- Email: [email protected]
Yohkoh
Pergamon
Adv. Syace Res. Vol. 20, No. 12, pp. 2319-2322, 1997 01997 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/97 $17.00 + 0.00 PII: SO273-1177(97)01052-l
LINE SHIFTS OBSERVED
BY BCSNOHKOH
W. Q. Gan* and T. Watanabe**
*Purple Mountain Observatory Nanjing 210008, China **National Astronomicd Observatoy, Mikate, To&v0 181, Japan
ABSTRACT Using the observational data of Bragg crystal spectromater on Yohkoh for the period from October, 1991 to the end of 1995, we have made a statistical study for the flares which show to be in a single loop or a single loop dominated in the SXT images. It is found that although the blue asymmetry is very common during the impulsive phase, the number of events with great total blueshift is very small. It is also found that the blueshift for most flares appears at the early impulsive phase and is temporally correlated with the broadening of the line. 0 1997 COSPAR. Publishedby Elsevier Science Ltd 1.
INTRODUCTION
It is still an open question for the role of chromospheric evaporation in a solar flare. At least there are some differences between the theoretical models and observations. Such differences were ever explained by Li et al. (1989), Antonucci (1989), and Gan et al. (1992) that the temporal resolution in the early observations was too low to catch up the early impulsive phase. The Bragg crystal spectrometer (BCS) on Yohkoh has been designed to greatly improve the temporal resolution (Culhane et al. 1991) and it should be able to provide a judgement about whether or not there is a dominant blueshift of the soft X-ray lines at the early impulsive phase. Mariska et al. (1993) made a statistical study based on the first year observations of Yohkoh. Bentley et al. (1994) analysed the temporal relationship between the hard X-ray burst and the CaXIX w blueshift. Besides the statistical studies, there is also a great deal of work for an individual event, which usually has a rather strong blueshift (e.g., Culhane et ~2. 1994; Doschek et al. 1994; Fludra et al. 1995; Gopalswamy et al. 1995). However, to what extent the evaporation model is correct, and to what degree the existing hydrodynamic model can explain the observation, the answers to these questions should be based on the detailed review of the observation by BCS/Yohkoh. Although Mariska et al. (1993) made a comprehensive statistical study, we have noted that the morphology of solar flares observed by Soft X-ray Telescope (SXT, see Tsuneta et al. 1991) on board Yohkoh has not been taken into account. In addition, the centriod shift, taken by them to characterize the line shift, can hardly distinguish the blue-asymmetry from total blueshift of the line. In this paper, we use a more direct method to reflect the line shift and include the consideration of soft X-ray morphology. The results are expected to provide a platform for the construction of future hydrodynamic models. 2. THE SAMPLES
2319
W. Q. Gan and T. Watanabe
2320
The soft X-ray morphology of solar flares observed by SXT is manifold. As the first criterion, we select such flares that the SXT image shows to have only a single loop or a single loop dominated at least during the impulsive phase. We exclude other flares with multi-loops or rather complex morphology in soft X-rays. As the second criterion, we select those flares which were effectively observed by BCS. Besides, the flux of CaXIX spectra should be rather strong, avoiding much fluctuations. We scan all of the flares observed by Yohkoh from its earliest avaiable data to the end of 1995. Among them we find about 200 flares that satisfy the criteria. After eliminating the spatial effects on the spectral asymmetries, we obtain finally 158 samples. 3. THE RESULTS
In order to display more directly the shift of CaXIX resonance line, we use the distance from the rest wavelength to the bisectors of the line profile at one third peak intensity (4X1/s), half and peak intensity (4x1) peak intensity (4x,,,), to represent the line shift, and we take these three parameters only when the line shift is at its maximum. This method can explicitly demonstrate whether it is a total shift or only an asymmetry. Figure 1 shows the frequency distribution of 4&/s, 4Ai,s, and 4x1 for the disk events with heliocentric angle less than 60”. We may see from it that during the rising phase of flares, blue wing emission is very common. The ratio of the events with blueshift greater than 0.3 rnA at 4x1,s is about 60%. At 4X1,,, and 4Ai, this ratio decreases to 45% and 15%. Only in a very few events is the great total line shift (~2 m?l) seen. Another notable characteristic is that about 10% of the disk events show obvious redshift at 4x1,s. This ratio seems to be larger than that found by Mariska ct al. (1993). 3.2 Heliographic
distribution
of the blueshift
We extract such events that have blueshift or nonshift, and average them over each 10” interval of the angle. Figure 2 presents the averaged blueshift
1
_,,I
3.1 The line shift of disk events i:i
-3
-2
-1
0
1
2
Fig. 1. The frequency distribution of line shifts -manifested by 4&/s, 4X1/2, and 4x1 (in mA) for the disk events with heliocentric angle less than 60”.
“,< I
1.4
.
1.2
.
1
.
5 g 0.8
$ 0.6 . g 0.4 . $ 3 0.2 . _-
0 -0.2 I-0.4 ’ 0
I 10
20
30
40 50 60 Heliocentric angle
70
60
90
Fig. 2. The average blueshift (in m.A) as a function’of heliocentric angle (in degrees), together with fla error bars.
2321
Line Shifts Observed by BCSNohkoh
0.4 Relative
0.6 intensity
0.8
1.0
Fig. 3. The distribution of the number of events with the relative intensity, which is defined as the ratio of the count rate in the CaXIX light curve where the line shift is at the maximum to the peak count rate of the light curve.
I...., 3
4 Maximum
5
6
7
8
line width
Fig. 4. The frequency distribution of the maximum line width (in ma) (see text).
1.0
3.5 1.5 2.0 2.5 3.0 Relative maximum width
Fig. 5. The frequency distribution tive line width (see text).
4.0
of the rela-
of A&/s as a function of heliographic angle, together with the flu error bars. It seems that from the disk center to the limb, the blueshift has a tendency of decrease, just as found by Mariska et al. (1993). 3.3 Temporal behavior of maximum
blueshift
We use IblIP to reflect the temporal behavior of the maximum blueshift, where 16 represents the count rate in the CaXIX light curve where the blueshift of Ax 11s is at the maximum, and I, the peak count rate of the light curve. Figure 3 gives the distribution of the number of events with &/I,. It shows that over 80% of the samples arrive at the shift maximum before I&/I, becoming greater than 0.5. The events in which the line shift maximum is after the maximum phase of the flare have never appeared in our samples. Figure 3 also demontrates that I&=0.25 is a good average value to reflect the shift maximum, as taken by Mariska et al. (1993). But if we take the shift just at 16/1,=0.25, we lose quite a number of events because they reach the shift maximum at other times. We analyse the duration
of the blueshift as well. The blueshift for most flares appears at the early
2322
W. Q. Gan and T. Watanabe
impulsive phase and continues from .ten to several tens of seconds. There are only about 5% of the events in which the blueshift can last until the maximum phase of the flare.. After the maximum phase, nearly no blueshifts have been seen in our samples. 3.4 The widths of the line We use the width at the half peak intensity of the line profile to represent the line width and we seek the maximum width for each flare. Figure 4 exhibits the number of events versus the maximum line widths. Generally speaking, the maximum line width is less than 5.5 mA. Only 7% of the events have the width greater than 5.5 mA. Among the samples, the greatest line width is 7.5 rn.A. To better display the broadening, we show in Figure 5 the ratio of the maximum line width to the line width at the peak total intensity. It shows that this ratio is mostly less than 2 and the maximum in our samples is about 3.6. We find that for most flares (- 85%) the time of the maximum line width is very close to the time of the maximum line shift; there are 13% of the events in which the maximum line shift is ahead of the maximum line width; the events in which the maximum shift is behind of the maximum line width seem to be very few. ACKNOWLEDGMENTS The data used in this paper are taken by the Yohkoh mission of ISAS, Japan, which was the joint efforts of Japanese, US, and UK scientists. REFERENCES Antonucci,E., Solar Flare Spectral Diagnosis: Present and Future, Sol. Whys., 121, 31 (1989). Bentley,R.D:, G.A. Doschek, G.M. Simnett, M.L. Rilee, J.T. Mariska, et al., The Correlation of Solar Flare Hard X-rays with Doppler Blueshifted Soft X-ray Flare Emission, ApJ, 421, L55 (1994). Culhane,J.L., E. Hiei, G.A. Doschek, A.M. Cruise, Y. Ogawara, et al., The Bragg Crystal Spectrometer for SOLAR-A, Sol. Phys., 136, 89 (1991). Culhane,J.L., A.T. Phillips, M. Inda-Koide, T. Kosugi, A. Fludra, et aZ., Yohkoh Obseravtions of the Creation of High-temperature Plasma in the Flare of 16 Dec., 1991, Sol: Phys.,.l53, 307 (1994). Doschek,G.A., J.T. Mariska, K.T. Strong, R.D. Bentley, C.M. Brown, et al., 91.11.9 Flare at 03:02 UT: Observations from Yohkoh, ApJ, 431, 888 (1994). Fludra,A., J.G. Doyle, T. Metcalf, J.R. Lemen, K.J.H. Phillips, et al, Evolution of Two Small Solar Flares, A&A, 303, 914 (1995). Gan,W.&., E. Rieger, C. Fang, and H.Q. Zhang, Spectral Diagnostics for a Thermal Hydrodynamical Model of a Solar Flare Loop, ABA, 266, 573 (1992). Gopalswamy,N., J.P. Raulin, M.R. Kundu, N. Nitta, J.R. Lemen, et al., VLA and Yohkoh Observations of an Ml.5 Flare, ApJ, 455, 715 (1995). Li,P., A.G. Emslie, and J.T. Mariska, Soft X-ray Diagnostics of Electron-heated Solar Flare Atmospheres, ApJ, 341, 1075 (1989). Mariska,J.T., G.A. Doschek, and R.D. Bentley, Flare Plasma Dynamics Observed with the Yohkoh BCS I. Properties of the CaXIX Resonance Line, ApJ, 419, 418 (1993). Tsuneta,S., L. Acton,. M. Bruner, J. Lemen, W. Brown, et al., The Soft X-Ray Telescope for the SOLAR-A Mission, Sol. Phys., 136, 37 (1991).
Adv. Syace Res. Vol. 20, No. 12, pp. 2319-2322, 1997 01997 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/97 $17.00 + 0.00 PII: SO273-1177(97)01052-l
LINE SHIFTS OBSERVED
BY BCSNOHKOH
W. Q. Gan* and T. Watanabe**
*Purple Mountain Observatory Nanjing 210008, China **National Astronomicd Observatoy, Mikate, To&v0 181, Japan
ABSTRACT Using the observational data of Bragg crystal spectromater on Yohkoh for the period from October, 1991 to the end of 1995, we have made a statistical study for the flares which show to be in a single loop or a single loop dominated in the SXT images. It is found that although the blue asymmetry is very common during the impulsive phase, the number of events with great total blueshift is very small. It is also found that the blueshift for most flares appears at the early impulsive phase and is temporally correlated with the broadening of the line. 0 1997 COSPAR. Publishedby Elsevier Science Ltd 1.
INTRODUCTION
It is still an open question for the role of chromospheric evaporation in a solar flare. At least there are some differences between the theoretical models and observations. Such differences were ever explained by Li et al. (1989), Antonucci (1989), and Gan et al. (1992) that the temporal resolution in the early observations was too low to catch up the early impulsive phase. The Bragg crystal spectrometer (BCS) on Yohkoh has been designed to greatly improve the temporal resolution (Culhane et al. 1991) and it should be able to provide a judgement about whether or not there is a dominant blueshift of the soft X-ray lines at the early impulsive phase. Mariska et al. (1993) made a statistical study based on the first year observations of Yohkoh. Bentley et al. (1994) analysed the temporal relationship between the hard X-ray burst and the CaXIX w blueshift. Besides the statistical studies, there is also a great deal of work for an individual event, which usually has a rather strong blueshift (e.g., Culhane et ~2. 1994; Doschek et al. 1994; Fludra et al. 1995; Gopalswamy et al. 1995). However, to what extent the evaporation model is correct, and to what degree the existing hydrodynamic model can explain the observation, the answers to these questions should be based on the detailed review of the observation by BCS/Yohkoh. Although Mariska et al. (1993) made a comprehensive statistical study, we have noted that the morphology of solar flares observed by Soft X-ray Telescope (SXT, see Tsuneta et al. 1991) on board Yohkoh has not been taken into account. In addition, the centriod shift, taken by them to characterize the line shift, can hardly distinguish the blue-asymmetry from total blueshift of the line. In this paper, we use a more direct method to reflect the line shift and include the consideration of soft X-ray morphology. The results are expected to provide a platform for the construction of future hydrodynamic models. 2. THE SAMPLES
2319
W. Q. Gan and T. Watanabe
2320
The soft X-ray morphology of solar flares observed by SXT is manifold. As the first criterion, we select such flares that the SXT image shows to have only a single loop or a single loop dominated at least during the impulsive phase. We exclude other flares with multi-loops or rather complex morphology in soft X-rays. As the second criterion, we select those flares which were effectively observed by BCS. Besides, the flux of CaXIX spectra should be rather strong, avoiding much fluctuations. We scan all of the flares observed by Yohkoh from its earliest avaiable data to the end of 1995. Among them we find about 200 flares that satisfy the criteria. After eliminating the spatial effects on the spectral asymmetries, we obtain finally 158 samples. 3. THE RESULTS
In order to display more directly the shift of CaXIX resonance line, we use the distance from the rest wavelength to the bisectors of the line profile at one third peak intensity (4X1/s), half and peak intensity (4x1) peak intensity (4x,,,), to represent the line shift, and we take these three parameters only when the line shift is at its maximum. This method can explicitly demonstrate whether it is a total shift or only an asymmetry. Figure 1 shows the frequency distribution of 4&/s, 4Ai,s, and 4x1 for the disk events with heliocentric angle less than 60”. We may see from it that during the rising phase of flares, blue wing emission is very common. The ratio of the events with blueshift greater than 0.3 rnA at 4x1,s is about 60%. At 4X1,,, and 4Ai, this ratio decreases to 45% and 15%. Only in a very few events is the great total line shift (~2 m?l) seen. Another notable characteristic is that about 10% of the disk events show obvious redshift at 4x1,s. This ratio seems to be larger than that found by Mariska ct al. (1993). 3.2 Heliographic
distribution
of the blueshift
We extract such events that have blueshift or nonshift, and average them over each 10” interval of the angle. Figure 2 presents the averaged blueshift
1
_,,I
3.1 The line shift of disk events i:i
-3
-2
-1
0
1
2
Fig. 1. The frequency distribution of line shifts -manifested by 4&/s, 4X1/2, and 4x1 (in mA) for the disk events with heliocentric angle less than 60”.
“,< I
1.4
.
1.2
.
1
.
5 g 0.8
$ 0.6 . g 0.4 . $ 3 0.2 . _-
0 -0.2 I-0.4 ’ 0
I 10
20
30
40 50 60 Heliocentric angle
70
60
90
Fig. 2. The average blueshift (in m.A) as a function’of heliocentric angle (in degrees), together with fla error bars.
2321
Line Shifts Observed by BCSNohkoh
0.4 Relative
0.6 intensity
0.8
1.0
Fig. 3. The distribution of the number of events with the relative intensity, which is defined as the ratio of the count rate in the CaXIX light curve where the line shift is at the maximum to the peak count rate of the light curve.
I...., 3
4 Maximum
5
6
7
8
line width
Fig. 4. The frequency distribution of the maximum line width (in ma) (see text).
1.0
3.5 1.5 2.0 2.5 3.0 Relative maximum width
Fig. 5. The frequency distribution tive line width (see text).
4.0
of the rela-
of A&/s as a function of heliographic angle, together with the flu error bars. It seems that from the disk center to the limb, the blueshift has a tendency of decrease, just as found by Mariska et al. (1993). 3.3 Temporal behavior of maximum
blueshift
We use IblIP to reflect the temporal behavior of the maximum blueshift, where 16 represents the count rate in the CaXIX light curve where the blueshift of Ax 11s is at the maximum, and I, the peak count rate of the light curve. Figure 3 gives the distribution of the number of events with &/I,. It shows that over 80% of the samples arrive at the shift maximum before I&/I, becoming greater than 0.5. The events in which the line shift maximum is after the maximum phase of the flare have never appeared in our samples. Figure 3 also demontrates that I&=0.25 is a good average value to reflect the shift maximum, as taken by Mariska et al. (1993). But if we take the shift just at 16/1,=0.25, we lose quite a number of events because they reach the shift maximum at other times. We analyse the duration
of the blueshift as well. The blueshift for most flares appears at the early
2322
W. Q. Gan and T. Watanabe
impulsive phase and continues from .ten to several tens of seconds. There are only about 5% of the events in which the blueshift can last until the maximum phase of the flare.. After the maximum phase, nearly no blueshifts have been seen in our samples. 3.4 The widths of the line We use the width at the half peak intensity of the line profile to represent the line width and we seek the maximum width for each flare. Figure 4 exhibits the number of events versus the maximum line widths. Generally speaking, the maximum line width is less than 5.5 mA. Only 7% of the events have the width greater than 5.5 mA. Among the samples, the greatest line width is 7.5 rn.A. To better display the broadening, we show in Figure 5 the ratio of the maximum line width to the line width at the peak total intensity. It shows that this ratio is mostly less than 2 and the maximum in our samples is about 3.6. We find that for most flares (- 85%) the time of the maximum line width is very close to the time of the maximum line shift; there are 13% of the events in which the maximum line shift is ahead of the maximum line width; the events in which the maximum shift is behind of the maximum line width seem to be very few. ACKNOWLEDGMENTS The data used in this paper are taken by the Yohkoh mission of ISAS, Japan, which was the joint efforts of Japanese, US, and UK scientists. REFERENCES Antonucci,E., Solar Flare Spectral Diagnosis: Present and Future, Sol. Whys., 121, 31 (1989). Bentley,R.D:, G.A. Doschek, G.M. Simnett, M.L. Rilee, J.T. Mariska, et al., The Correlation of Solar Flare Hard X-rays with Doppler Blueshifted Soft X-ray Flare Emission, ApJ, 421, L55 (1994). Culhane,J.L., E. Hiei, G.A. Doschek, A.M. Cruise, Y. Ogawara, et al., The Bragg Crystal Spectrometer for SOLAR-A, Sol. Phys., 136, 89 (1991). Culhane,J.L., A.T. Phillips, M. Inda-Koide, T. Kosugi, A. Fludra, et aZ., Yohkoh Obseravtions of the Creation of High-temperature Plasma in the Flare of 16 Dec., 1991, Sol: Phys.,.l53, 307 (1994). Doschek,G.A., J.T. Mariska, K.T. Strong, R.D. Bentley, C.M. Brown, et al., 91.11.9 Flare at 03:02 UT: Observations from Yohkoh, ApJ, 431, 888 (1994). Fludra,A., J.G. Doyle, T. Metcalf, J.R. Lemen, K.J.H. Phillips, et al, Evolution of Two Small Solar Flares, A&A, 303, 914 (1995). Gan,W.&., E. Rieger, C. Fang, and H.Q. Zhang, Spectral Diagnostics for a Thermal Hydrodynamical Model of a Solar Flare Loop, ABA, 266, 573 (1992). Gopalswamy,N., J.P. Raulin, M.R. Kundu, N. Nitta, J.R. Lemen, et al., VLA and Yohkoh Observations of an Ml.5 Flare, ApJ, 455, 715 (1995). Li,P., A.G. Emslie, and J.T. Mariska, Soft X-ray Diagnostics of Electron-heated Solar Flare Atmospheres, ApJ, 341, 1075 (1989). Mariska,J.T., G.A. Doschek, and R.D. Bentley, Flare Plasma Dynamics Observed with the Yohkoh BCS I. Properties of the CaXIX Resonance Line, ApJ, 419, 418 (1993). Tsuneta,S., L. Acton,. M. Bruner, J. Lemen, W. Brown, et al., The Soft X-Ray Telescope for the SOLAR-A Mission, Sol. Phys., 136, 37 (1991).