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Solar maximum mission experiment: Early results of the hard X-ray imaging experiment
266. © COSPAR, 1981. Printed in Great Britain.
Adv. Space Res. Vol.1, pp.255—
0273l177/81/04010255$o5.oo/o
SOLAR MAXIMUM MISSION EXPERIMENT: EARLY RESULTS OF THE HARD X-RAY IMAGING EXPERIMENT G.M. Simnett,2 A. Boelee,’ C. P. Charlton,2 C. de Jager,’ A. Duijveman,’ R. Fryer,2 M. Galama,’ R. Hoekstra,’ P. Hoyng,’ J. P. Imhof,’ H. Lafleur,’ M. E. Machado,3 H.V.A.M. Maseland,’ W. A. Mels,’ A. Schadee,’ J. Schrijver,’ Z. Svestka,’ H. F. van Beck,’ P. van Rens,’ J.J.M. van der Laan,’ W. van Tend,’ F. Werkhoven,’ G. Wiersma,’ A.P. Willmore,2 J.W.G. Wilson2 and W. Zandee’ ‘The Astronomical Institute at Utrecht, Utrecht, The Netherlands 25pa~Research Department, University ofBirmingham, UK ~Observatorio de Fisica Cosmica-CNIE, San Miguel, Argentina ABSTRACT We have selected four widely different flares from the early period of operations of the Hard X—Ray Imaging Spectrometer (HXIS) on SMM to illustrate the characteristic imaging properties of this experiment. For the small flare of April 4, 1980, we demonstrate the Instrument’s capability for locating a compact source. In the weak, but extensive, flare of April 6 we show how well the instrument can display spatial structure, and also the low level of the instrument background. In the lB flare of April 7 we are able to locate positions of the X—ray emission in the soft and hard channels, and estimate the positional variations of the emission patches. Finally, in the IN flare of April 10, which produced the strongest hard X—ray burst we have seen so far, we repeat some of the studies made for the April 7 event, and also demonstrate the capability of the HXIS instrument to study the development, with high time resolution, of individual 8”x8 elements of the flare. INTRODUCTION We report here the first results of observations of the Hard X—Ray Imaging Spectrometer (HXIS) on board the Solar Maximum Mission spacecraft. The instrument Is a major advance over previous satellite detectors, because it images solar X—ray sources at much higher energies than had hitherto been possible [1,2,3]. Previously, the best observed position of a hard X—ray solar burst had been from a balloon flight at the time of the last solar maximum [4]. The first results illustrate how well the response matches its specifications. A description of HXIS together with its scientific objectives has been published elsewhere [5]. The principal features are continuous Imaging capability over approximately 2’40° (8” resolution) and over 6’24” (32” resolution) in six energy bands selected from the broad range 3.5—30 keV. The time resolution of the images is 0.5—7s depending on the mode of operation, although during the period of the observations reported here time resolution less than 1.5s was not attempted. We document the Instrument response to X—rays by reference to four solar flares occurring between April 4 and April 10, 1980. Each flare was chosen to emphasize
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different features of the instrument. It is not our intent to discuss here in detail the scientific aspects of any of these events. An indepth scientific analysis will be published elsewhere. THE INSTRUMENT The instrument consists of an imaging collimator, a position sensitive detector system with attached analog electronics, and data handling electronics having two microprocessors. The detector system and analog electronics are capable of registering X—ray photons at rates up to 18,000 counts per second overall. The data handling electronics accumulates these data and assembles a compressed data stream for the spacecraft telemetry system. A Solar Limb Sensor is built into the instrument, enabling a cross correlation between the boresight of HXIS and the spacecraft Fine Pointing Sun Sensor to an accuracy better than 2”. SOLAR FLARE OBSERVATIONS April 4,
21:42 UT
The April 4 event was optically a subflare in active region 2363, at N29 W45 on the solar disc. We have selected this flare on account of its compactness to illustrate the potential of the instrument for imaging spatially small sources. The event was structureless as seen by HXIS, with a soft spectrum and a symmetrical intensity—time profile. Negligible numbers of X—rays were detected above 11 keV. Figure IA shows a representation of a 40” x4O’~ section of the HXIS fine field of of view, as seen in the three energy bands 3.5—5.5 keV, 5.5—8 keV and 8—30 keV. (Throughout this paper we will refer to energy bands in this way; however, these are the nominal energy bands, and we have made no attempt to account for the detector response and the collimator response.)
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DERIVED INTENSITIES Figure 1. Counts in a 40”x40” section of the HXIS fine field of view, in three energy bands: (A) as recorded in the flare of April 4; (B) as expected under the assumption that all the counts were spread uniformly throughout the three brightest elements. The accumulation time is 50 seconds.
It is apparent from Figure 1A that the majority of counts observed from the snail flare were seen in just three 8”x8” image elements. The intensities of the surrounding elements are shown for comparison. We now show how a consideration of
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the triangular response of the collimator [6] can be used to deduce that in fact the source size within each of the three brightest elements must be less than 8”x8”: Suppose the contents of the three brightest elements were spread equally throughout these elements, and that there was no source outside these elements. By using the known response of the collimator, we can now deduce what the instrument would see in all elements. This is shown in Figure lB for the three we have assumed to encompass the source. If we examine the derived intensities, it is apparent that those observed are considerably less than those derived. The explanation for this is that the source must have been generally brighter towards the center of the elements than at the edges. April 6, 21:13 UT The April 6 flare we have chosen was the first flare we saw in the active region 2372 at N1O EO5, from which we have data on at least ten different flares (among them also those discussed in the following two sections). It was a class iN optical flare, reaching Ml on the soft X—ray i—8A classification according to the flux recorded by GOES, observed in the decay phase. The flare was in progress as the SMM came Out of eclipse.
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Figure 2A shows the full images of the flare in the HXIS fine field of view for 3.5—5.5 key, 5.5—8 keV and 8—16 key. The images represent an integration of forty— eight 4.6 sec accumulations equally distributed between 21:41:31 and 21:48:44 UT. It is clear from these images that this was a weak X—ray source, but rather extensive, covering an area of over Pxl’ with emission > 10% of the peak brightness. Apparently, the HXIS fine field of view encompassed the whole of the flaring region. Figure 2B shows a similar data set for the period 21 :48:44 — 21:59:58 UT. Such integrated images can be compared with Ha pictures of the flare or with a magnetogram (as we demonstrate for the next event) to see the location of the X—ray emission in the active region. At higher energies, some detailed fine structure is becoming apparent, but, of course, for flares of this relatively low X—ray intensity all fast changes are lost in the integration process. Nevertheless, from a detailed examination of many contiguous data sets it is possible to place limits on the motion of the brightest point in the field of view, due account having been taken of spacecraft motions. Figure 3 shows two sets of histograms taken through the brightest point in the E—W and N—S directions. In the N—S scans there is some indication that the brightest point nay have moved some ten arc—seconds. Counting statistics are omitted, but they are typically 10% at the peak of each histogram. While we do not intend to discuss possible motion of the source in this flare, the data sets shown in Figure 3 demonstrate the potential the Instrument has for this type of analysis even for weak flares.
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05:18 UT of the April 7 event we discuss was observed from 05:40:16 UT, at which event was in its declining phase, until the end of the orbit, at 06:37 UT, X—ray intensity had virtually regained its background level. The flare
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was viewed almost entirely within the fine field of view of HXIS (Figure 4), and was remarkable for several reasons: (1) There were two distinct bright regions separated by approximately 48” at 3.55.5 keV at the start of our observations; the intensity minimum at 5.5—8 keV being only 6.5% of the average of the two peak pixels at this energy. (2) The intensity of the two points decayed monotonically, but differently, throughout this event, the decay of the western area being over a factor of two faster. (3) The energy spectrum of the two points evolved very diff— erently; the spectrum of the faster decaying point softened more rapidly than that of the easterly point. (4) The centroid of the X—ray emission moved in a general southerly direction, with a slight additional westerly motion of the eastern point and a slight easterly motion of the western point. (5) The centroid of the emissions was not spatially coincident at either point for different energies. (6) There was evidence of a third local intensity maximum, although of considerably lower brightness, approximately 32” to the south of the line joining the two brightest points: this was more pronounced at energies above 10 keV, and late in the decay this became the brightest point in our fine field of view at 8—11.5 keV.
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This flare, of optical importance 1B, occurred in the same region 2372, at N1O EOO, as the flares discussed in the preceding and following sections. The Ha maximum was at 05:38 UT i.e. very shortly before our X—ray observations began. The X—ray burst, classified as M8 in soft X—rays, was actually made up of two components: one coming from the flare described here and another from a 28 flare in region 2362 at N26 W71 which had its Ha maximum at 05:33 UT. In any case, however, the X—ray burst from the studied flare was very strong, long—lasting, and indeed one of the hardest events detected by HXIS during the first three months of operations. The intensity—time profiles over the whole coarse field of view in all the six nominal energy bands are shown in Figure 5A. The decay is approximately exponential, but the decay constant is about 50% greater at 11.5—16 key. However, when the event is examined in more detail the two bright regions, added in Figure 5A, behave differently. There has also been a significant movement in a general southerly direction of both bright points, taking the easterly, more intense point over the edge of the HXIS fine field of view. Another significant development in the later phase was the appearance on the southerly edge of the field of view of a third local intensity maximum, which then became the brightest point in the 8.0— 11.5 key band. Thus a much more detailed analysis of X—ray emission from the individual elements of the flare is needed which will be the subject of later publications. 590 l:~
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It is of interest to compare the location of the X—ray emission with a nagnetogram as we have done in Figure 6. We believe the two bright emission patches are the footprints of arcades of loops that bridge regions of opposite polarity inside the active region, similar to those commonly observed in flares during the Skylab mission [3]. The hardest emission (up to 16 key in our images) occurred in the eastern loop, rather in its top portion than near any of the footpoints. This might indicate preference for a thermal model of the hard X—ray burst, but because of the relatively low energy range involved (16—22 key) and late time in the flare development (4 mm after Ha maximum) one should be cautious in drawing any definite conclusions from this observation (note that data on April 10 indicates a thick—target model, cf next section).
Figure 6. The position of the flare of April 7th, 1980, at 5:43 UT projected upon a Marshall Space Flight Center magnet— ogram. The emission in the lower energy range (3.5—8.0 key) is outlined. The triangle marks the position of the most intense emission recorded in the 16—22 keV range.
We have noted that the bright regions appear to move across our field of view. In Figure 7 we have attempted to estimate the apparent velocity of the motion. The image elements around the easterly bright point were divided into three groups of two and one single point as outlined in the inset in Figure 7. This inset is identical with the area marked in the 22—30 key picture in Figure 4. The percentage of the summed counts in this group for each sub—set of elements is plotted as a function of time. The motion of the source may now be gauged from the appearance of the curves.
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By comparing the times at which in neighboring groups of image elements maximum relative brightness was reached, or at which the same relative intensity was obtained (e.g. 50%, either on the increasing slopes or on the decreasing slopes of the curves), one may estimate the time needed for the point where a certain relative brightness is reached, to traverse the distance between two image elements. Thus an apparent velocity has been derived which was initially — 20 km/s and decreased thereafter to 10 km/s. It is evidently uncertain whether these velocities represent true material motion, energy transfer, or just a change of the physical conditions in the area considered. For example, we may be observing the sequential brightenings of an arcade of loops, as seen earlier by Skylab [3]. April 10, 09:14 UT The April 10 event, in common with those on April 6 and 7, occurred in region 2372, then at Ni2 W42. The flare was iN optical class and M4 according to the 1—8A flux. The Ha flare started at 09:14 UT, reached maximum at 09:23 UT and finished at 09:45 UT. There was an associated microwave burst, which reached a maximum of 180 sfu in 8.8 GHz at 09:21 UT. Figure 5B shows the time evolution of the intensity in our six energy channels. From the number of counts per second observed in the high energy bands, we can see the harder nature of the X—ray spectrum in this flare, especially at the time of a short lived hard X—ray burst (duration FWHM approximately 81s) that reached maximum intensity at O9:18:37±2s, and which was also recorded by the HXRBS on SMM at higher energies (25—300 keV). The light curves of Figure 5B also show some pre— flare intensity oscillations that can be seen in all energy bands having good counting statistics.
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Figure 8 shows the spatial structure of this event, as seen in the fine field of view images at the time of the short—lived hard X—ray burst mentioned above. The X—ray emission was located in the western portion of the active region, close to the place where the western emission patch was observed in the April 7 event (cf. Figure 6).
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Preliminary comparison with magnetic field maps again indicates that the emission at lower energies (3.5—8.0 keV) overlays opposite polarity regions. The brightest emission at these softer energies is concentrated in eight contiguous pixels centered around a single maximum. At energies between 16—30 key, on the other hand, maximum intensity was displaced by one resolution element to the south. There is some evidence in them for two distinct emission patches in the ±(EW)direction separated by one or two 8” elements. Later in the flare, when the short—lived burst disappears, the intensity in these energies decreases considerably and the overall intensity at all energies becomes more homogeneous over the whole flare area. A tentative interpretation of the observations at the time of the X—ray burst would imply that the flare consisted of a loop or loop system and the hardest X—ray maxima were located at lower altitudes in the solar atmosphere than that of the softer emission, hence contrary to our conclusions from the flare of April 7. A much more sophisticated analysis is apparently needed to solve this important problem of the location of hard X—ray bursts. One has to realize that all the flares we have seen so far, including the events of April 7 and 10 discussed here, are only moderate flares with relatively weak hard X—ray bursts. The point—like nature of the source is illustrated by the histograms plotted in Figure 9, which are taken close to the maximum intensity in the 3.5—5.5 key band
G.M. Simnett et al.
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in tha time interval 09:20:12 — 09:21:07 UT. For comparison, histograms of the intensity in the E—W direction in the 16—22 key band are drawn both for the image slice through the brightest point in the 3.5—5.5 key band and for the parallel image slice immediately to the south, which contains the brightest point in the hard X—ray field of view. It is apparent that the source width is narrow, <16”, that the cores of the emission at different energies are not spatially coincident, and that there is a tendency for the profiles at higher energies to be sharper.
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The core of the soft X—ray emission was displaced approximately 12” from the brightest point at higher energies; in addition, referring to emission above 16 key, there is some evidence that there was a second bright point separated by ‘~ 16” in the westerly direction from the brightest high energy point.
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One of the interesting features of HXIS is its ability to derive an intensity—tine curve for a given point. Figure 10 presents such curves for the two high energy maxima, I and II, and for the low energy maximum, III. Pre—flare oscillations are clearly visible in region II, while only slight indications are seen in regions I and III. We note the faster decay of the soft X—ray bright point; however if the total flare region is considered, the soft X—ray emission has the slower decay. a would indicate a gradual energy transfer from the local bright points to produce larger volume of cooler plasma which slowly regains thermal equilibrium with the ambient medium. SUMMARY We have endeavored to illustrate the excellent imaging properties of the Hard X— ray Imaging Spectrometer on SMM. From the results we have presented here from four widely differing solar flares, it is clear that the potential for new and interesting solar physics studies is very high. ACKNOWLEDGEMENTS The development and construction of the Hard X—ray Imaging Spectrometer was made possible by support given by the Netherlands Ministry for Education and Science through the Committee for Geophysics and Space Research of the Royal Netherlands Academy of Arts and Sciences, and the Science Research Council of the United Kingdom. Their support is gratefully acknowledged. We extend our thanks to our .iany coworkers in the Utrecht Space Research Laboratory, and the Birmingham Department of Space Research, to Netherlands Aircraft Factories Fokker—VFW, NV Phillips Gloeilampenfabrieken, PYE Telecommunications, Cambridge, the Technical University of Eindhoven and TDS Circuits Blackburn, as well as to the many others in the Netherlands, the United Kingdom and the USA on whose untiring efforts the success of the experiment and SMM has depended. W. v.T. wishes to acknowledge the Netherlands Organization for the Advancement of Pure Research (ZWO). JASA 1,13-A
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References:
1. Vaiana, G.S., A.S. Krieger, and S.F. Timothy: 1973, Solar Phys. 32,81. 2. Neupert, W.M., R.J. Thomas, and R.D. Chapman: 1974, Solar Phys. 34, 349. 3. Sturrock, P.A. (ed.), 1979 Proceedings of the Second Skylab Workshop, on Solar Flares, Boulder, Colorado, U.S.A. 4. Takakura, T., K. Ohki, N. Shibuya, M. Fujii, H. Matsuoka, S. Miyamoto, J. Nishimura, H. Oda, Y.Ogawara and S. Ota: 1971,Solar Phys. 16, 454. 5. Van Beek, H.F.,: 1976, Space Science Instr. 2, 197. 6. Van Beek, H.F., P. Hoyng, H. Lafleur, and G.M. Simnett: 1980, Solar Phys. 65, 39.
Adv. Space Res. Vol.1, pp.255—
0273l177/81/04010255$o5.oo/o
SOLAR MAXIMUM MISSION EXPERIMENT: EARLY RESULTS OF THE HARD X-RAY IMAGING EXPERIMENT G.M. Simnett,2 A. Boelee,’ C. P. Charlton,2 C. de Jager,’ A. Duijveman,’ R. Fryer,2 M. Galama,’ R. Hoekstra,’ P. Hoyng,’ J. P. Imhof,’ H. Lafleur,’ M. E. Machado,3 H.V.A.M. Maseland,’ W. A. Mels,’ A. Schadee,’ J. Schrijver,’ Z. Svestka,’ H. F. van Beck,’ P. van Rens,’ J.J.M. van der Laan,’ W. van Tend,’ F. Werkhoven,’ G. Wiersma,’ A.P. Willmore,2 J.W.G. Wilson2 and W. Zandee’ ‘The Astronomical Institute at Utrecht, Utrecht, The Netherlands 25pa~Research Department, University ofBirmingham, UK ~Observatorio de Fisica Cosmica-CNIE, San Miguel, Argentina ABSTRACT We have selected four widely different flares from the early period of operations of the Hard X—Ray Imaging Spectrometer (HXIS) on SMM to illustrate the characteristic imaging properties of this experiment. For the small flare of April 4, 1980, we demonstrate the Instrument’s capability for locating a compact source. In the weak, but extensive, flare of April 6 we show how well the instrument can display spatial structure, and also the low level of the instrument background. In the lB flare of April 7 we are able to locate positions of the X—ray emission in the soft and hard channels, and estimate the positional variations of the emission patches. Finally, in the IN flare of April 10, which produced the strongest hard X—ray burst we have seen so far, we repeat some of the studies made for the April 7 event, and also demonstrate the capability of the HXIS instrument to study the development, with high time resolution, of individual 8”x8 elements of the flare. INTRODUCTION We report here the first results of observations of the Hard X—Ray Imaging Spectrometer (HXIS) on board the Solar Maximum Mission spacecraft. The instrument Is a major advance over previous satellite detectors, because it images solar X—ray sources at much higher energies than had hitherto been possible [1,2,3]. Previously, the best observed position of a hard X—ray solar burst had been from a balloon flight at the time of the last solar maximum [4]. The first results illustrate how well the response matches its specifications. A description of HXIS together with its scientific objectives has been published elsewhere [5]. The principal features are continuous Imaging capability over approximately 2’40° (8” resolution) and over 6’24” (32” resolution) in six energy bands selected from the broad range 3.5—30 keV. The time resolution of the images is 0.5—7s depending on the mode of operation, although during the period of the observations reported here time resolution less than 1.5s was not attempted. We document the Instrument response to X—rays by reference to four solar flares occurring between April 4 and April 10, 1980. Each flare was chosen to emphasize
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different features of the instrument. It is not our intent to discuss here in detail the scientific aspects of any of these events. An indepth scientific analysis will be published elsewhere. THE INSTRUMENT The instrument consists of an imaging collimator, a position sensitive detector system with attached analog electronics, and data handling electronics having two microprocessors. The detector system and analog electronics are capable of registering X—ray photons at rates up to 18,000 counts per second overall. The data handling electronics accumulates these data and assembles a compressed data stream for the spacecraft telemetry system. A Solar Limb Sensor is built into the instrument, enabling a cross correlation between the boresight of HXIS and the spacecraft Fine Pointing Sun Sensor to an accuracy better than 2”. SOLAR FLARE OBSERVATIONS April 4,
21:42 UT
The April 4 event was optically a subflare in active region 2363, at N29 W45 on the solar disc. We have selected this flare on account of its compactness to illustrate the potential of the instrument for imaging spatially small sources. The event was structureless as seen by HXIS, with a soft spectrum and a symmetrical intensity—time profile. Negligible numbers of X—rays were detected above 11 keV. Figure IA shows a representation of a 40” x4O’~ section of the HXIS fine field of of view, as seen in the three energy bands 3.5—5.5 keV, 5.5—8 keV and 8—30 keV. (Throughout this paper we will refer to energy bands in this way; however, these are the nominal energy bands, and we have made no attempt to account for the detector response and the collimator response.)
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DERIVED INTENSITIES Figure 1. Counts in a 40”x40” section of the HXIS fine field of view, in three energy bands: (A) as recorded in the flare of April 4; (B) as expected under the assumption that all the counts were spread uniformly throughout the three brightest elements. The accumulation time is 50 seconds.
It is apparent from Figure 1A that the majority of counts observed from the snail flare were seen in just three 8”x8” image elements. The intensities of the surrounding elements are shown for comparison. We now show how a consideration of
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the triangular response of the collimator [6] can be used to deduce that in fact the source size within each of the three brightest elements must be less than 8”x8”: Suppose the contents of the three brightest elements were spread equally throughout these elements, and that there was no source outside these elements. By using the known response of the collimator, we can now deduce what the instrument would see in all elements. This is shown in Figure lB for the three we have assumed to encompass the source. If we examine the derived intensities, it is apparent that those observed are considerably less than those derived. The explanation for this is that the source must have been generally brighter towards the center of the elements than at the edges. April 6, 21:13 UT The April 6 flare we have chosen was the first flare we saw in the active region 2372 at N1O EO5, from which we have data on at least ten different flares (among them also those discussed in the following two sections). It was a class iN optical flare, reaching Ml on the soft X—ray i—8A classification according to the flux recorded by GOES, observed in the decay phase. The flare was in progress as the SMM came Out of eclipse.
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Figure 2A shows the full images of the flare in the HXIS fine field of view for 3.5—5.5 key, 5.5—8 keV and 8—16 key. The images represent an integration of forty— eight 4.6 sec accumulations equally distributed between 21:41:31 and 21:48:44 UT. It is clear from these images that this was a weak X—ray source, but rather extensive, covering an area of over Pxl’ with emission > 10% of the peak brightness. Apparently, the HXIS fine field of view encompassed the whole of the flaring region. Figure 2B shows a similar data set for the period 21 :48:44 — 21:59:58 UT. Such integrated images can be compared with Ha pictures of the flare or with a magnetogram (as we demonstrate for the next event) to see the location of the X—ray emission in the active region. At higher energies, some detailed fine structure is becoming apparent, but, of course, for flares of this relatively low X—ray intensity all fast changes are lost in the integration process. Nevertheless, from a detailed examination of many contiguous data sets it is possible to place limits on the motion of the brightest point in the field of view, due account having been taken of spacecraft motions. Figure 3 shows two sets of histograms taken through the brightest point in the E—W and N—S directions. In the N—S scans there is some indication that the brightest point nay have moved some ten arc—seconds. Counting statistics are omitted, but they are typically 10% at the peak of each histogram. While we do not intend to discuss possible motion of the source in this flare, the data sets shown in Figure 3 demonstrate the potential the Instrument has for this type of analysis even for weak flares.
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05:18 UT of the April 7 event we discuss was observed from 05:40:16 UT, at which event was in its declining phase, until the end of the orbit, at 06:37 UT, X—ray intensity had virtually regained its background level. The flare
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was viewed almost entirely within the fine field of view of HXIS (Figure 4), and was remarkable for several reasons: (1) There were two distinct bright regions separated by approximately 48” at 3.55.5 keV at the start of our observations; the intensity minimum at 5.5—8 keV being only 6.5% of the average of the two peak pixels at this energy. (2) The intensity of the two points decayed monotonically, but differently, throughout this event, the decay of the western area being over a factor of two faster. (3) The energy spectrum of the two points evolved very diff— erently; the spectrum of the faster decaying point softened more rapidly than that of the easterly point. (4) The centroid of the X—ray emission moved in a general southerly direction, with a slight additional westerly motion of the eastern point and a slight easterly motion of the western point. (5) The centroid of the emissions was not spatially coincident at either point for different energies. (6) There was evidence of a third local intensity maximum, although of considerably lower brightness, approximately 32” to the south of the line joining the two brightest points: this was more pronounced at energies above 10 keV, and late in the decay this became the brightest point in our fine field of view at 8—11.5 keV.
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Figure 4. Images of the flare of April 7 at 05:42:59 UT in six different energy channels. The count numbers (which, for a weak flare, we showed directly in Figure 2) are now reprent— ed by various grades of shading, in 10 steps by multiples of two, from 0—1 to >288 counts per 8”x8” element. The area marked in the 22—30 key energy band shows the boundary of the 7 elements studied in Figure 7; the black square immediately above this image is the position of the weak impulsive burst discussed in the text.
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This flare, of optical importance 1B, occurred in the same region 2372, at N1O EOO, as the flares discussed in the preceding and following sections. The Ha maximum was at 05:38 UT i.e. very shortly before our X—ray observations began. The X—ray burst, classified as M8 in soft X—rays, was actually made up of two components: one coming from the flare described here and another from a 28 flare in region 2362 at N26 W71 which had its Ha maximum at 05:33 UT. In any case, however, the X—ray burst from the studied flare was very strong, long—lasting, and indeed one of the hardest events detected by HXIS during the first three months of operations. The intensity—time profiles over the whole coarse field of view in all the six nominal energy bands are shown in Figure 5A. The decay is approximately exponential, but the decay constant is about 50% greater at 11.5—16 key. However, when the event is examined in more detail the two bright regions, added in Figure 5A, behave differently. There has also been a significant movement in a general southerly direction of both bright points, taking the easterly, more intense point over the edge of the HXIS fine field of view. Another significant development in the later phase was the appearance on the southerly edge of the field of view of a third local intensity maximum, which then became the brightest point in the 8.0— 11.5 key band. Thus a much more detailed analysis of X—ray emission from the individual elements of the flare is needed which will be the subject of later publications. 590 l:~
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It is of interest to compare the location of the X—ray emission with a nagnetogram as we have done in Figure 6. We believe the two bright emission patches are the footprints of arcades of loops that bridge regions of opposite polarity inside the active region, similar to those commonly observed in flares during the Skylab mission [3]. The hardest emission (up to 16 key in our images) occurred in the eastern loop, rather in its top portion than near any of the footpoints. This might indicate preference for a thermal model of the hard X—ray burst, but because of the relatively low energy range involved (16—22 key) and late time in the flare development (4 mm after Ha maximum) one should be cautious in drawing any definite conclusions from this observation (note that data on April 10 indicates a thick—target model, cf next section).
Figure 6. The position of the flare of April 7th, 1980, at 5:43 UT projected upon a Marshall Space Flight Center magnet— ogram. The emission in the lower energy range (3.5—8.0 key) is outlined. The triangle marks the position of the most intense emission recorded in the 16—22 keV range.
We have noted that the bright regions appear to move across our field of view. In Figure 7 we have attempted to estimate the apparent velocity of the motion. The image elements around the easterly bright point were divided into three groups of two and one single point as outlined in the inset in Figure 7. This inset is identical with the area marked in the 22—30 key picture in Figure 4. The percentage of the summed counts in this group for each sub—set of elements is plotted as a function of time. The motion of the source may now be gauged from the appearance of the curves.
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By comparing the times at which in neighboring groups of image elements maximum relative brightness was reached, or at which the same relative intensity was obtained (e.g. 50%, either on the increasing slopes or on the decreasing slopes of the curves), one may estimate the time needed for the point where a certain relative brightness is reached, to traverse the distance between two image elements. Thus an apparent velocity has been derived which was initially — 20 km/s and decreased thereafter to 10 km/s. It is evidently uncertain whether these velocities represent true material motion, energy transfer, or just a change of the physical conditions in the area considered. For example, we may be observing the sequential brightenings of an arcade of loops, as seen earlier by Skylab [3]. April 10, 09:14 UT The April 10 event, in common with those on April 6 and 7, occurred in region 2372, then at Ni2 W42. The flare was iN optical class and M4 according to the 1—8A flux. The Ha flare started at 09:14 UT, reached maximum at 09:23 UT and finished at 09:45 UT. There was an associated microwave burst, which reached a maximum of 180 sfu in 8.8 GHz at 09:21 UT. Figure 5B shows the time evolution of the intensity in our six energy channels. From the number of counts per second observed in the high energy bands, we can see the harder nature of the X—ray spectrum in this flare, especially at the time of a short lived hard X—ray burst (duration FWHM approximately 81s) that reached maximum intensity at O9:18:37±2s, and which was also recorded by the HXRBS on SMM at higher energies (25—300 keV). The light curves of Figure 5B also show some pre— flare intensity oscillations that can be seen in all energy bands having good counting statistics.
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Figure 8 shows the spatial structure of this event, as seen in the fine field of view images at the time of the short—lived hard X—ray burst mentioned above. The X—ray emission was located in the western portion of the active region, close to the place where the western emission patch was observed in the April 7 event (cf. Figure 6).
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Preliminary comparison with magnetic field maps again indicates that the emission at lower energies (3.5—8.0 keV) overlays opposite polarity regions. The brightest emission at these softer energies is concentrated in eight contiguous pixels centered around a single maximum. At energies between 16—30 key, on the other hand, maximum intensity was displaced by one resolution element to the south. There is some evidence in them for two distinct emission patches in the ±(EW)direction separated by one or two 8” elements. Later in the flare, when the short—lived burst disappears, the intensity in these energies decreases considerably and the overall intensity at all energies becomes more homogeneous over the whole flare area. A tentative interpretation of the observations at the time of the X—ray burst would imply that the flare consisted of a loop or loop system and the hardest X—ray maxima were located at lower altitudes in the solar atmosphere than that of the softer emission, hence contrary to our conclusions from the flare of April 7. A much more sophisticated analysis is apparently needed to solve this important problem of the location of hard X—ray bursts. One has to realize that all the flares we have seen so far, including the events of April 7 and 10 discussed here, are only moderate flares with relatively weak hard X—ray bursts. The point—like nature of the source is illustrated by the histograms plotted in Figure 9, which are taken close to the maximum intensity in the 3.5—5.5 key band
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in tha time interval 09:20:12 — 09:21:07 UT. For comparison, histograms of the intensity in the E—W direction in the 16—22 key band are drawn both for the image slice through the brightest point in the 3.5—5.5 key band and for the parallel image slice immediately to the south, which contains the brightest point in the hard X—ray field of view. It is apparent that the source width is narrow, <16”, that the cores of the emission at different energies are not spatially coincident, and that there is a tendency for the profiles at higher energies to be sharper.
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The core of the soft X—ray emission was displaced approximately 12” from the brightest point at higher energies; in addition, referring to emission above 16 key, there is some evidence that there was a second bright point separated by ‘~ 16” in the westerly direction from the brightest high energy point.
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One of the interesting features of HXIS is its ability to derive an intensity—tine curve for a given point. Figure 10 presents such curves for the two high energy maxima, I and II, and for the low energy maximum, III. Pre—flare oscillations are clearly visible in region II, while only slight indications are seen in regions I and III. We note the faster decay of the soft X—ray bright point; however if the total flare region is considered, the soft X—ray emission has the slower decay. a would indicate a gradual energy transfer from the local bright points to produce larger volume of cooler plasma which slowly regains thermal equilibrium with the ambient medium. SUMMARY We have endeavored to illustrate the excellent imaging properties of the Hard X— ray Imaging Spectrometer on SMM. From the results we have presented here from four widely differing solar flares, it is clear that the potential for new and interesting solar physics studies is very high. ACKNOWLEDGEMENTS The development and construction of the Hard X—ray Imaging Spectrometer was made possible by support given by the Netherlands Ministry for Education and Science through the Committee for Geophysics and Space Research of the Royal Netherlands Academy of Arts and Sciences, and the Science Research Council of the United Kingdom. Their support is gratefully acknowledged. We extend our thanks to our .iany coworkers in the Utrecht Space Research Laboratory, and the Birmingham Department of Space Research, to Netherlands Aircraft Factories Fokker—VFW, NV Phillips Gloeilampenfabrieken, PYE Telecommunications, Cambridge, the Technical University of Eindhoven and TDS Circuits Blackburn, as well as to the many others in the Netherlands, the United Kingdom and the USA on whose untiring efforts the success of the experiment and SMM has depended. W. v.T. wishes to acknowledge the Netherlands Organization for the Advancement of Pure Research (ZWO). JASA 1,13-A
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References:
1. Vaiana, G.S., A.S. Krieger, and S.F. Timothy: 1973, Solar Phys. 32,81. 2. Neupert, W.M., R.J. Thomas, and R.D. Chapman: 1974, Solar Phys. 34, 349. 3. Sturrock, P.A. (ed.), 1979 Proceedings of the Second Skylab Workshop, on Solar Flares, Boulder, Colorado, U.S.A. 4. Takakura, T., K. Ohki, N. Shibuya, M. Fujii, H. Matsuoka, S. Miyamoto, J. Nishimura, H. Oda, Y.Ogawara and S. Ota: 1971,Solar Phys. 16, 454. 5. Van Beek, H.F.,: 1976, Space Science Instr. 2, 197. 6. Van Beek, H.F., P. Hoyng, H. Lafleur, and G.M. Simnett: 1980, Solar Phys. 65, 39.