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2005 年 6 月 6 日 太陽雑誌会 西田. A study of flare-associated X-ray plasma ejections. I. Association with coronal mass ejections. Yeon-Han Kim, Y.-J. Moon, K.-S. Cho, Kap-Sung Kim, and Y. D. Park, 2005, ApJ, 622, 1240-1250. Abstract.
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2005年6月6日 太陽雑誌会 西田 A study of flare-associated X-ray plasma ejections.I. Association with coronal mass ejections Yeon-Han Kim, Y.-J. Moon, K.-S. Cho, Kap-Sung Kim, and Y. D. Park, 2005, ApJ, 622, 1240-1250
Abstract • The authors have made a statistical study of the relation ship between flare-associated X-ray plasma ejections and coronal mass ejections (CMEs). • In 279 limb flares observed by Yohkoh/SXT, • 69% of the events with plasmoid ejections are associated with CMEs, observed by SOHO/LASCO, • 84% of the events without plasmoid ejections have no related CMEs. • X-ray plasma ejections occur nearly simultaneously with HXR peak. • 80% of the CMEs are preceded X-ray plasma ejections, by approximately 20 minutes on average.
X-ray plasma ejections • Outside main flare loops • Around the impulsive phase of flares • Bloblike, looplike, jetlike, or complex in shape • Found in both LDE and impulsive flares
Three stages of kinematic evolution • Ohyama & Shibata (1997) found that the ejected material was already heated to 10MK before the start o the ejection and that its temperature was nearly the same as that of the flare loop. • The three stages of kinematic evolution: preflare rise, main rise, and gradual propagation. • This is very similar to the kinematic evolution of CMEs. (Zhang et al. 2001a) • Owing to their kinematic and morphological likeness to CMES, X-ray plasma ejections have sometimes been regarded as possibly being direct signatures of CMEs.
Onset of CMEs • The onset and origin of CMEs are still not well understood. • In order to understand the mechanism of CME launch, one needs to observe their early signature in the low corona. • There are several candidate CME early signatures, such as filament eruptions, sigmoid-to-arcade events, flare-associated X-ray plasma ejections, and coronal dimmings.
Relation between flare-associated X-ray ejections and CMEs • Nitta & Akiyama (1999) made the first attempt to correlate flare-associated plasma ejections and CMEs, using 17 limb flares. • No CME around the flare time → No X-ray ejection • Eight flares with CMEs (all but one) → X-ray ejection • The authors made statistical extension of Nitta & Akiyama (1999). • In addition, they examine the difference in onset time between X-ray plasma ejections and their associated phenomena and pay special attention to comparing the event times between the X-ray plasma ejections and the CMEs when the CMEs are extrapolated into the Yohkoh field of view.
2.1. Data • For the identification of X-ray plasma ejections, they used all flare-mode data of Yohkoh/SXT with high temporal resolution. • The flare time was taken by GOES and Yohkoh/HXT. • The flare location comes from the list of optical flares at NGDC or SXT images. • They used CME catalogue (Yashiro et al. 2004) and raw LASCO data observed in order to identify the position and speed of the CMEs. • They also used the 195Å (Fe XII) SOHO/EIT images, since this channel allows one to see both the prominence and coronal structures in CMEs (Dere et al. 1997).
2.2. Event selection • They consider 279 limb flares whose longitudes are larger than 60° from all flare-mode data in Yohkoh/SXT from 1999 April to 2001 March. • The identification procedure whether each flares accompanied an X-ray plasma ejection: • Make movie files of SXT. • Identify large-scale plasma ejection around the impulsive phase looking half- and quarter-resolution movies. • For the events without large-scale plasma ejections, they examined full-resolution movies. • They found many confined ejections that did not show any eruptive motion in the half- and quarter-resolution images but did appear in the full-resolution images. • As a result of this analysis, they found a total of 137 flares with X-ray plasma ejections. • The identified event times for some X-ray plasma ejections may be a little later than the real onset times, because of their apparently being hidden by flare loops.
2.2. Event selection • They established associations between the flares and the LASCO CMEs according to temporal and spatial proximity; that is, the flare start time is within 1 hr of the CME onset time extrapolated at 1.1 R⊙ using the constant-speed method, and the flare position angle is within the angular extent of the associated CME. • We excluded data from the period when LASCO made no observations because of its operational condition.
2.3. Morphological classification • They classified the X-ray plasma jections into five groups according to their shape: • Loop-type (60 events) • Shape of loops. • Spray-type (40) • Continuous stream of plasma without any typical shape. • Jet-type (11) • Collimated motions of plasma. • Confined ejection (18) • Limited plasma motion near the flaring site that is usually seen only in the full-resolution flare-mode movie. • Other (8)
Figure 2 • Typical example of a loop-type X-ray plasma ejection, associated with an M2.4 X-ray flare on 1999 July 25. • Initial speed = about 112 km/s
Figure 3 • Running-difference images of the CME associated with the plasma ejection shown in Fig. 2. • The central circle drawn in the top left panel indicates the solar disk, and the small box represents the Yohkoh field of view, which corresponds to 10′4 × 10′4. • X-ray plasma ejection is quite similar to that of the associated CME. ↓ Such a similarity may argue for the possibility that X-ray plasma ejections are early signatures of CMEs.
Figure 4 • A jet-type X-ray plasma ejection and its associated CME on 2000 October 26. • Top: Half-resolution SXT flare-mode images. • Bottom: The corresponding LASCO C2 and EIT images. • It is interesting that this ejection was accompanied by quite a narrow CME compared with the one associated with the loop-type ejection on 1999 July 25.
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3.1. CME Association • About half (137/279) of the flares have associated plasma ejections within the sensitivity of the Yohkoh SXT. • While 69% (95/137) of the X-ray plasma ejections are associated with CMEs, 31% (42/137) have no CMEs. • Of the events without plasma ejections, only 16% (23/142) are related to CMEs, and 84% (119/142) do not have associated CMEs. • On the other hand, it is also found that 81% (95/118) of the flares associated with LASCO CMEs are related to X-ray plasma ejections. • Our results support Nitta & Akiyama (1999), who found a close correlation between the presence or absence of X-ray plasma ejections and CMEs using a sample of 17 limb flares.
Flare-strength dependence of the association between X-ray plasma ejections and CMEs • Stronger flares with CMEs are more closely associated with X-ray plasma ejections. • It is also found that for LDEs, all flare-associated CMEs have associated X-ray plasma ejections regardless of their strength. It it known that LDEs are highly correlated with CMEs and filament eruptions. • The association of non-LDE flares with CMEs varies with flare strength. • Thus, Nitta (2002) proposed that LDEs may be a part of the CME process that commence as a result of large-scale instability or loss of equilibrium and that non-LDE flares are something else that could occur without CMEs.
Morphological dependence of the CME associations • Loop-type, spray-type, and jet-type of plasma ejections show a relatively high association with CMEs; in paticular, the jet type plasma ejections are all associated with CMEs. • The morphology of an X-ray plasma ejection will be affected by the magnetic topology of the flaring site. • Such a close correlation may imply that an open field structure near a flaring site makes a better environment or producing a CME.
3.2. Temporal relationship Figure 5 Time differences between X-ray plasma ejection start and HXR flare peak (93 events). • The time differences fall within 10 minutes for most events. • The mean time difference is only about -2 minutes, implying that X-ray plasma ejections are nearly coincident with HXR flare peak times. • Our results also support the theory that X-ray plasma ejections are probably due to magnetic reconnection.
3.2. Temporal relationship Figure 6 Comparison of the event times of the X-ray plasma ejections and the CME event times extrapolated into the Yohkoh field of view. (43 events). • They compared the event times of the X-ray plasma ejections with the extrapolated CME-front times at the same location in the Yohkoh field of view. • They find that the extrapolated CME fronts in most cases (35/43) preceded the expanding fronts of the X-ray plasma ejections, by about 20 minutes on average. • In addition, for about 28% of the events (12/43) both fronts are coincident to within 10 minutes.
3.2. Temporal relationship • They note several reports of strong accelerations in the lower corona (e.g., Zhang et al. 2001a) • If we were to consider such accelerations, the CME event times would be even earlier. • From Figure 7, they find that the CME was strongly accelerated below 2R⊙. • As a result, the real onset time is found to be much earlier than the onset time predicted by the constant-speed method. • Statistically speaking, the fronts of X-ray plasma ejections seem to represent the CME the CMEs’ internal structures rather than early signatures of CME fronts. GOES CME (C2, C3) CME (C1) Extrapolated CME time Figure 7 Height-time behavior of a well-observed CME on 1998 June 11 from the lower corona to the higher corona.
3.3. Flare association • Shibata (1995) have argued that X-ray plasma ejections are a universal phenomenon in solar flares. • Several observations of each LDEs and impulsive flares are consistent with the predictions of CSHKP-type flare models, in which magnetic reconnections occur in the vertical current sheet above flare loops.
3.3. Flare association • Occurrence rate of flare associated X-ray plasma ejections are 35-40% (Nitta 1996), 20-35% (Akiyama), and 43-46% (Ohyama & Shibata 2000). • Although X-ray plasma ejections were originally found around the flare impulsive phase (shibata et al. 1995), SXT flare-mode observations occasionally started too late to catch this phase. • 63-70% with observations that started before the HXR peak time (Ohyama & Shibata 2000) • It is difficult to detect X-ray plasmoids in the weaker flares and proposed that X-ray plasma ejections are a general phenomenon associated with solar flares. • The authors’ results supports it.
Summary and conclusion • In this work, we have carried out a comprehensive statistical study in order to understand the relationship between flare associated X-ray plasma ejections and CMEs, using data from 1999 April to 2001 March. • There have been several studies of the close relationship between flares and CMEs. • A detailed discussion will be presented in a separate paper.