analyses of the 1992 February 21 Ñare (an LDE-type Ñare or 1992 January 13 Ñare (an impulsive-type Ñare; Masuda et

Transcription

1 THE ASTROPHYSICAL JOURNAL, 483:57È514, 1997 July 1 ( The American Astronomical Society. All rights reserved. Printed in U.S.A. MOVING PLASMOID AND FORMATION OF THE NEUTRAL SHEET IN A SOLAR FLARE SAKU TSUNETA National Astronomical Observatory, Mitaka, Tokyo 181, Japan Received 1996 October 28; accepted 1997 January 31 ABSTRACT A spectacular erupting feature with a plasmoid-like structure is observed before and during the solar Ñare that occurred on the limb on 1991 December 2 with the Yohkoh soft X-ray telescope. The rise of a loop structure starts D1 min before the Ñare, evolving to a plasmoid-like structure in the impulsive phase of the Ñare. The speed of the rising loop (plasmoid) is almost constant (D96 km s~1) throughout the observation. A clear X-shaped structure is formed underneath the rising plasmoid, and a bright soft X-ray loop is formed below the X-point. The X-shaped structure indicates a magnetic neutral point with a large-scale magnetic separatrix structure. Inverse-VÈshaped high-temperature ridges are located above the soft X-ray loop and below the X-point. We interpret these as reconnected loops heated by slow shocks. A moving high-temperature (15 MK) source is found, coincident in position with the rising structure above the X-point. A hard X-ray source (33È53 kev) is located at the top of the soft X-ray Ñare loop. These two compact high-temperature sources located above and below the X-point would be formed by fast shocks due to the symmetric reconnection outñows both upward and downward from the X-point. Subject headings: MHD È Sun: Ñares È Sun: X-rays, gamma rays 1. INTRODUCTION The Yohkoh soft X-ray telescope (SXT) allows us for the Ðrst time to perform continuous soft X-ray observations from before the onset of a Ñare, through its development, and during the recovery of the corona after the end of the Ñare, with high time (2 s) and spatial (D3A) resolution (Ogawara et al and references therein). Detailed analyses of the 1992 February 21 Ñare (an LDE-type Ñare or a cusp-type Ñare; MacCombie & Rust 1979, Tsuneta et al. 1992a, Tsuneta 1996, Forbes & Acton 1996), and the 1992 January 13 Ñare (an impulsive-type Ñare; Masuda et al. 1994, Shibata et al. 1995, Tsuneta et al. 1997) demonstrated that magnetic reconnection (Sweet 1958; Parker 1957; Petschek 1964) occurs in both impulsive and LDE types of Ñares (Carmichael 1964; Sturrock 1966; Hirayama 1974; Kopp & Pneuman 1976; Hirayama 1991; Gosling 1993; Hundhausen 1996). In particular, the critical importance of slow shocks in converting magnetic energy to plasma kinetic and thermal energies was pointed out theoretically (Petschek 1964; Cargill & Priest 1983) and then observationally conðrmed (Tsuneta 1996; Tsuneta et al. 1997). These new observations are also consistent with the in situ detection of slow shocks in the geomagnetic tail (Feldman et al. 1994a, 1994b; Saito et al. 1996). The formation of large-scale cusp structures with intense heating also occurs away from active regions (giant cusp; Tsuneta et al. 1992b, Hiei, Hundhausen, & Sime 1993, Hanaoka et al. 1994, McAllister et al. 1996), and in active regions (minicusp; Yoshida & Tsuneta 1996). The giant cusps in the quiet Sun, the cusps seen in LDE Ñares, and the minicusps in active regions are apparently governed by the same physical process: magnetic reconnection. Numerical simulations on magnetic reconnection have extensively been carried out by Forbes & Malherbe (1991), Lee & Yan (1992), Magara et al. (1996a), Ugai (1996), and Yokoyama & Shibata (1996). In particular, Yokoyama & Shibata (1996) performed a realistic simulation by introducing the e ect of heat conduction along magnetic Ðelds. These simulations have striking agreement but also some discrepancies with the new observations, allowing us for the Ðrst time to make a detailed comparison between the observations and the models. These models mainly concentrate on magnetic reconnection after it starts, and do not reveal how the neutral sheet where reconnection takes place is dynamically formed. Any magnetic structure emerging from the convection zone of the Sun has the form of a closed-loop structure. On the other hand, these Yohkoh observations require that a largescale (13È15 km) neutral sheet structure be prepared before or at the onset of Ñares. Yohkoh observations so far have shown many examples of erupting (rising) structures for both LDE-type (Tsuneta et al. 1992a) and impulsive-type Ñares (Shibata et al. 1995; Tsuneta 1993; Ohyama & Shibata 1996) around the start and peak of the soft X-ray Ñares. Nitta (1996) showed from systematic analyses of many Ñares observed with Yohkoh that most of the Ñares are indeed associated with erupting features in soft X-rays. It is, however, not clear whether these eruptions are the result of reconnection (related to the fast outñow), or are related to the cause of reconnection. This paper reports on the observations of an erupting structure beginning before the start of a Ñare, and describes the global structure around the reconnection X- point. 2. GLOBAL EVOLUTION OF X-RAY STRUCTURE Figure 1 shows the time proðles of a Ñare that occurred on the east solar limb on 1991 December 2 (M3.6 in GOES classiðcation). The Ñare has impulsive spikes above 23 kev at around 4:54 UT. The hard X-ray images taken with the hard X-ray telescope (HXT) aboard Yohkoh do not show any footpoint emission (see Fig. 5), indicating that the hard X-ray footpoints are occulted by the solar limb. The long soft ÏÏ hard X-ray emission with relatively weak impulsive spikes may be related to the occultation of the hard X-rays from the footpoints. The clear loop structure in the soft 57

2 58 TSUNETA Vol. 483 FIG. 1.ÈX-ray time proðle of the 1991 December 2 Ñare from 3 to 33 kev. Energy bands are (from above) 3È15 kev, 15È4 kev (soft X-ray spectrometer), 14È23 kev (HXT L channel), 23È33 kev (HXT M1 channel). X-ray images (Fig. 4) does not imply that this Ñare is classi- Ðed as an LDE Ñare. Fig. 5 of Tsuneta (1993) shows schematic evolution of the soft X-ray Ñare PreÑare Evolution Figure 2 (Plate 13) shows in panels 1È3 the preñare images several minutes before the start of the hard X-ray Ñare. A steady overlying loop with a height of D5 ] 14 km is seen above the compact bright point, which is too small to be resolved, about 1 hour before the Ñare (4:35È4:46 UT). This overlying steady structure and the pointlike saturated source on the limb are clearly seen in Figure 2, panels 1 and 2. A distinct rising loop structure with speed of D96 km s~1 appears around 1 minutes prior to the Ñare at around 4:39È4:44 UT (Fig. 2, panel 2). (The rising loop structure is marked with an arrow in Fig. 2, panel 4.) This erupting loop is located under the steady overlying loop and above the compact bright point, and the steady overlying loop structure does not move even after the start of the rise of the loop underneath the steady loop. The compact bright point located below the overlying loop increases its soft X-ray intensity as the loop above starts to rise at around 4: 39È4: 43 UT. This leads to a more explosive increase of the soft X-ray intensity (start of the Ñare at D4:5:37 UT). The erupting loop continues to increase its height in panels 1È3 of Figure 2, and appears to collide with the steady structure at around 4:56 UT (Fig. 2, panel 7) Flare Evolution Panels 4È15 of Figure 2 are composite images created from four di erent images taken at nearby times; one fullresolution (2A.45) image, one half-resolution (4A.9) image, and two quarter-resolution (9A.8) images, with di erent exposure times, to have Ðeld of view and dynamic range as wide as possible. Only the full-resolution images have exposure times optimized for the bright core with on-board automatic exposure control (AEC) (Tsuneta et al. 1991). The half- and quarter-resolution images have prespeciðed Ðxed exposure times. The Ðeld of view is 1@.5 for quarterresolution images, 5@.2 for a half-resolution image, and 2@.6 for a full-resolution image. The higher resolution data with longer exposure times are used pixel by pixel, whenever valid (nonsaturated) data are available. The boundary of the full- and half-resolution images is slightly visible. The full-resolution images located in the center of each image have a lower signal-to-noise ratio for faint features, because the exposure time is optimized for the bright core with AEC. Such composite images allow us to see not only the bright soft X-ray loop with high resolution but also the dim global structure around the Ñare loop with size as large as 1@.5. The rising loop seen in the preñare phase continues to rise in the impulsive phase of the Ñare. Panels 4È7 of Figure 2 and panel 1 of Figure 4 clearly show the continuous rise of the loop structure in the impulsive phase. At the same time, a bright soft X-ray loop is formed underneath the rising loop structure. In Figure 2, panels 9È12, the rising loop structure evolves to a spectacular circular-shaped blob or plasmoid structure, although the transition from the initial rising loop structure in the preñare phase to the plasmoidlike structure in the decay phase is not completely clear. The rising plasmoid is most clearly seen in the late phase (Fig. 2, panels 12È15). (The plasmoid is indicated by an arrow in Fig. 2, panel 15.) We also notice a distinct X-shaped structure below the plasmoid. The X-shaped structure indicates a reconnection X-point with large-scale separatrix structure. A bright loop (Ñare loop) is always seen below the X-point. The face-on ÏÏ rising loop structure appears to be slightly tilted with respect to the Ñare loop in panel 1 of Figure 4. This structure may be tilted further toward the line-of-sight direction, to appear almost edge-on ÏÏ in the impulsive phase. Thus, the top of the rising loop structure could be transformed into a plasmoid appearance due to line-of-sight e ects. A large circular thin loop that goes through the plasmoid is seen in Figure 2, panels 1È14. One of the footpoints can be clearly seen (Fig. 2, lower right corner) and is apparently located far outside the active region. The portion of the loop near the footpoint (marked by an arrow in Fig. 2, panel 11) increases its soft X-ray intensity from the footpoint side, possibly owing to chromospheric evaporation in response to heat input along the Ðeld line. Since this connectivity (the plasmoid and the footpoint far outside the active region) is not seen in the preñare images, the plasmoid may involve such large-scale Ðeld lines above the active region as it moves upward, and may produce heat with magnetic reconnection, which is di erent from the primary magnetic reconnection to produce Ñares PostÑare Evolution Panel 16 of Figure 2 was taken about 2 hours after the start of the hard X-ray Ñare. A Ñare loop with larger size is

3 No. 1, 1997 PLASMOID RISE AND FORMATION OF NEUTRAL SHEET 59 clearly seen. This is naturally explained by successive continuous reconnection at the X-point. A bright vertical structure is located just on top of the rising postñare X-ray loop. This structure rises with the postñare loop and is perhaps related to the remnant of the X-point structure. The vertical structure appears to be connected to other regions within the active region (e.g., Uchida 1996) Rising Plasmoid Figure 3 (upper panel) shows the height of the rising loop above the X-point (later seen as the plasmoid-like structure) and the height of the Ñare loop below the X-point. The rising loop is smoothly connected to the plasmoid-like structure in height and speed. The speed is almost constantly D96 km s~1 until the Ñare loop disappears at the edge of the CCD. This indicates that the same physical structure continuously rises with almost constant speed. The rising speed of the Ñare loop is higher (18 km s~1) only during the hard X-ray Ñare in the HXT L channel (14È23 kev), and becomes much smaller (1È2 kms~1) after the end of the hard X-ray Ñare in the HXT L channel. We estimate the thermal energy content of the plasmoid to be about 2 ] 129 ergs, and the kinetic energy to be about 8 ] 127 ergs. The mass carried with the plasmoid is Height (km) Soft X-ray Intensity (DN/sec/pixel) Rising Loop/Plasmoid HXT L-channel Flare Loop 4: 5: 6: 7: 8: 9: Start Time (2-Dec-91 3:35:23) Flare Loop Rising Loop/Plasmoid 4: 5: 6: 7: 8: 9: Start Time (2-Dec-91 3:35:23) FIG. 3.ÈUpper panel: Heights of the rising loop and the soft X-ray Ñare loop as a function of time. The time proðle of the HXT L channel (14È23 kev) is also shown. Note that the rise of the loop starts before the start of the hard X-ray Ñare. L ower panel: Soft X-ray intensities of the rising loop and the Ñare loop. The intensity of the rising loop decreases rapidly with increasing height. The erupting structure is about 2 orders of magnitude fainter than that of the Ñare loop. about 6 ] 113 g. (The line-of-sight depth of the plasmoid is assumed to be 14 km.) The plasma density of the plasmoid is about 11 cm~3, and the pressure about 45 dyn cm~2. The kinetic and thermal energies of the plasmoid appear signiðcant. Figure 3 (lower panel) shows the time proðles of the peak intensities of the Ñare loop and the rising structure. The intensities of the Ñare loop and the rising structure are almost the same around the start of the Ñare. The peak intensity of the rising structure then decreases constantly with increasing height, while that of the Ñare loop increases rapidly. The intensity of the plasmoid is about 2 orders of magnitudes fainter than that of the Ñare loop in the peak and decay phases. Note that images in Figure 2 are shown with logarithmic X-ray intensity to have a wider dynamic range, and that the color table is optimized in each image to see faint structures as clearly as possible. 3. TEMPERATURE STRUCTURE Figure 4 (Plate 14) shows temperature maps of the region around the Ñare loop obtained from two full-resolution images (64 ] 64 pixels) taken with two broadband Ðlters (thick aluminum and beryllium Ðlters). This combination of the Ðlters is suitable for the diagnostics of a Ñare plasma above 1 MK. (The region outside the 64 ] 64 fullresolution pixel area in Figure 2 cannot be converted to temperatures because of the single-ðlter observation.) Several exposures are summed, to improve the signal-tonoise ratio, and the time resolution is about 2 minutes. Although we have a moving feature in the images, the summation does not a ect the accuracy of the derived temperatures, because (1) the two Ðlters alternate, exposure by exposure, and cancel the slow time variation, and (2) the size of the moving feature is much larger than the distance over which the source moves during the time interval between the adjacent exposures. The region inside the limb is not converted to the temperature owing to the very low signalè toèphoton-noise ratio even after the summation. The pixels which are heavily contaminated ([4%) by the scattering photons from the bright Ñare loop are also not converted to temperatures (See Tsuneta et al for the estimation of the scattering e ect.) These pixels are also blacked out in Figure 4. Smoothing with a width of 3 pixels is done for the pixels whose intensity is smaller than 1, DN~1 pixel~1 in the summed X-ray map. Smoothing with a width of 5 pixels is done for the pixels whose intensity is smaller than 25 DN~1 pixel~1, 7 pixels for intensities smaller than 625 DN~1 pixel~1, and 9 pixels for intensities smaller than 156 DN~1 pixel~1. This arrangement improves the signalètoèphoton-noise ratio for faint regions by sacriðcing the spatial resolution High-T emperature Ridge Figure 4 clearly shows that the region above the bright Ñare loop (but below the rising loop) has higher temperatures reaching 2 MK. The overall feature is quite similar to the temperature distributions of the 1992 February 21 Ñare (LDE Ñare) and the 1992 January 13 Ñare (impulsive Ñare), and strongly supports the conclusion that magnetic reconnection heats the reconnected loops. These reconnected loops with high temperatures are dark in X-rays. The gradual increase of the plasma density due to chromospheric evaporation and due to the fast outñow from the reconnection site forms the bright soft X-ray loop.

4 51 TSUNETA Vol. 483 The time delay for the density increase due to the evaporation places the bright soft X-ray loop below the dim hightemperature ridges High-T emperature Moving Source The temperature maps also show that the compact source some 25, km high above the Ñare loop has a higher temperature (15 MK) with respect to the surrounding region (5È1 MK) (Fig. 4, panels 2È7). The hightemperature region is located at the top of the rising loop structure, and has a higher plasma density (see pressure and X-ray maps in Fig. 4). This compact high-temperature source moves upward with the rising loop structure above the X-point and would correspond to the other half of the reconnected loops (U-loops or inverse loops) moving upward with the outñow from the reconnection point Hard X-Ray Images Figure 5 (Plate 15) shows the contours of the HXT images (M2 channel [33È53 kev], M1 channel [23È33 kev], and L channel [14È23 kev]) overlaid on the temperature, density, pressure, and X-ray maps. The usual footpoint sources are not seen because of the occultation by the limb (Masuda et al. 1994), and only a single source is seen throughout the Ñare. The locations and shapes of the hard X-ray images do not change much with energy. The hard X-ray source is located just at the top of the bright Ñare loop throughout the Ñare duration. The loop-top hard X-ray source increases its height with a speed of D1 km s~1, which is similar to the speed of the Ñare-loop rise. The e ective temperature of the loop-top source obtained from the ratio of the HXT M1 and M2 channels is 9 MK, and that obtained from the M1 and L channels is 58 MK. The total emission measure of the hard X-ray source is 4 ] 145 cm~3 from the M1 and M2 channels, and 2 ] 146 cm~3 from the L and M1 channels (Masuda 1994). The HXT emission measure (58È9 MK) is much smaller than the SXT emission measure (1È15 MK) located at the same position (Fig. 7). Note that the hard X-ray source is located where the temperature obtained from SXT is not the highest. Rather, it is located in the lower temperature region. This tendency is the same as that in the 1992 January 13 Ñare (Tsuneta et al. 1997) and other LDE-type Ñares (Sato 1997). This means that a high-temperature source (e ective temperature 58È9 MK) with small emission measure is embedded in the cooler plasma (1È15 MK) with a much larger emission measure. 4. DISCUSSIONS AND SUMMARY 4.1. Evidence of Magnetic Reconnection The observations presented in this paper provide ample pieces of evidence to support the idea that magnetic reconnection is the heating source for this Ñare. Figure 8 shows the schematic magnetic and temperature structures. 1. Large-scale X-structure is seen above the Ñare loop. The X-point has the conðguration of a magnetic neutral point, and the large-scale X-structure would then reñect the magnetic separatrix lines. 2. A bright soft X-ray loop (Ñare loop) is formed underneath the X-point. Hard X-ray emissions also come from the top of the soft X-ray loop. The Ñare loop is about 1 times brighter in soft X-rays than the rising loop structure. This clearly indicates that only the reconnected loop structure is heated up, and becomes bright in hard and soft X-rays. 3. The height of the Ñare loop and the separation of the footpoint increase as the X-point rises. This is naturally explained by successive reconnection at the X-point. 4. The plasma temperature is higher above the Ñare loop, and peaks where the X-ray intensity is only 1% of the peak X-ray intensity of the Ñare loop. The hot ridge structure is located on either side above the Ñare loop but below the rising structure and the X-point. This is explained with heating due to the large-scale slow shocks attached to the X-point. There must be (less dense) symmetrical hot ridges heated by another pair of the slow shocks above the X- point. This is not clearly seen by SXT, probably owing to the contamination of the line-of-sight corona with lower temperature (see the Appendix of Tsuneta et al for details). The temperature of the plasma heated by the slow shocks reaches only 1È2 MK for the Ñares so far analyzed (Tsuneta 1996; Tsuneta et al. 1997), so that it is also not seen by HXT. 5. A new compact hot source above the X-point is discovered. It coincides in position with the rising loop structure (plasmoid) and moves upward with it. The hot source is probably related to the reconnected Ðeld lines going upward from the X-point and is heated by the fast shock due to the collision of the upward fast outñow from the reconnection point with the rising loop structure. 6. A hard X-ray source (33È53 kev) is located at the top of the Ñare loop. Although this location is lower (with respect to the bright Ñare loop) than that of the loop-top hard X-ray source of the 1992 January 13 Ñare, the nature of the loop-top source would be essentially the same in our interpretation. The loop-top hard X-ray source is heated by the fast shock due to the collision of the downward fast outñow from the reconnection point with the reconnected Ñare loops Hot Ridge Structure below the X-Point The two hot sources above and below the X-point and the inverse-v hot ridges below the X-point (above the Ñare loop) constitute the key temperature structures around the reconnecting X-point (Fig. 8). The temperature, density, and pressure distributions along the lines indicated in Figure 6 (Plate 16) are shown in Figure 7. (The line-of-sight thickness is assumed to be 14 km.) Some of the lines in Figure 6 go through the compact hot source and the hot ridge structures. Figure 6 (1) shows the two humps and the cool channel in between them in the temperature distribution. This is quite similar to the temperature structure of the LDE Ñares (Tsuneta 1996). The cool channel is interpreted as due to the conduction cooling along the reconnected Ðeld lines. Figure 6 (3) shows that the temperature of the hot source is 17 MK. Figure 6 (5) shows that the region in between the compact hot source and the hot ridges has lower temperature (1 MK), indicating that the apparent temperature of the X-point is lower as compared with those of the hot sources. This may be due to the e ect of the active region corona with lower temperatures, as will be discussed below. As is clearly shown in Figure 4, the hot ridges above the Ñare loop are asymmetric in temperature, especially in the decay phase of the Ñare. The northern ridge has higher temperature around 15È2 MK, while the southern ridge has 1È15 MK. If this asymmetry is real, a simple sym-

5 No. 1, 1997 PLASMOID RISE AND FORMATION OF NEUTRAL SHEET 511 MK (1) Temperature cm (1) Density dyn cm (1) Pressure 5 MK (2) cm (2) dyn cm (2) 5 MK (3) cm (3) dyn cm (3) 5 MK (4) Temperature cm (4) Density dyn cm (4) Pressure 5 MK (5) cm (5) dyn cm (5) 5 MK (6) cm (6) dyn cm (6) 5 FIG. 7.ÈDistributions of temperature, density, and pressure along the lines indicated in Fig. 6. L ines 1È3: horizontal cuts. L ines 4È6: vertical cuts. metric conðguration of magnetic reconnection cannot explain this asymmetry. We must consider the e ect of the line-of-sight active-region corona with lower temperatures, when we interpret the temperatures derived with the Ðlterratio analysis (Yoshida 1996). As an exercise to see this e ect, we assume that 2 MK plasmas symmetrically exist on either side above the Ñare loop, and that there is 3 MK (active region) plasma with an emission measure 5% of the 2 MK component along the line of sight only in the southern ridge. Note that the region where the high-

6 512 TSUNETA Vol. 483 FIG. 8.ÈSchematic magnetic and temperature structure. The hot ridges located below the X-point are heated by the pair of slow shocks and are clearly seen in the SXT temperature maps. There must be (less dense) symmetrical hot ridges heated by another pair of the slow shocks located above the X-point. This is not seen by SXT, probably owing to the contamination of the line-of-sight corona with lower temperature. The downward and upward reconnection outñows collide with the bright soft X-ray loop below the X-point and with the rising plasmoid above the X-point, and form fast (perpendicular) shocks. The hot source heated by the fast shock below the X-point is seen by HXT (5È9 MK), and that above the X-point is seen by SXT (15 MK). temperature ridges are located has about 1% of the peak soft X-ray intensity (the top of the Ñare loop), and the contribution from the line-of-sight corona may have to be taken into account. The derived temperature of the southern ridge decreases by about 4 MK, even if its true temperature is 2 MK, according to Figure 7 of Tsuneta et al. (1997). It is thus not clear whether this asymmetry is real or not, from the available data set. The reason why we do not see any ridge structure symmetrically above the X-point may be the same. The plasma density of the hot plasma there is lower (Fig. 7), and the high-temperature ridge structure may be completely erased by the line-of-sight activeregion corona Compact Hot Sources Hot Source above the X-Point The high-temperature ridge-structures below the X-point are quite similar to the ridge structures observed in most of the LDE Ñares and some impulsive Ñares, while a compact hot source above the X-point is reported here for the Ðrst time. This hot source is di erent from the loop-top hard X-ray source (Masuda et al. 1994; Tsuneta et al. 1997), because (1) no hard X-ray source is seen at the position of the compact hot source, (2) the temperature of the hot source is much lower than that of the loop-top hard X-ray source, and (3) the hot source is clearly located above the reconnection X-point, whereas the loop-top hard X-ray source reported by Masuda et al. (1994) is located below the X-point, or at least around the X-point. If the hot source is heated by the stationary slow shocks, its speed of rise is similar to that of the Ñare loop (18 km s~1). The hot source coincides in position with the rising loop structures, and moves upward with much faster speed as shown in Figure 4, panels 2È8. Since the rise speed of the plasmoid (96 km s~1) is much smaller than the local sound (D3 km s~1) and magnetosonic (D1 km s~1 or more) speeds, the rising loop itself does not form a bow shock. Thus, the hot compact source above the X-point is most probably heated by the fast-mode (perpendicular) shock due to the collision of the upward fast outñow from the X-point with the rising loop structure Hot Source below the X-Point On the downward side of the reconnection outñow from the X-point, the hard X-ray source (33È53 kev) is located around the top of the bright Ñare loop. It is embedded in the lower temperature plasma at the loop top. This situation is

7 No. 1, 1997 PLASMOID RISE AND FORMATION OF NEUTRAL SHEET 513 essentially the same as the 1992 January 13 Ñare. The only di erence between these two Ñares is the relative location of the hard X-ray source with respect to the soft X-ray Ñare loop. The hard X-ray source indicates fast shock heating due to the collision of the downward fast outñow from the X-point with quasi-stationary reconnected Ðeld lines (Masuda et al. 1994). This interpretation is also consistent with the fact that the hard X-ray source moves upward with increasing height of the soft X-ray Ñare loop. The actual location of the fast shock may depend on the location and shape of the reconnected Ðeld lines, the magnetosonic speed of the outñow plasma, the spatial distribution of the outñow, and the speed of chromospheric evaporation. These parameters may cause some variety on the location of the loop-top ÏÏ hard X-ray source with respect to the Ñare (reconnected) loops. Therefore, it appears from the hard and soft X-ray observations that there are two symmetric compact hot sources above and below the X-point. This indicates the symmetric fast outñows from the X-point both upward and downward. The resultant fast-shock heating with the rising loop located above the X-point, and with the reconnected steady loops located below the X-point, creates the hot plasmas above and below the X-point. The reason why the hot source (hard X-ray source) below the X-point has much higher temperature may be the higher Mach number of the downward outñow due to a lower plasma temperature, because the temperature of the plasma heated by fast shock critically depends on the Mach number of the upstream side of the shock. The downward reconnected loops are anchored to the photosphere in a small distance, and the plasma heated by reconnection would be cooled faster than the hot plasma located above Hot Ridge Structure above the X-Point In addition to these hot sources, we have found an inverse-vèshaped ridge structure in temperature above the soft X-ray Ñare loop as discussed in 4.2. Since the above observations of the hot sources indicate the symmetric Ñow and magnetic structure around the X-point, we should have observed a symmetric V-shaped ridge structure in temperature above the X-point. Such a V-shaped ridge structure is not seen above the X-point, although the region has somewhat higher temperatures (D1 MK; Fig. 4). As we have discussed in the previous section, the hot ridge structure may not be seen, owing to the lower density and the resultant higher contamination from the lower temperature line-of-sight corona. The only di erence between the upward U-loops and the downward loops is that only the downward loops are a ected by chromospheric evaporation in response to the heat input along the reconnected Ðeld lines. As a result of evaporation, only these downward loops are signiðcant in the soft X-ray images Eruptive Structure: Cause or Result of Reconnection? The evolution of the X-ray morphology can be summarized as follows (Fig. 5 of Tsuneta 1993). A steady overlying loop continues to be seen from about 1 hour prior to the Ñare. About 1 minutes before the Ñare, a distinct loop structure starts to rise. This is associated with the increase in the intensity of the compact source located below the erupting structure (preñare brightening). This may indicate that the slow reconnection starts with the eruption. In any case, this is followed by the start of the hard X-ray Ñare (the start of the primary reconnection) in 1 minutes. It appears that this loop evolves to become the rising plasmoid of the hard X-ray decay phase. The rising blob in this Ñare appears to be a side view of the rising loop system sheared with respect to the Ñare loop. The soft X-ray (Ñare) loop, which is about 1 times brighter than the rising structure, is formed underneath the X-point. The height of the Ñare loop and the separation of the footpoint increase as the plasmoid rises. This sequence of evolution clearly indicates that the eruption started before the start of the Ñare (primary reconnection) and is related to the cause of the Ñare rather than being a result of reconnection such as the fast outñow from the X-point. Although the apparent structure of the plasmoid discovered here appears similar to the plasmoid structure frequently observed in the geomagnetic tail (Mukai et al. 1996), the two may be di erent: the plasmoid in the geomagnetic tail is purely the result of magnetic reconnection. If reconnection spontaneously starts at a point, and the observed upward motion of the loop structure is driven by the fast outñow from the reconnection X-point, the speed of the eruption should have decreased as the height of the moving plasmoid increased from 1.5 ] 14 to 2.5 ] 15 km above the solar limb: the speed is almost constant throughout the evolution. We argue from these observations that the eruption is driven by a di erent mechanism, and that the formation of the X-point and the subsequent magnetic reconnection are ultimately driven by the upward motion of the magnetic structure. The classical tether cutting models (e.g., Hirayama 1974, 1991) assume that the rising loop structure stretches the steady overlying structure upward and creates an X-point structure beneath it, and that reconnection at the X-point in turn disconnects the rising component from the stretched overlying Ðelds and allows the rising Ñux tube to go up further. The morphological evolution of this Ñare appears consistent with this picture. We, however, emphasize that we have not yet understood how the eruption takes place and how the X-type neutral point is formed as a result (Tsuneta 1993; Kusano, Suzuki, & Nishikawa 1996), and that there may be other possibilities suggested by Magara, Shibata, & Yokoyama (1996b), Uchida (1996), and Low (1996). One important clue to these questions is their scaleinvariant nature: the eruption and subsequent formation of the cusp structure are seen in the quiet Sun (giant cusp; Tsuneta et al. 1992a, 1992b, Hiei et al. 1993, Hanaoka et al. 1994, McAllister et al. 1996) as well as in active regions (minicusp; Yoshida & Tsuneta 1996). The size of the giant cusps is as large as the solar radius, and the minicusps as small as 14 km. If the eruption is related to the photospheric shear motion, that photospheric motion should have had the same scale-invariant nature. The author thanks S. Masuda for processing the hard X-ray images presented in this paper, and T. Magara and H. Hudson for discussions.

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9 7 x 1 4 km (1) 4:39:43 (2) 4:43:59 (3) 4:48:15 (4) 4:5:37 (5) 4:52:3 (6) 4:53:31 (7) 4:56:11 (8) 4:58:55 (9) 5:7:31 (1) 5:1:17 (11) 5:11:41 (12) 5:17:1 (13) 5:18:19 (14) 5:19:37 (15) 5:2:55 (16) 6:42:21 FIG. 2.ÈPanels 1È3: PreÑare X-ray images of the 1991 December 2 Ñare taken with a thin aluminum Ðlter. The Ðeld of view is 1@.5, and the pixel size is 2A.4. North is to the right, and east is up. The logarithmic X-ray intensity is shown to see both the Ñare loop and the faint structure above the Ñare loop. The color table is optimized in each panel to see the structure of interest most clearly. An expanding loop structure is indicated with an arrow in panel 4. A vertical line in panel 5 is an artifact. The missing data on the right-hand side of panel 5 is due to the telemetry loss. Panels 4È15: Composite images consisting of four di erent images with thin aluminum Ðlter. A leg of the large-scale loop that goes through the apparent plasmoid is indicated by a white arrow in panel 11. The plasmoid is indicated by a white arrow in panel 15. Panel 16: A postñare X-ray image taken with a thin aluminum Ðlter. TSUNETA (see 483, 58) PLATE 13

10 FIG. 4.ÈUpper three rows: Temperature maps obtained with the pair of broadband Ðlters (thick aluminum and beryllium). The original 64 ] 64 pixel full-resolution images are trimmed to 64 (north-south) ] 41 pixel (east-west). The pixel size is 2A.4. North is to the right, and east is up. Contours of X-ray maps with levels of 1%, 1%, and 5% of the peak intensity in each map are also shown. Middle row: Pressure maps obtained from the derived temperatures and emission measures for panels 1, 4, 8, and 12. The logarithmic pressure is shown so that both the Ñare loop and the faint structure can be seen above the Ñare loop. L ower panel: X-ray maps (thin aluminum) for panels 1, 4, 8, and 12. The logarithmic intensity is shown so that both the Ñare loop and the faint structure can be seen above the Ñare loop. TSUNETA (see 483, 59) PLATE 14

11 FIG. 5.ÈHard X-ray contours overlaid on (a) temperature, (b) temperature (same as [a]), (c) pressure, (d) plasma density, and (e) soft X-ray maps at four di erent times: (1) 4:52:49È4:54:59 UT, (2) 4:54:59È4:56:53 UT, (3) 4:56:53È4:59:1 UT, and (4) 5:5:1È5:7:37 UT. The temperature maps (a) and (b) are the same. The contours on rows (b), (c), (d), and (e) are the HXT images of the L channel (14È23 kev). Panels 1 and 2 of the top row have the contours of the HXT M2 channel (33È53 kev), panel 3 the M1 channel (23È33 kev), and panel 4 the L channel (14È23 kev). TSUNETA (see 483, 51) PLATE 15

12 FIG. 6.ÈEnlarged map of Fig. 4 (3): (a) Temperature. (b) Pressure. (c) Temperature (same as [a]). The temperature, pressure, and density proðles along the lines indicated are shown in Fig. 7. (d) Plasma density. TSUNETA (see 483, 51) PLATE 16