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Do all flares occur within a hierarchy of magnetic loops?
99—202, 1984 Ath. Space No.7, Printed in Res. Great Vol.4, Britain. All pp.l rights reserved.
0273—1177/84 $0.00 + .50 Copyright 0 COSPAR
DO ALL FLARES OCCUR WITHIN A HIERARCHY OF MAGNETIC LOOPS? R. A. Harrison and G. M. Simnett Department of Space Research, University of Birmingham, B15 2TT, England
ABSTRACT X—ray events observed by the Hard X—ray imaging Spectrometer on the Solar Maximum Mission frequently indicate the following scenario for solar flares: The initial energy release occurs In a compact magnetic loop and during the impulsive phase may spread rapidly to Involve a larger structure. In later phases the soft X—ray emission is from a much larger structure encompassing these initial features and. overlying them all is a huge loop with footpoints separated by up to several hundred thousand km.. In the light of these observations, we believe a flare model involving a single magnetic loop is rarely. if ever appropriate. INTRODUCTION In the study of the solar flare, the process is often modelled as though the bulk of the activity occurs in a simple magnetic loop. Provided the major part of the energy is released or confined Within a simple loop. this assumption is justified. However, from a study of many flares observed by the Hard X—ray imaging Spectrometer (HX1S) on the Solar Maximum Mission. it appears that the scenario is often, if not generally, rather different: namely the Initial energy release Is in a relatively compact magnetic ioop and then spreads rapidly, within tens of seconds to involve a somewhat larger structure, with continued energy release. Hot plasma then expands Into a larger structure encompassing these features and, in many events, there is evidence for huge magnetic loops which may link distant parts of an active region, or even link active regions together. If this is the general situation, then such a hierarchical magnetic structure should be considered when studying the energy build—up prior to the flare and the subsequent flare evolution. For example. the bulk of the energetic particles accelerated in the early stages of flares may not escape along open field lines, but may be trapped in the overlying structures. With this in mind, it is the purpose of this paper to show how common such complex magnetic loop systems are. OBSERVATIONS We have noted that in some flares the initial soft X—ray brightening does not occur in precisely the same location as the impulsive phase. it is as if the first energy release suddenly involves a much larger structure, with presumably more potential energy. Then, as the impulsive phase progresses. frequently the soft X—ray emitting structure spreads to encompass a linear dimension well in excess of that inferred from the initial phases of the impulsive burst. The logical conclusion is that it has spread to a larger overlyIng loop.
______
______ ~
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Fig. 1. The large X—ray emitting arch observed following the 5 Nov. 1980 flare. A 3.5—5.5 key. bOOs accumulation has been taken from t99%90%80%7O%55%
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R.A. Harrison and G.M. Simnett
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35-55 keV
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3’5- 5’5 keV (I) [18]:
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I
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Fig.2. X—ray images taken during the 11 Oct.1980 flare. The accumulation start times (UT) and durations are: (a) 17:40:29 for 32.3s (b) 17:41:42 for 8. bs Cc) 17:41:58 for 35.5s Cd) 17:43:58 for 59.9s Ce) 17:39:50 for 3b.7s (f) 17:40:48 for 15.8s (g) 17:42:53 for 18.4s (h) 17:43:48 for 18.4s Ci) 17:04:43 for 22.5s (j) 17:07:29 for 58.08 Ck) 17:12:55 for 100.Os The energies and intensity levels are given. Contour levels are taken at 99%. 90%. 80%. 70%, 55%. 40% and 25% of these maxima. Additional levels are plotted at 15% (a.b.c.d.e.f.g.h.J), 7% Ca.b.c.d,f,g,h) and 3% Cb,c.d.g.h). Note the larger scale of Images Ci). (j) and (k). The images are deconvolved for the HXIS collimator response. For the 5 .iuiy 1980’ flare /1/ a comparison of Ha and hard X—ray Images led to the identification of the primary energy release site, from which point a compact magnetic loop filled with hot plasma. in the later phase of the flare, the soft X—ray emission was from a much larger structure, and It was concluded /1/ that this was a magnetic loop overlying the origInal loop. Again. in the 21 May and 5 Nov. flares /2.3/ there was evidence for several magnetic structures of differing scale, in both of these flares, at the peak of the Impulsive phase (note: ngi the onset) a hard X—ray burst was seen from a point well removed from the primary energy release site, In the case of the 21 May flare, we interpret the westerly footpolnt identified by Hoyng et al. /2/ as the remote point. Also, following both flares a large X-ray emitting arch was visible: that following the 5 Nov. flare Is shown in Figure 1. Very similar characteristics were visible from the flare on 12 Nov. reported by Rust et al. /4/. As might be imagined . in not every flare does the soft X—ray emitting plasma break out of the magnetic structure, identified by the hard X—ray footpolnts. into a larger structure. For this to occur the thermal pressure of the plasma must overcome the magnetic pressure. and we see no reason why this ‘As all the events we discuss occurred In 1980.
the year will be omitted in future.
A Hierarchy of Magnetic Loops?
201
should be inevitable. An example where the flare is contained Is that of 7 May. where the thermal X—ray emitting plasma is contained within a loop of scale 16 in which both footpoints are resolved at the time of the hard X—ray burst. However, the soft X—ray enhancement prior to the impulsive phase was definitely removed to one side of the loop, consistent with it being in a small, unresolved structure. The hard X—ray burst of the 18 Nov. west—limb flare /5/ initially originates from a compact location just on the solar disc. As the burst progresses. the centroid rises to an altitude of (~Q4km presumably guided within a magnetic loop feature. After the bulk of the hard X—ray burst, the soft X—ray emitting plasma expands Into a larger overlying loop of footpoint separation “50 which is easily resolved against the dark sky. The above events are all discussed in the literature. However, there are many other so far unpublished examples where multiple structures are visible. Figure 2C a—h) illustrates the development of the flare of 11 Oct. and indicates initial activity in a loop of dimension “16 with a subsequent expansion into an ajoining loop system of about 3O dimension. The footpolnts of the smaller loop are resolved in Figures 2(a) and 2(b). as seen in 8.0—16 key X—rays. Figure 2(e). taken in 3.5—5.5 key X—rays. Indicates that the footpoints are. in fact, hotter than the loop—top. The bright patch to the eastern side of Figure 2(a) is significant and is found to be consistent with subsequent activity. The western footpoint of the small ioop remains static but decays as the brightening associated with the eastern footpoint. moves to the east. By the time of Figure 2(c) a new feature is resolved, linking the extreme eastern bright patch. previously mentioned, to the vicinity of the eastern footpolnt of the first loop. This picture is confirmed in Figure 2(d) and the softer images of Figures 2(g) and 2(h). Comparing, for example. Figures 2(h) and 2(d) we note this second, larger loop, is hotter near the top. One of the best examples we have of a huge loop is shown In Figure 3 for the late stage of the 12 Nov. flare. The primary flare loop is contained In the bright central patch but a clear extension of bright material Is seen reaching to the southern edge of the field of view and 5km.curving back to the north in the south—western corner. This ioop has a footpoint separation of “2. 5xlO in the cases discussed above the large loops have been easily resolved but In many events soft X—ray emission is only detected from the footpoints and the presence of a large loop can only be inferred from simultaneous activity from the footpolnts. An example of such an event Is the 5 July flare /1/. PrIor to the flare event, simultaneous brightenings at the flare site and from a patch “1. 3’ distant are interpreted as being evidence for the existance of a ioop of footpoint separation 6xlO4km overlying the previously described magnetic structure. Similar distant brightenings are observed in association with the 11 Oct. flare and these are illustrated In Figure 2(1—k). Some 38 minutes prior to the flare, three areas display simultaneous bursts of X—ray activity for “10 minutes (see Simnett and Harrison /6/) and it is during this activity that the accumulations for Figure 2(i—k) were made. The features Illustrated in Figure 2(a—h) . at the primary flare site, are all contained within the bright patch to the east of the Figures 2(i). (j) and (K). These features are consistent with loops connectIng to the two far points, from the flare—site, of footpolnt separation about 6xlO4km and 9xbO4km. On 29 June three flares occurred on the solar west limb. Throughout the entire period a system of soft X—ray emitting coronal loops were imaged against the dark sky. overlying the flare sites at altitudes of up to 1. SxlO5km. Harrison et al. /7/ demonstrate that these structures are active region interconnecting loops similar to those observed during the Skylab period /8/. SMM had observed the complex of active regions concerned since 24 June. An analysis of some of the X—ray events seen on that day /9/ has demonstrated quite clearly the complexity of the magnetic loop structure seen within one region pius the long, high coronal link to a separate region 1. 4xlO5km away. it is quite possible that very stable large loops were present in November also, as Svestka et al. /10/ observed a large coronal arch on 6 Nov. above the same active region we showed in Figure 1 some six days later.
N
64”
FIg. 3. The large X—ray emitting arch observed following the 12 Nov. 1980
202
R.A. Harrison and G.M. Simnett
To determine how common the observation of a loop hierarchy is. we have made a systematic study of the 36 active centres observed by HXIS during 1980. The use of the words ‘active centre’ rather than ‘active region’ is deliberate since some of the latter are so closely associated that we do not consider them to be independent. We chose to examine flares of importance )C5 on the GOES soft X—ray scale. which gives flares which can be examined spatIally and temporally with a high signal to noise level. Only 19 of these centres had flares of )C5 which were observed without serious interruption by spacecraft night or the South Atlantic Anomaly. However, in all 19 there was good evidence in at least one flare of the magnetic topology illustrated by the examples above. CONCLUSIONS We have given a number of examples of flares seen by HXIS where there is unambiguous evidence for a hierarchy of magnetic loops. There are many more examples which we have found including one for each of the 19 active centres examined In a systematic study. There are a number of examples of regions with homologous flares, although the signatures of X—ray emission from all the scales of magnetic loop are not necessarily present in every case. However, this does not mean that the hierarchical structures do not exist, merely that the energy release did not produce the right conditions to produce X—ray emitting plasma in them all. The phenomena we describe are so common that we feel it is highly relevant to ask: ‘do all flares occur within a hierarchy of magnetic loops?’, with a hlgn expectation of obtaining a confirmatory reply.
REFERENCES 1. R.A.HarrIson. (1983) 2.
G,M.Simnett.
P.Hoyng.
H.LaFieur and H.F.van Beak. Solar Phvs. 84.237
P. Hoyng. A. DuIjveman. M. E. Machado. D. M. Rust. Z. Svestka. A. Boeiee. C. de Jager. K. J. Frost. H. LaFieur. G. M. Slmnett. H. F. van Beek and B. E. Woodgate. Astroohvs. J~246. L155 (1981)
3. A. Duijveman.
P. Hoyng and M. E. Machado.
4.
D.M.Rust.
G.M.Simnett and D.F.Smith.
5.
G.M.Simnett and K.T.Strong.
6.
G.M.Simnett and R.A.Harrison.
Solar Phvs.
Astroohvs.,J.
Astroohvs.J.
81.
137 (1982)
submitted (1984)
Sept. 15 issue (1984)
this Issue (1984)
7. R.A.Harrlson. G.M.Simnett. P.Hoyng. H.LaFieur and H.F.van Beek. Proc. SCOSTEP/STIP Symp. on Solar/interplanetary intervals. Maynooth. Eire. Aug. 1982 (1983) 8.
R.C.Chase.
A.S.Krleger.
Z.Svestka and G.S.Vaiana.
9.
G.M.Slmnett. R.A.HarrIson. P.Hoyng and H.F.van Beek. Proc. SCOSTEP/STIP Symp. Solar/interplanetary Intervals. Maynooth. Eire. Aug. 1982 (1983)
10. Z. Svestka. B. R. Dennis. M. Pick. Solar Phvs. 80. 143 (1982)
~
A. RaouIt. C. G. Rapley.
Res,
16.
917 (1976) on
A. T. Stewart and B. E. Woodgate.
0273—1177/84 $0.00 + .50 Copyright 0 COSPAR
DO ALL FLARES OCCUR WITHIN A HIERARCHY OF MAGNETIC LOOPS? R. A. Harrison and G. M. Simnett Department of Space Research, University of Birmingham, B15 2TT, England
ABSTRACT X—ray events observed by the Hard X—ray imaging Spectrometer on the Solar Maximum Mission frequently indicate the following scenario for solar flares: The initial energy release occurs In a compact magnetic loop and during the impulsive phase may spread rapidly to Involve a larger structure. In later phases the soft X—ray emission is from a much larger structure encompassing these initial features and. overlying them all is a huge loop with footpoints separated by up to several hundred thousand km.. In the light of these observations, we believe a flare model involving a single magnetic loop is rarely. if ever appropriate. INTRODUCTION In the study of the solar flare, the process is often modelled as though the bulk of the activity occurs in a simple magnetic loop. Provided the major part of the energy is released or confined Within a simple loop. this assumption is justified. However, from a study of many flares observed by the Hard X—ray imaging Spectrometer (HX1S) on the Solar Maximum Mission. it appears that the scenario is often, if not generally, rather different: namely the Initial energy release Is in a relatively compact magnetic ioop and then spreads rapidly, within tens of seconds to involve a somewhat larger structure, with continued energy release. Hot plasma then expands Into a larger structure encompassing these features and, in many events, there is evidence for huge magnetic loops which may link distant parts of an active region, or even link active regions together. If this is the general situation, then such a hierarchical magnetic structure should be considered when studying the energy build—up prior to the flare and the subsequent flare evolution. For example. the bulk of the energetic particles accelerated in the early stages of flares may not escape along open field lines, but may be trapped in the overlying structures. With this in mind, it is the purpose of this paper to show how common such complex magnetic loop systems are. OBSERVATIONS We have noted that in some flares the initial soft X—ray brightening does not occur in precisely the same location as the impulsive phase. it is as if the first energy release suddenly involves a much larger structure, with presumably more potential energy. Then, as the impulsive phase progresses. frequently the soft X—ray emitting structure spreads to encompass a linear dimension well in excess of that inferred from the initial phases of the impulsive burst. The logical conclusion is that it has spread to a larger overlyIng loop.
______
______ ~
N
~
64 W
Fig. 1. The large X—ray emitting arch observed following the 5 Nov. 1980 flare. A 3.5—5.5 key. bOOs accumulation has been taken from t99%90%80%7O%55%
~
~ 199
40%25%
has
200
R.A. Harrison and G.M. Simnett
8.0 16 keV
35-55 keV
-
[-~~ ~ --
~-
N
~
(b) (20] ~-1
~
-
r
16”
-
r-
~
(e) (23]’ -~-~
-
-
~-
‘-~-~—~
(d)1354J~ i -—‘——~—I——i—- —H—~-—’—~—~.
I i_~_
--
-—~—+—
_I
-.
i1I-t
~
___
I’ll
(h) (9744
(g) (514)1 -~—
--
~
:~JP: ~L~j?
(c) 126 9J~
—~
______
3’5- 5’5 keV (I) [18]:
N
I
E’—.j—’
:j~r
(J)1311
(K)1261 I 64”
j’~II~ZII
Fig.2. X—ray images taken during the 11 Oct.1980 flare. The accumulation start times (UT) and durations are: (a) 17:40:29 for 32.3s (b) 17:41:42 for 8. bs Cc) 17:41:58 for 35.5s Cd) 17:43:58 for 59.9s Ce) 17:39:50 for 3b.7s (f) 17:40:48 for 15.8s (g) 17:42:53 for 18.4s (h) 17:43:48 for 18.4s Ci) 17:04:43 for 22.5s (j) 17:07:29 for 58.08 Ck) 17:12:55 for 100.Os The energies and intensity levels are given. Contour levels are taken at 99%. 90%. 80%. 70%, 55%. 40% and 25% of these maxima. Additional levels are plotted at 15% (a.b.c.d.e.f.g.h.J), 7% Ca.b.c.d,f,g,h) and 3% Cb,c.d.g.h). Note the larger scale of Images Ci). (j) and (k). The images are deconvolved for the HXIS collimator response. For the 5 .iuiy 1980’ flare /1/ a comparison of Ha and hard X—ray Images led to the identification of the primary energy release site, from which point a compact magnetic loop filled with hot plasma. in the later phase of the flare, the soft X—ray emission was from a much larger structure, and It was concluded /1/ that this was a magnetic loop overlying the origInal loop. Again. in the 21 May and 5 Nov. flares /2.3/ there was evidence for several magnetic structures of differing scale, in both of these flares, at the peak of the Impulsive phase (note: ngi the onset) a hard X—ray burst was seen from a point well removed from the primary energy release site, In the case of the 21 May flare, we interpret the westerly footpolnt identified by Hoyng et al. /2/ as the remote point. Also, following both flares a large X-ray emitting arch was visible: that following the 5 Nov. flare Is shown in Figure 1. Very similar characteristics were visible from the flare on 12 Nov. reported by Rust et al. /4/. As might be imagined . in not every flare does the soft X—ray emitting plasma break out of the magnetic structure, identified by the hard X—ray footpolnts. into a larger structure. For this to occur the thermal pressure of the plasma must overcome the magnetic pressure. and we see no reason why this ‘As all the events we discuss occurred In 1980.
the year will be omitted in future.
A Hierarchy of Magnetic Loops?
201
should be inevitable. An example where the flare is contained Is that of 7 May. where the thermal X—ray emitting plasma is contained within a loop of scale 16 in which both footpoints are resolved at the time of the hard X—ray burst. However, the soft X—ray enhancement prior to the impulsive phase was definitely removed to one side of the loop, consistent with it being in a small, unresolved structure. The hard X—ray burst of the 18 Nov. west—limb flare /5/ initially originates from a compact location just on the solar disc. As the burst progresses. the centroid rises to an altitude of (~Q4km presumably guided within a magnetic loop feature. After the bulk of the hard X—ray burst, the soft X—ray emitting plasma expands Into a larger overlying loop of footpoint separation “50 which is easily resolved against the dark sky. The above events are all discussed in the literature. However, there are many other so far unpublished examples where multiple structures are visible. Figure 2C a—h) illustrates the development of the flare of 11 Oct. and indicates initial activity in a loop of dimension “16 with a subsequent expansion into an ajoining loop system of about 3O dimension. The footpolnts of the smaller loop are resolved in Figures 2(a) and 2(b). as seen in 8.0—16 key X—rays. Figure 2(e). taken in 3.5—5.5 key X—rays. Indicates that the footpoints are. in fact, hotter than the loop—top. The bright patch to the eastern side of Figure 2(a) is significant and is found to be consistent with subsequent activity. The western footpoint of the small ioop remains static but decays as the brightening associated with the eastern footpoint. moves to the east. By the time of Figure 2(c) a new feature is resolved, linking the extreme eastern bright patch. previously mentioned, to the vicinity of the eastern footpolnt of the first loop. This picture is confirmed in Figure 2(d) and the softer images of Figures 2(g) and 2(h). Comparing, for example. Figures 2(h) and 2(d) we note this second, larger loop, is hotter near the top. One of the best examples we have of a huge loop is shown In Figure 3 for the late stage of the 12 Nov. flare. The primary flare loop is contained In the bright central patch but a clear extension of bright material Is seen reaching to the southern edge of the field of view and 5km.curving back to the north in the south—western corner. This ioop has a footpoint separation of “2. 5xlO in the cases discussed above the large loops have been easily resolved but In many events soft X—ray emission is only detected from the footpoints and the presence of a large loop can only be inferred from simultaneous activity from the footpolnts. An example of such an event Is the 5 July flare /1/. PrIor to the flare event, simultaneous brightenings at the flare site and from a patch “1. 3’ distant are interpreted as being evidence for the existance of a ioop of footpoint separation 6xlO4km overlying the previously described magnetic structure. Similar distant brightenings are observed in association with the 11 Oct. flare and these are illustrated In Figure 2(1—k). Some 38 minutes prior to the flare, three areas display simultaneous bursts of X—ray activity for “10 minutes (see Simnett and Harrison /6/) and it is during this activity that the accumulations for Figure 2(i—k) were made. The features Illustrated in Figure 2(a—h) . at the primary flare site, are all contained within the bright patch to the east of the Figures 2(i). (j) and (K). These features are consistent with loops connectIng to the two far points, from the flare—site, of footpolnt separation about 6xlO4km and 9xbO4km. On 29 June three flares occurred on the solar west limb. Throughout the entire period a system of soft X—ray emitting coronal loops were imaged against the dark sky. overlying the flare sites at altitudes of up to 1. SxlO5km. Harrison et al. /7/ demonstrate that these structures are active region interconnecting loops similar to those observed during the Skylab period /8/. SMM had observed the complex of active regions concerned since 24 June. An analysis of some of the X—ray events seen on that day /9/ has demonstrated quite clearly the complexity of the magnetic loop structure seen within one region pius the long, high coronal link to a separate region 1. 4xlO5km away. it is quite possible that very stable large loops were present in November also, as Svestka et al. /10/ observed a large coronal arch on 6 Nov. above the same active region we showed in Figure 1 some six days later.
N
64”
FIg. 3. The large X—ray emitting arch observed following the 12 Nov. 1980
202
R.A. Harrison and G.M. Simnett
To determine how common the observation of a loop hierarchy is. we have made a systematic study of the 36 active centres observed by HXIS during 1980. The use of the words ‘active centre’ rather than ‘active region’ is deliberate since some of the latter are so closely associated that we do not consider them to be independent. We chose to examine flares of importance )C5 on the GOES soft X—ray scale. which gives flares which can be examined spatIally and temporally with a high signal to noise level. Only 19 of these centres had flares of )C5 which were observed without serious interruption by spacecraft night or the South Atlantic Anomaly. However, in all 19 there was good evidence in at least one flare of the magnetic topology illustrated by the examples above. CONCLUSIONS We have given a number of examples of flares seen by HXIS where there is unambiguous evidence for a hierarchy of magnetic loops. There are many more examples which we have found including one for each of the 19 active centres examined In a systematic study. There are a number of examples of regions with homologous flares, although the signatures of X—ray emission from all the scales of magnetic loop are not necessarily present in every case. However, this does not mean that the hierarchical structures do not exist, merely that the energy release did not produce the right conditions to produce X—ray emitting plasma in them all. The phenomena we describe are so common that we feel it is highly relevant to ask: ‘do all flares occur within a hierarchy of magnetic loops?’, with a hlgn expectation of obtaining a confirmatory reply.
REFERENCES 1. R.A.HarrIson. (1983) 2.
G,M.Simnett.
P.Hoyng.
H.LaFieur and H.F.van Beak. Solar Phvs. 84.237
P. Hoyng. A. DuIjveman. M. E. Machado. D. M. Rust. Z. Svestka. A. Boeiee. C. de Jager. K. J. Frost. H. LaFieur. G. M. Slmnett. H. F. van Beek and B. E. Woodgate. Astroohvs. J~246. L155 (1981)
3. A. Duijveman.
P. Hoyng and M. E. Machado.
4.
D.M.Rust.
G.M.Simnett and D.F.Smith.
5.
G.M.Simnett and K.T.Strong.
6.
G.M.Simnett and R.A.Harrison.
Solar Phvs.
Astroohvs.,J.
Astroohvs.J.
81.
137 (1982)
submitted (1984)
Sept. 15 issue (1984)
this Issue (1984)
7. R.A.Harrlson. G.M.Simnett. P.Hoyng. H.LaFieur and H.F.van Beek. Proc. SCOSTEP/STIP Symp. on Solar/interplanetary intervals. Maynooth. Eire. Aug. 1982 (1983) 8.
R.C.Chase.
A.S.Krleger.
Z.Svestka and G.S.Vaiana.
9.
G.M.Slmnett. R.A.HarrIson. P.Hoyng and H.F.van Beek. Proc. SCOSTEP/STIP Symp. Solar/interplanetary Intervals. Maynooth. Eire. Aug. 1982 (1983)
10. Z. Svestka. B. R. Dennis. M. Pick. Solar Phvs. 80. 143 (1982)
~
A. RaouIt. C. G. Rapley.
Res,
16.
917 (1976) on
A. T. Stewart and B. E. Woodgate.