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, 年 6 月 6 日 太陽雑誌会 西田

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.

1. Introduction

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. Data and analysis

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 195 Å (Fe XII) SOHO/EIT images, since this channel allows one to see both the prominence and coronal structures in CMEs (Dere et al. 1997).

Figure 1

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: 1.Make movie files of SXT. 2.Identify large-scale plasma ejection around the impulsive phase looking half- and quarter-resolution movies. 3.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.

Table 1 Continue …

3. Results and discussion

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 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. Figure 5 Time differences between X-ray plasma ejection start and HXR flare peak (93 events).

3.2. Temporal relationship 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. 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).

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. Figure 7 Height-time behavior of a well- observed CME on 1998 June 11 from the lower corona to the higher corona. GOES CME (C1) CME (C2, C3) Extrapolated CME time

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 % 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.

4. Summary and conclusion

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.

Nitta & Akiyama (1999)