Alexi Glover, Sarah Matthews, Kuniko Hori, and Spiros Patsourakos for useful discussions and comments. We acknowledge the Solar UK Research Facility1 for providing data for use in this publication. SOHO is a project of international cooperation between ESA and NASA. TRACE is a mission of the StanfordLockheed Institute for Space Research and part of the NASA Small Explorer program. The Yohkoh SXT is a collaborative project of the Lockheed Palo Alto Research Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo, supported by NASA and ISAS. 1
See http://surfwww.mssl.ucl.ac.uk/surf.
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