- Email: [email protected]
X-ray eruptive structures associated with small flares
Adv. Space Res. Vol. 26, NO. 3, pp. 465468,2ooO Q 2000 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/00 $20.00 + 0.00
Pergamon www.elsevier.nl/locate/asr
PII: SO273-1177(99)01107-2
X-RAY ERUPTIVE STRUCTURES WITH SMALL FLARES S. Akiyama’
and H. Hara2
’ Departm e n t of Astronomical Mitaka, ‘National
ASSOCIATED
Tokyo 181-8588, Astronomical
Science,
Graduate
University for Advanced Studies,
%I?l-1 Osawa,
Japan Observatory,
Z-21-1 Osawa, Mitaka,
Tokyo 181-8588,
Japan
ABSTRACT We searched for X-ray eruptive structures above the flare loops, whose soft X-ray flux at the peak phase ranges from class A to C in the GOES classification, using the Yohkoh soft X-ray observations in 1996 when the background corona was very weak. We found 12 plasmoid-like or loop-like ejecta out of 43 flares studied. Existence of these eruptive structures implies that the magnetic reconnection model, which was strongly supported by Yohkoh observations of flares of a narrow range of X-ray peak flux, can explain the solar flares covering more than five orders of magnitude in the soft X-ray flux. We also found that the projected outgoing velocities of the ejecta are systematically slower in Q 2000 COSPAR.Published by Elsevier Science Ltd. small flares than in large flares.
INTRODUCTION There are two kinds of solar flares. One is the long duration event (LDE) flare, and the other is the impulsive flare which has a short lifetime. Filament eruptions and the subsequent expansion of bright pairs of ribbons seen in Ha for LDE flares led to a magnetic reconnection model called CSHKP model (Carmichael 1964; Sturrock 1966; Hirayama 1974; Kopp and Pneuman 1976). Soft X-ray observations by Yohkoh revealed that the LDE flares often have clear cusp-shaped X-ray structures at the loop top, and that the outer part of cusp structures has a higher temperature, suggesting the magnetic reconnection process in solar flares (Tsuneta et al. 1992). However, other mechanisms have been searched for understanding impulsive flares, because there was no direct evidence in those events for the magnetic reconnection. Discovery of the hard X-ray source above the soft X-ray loop in some impulsive flares (Masuda et al. 1994) led to a unified model which explains both LDE and impulsive flares by a single mechanism, that is, the magnetic reconnection. This idea was strongly supported by the detection of plasmoidlike or loop-like faint X-ray plasma ejecta above energetic impulsive flare loops (Shibata et al. 1995). A slow rise of the ejecta before the flare onset and subsequent sudden acceleration at the impulsive phase (Tsuneta 1997; Ohyama and Shibata 1997) indicate an important role of the magnetic structure of the ejecta in the reconnection process (Magara et al. 1997).
465
466
S. Akiyama and H. Hara
In this paper we pay attention to plasma ejecta in less energetic flares, which belong to class A-C in the GOES classification, to make clear if these flares are also explained by the magnetic reconnection model like as energetic flares. DATA AND ANALYSIS In order to find plasma ejecta above flare loops associated with weak X-ray flares, we used the full frame images (FFIs) obtained in 1996 with Y&oh/SXT. The FFIs are taken when the Yohlcoh is in the QUIET mode, not in the FLARE mode which is automatically triggered for the flare observations by the soft and hard X-ray spectrometers on board YohlEoh. Therefore FFIs show weaker flare events, below about C level in the GOES classification. The data in the QUIET mode have been barely used for flare studies, especially for the study of plasma ejecta above flare loops. Variation of a large-scale coronal structure around the flare site can be seen because of wide observation region of FFIs. There are a few advantages of using FFIs obtained in 1996 for the flare study: (1) Faint X-ray structures around flare loops are easily detected because of long exposures for the FFIs and a very weak background emission in the solar activity minimum phase. (2) A study of low-temperature coronal structures and coronal mass ejection around the flare site is possible because of the presence of SOHO. We detected 43 GOES X-ray flares with bright flaring loops and found X-ray plasma ejecta in 12 cases. The velocity of ejecta which is a component projected into the plane of the sky were estimated from the proper motion of the brightest point at the front part of the outgoing ejecta along the line should be from a point to point. The error of the velocity is estimated from a difference among the velocities derived from different parts of the ejecta. RESULTS
AND DISCUSSION
We searched for X-ray plasma ejecta in 43 flares which we selected and found 12 events. There seems to be two types of X-ray ejecta ; one is loop-like as seen in the left panel of Figure 1, and the other is a blob, we term a plusmoid, which is similar to the eruptive structure as shown in the left panel of Figure 2. The X-ray fluxes of the 12 events range from class A to C in the GOES classification. The existence of these plasma ejecta above flare loops indicates that the magnetic reconnection is functioning irrespective of the magnitude of energy release represented by the X-ray flux. As for the projected velocity of the ejecta in the case of 1996 August 31 flare shown in the right panel of Figure 1, a speed of N 20 km s-l was found. This is much lower than the velocities of ejecta reported in the previous studies (Shibata et al. 1995; Hudson et al. 1996; Tsuneta 1997; Ohyama and Shibata 1997). The flares in the previous studies, the 1991 December 2 flare in the right panel of Figure 2 and others, were re-analyzed using our method and the projected
X-Ray
GOES X-Rays
I
I
lE-31
Eruptive
I
Structures
Associated
With
Small
Flares
I Flux of Flare
-
and Distance
Velocitv
of Eiecto
Velocity of Eject0 .
q
0
Distance
of Eject0
A
I
I
Start
I
20100
l&O0
Time
(31 Au-96
I
18:OO
17530
22:oo
Start
16:Oo:OO)
19:OO
2o:OO 21:oo 16:29:41)
Time (31 -Aug-96
Fig. 1. Loop-like ejecta associated with small X-ray flux in the 1996 August 31 flare [Left] Soft X-ray structures of ejecta and GOES time profile. The ejected loopsare indicated arrow. [Right] T’rme profiles of the soft X-ray flux, the velocity, and the distance of ejection. ( x ), a square ( 17 ) and a triangle ejecta,
and the distance
of ejecta
by an A cross
( n ) symbol stand for the soft X-ray flux, the velocity of
from starting
point, respectively.
GOES X-Roys
Flyx of Flare 10
‘E::~
. .
. ”
-
Velocity
‘.
‘1..
x
ond Distance
1.
Flux of Flare
0 Velocity Y
1
I
\
I
of Ejecto
I
t
E P
I
B 5 G
05:20 Start Time
0340 (OZ-k-91
O&O0 04w:oZ)
06:20
-lSO’;-
-50
.
106.__.
4
I I
-10
.
3-8
z o 8
. t. . 0 2 2 6, ,o - lOOiJ--0 . 8 . z-4 ‘w’ x. B .=
E
05:oo
200
.
of Eject0
A Distance of Eject0
T \
‘.
3. z-25
:: 2
.
04% Start Time
.
.
I
.
05:OO (02~Dee-91
.
I 05:O4 04:52:05)
.
,:O
-0
05:08
Fig. 2. Blob-like ejecta associated with large X-ray flux in the 1991 December 2 flare [Left] Soft X-ray structures of ejecta and GOES time profile. The ejected blob is indicated by an arrow. [Right] Time profiles of the soft X-ray flux, the velocity, and the distance of ejection. The symbols are the same as in fig. 1.
468
S. Akiyama and H.Hara
Velocity
-
X-ray
Flux I
16%566
F
E6ONl6 0
F
-5
E b 0
60666
&Jpp
w6667
W5lNl3++f6666 E’+=
1o-6-
~6657
&I5
2
+
w72517*
LL 2 ;
it
&N10
25
E
w67616 CI
WEON
_ :
of Flares waOM
+mffsJ +am
w6&6 A6525
572~26
10-7:
X E55Nll +
I
10-8 10 Maximum
100 Velocity of Ejecta
1000 [km/s]
Fig. 3. The outgoing velocities of X-ray ejecta and soft X-ray fluxes of flares Plus ( + ) and square ( 0 ) symbol stand for events of this study and previous studies, respectively (Shibata et al. 1995, Tsuneta 1997, Ohyama and Shibata 1997 ). The letters of each symbols indicate the location of flare onset in the heliocentric coordinate.
SUMMARY 1. X-ray plasma ejecta were found in 12 case out of 43 flares of GOES class A-C. This indicates that the magnetic reconnection of the magnitude
is functioning
as the fundamental
physical mechanism
irrespective
of energy release.
2. The velocities of ejecta for weak flares are generally lower than those for major flares. This may reflect information
on the reconnection
process around the site of energy release.
REFERENCES Carmichael,
H., in AAS-NASA
Symp. on the Physics of Solar Flares, ed. W. N. Hess, NASA-SP
50,
451 (1964) Hirayama, T., Solar Phys., 34, 323 (1974) Hudson, H. S., Acton, L. W. and Freeland, S. L., ApJ, 470, 629 (1996) Kopp, R. A., and Pneuman
G. W., Solar Phys., 50, 85 (1976)
Magara, T., Shibata, K. and Yokoyama, T., ApJ, 487, 437 (1977) Masuda, S., Kosugi, T., Hara, H., Tsuneta, S. arid Ogawara, Y., Nature, 371, 495 (1994) Ohyama, M., and Shibata, K., PASJ, 49, 249 (1997) Shibata, K., Masuda, S., Shimojo,
M., Hara, H., Yokoyama, T., Tsuneta, S., Kosugi, T. and Ogawara,
Y., ApJ, 451, L83 (1995) Sturrock, P. A., Nature, 211, 695 (1966) Tsuneta, S., Hara, H., Shimizu, T., Acton, PASJ, 44, L63 (1992) Tsuneta, S., ApJ, 484, 507 (1997)
L. W., Strong, K. T., Hudson, H. S. and Ogawara, Y.,
Pergamon www.elsevier.nl/locate/asr
PII: SO273-1177(99)01107-2
X-RAY ERUPTIVE STRUCTURES WITH SMALL FLARES S. Akiyama’
and H. Hara2
’ Departm e n t of Astronomical Mitaka, ‘National
ASSOCIATED
Tokyo 181-8588, Astronomical
Science,
Graduate
University for Advanced Studies,
%I?l-1 Osawa,
Japan Observatory,
Z-21-1 Osawa, Mitaka,
Tokyo 181-8588,
Japan
ABSTRACT We searched for X-ray eruptive structures above the flare loops, whose soft X-ray flux at the peak phase ranges from class A to C in the GOES classification, using the Yohkoh soft X-ray observations in 1996 when the background corona was very weak. We found 12 plasmoid-like or loop-like ejecta out of 43 flares studied. Existence of these eruptive structures implies that the magnetic reconnection model, which was strongly supported by Yohkoh observations of flares of a narrow range of X-ray peak flux, can explain the solar flares covering more than five orders of magnitude in the soft X-ray flux. We also found that the projected outgoing velocities of the ejecta are systematically slower in Q 2000 COSPAR.Published by Elsevier Science Ltd. small flares than in large flares.
INTRODUCTION There are two kinds of solar flares. One is the long duration event (LDE) flare, and the other is the impulsive flare which has a short lifetime. Filament eruptions and the subsequent expansion of bright pairs of ribbons seen in Ha for LDE flares led to a magnetic reconnection model called CSHKP model (Carmichael 1964; Sturrock 1966; Hirayama 1974; Kopp and Pneuman 1976). Soft X-ray observations by Yohkoh revealed that the LDE flares often have clear cusp-shaped X-ray structures at the loop top, and that the outer part of cusp structures has a higher temperature, suggesting the magnetic reconnection process in solar flares (Tsuneta et al. 1992). However, other mechanisms have been searched for understanding impulsive flares, because there was no direct evidence in those events for the magnetic reconnection. Discovery of the hard X-ray source above the soft X-ray loop in some impulsive flares (Masuda et al. 1994) led to a unified model which explains both LDE and impulsive flares by a single mechanism, that is, the magnetic reconnection. This idea was strongly supported by the detection of plasmoidlike or loop-like faint X-ray plasma ejecta above energetic impulsive flare loops (Shibata et al. 1995). A slow rise of the ejecta before the flare onset and subsequent sudden acceleration at the impulsive phase (Tsuneta 1997; Ohyama and Shibata 1997) indicate an important role of the magnetic structure of the ejecta in the reconnection process (Magara et al. 1997).
465
466
S. Akiyama and H. Hara
In this paper we pay attention to plasma ejecta in less energetic flares, which belong to class A-C in the GOES classification, to make clear if these flares are also explained by the magnetic reconnection model like as energetic flares. DATA AND ANALYSIS In order to find plasma ejecta above flare loops associated with weak X-ray flares, we used the full frame images (FFIs) obtained in 1996 with Y&oh/SXT. The FFIs are taken when the Yohlcoh is in the QUIET mode, not in the FLARE mode which is automatically triggered for the flare observations by the soft and hard X-ray spectrometers on board YohlEoh. Therefore FFIs show weaker flare events, below about C level in the GOES classification. The data in the QUIET mode have been barely used for flare studies, especially for the study of plasma ejecta above flare loops. Variation of a large-scale coronal structure around the flare site can be seen because of wide observation region of FFIs. There are a few advantages of using FFIs obtained in 1996 for the flare study: (1) Faint X-ray structures around flare loops are easily detected because of long exposures for the FFIs and a very weak background emission in the solar activity minimum phase. (2) A study of low-temperature coronal structures and coronal mass ejection around the flare site is possible because of the presence of SOHO. We detected 43 GOES X-ray flares with bright flaring loops and found X-ray plasma ejecta in 12 cases. The velocity of ejecta which is a component projected into the plane of the sky were estimated from the proper motion of the brightest point at the front part of the outgoing ejecta along the line should be from a point to point. The error of the velocity is estimated from a difference among the velocities derived from different parts of the ejecta. RESULTS
AND DISCUSSION
We searched for X-ray plasma ejecta in 43 flares which we selected and found 12 events. There seems to be two types of X-ray ejecta ; one is loop-like as seen in the left panel of Figure 1, and the other is a blob, we term a plusmoid, which is similar to the eruptive structure as shown in the left panel of Figure 2. The X-ray fluxes of the 12 events range from class A to C in the GOES classification. The existence of these plasma ejecta above flare loops indicates that the magnetic reconnection is functioning irrespective of the magnitude of energy release represented by the X-ray flux. As for the projected velocity of the ejecta in the case of 1996 August 31 flare shown in the right panel of Figure 1, a speed of N 20 km s-l was found. This is much lower than the velocities of ejecta reported in the previous studies (Shibata et al. 1995; Hudson et al. 1996; Tsuneta 1997; Ohyama and Shibata 1997). The flares in the previous studies, the 1991 December 2 flare in the right panel of Figure 2 and others, were re-analyzed using our method and the projected
X-Ray
GOES X-Rays
I
I
lE-31
Eruptive
I
Structures
Associated
With
Small
Flares
I Flux of Flare
-
and Distance
Velocitv
of Eiecto
Velocity of Eject0 .
q
0
Distance
of Eject0
A
I
I
Start
I
20100
l&O0
Time
(31 Au-96
I
18:OO
17530
22:oo
Start
16:Oo:OO)
19:OO
2o:OO 21:oo 16:29:41)
Time (31 -Aug-96
Fig. 1. Loop-like ejecta associated with small X-ray flux in the 1996 August 31 flare [Left] Soft X-ray structures of ejecta and GOES time profile. The ejected loopsare indicated arrow. [Right] T’rme profiles of the soft X-ray flux, the velocity, and the distance of ejection. ( x ), a square ( 17 ) and a triangle ejecta,
and the distance
of ejecta
by an A cross
( n ) symbol stand for the soft X-ray flux, the velocity of
from starting
point, respectively.
GOES X-Roys
Flyx of Flare 10
‘E::~
. .
. ”
-
Velocity
‘.
‘1..
x
ond Distance
1.
Flux of Flare
0 Velocity Y
1
I
\
I
of Ejecto
I
t
E P
I
B 5 G
05:20 Start Time
0340 (OZ-k-91
O&O0 04w:oZ)
06:20
-lSO’;-
-50
.
106.__.
4
I I
-10
.
3-8
z o 8
. t. . 0 2 2 6, ,o - lOOiJ--0 . 8 . z-4 ‘w’ x. B .=
E
05:oo
200
.
of Eject0
A Distance of Eject0
T \
‘.
3. z-25
:: 2
.
04% Start Time
.
.
I
.
05:OO (02~Dee-91
.
I 05:O4 04:52:05)
.
,:O
-0
05:08
Fig. 2. Blob-like ejecta associated with large X-ray flux in the 1991 December 2 flare [Left] Soft X-ray structures of ejecta and GOES time profile. The ejected blob is indicated by an arrow. [Right] Time profiles of the soft X-ray flux, the velocity, and the distance of ejection. The symbols are the same as in fig. 1.
468
S. Akiyama and H.Hara
Velocity
-
X-ray
Flux I
16%566
F
E6ONl6 0
F
-5
E b 0
60666
&Jpp
w6667
W5lNl3++f6666 E’+=
1o-6-
~6657
&I5
2
+
w72517*
LL 2 ;
it
&N10
25
E
w67616 CI
WEON
_ :
of Flares waOM
+mffsJ +am
w6&6 A6525
572~26
10-7:
X E55Nll +
I
10-8 10 Maximum
100 Velocity of Ejecta
1000 [km/s]
Fig. 3. The outgoing velocities of X-ray ejecta and soft X-ray fluxes of flares Plus ( + ) and square ( 0 ) symbol stand for events of this study and previous studies, respectively (Shibata et al. 1995, Tsuneta 1997, Ohyama and Shibata 1997 ). The letters of each symbols indicate the location of flare onset in the heliocentric coordinate.
SUMMARY 1. X-ray plasma ejecta were found in 12 case out of 43 flares of GOES class A-C. This indicates that the magnetic reconnection of the magnitude
is functioning
as the fundamental
physical mechanism
irrespective
of energy release.
2. The velocities of ejecta for weak flares are generally lower than those for major flares. This may reflect information
on the reconnection
process around the site of energy release.
REFERENCES Carmichael,
H., in AAS-NASA
Symp. on the Physics of Solar Flares, ed. W. N. Hess, NASA-SP
50,
451 (1964) Hirayama, T., Solar Phys., 34, 323 (1974) Hudson, H. S., Acton, L. W. and Freeland, S. L., ApJ, 470, 629 (1996) Kopp, R. A., and Pneuman
G. W., Solar Phys., 50, 85 (1976)
Magara, T., Shibata, K. and Yokoyama, T., ApJ, 487, 437 (1977) Masuda, S., Kosugi, T., Hara, H., Tsuneta, S. arid Ogawara, Y., Nature, 371, 495 (1994) Ohyama, M., and Shibata, K., PASJ, 49, 249 (1997) Shibata, K., Masuda, S., Shimojo,
M., Hara, H., Yokoyama, T., Tsuneta, S., Kosugi, T. and Ogawara,
Y., ApJ, 451, L83 (1995) Sturrock, P. A., Nature, 211, 695 (1966) Tsuneta, S., Hara, H., Shimizu, T., Acton, PASJ, 44, L63 (1992) Tsuneta, S., ApJ, 484, 507 (1997)
L. W., Strong, K. T., Hudson, H. S. and Ogawara, Y.,