- Email: [email protected]
The SOLAR-A soft X-ray telescope experiment
Adv. Space Res. Vol. 8, No. 11, pp. (11)93—(11)99, 1988 Printed in Great Britain. All rights reserved.
0273—1177/88 $0.00 + .50 Copyright © 1989 COSPAR
THE SOLAR-A SOFT X-RAY TELESCOPE EXPERIMENT L. Acton,* M. Bruner,* W. Brown,* J. Lemen,* T. Hirayama,** S. Tsuneta,** T. Watanabe** and Y. Ogawara*** *Lockheed Palo Alto Research Laboratory, Palo Alto, CA 94304, U.S.A. * * National Astronomical Observatory of Japan, Mitaka, Tokyo, Japan * * * Institute of Space and Astronautical Sciences, Sagamihara, Japan
Abstract The Japanese SOLAR-Amission for the study of high energy solarphysics is timed to observe the sun during the next activity maximum. This small spacecraft includes a carefully coordinated complement of instruments for flare studies. In particular, the soft X-ray telescope (SXT) will provide x-ray images of flares with higher sensitivity and time resolution than have been available before. This paper describes the scientific capabilities of the SXT and illustrates it application to the study of an impulsive compact flare. THE SOLAR-A MISSION The principal motivation for the Japanese SOLAR-A mission is to further pursue the study of high energy processes in solar flares. The very fruitful Hinotori (Japan) and Solar Maximum (U.S.) missions at the time of the last solar maximum (solar cycle 21) have provided the experience for definition of a new scientific earth satellite with an emphasis on imaging of soft and hard x-rays with spectroscopy to determine the high energy properties of the features observed. The SOLAR-A satellite is to be launched in Aug/Sept 1991 from Japan’s Kagoshima Space Center and is intended to be operated for at least 3 years. It is a fully stabilized fine-pointing spacecraft with onboard computer control and data storage in a non-volatile bubble memory. The mission is an international cooperation with the United States and the United Kingdom. It is managed by the Japanese Institute for Space and Astronautical Sciences and is mOre fully described by Ogawara Ill. A summary ofthe SOLAR-A payload is given in Table 1. These instruments have been chosen with a view to achieving good coordinated measurements of solar high energy phenomena within the constraints of the 420 Kg spacecraft. This technical information is preliminary and subject to change as the SOLAR-A instruments and spacecraft are still under development. As compared to the Hinotori and SMM payloads, SOLAR-A provides several important enhancements for flare observing. All of the instruments provide full-disk coverage, an advantage in capturing major flares, the Hard Xray Telescope has improved high energy response and angular resolution, while the Bent Crystal Spectrometer has increased sensitivity and broader temperature coverage. The Soft X-ray Telescope will provide broad-band images, unavailable in a flare mission since SKYLAB, with good angular and time resolution. Aspect sensors, operating atvisible wavelengths, will provide the reference information necessary to align SXT, HXT, and ground based images to arcsecond accuracy. Finally, the SOLAR-A Wide Band Spectrometer yields spectral data from soft x-ray to gamma ray energies. The SOFT X-RAY TELESCOPE The SOLAR-A Soft X-ray Telescope (SXT) scientific consortium includes the National Astronomical Observatory of Japan (NAOJ), Lockheed Palo Alto ResearchLaboratory, University of Hawaii, Stanford University, and the University of California atBerkeley. For the SXT instrument the data handling and control electronics and spacecraft interface management are the responsibility of NAOJ while the telescope, detector and associated control electronics are provided by Lockheed under contract to NASA Marshall Space Flight Center. SXT hardware development is funded out of the NASA Explorer budget. Bruner, et al. /2/describe the design and capabilities of the SXT in detail. The principal optical elements of the SXT (Figure 1) comprise a glancing incidence x-ray mirror, a concentric and coaligned optical lense and filter assembly, a metering tube and mounting structure, a pair of 6 position filter wheels, a rotating shutter, and a charge coupled device (CCD) camera. The ultimate angularresolution of the SXT is a complicated function of mirror figure, scattering, diffraction, pixel size, and charge splitting between pixels. (11)93
(11)94
L.
Acton et al.
TABLE 1 SOLAR-A Instruments Hard X-ray Telescope P1: Instrument: Energy range: Angular resolution: Effective area: Time resolution: Soft X-ray Telescope P1: Prof. T. Hirayama U.S. P1 to NASA: Instrument: Wavelength range: Spectral discrimination: Angular resolution: Time resolution:
Prof. K. Kai (National Astron. Obs. of Japan) Fourier Synthesis Telescope 10 - 100 KeV 7 arcsec 2 avg. x 64 elements 1.5 cm <1 sec (Japan-US collaboration) (National Astron. Obs. ofJapan) Dr. L. W. Acton (Lockheed) Glancing incidence mirror/CCD sensor Co-aligned optical telescope using same CCD 3-60 A 4600-4800 A, 4308 A Filters 2-4 arcsec 2 sec
Bragg Crystal Spectrometer (Japan-UK collab. including NRL & NBS in US) P1: Prof. E. Hiei (National Astron. Obs. of Japan) UK P1 to SERC: Prof. J. L. Culhane (MSSL) U.S. (NRL) Dr. G. Doschek U.S. (NBS) Dr. R. Deslattes Instrument: Bent Crystal Spectrometers Spectral lines: S XV, Ca XIX, Fe XXV, Fe XXVI Spectral resolution: 1/3000 - 1/8000 Angular resolution: Full disk Time resolution: <1 sec Wide Band Spectrometer P1: Instrument:
Prof. J. Nishimura (ISAS) Soft X-ray / gas prop. counter Hard X-ray / Nal scint. counter Gamma-ray / BGO scint. counter 2 KeV - 50 MeV <1 sec
Energy range: Time resolution:
.5 1ciLAR_A.,’.S’,YT
FtLTER
ZNSTR/JAIE/VT ASSEMBL K S%T-230/8
REV C
8~~’ 26 ~
RFT SUPPORT
nrr
FLamE
Fig. 1. Principal components of the Soft X-ray Telescope instrument.
r CCO
The Solar-A Soft X-Ray Telescope
(11)95
The CCD pixels subtend 2.44 arcsec on the sun. Most of the other effects are wavelength dependent and will not be fully known until the instrument is built and calibrated. The effective angular resolution will depend upon the contrast of the scene but should be better than 5 arcsec over most of the sun. The SXT will utilize a 1024 x 1024 CCD in a 1550mm focal length system. Either X-ray or optical images will be taken, depending upon the position of the filter wheels. The arrangement of the image on the CCD is illustrated in Figure 2.
N —
_____
E
—
“Full Frame” Image Partial Frame Image Region of Interest
_________________
Fig. 2. Arrangement ofthe solar image on the CCD. The partial frame image (64 x 64) pixels may be taken anywhere in the 1024 x 1024 pixel area while the full frame image (1024 x 512 pixels) may be placed anywhere in the N-S direction. The use ofthe full frame (FF1) and partial frame (PFI) images depends primarily upon trading time resolution for field-of-view. It is also possible to increase the field-of-view at the expense of angularresolution, while maintaining time resolution, by summing pixels 2 x 2 or 4 x 4 as they are read out from the camera. Partial frame images typically are positioned automatically or by command to observe particular solar features (e.g., active regions) which are called regions of interest (ROI). Automatic positioning is based upon locating the brightest Xray features. Table 2 summarizes the size ofthe FF1 and PFI and the time resolution available with each for flare observing. Up to four ROI can be monitoredsequentially. The control and observing choices are somewhat more complex than this limited description implies /2/. SXT spectral discrimination and the ability to diagnose temperatures is through the use of transmission filters. The final selection of filters has not been made but a set presently under consideration is listed in Table 3. TABLE 2 SXT Image Parameters Type
No. Pixels
Pixel Sum
Field-of-View
Time resol.
FF1
1024x512 512x512 256x 256
lxi 2x2 4x4
41.6’ x 20.8’ 41.6’ x 41.6’ 41.6’ x 41.6’
256s 128s 32s
PFI
64x64
lxi 2x2 4 x4
2.6’ x 2.6’ 5.2’ x 5.2’ 10.4 ‘ x 10.4’
2s 2s 2s
64x64 64 x 64
TABLE 3 SXT Filter Wheels Filter Wheel 1 Filter
Code
Open Al(1200A) 10% mask Optical (200 A @ 4700 A) Optical (30 A @ 4308 A) Optical diffuser
Cop Cal Msk
Filter Wheel 2 Filter Open Al(1200A) To be determined Al-Mg-Mn Al (12 ~.tm) Be (100 ~Lm)
Code Cop Cal Cds Hal Hbe
Note: The codes “C” and “H” indicate filters designed for cooler or hotter plasma. The compoundfilter Al(3000 A)-Mg(2000 A)-Mn(600 A) is calledby its manufacturer (Luxel, Inc.) a “Dagwood sandwich”, hence Cds.
(11)96
L.
Acton et a!.
The sensitivity of the instrument for solar observing is conveniently described as the signal (charge) produced in the 3. Figure 3 CCD per unit time for an imaged feature of a standard emission measure, chosen here as jo44 cm presents this information for five filters.
io~ 0
1:
/ .-
1/f
-~Cop ~
....
7~6?!
—...
Z~ =~ 0mg ~Cds
______
Log I Fig. 3. Sensitivity of SXT as a function ofplasma temperatue for an emission measure of iø~. Solar spectrum taken from Mewe, et al. /3,4/ The ability to measure flare temperatures with SXT comes from the temperature dependence of the instrument sensitivity through different filters. By taking the ratio of signals from the same feature as observed through different filters it is possible to estimate effective or isothermal temperatures over a wide range. This is illustrated in Figure 4 for two different pairs of ifiters, one appropriate for flares and the other for quiet corona and active regions. For exposures in which the CCD is operating at >0.1 of full well (full well is approximately 2 x io~ electrons) simulations indicate that isothermal temperatures can be determined to better than 0.1 in the logarithm. 3.
~2
__
/f
~-
Hbe/Hal 3*Cds/Cal
01/
:• —o—c—~— 5.5
6.5
7.5
Log T
Fig. 4. Diagnostic filter combinations. Filter properties are given in Table 3. Thick (H) ratios are for exposures of the same duration whereas the C curve represents the case for a Cdi exposure 3 times longer than Cal. The instruments and data system of SOLAR-A are able to respond to flare flags based on the counting rates of the wide band spectrometer. When no flare is in progress, termed “quiet mode”, the SXT will typically be interleaving FFIs every 4.3 mm with PFIs which are taken every 8 sec. Up to 8 different regions of interest may be observed in rotation with the PFIs. At the time of a flare flag the FFIs are abandoned and all attention is given to the ROl containingthe flare, which is located based on x-ray brightness by on-board examination of a special x-ray image. All of this happens within approximately 10 sec following the flare flag.
The Solar-A Soft X-Ray Telescope
(11)97
FLARE OBSERVING WITH SXT The SXT experiment will represent a substantial advance in sensitivity and time resolution over earlier solar X-ray telescopes. Although the angular resolution of the SXT is inferior to some other recent instruments it is, at 2-4 arcsec, a very powerful research tool. It will contribute to research on coronal holes, global magnetic evolution, Xray bright points, evolution of active regions, and many other phenomena. It is hoped that solar oscillation studies can be carried out with the SXT optical telescope which is primarily designed to serve as an aspect and alignment sensor. However, the primary purpose of SXT and SOLAR-Ais to concentrate on high energy solar physics, especially flares. Earlier work has provided a strong observational framework for our research. Indeed, a major contribution of SXT will be to clarify and quantify many details which are left confusing and ambiguous by previous work. We will examine a singleexample to elucidate some of the questions SXT will address and illustrate how an observing sequence might be constructed for study of an impulsive compact flare. Linford and Wolfson /5/ have studied an impulsive compact flare which was well-observed by SMM. This event [21 May 1985, 0954 UT] produced a multiple-spike hard x-ray burst of about 2 minutes total duration and a soft xray burst which rose and decayed in less than 10 minutes (Figure 5). Soft x-ray images taken late in the impulsive phase and Mg XI spectra from the soft x-ray maximum onwards were used to estimate the geometry and density of the flare. The x-ray image, built up over 13 sec by rastering the 15’ (FWHM) field-of-view of the Flat Crystal Spectrometer (FCS), suggested the occurrence of at least 3 small (<7’) hot (>10 MK) kernels connected by a larger (approx 28” x 14”), cooler (<10 MK) structure which increased in volume with time. Although they note discrepancies and the possibility of alternate interpretations Linford and Wolfson conclude that the simplest interpretation of their observations yields a single long thin loop ofquite high
52
—
Hard X-ray (25
—
Ca XIX (10-20 MK)
- -.
Mg Xl (5-10 MK)
~IT: i:
-
407 keV)
Fe XXV kern&sT~,
Minutes past 0900 UT
Fig. 5. X-ray light curves of an impulsive compact flare. Arbitrary intensity scale. SXT will change to flare mode at flare trigger. Thick (H) and thin (C) SXT filters will respond to Ca XIX and Mg XI intensities while HXT wifi image hard X-ray emission. Different instruments of the SOLAR-A wide band spectrometer will generate light curves ofthe total event similar to these. The SOLAR-A bent crystal spectrometer will measure dominant line profiles and shifts without spatial resolution in X-ray emission lines of S XV, Ca XIX, Fe XXV and Fe XXVI. 3) which is heated at the two footpoints by non-thermal particles. They derive a loop cross density (>10 12 cm section diameter of 150 km from the length (2 x iø~km), emission measure (3 x i048 cm3), and measured electron density. This flare, even though of short duration, appeared to require post-impulsive-phase heating to maintain the soft x-ray source. Although the SMM results, especially the density measurements from Mg XI spectra, are an important contribution to the physical description of a flare ofthis type they leave a number ofquestions open. For example: 1. Questions concerning the topology of the flare: a. Does the single uniform loop adequately describe the flare? The SMM data are not adequate to be defmite on this point. Many lines of evidence, e.g., chromospheric brightenings, indicate that even compact flares may be asymmetric and complex. b. Do individual hardx-ray spikes typically come from different locations? Time correlations between hard Xray bursts and spatially-resolved UV bursts lead Cheng, et al. /6/ to infer that this may often be the case. c. Are hard X-ray burst locations and high temperature soft x-ray locations coincident, do these sites serve as
L. Acton et a!.
(11)98
the source regions for all of the soft x-ray plasma, and how are they connected? d. How are the chromospheric flare brightenings which ~n~i correspond to hard X-ray bursts heated and how do they connect into the overall flare topology? 2. Questions concerning the time development of the flare: a. Are the photospheric, magnetic, and chromospheric changes comprising the build-up to the impulsive phase uniquely identifiable? At what phase do the first soft X-ray signatures appear? Recent high resolution movies with a CCD camera and tunable filter by Title, et al. [1/ indicate that great progress can be expected in this area when the highest quality visible data are combined with co-aligned X-ray movies and analyzed with powerful image processing and display techniques. b. Does the coronal topology of acompact flare change during the flare? If so, are changes limited to the impulsive phase? Do compact flares affect their surroundings? Can we observe changes in the connections between the brightest x-ray emitting volume(s) and surrounding structures? Do changes reflect concurrent modifications of the magnetic field configuration? c. Where does post-impulsive-phase heating take place? 3. Questions concerning physical conditions: a. Are there unique relationships between pre-flare coronal conditions oftemperature and density and flare production? b. Can we infer an upper limit to the density at primary energy release sites? c. How does the thermal structure (spatial distribution of differential emission measure) evolve as compared to the predictions of loop models? These are not fundamentally new questions and yet they remain important today. Table 4 is intended to illustrate an SXT experiment designed to observe aflare similar to the one discussed by Linford and Wolfson /5/. All ofthe physical parameters given are derived from their paper. In the period before the flare trigger the very small emission measure of the preflare loop require long exposures to effectively measure temperatures. Therefore,an image size of 4 PH by 4 PFI is chosen, yielding a frame frequency of 32 sec. [Note: This small emission measure per pixel seems inconsistent with other data on active regions./8/ We expect typical preflareexposures to be ofthe orderof I sec.] Following the trigger the SXT field of view is changed to a single PH centered on this small flare. In order to follow the thermal evolution of the event we have chosen to cycle through all four filters every 8 sec. For an experiment tailored to observe moving x-ray emission fronts we would choose to stay with a single filter, probably Cds, for a time resolution of 2 sec. Approximately once perminute a 4308 A picture for aspect and monitoring of magnetic changes in the active region would be inserted. Please note that the actual exposure times chosen here are primarily for illustration. The exposures will normally be chosen by the SXT automatic exposure control. CONCLUSION The SXT provides great scientific potential for solar studies although it has considerable operationalcomplexity in the choice of experiment sequences. This paper has attempted to summarize the key scientific considerations for its use. Along with the other instruments on SOLAR-A and modern ground based observations it should provide a rich harvest of advances in understanding of the active sun. ACKNOWLEDGEMENTS The authors are grateful to the Institute for Astronautical and Space Sciences and to NASA for initiating the SXT program. The program is supported at Lockheed under contract NAS8-37334 with NASA Marshall Space Flight Center and by the Lockheed Independent Research Program.
The Solar-A Soft X-Ray Telescope
(11)99
TABLE 4 Observing Parameters for Impulsive Compact Flare Pre-Flare
3)
EmMeas (cm T(MK) Density (cm3) Vol (cm-3) Projectedsize(”) No. pixels EM/pix (cm-3)
3.5E+43 2.8 1E+i0 3.5E+23 18 7.4 4.8E+42
Flare Fe XXV kernel 1 Fe XXV kernel 2 2.4E+47 1.5E+47 23 29 4E+12 4E+12 1.5E+22 9.6E-i-21 0.28x0.28 0.28x0.28 1 1 2.4E+47 1.5E+47
Electrons/pixel for exPosures given below Cal 18285 35890 Cds 9488 26080 Hbe x 122727 Hal x 70644
21908 15978 80699 43686
Exposure (sec) for chosen filters Pre-Flare Cal 30 Cds 30 Hbe x Hal x
Flare 0.001 0.001 0.02 0.02
Initial Flare 3E+48 8 4E+l2 3.5E÷23 18 7.4 4.1E+47
99561 66165 81634 157024
Decay phase 3E+48 5 1E÷12 3.5E+23 18 7.4 4.1E+47
127787 75957 27371 108147
REFERENCES 1.
Y. Ogawara, The Solar-A mission, Solar Phvs~113, #1/2, 361-370 (1987)
2. M.E. Bruner, L.W. Acton, W.A. Brown, R.A. Stern, T. Hirayama, S. Tsuneta, T. Watanabe, and Y. Ogawara, The Soft X-RayTelescope for the Solar-A mission, in: Proceedings of the 1988 Yosemite Conference on Outstanding Problems in Solar System Plasma Physics: Theory and Instrumentation, to be published as a monograph by the Am. Geophysical Union (1989) 3. R. Mewe, E.H.B.M. Gronenschild, and G.H.J. van den Oord, Calculated X-radiation from optically thin plasmas V., Astron. & Astro~hys.Supnl. Series 62, 197-254 (1985) 4. R. Mewe, J.R. Lemen, and G.H.J. van den Oord, Calculated X-radiation from optically thin plasmas VI., Astron. & Astrophvs. Sunpl. Series 63, 511 (1986) 5.
G.A. Linford, and C.J. Wolfson, Properties of an impulsive compact solar flare determined from Solar Maximum Mission X-ray measurements, Astrophvs. I. 331, 1036-1046 (1988)
6.
C.-C. Cheng, E. Tandberg-Hanssen, and L.E. Orwig, Correlated observations of impulsive UV and hard Xray bursts in solar flares from the Solar Maximum Mission, Astronhvs. J. 278, 853-862 (1984)
7.
A..M. Title, T.D. Tarbell, K.P. Topka, S.H. Ferguson, R.A. Shine, and the SOUP Team, Astrophvs. J. 15 Dec issue (1988)
8.
G.S. Vaiana, and R. Rosner, Ann. Rev. Asiron. and Astrophvs. 16, 393 (1978)
0273—1177/88 $0.00 + .50 Copyright © 1989 COSPAR
THE SOLAR-A SOFT X-RAY TELESCOPE EXPERIMENT L. Acton,* M. Bruner,* W. Brown,* J. Lemen,* T. Hirayama,** S. Tsuneta,** T. Watanabe** and Y. Ogawara*** *Lockheed Palo Alto Research Laboratory, Palo Alto, CA 94304, U.S.A. * * National Astronomical Observatory of Japan, Mitaka, Tokyo, Japan * * * Institute of Space and Astronautical Sciences, Sagamihara, Japan
Abstract The Japanese SOLAR-Amission for the study of high energy solarphysics is timed to observe the sun during the next activity maximum. This small spacecraft includes a carefully coordinated complement of instruments for flare studies. In particular, the soft X-ray telescope (SXT) will provide x-ray images of flares with higher sensitivity and time resolution than have been available before. This paper describes the scientific capabilities of the SXT and illustrates it application to the study of an impulsive compact flare. THE SOLAR-A MISSION The principal motivation for the Japanese SOLAR-A mission is to further pursue the study of high energy processes in solar flares. The very fruitful Hinotori (Japan) and Solar Maximum (U.S.) missions at the time of the last solar maximum (solar cycle 21) have provided the experience for definition of a new scientific earth satellite with an emphasis on imaging of soft and hard x-rays with spectroscopy to determine the high energy properties of the features observed. The SOLAR-A satellite is to be launched in Aug/Sept 1991 from Japan’s Kagoshima Space Center and is intended to be operated for at least 3 years. It is a fully stabilized fine-pointing spacecraft with onboard computer control and data storage in a non-volatile bubble memory. The mission is an international cooperation with the United States and the United Kingdom. It is managed by the Japanese Institute for Space and Astronautical Sciences and is mOre fully described by Ogawara Ill. A summary ofthe SOLAR-A payload is given in Table 1. These instruments have been chosen with a view to achieving good coordinated measurements of solar high energy phenomena within the constraints of the 420 Kg spacecraft. This technical information is preliminary and subject to change as the SOLAR-A instruments and spacecraft are still under development. As compared to the Hinotori and SMM payloads, SOLAR-A provides several important enhancements for flare observing. All of the instruments provide full-disk coverage, an advantage in capturing major flares, the Hard Xray Telescope has improved high energy response and angular resolution, while the Bent Crystal Spectrometer has increased sensitivity and broader temperature coverage. The Soft X-ray Telescope will provide broad-band images, unavailable in a flare mission since SKYLAB, with good angular and time resolution. Aspect sensors, operating atvisible wavelengths, will provide the reference information necessary to align SXT, HXT, and ground based images to arcsecond accuracy. Finally, the SOLAR-A Wide Band Spectrometer yields spectral data from soft x-ray to gamma ray energies. The SOFT X-RAY TELESCOPE The SOLAR-A Soft X-ray Telescope (SXT) scientific consortium includes the National Astronomical Observatory of Japan (NAOJ), Lockheed Palo Alto ResearchLaboratory, University of Hawaii, Stanford University, and the University of California atBerkeley. For the SXT instrument the data handling and control electronics and spacecraft interface management are the responsibility of NAOJ while the telescope, detector and associated control electronics are provided by Lockheed under contract to NASA Marshall Space Flight Center. SXT hardware development is funded out of the NASA Explorer budget. Bruner, et al. /2/describe the design and capabilities of the SXT in detail. The principal optical elements of the SXT (Figure 1) comprise a glancing incidence x-ray mirror, a concentric and coaligned optical lense and filter assembly, a metering tube and mounting structure, a pair of 6 position filter wheels, a rotating shutter, and a charge coupled device (CCD) camera. The ultimate angularresolution of the SXT is a complicated function of mirror figure, scattering, diffraction, pixel size, and charge splitting between pixels. (11)93
(11)94
L.
Acton et al.
TABLE 1 SOLAR-A Instruments Hard X-ray Telescope P1: Instrument: Energy range: Angular resolution: Effective area: Time resolution: Soft X-ray Telescope P1: Prof. T. Hirayama U.S. P1 to NASA: Instrument: Wavelength range: Spectral discrimination: Angular resolution: Time resolution:
Prof. K. Kai (National Astron. Obs. of Japan) Fourier Synthesis Telescope 10 - 100 KeV 7 arcsec 2 avg. x 64 elements 1.5 cm <1 sec (Japan-US collaboration) (National Astron. Obs. ofJapan) Dr. L. W. Acton (Lockheed) Glancing incidence mirror/CCD sensor Co-aligned optical telescope using same CCD 3-60 A 4600-4800 A, 4308 A Filters 2-4 arcsec 2 sec
Bragg Crystal Spectrometer (Japan-UK collab. including NRL & NBS in US) P1: Prof. E. Hiei (National Astron. Obs. of Japan) UK P1 to SERC: Prof. J. L. Culhane (MSSL) U.S. (NRL) Dr. G. Doschek U.S. (NBS) Dr. R. Deslattes Instrument: Bent Crystal Spectrometers Spectral lines: S XV, Ca XIX, Fe XXV, Fe XXVI Spectral resolution: 1/3000 - 1/8000 Angular resolution: Full disk Time resolution: <1 sec Wide Band Spectrometer P1: Instrument:
Prof. J. Nishimura (ISAS) Soft X-ray / gas prop. counter Hard X-ray / Nal scint. counter Gamma-ray / BGO scint. counter 2 KeV - 50 MeV <1 sec
Energy range: Time resolution:
.5 1ciLAR_A.,’.S’,YT
FtLTER
ZNSTR/JAIE/VT ASSEMBL K S%T-230/8
REV C
8~~’ 26 ~
RFT SUPPORT
nrr
FLamE
Fig. 1. Principal components of the Soft X-ray Telescope instrument.
r CCO
The Solar-A Soft X-Ray Telescope
(11)95
The CCD pixels subtend 2.44 arcsec on the sun. Most of the other effects are wavelength dependent and will not be fully known until the instrument is built and calibrated. The effective angular resolution will depend upon the contrast of the scene but should be better than 5 arcsec over most of the sun. The SXT will utilize a 1024 x 1024 CCD in a 1550mm focal length system. Either X-ray or optical images will be taken, depending upon the position of the filter wheels. The arrangement of the image on the CCD is illustrated in Figure 2.
N —
_____
E
—
“Full Frame” Image Partial Frame Image Region of Interest
_________________
Fig. 2. Arrangement ofthe solar image on the CCD. The partial frame image (64 x 64) pixels may be taken anywhere in the 1024 x 1024 pixel area while the full frame image (1024 x 512 pixels) may be placed anywhere in the N-S direction. The use ofthe full frame (FF1) and partial frame (PFI) images depends primarily upon trading time resolution for field-of-view. It is also possible to increase the field-of-view at the expense of angularresolution, while maintaining time resolution, by summing pixels 2 x 2 or 4 x 4 as they are read out from the camera. Partial frame images typically are positioned automatically or by command to observe particular solar features (e.g., active regions) which are called regions of interest (ROI). Automatic positioning is based upon locating the brightest Xray features. Table 2 summarizes the size ofthe FF1 and PFI and the time resolution available with each for flare observing. Up to four ROI can be monitoredsequentially. The control and observing choices are somewhat more complex than this limited description implies /2/. SXT spectral discrimination and the ability to diagnose temperatures is through the use of transmission filters. The final selection of filters has not been made but a set presently under consideration is listed in Table 3. TABLE 2 SXT Image Parameters Type
No. Pixels
Pixel Sum
Field-of-View
Time resol.
FF1
1024x512 512x512 256x 256
lxi 2x2 4x4
41.6’ x 20.8’ 41.6’ x 41.6’ 41.6’ x 41.6’
256s 128s 32s
PFI
64x64
lxi 2x2 4 x4
2.6’ x 2.6’ 5.2’ x 5.2’ 10.4 ‘ x 10.4’
2s 2s 2s
64x64 64 x 64
TABLE 3 SXT Filter Wheels Filter Wheel 1 Filter
Code
Open Al(1200A) 10% mask Optical (200 A @ 4700 A) Optical (30 A @ 4308 A) Optical diffuser
Cop Cal Msk
Filter Wheel 2 Filter Open Al(1200A) To be determined Al-Mg-Mn Al (12 ~.tm) Be (100 ~Lm)
Code Cop Cal Cds Hal Hbe
Note: The codes “C” and “H” indicate filters designed for cooler or hotter plasma. The compoundfilter Al(3000 A)-Mg(2000 A)-Mn(600 A) is calledby its manufacturer (Luxel, Inc.) a “Dagwood sandwich”, hence Cds.
(11)96
L.
Acton et a!.
The sensitivity of the instrument for solar observing is conveniently described as the signal (charge) produced in the 3. Figure 3 CCD per unit time for an imaged feature of a standard emission measure, chosen here as jo44 cm presents this information for five filters.
io~ 0
1:
/ .-
1/f
-~Cop ~
....
7~6?!
—...
Z~ =~ 0mg ~Cds
______
Log I Fig. 3. Sensitivity of SXT as a function ofplasma temperatue for an emission measure of iø~. Solar spectrum taken from Mewe, et al. /3,4/ The ability to measure flare temperatures with SXT comes from the temperature dependence of the instrument sensitivity through different filters. By taking the ratio of signals from the same feature as observed through different filters it is possible to estimate effective or isothermal temperatures over a wide range. This is illustrated in Figure 4 for two different pairs of ifiters, one appropriate for flares and the other for quiet corona and active regions. For exposures in which the CCD is operating at >0.1 of full well (full well is approximately 2 x io~ electrons) simulations indicate that isothermal temperatures can be determined to better than 0.1 in the logarithm. 3.
~2
__
/f
~-
Hbe/Hal 3*Cds/Cal
01/
:• —o—c—~— 5.5
6.5
7.5
Log T
Fig. 4. Diagnostic filter combinations. Filter properties are given in Table 3. Thick (H) ratios are for exposures of the same duration whereas the C curve represents the case for a Cdi exposure 3 times longer than Cal. The instruments and data system of SOLAR-A are able to respond to flare flags based on the counting rates of the wide band spectrometer. When no flare is in progress, termed “quiet mode”, the SXT will typically be interleaving FFIs every 4.3 mm with PFIs which are taken every 8 sec. Up to 8 different regions of interest may be observed in rotation with the PFIs. At the time of a flare flag the FFIs are abandoned and all attention is given to the ROl containingthe flare, which is located based on x-ray brightness by on-board examination of a special x-ray image. All of this happens within approximately 10 sec following the flare flag.
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FLARE OBSERVING WITH SXT The SXT experiment will represent a substantial advance in sensitivity and time resolution over earlier solar X-ray telescopes. Although the angular resolution of the SXT is inferior to some other recent instruments it is, at 2-4 arcsec, a very powerful research tool. It will contribute to research on coronal holes, global magnetic evolution, Xray bright points, evolution of active regions, and many other phenomena. It is hoped that solar oscillation studies can be carried out with the SXT optical telescope which is primarily designed to serve as an aspect and alignment sensor. However, the primary purpose of SXT and SOLAR-Ais to concentrate on high energy solar physics, especially flares. Earlier work has provided a strong observational framework for our research. Indeed, a major contribution of SXT will be to clarify and quantify many details which are left confusing and ambiguous by previous work. We will examine a singleexample to elucidate some of the questions SXT will address and illustrate how an observing sequence might be constructed for study of an impulsive compact flare. Linford and Wolfson /5/ have studied an impulsive compact flare which was well-observed by SMM. This event [21 May 1985, 0954 UT] produced a multiple-spike hard x-ray burst of about 2 minutes total duration and a soft xray burst which rose and decayed in less than 10 minutes (Figure 5). Soft x-ray images taken late in the impulsive phase and Mg XI spectra from the soft x-ray maximum onwards were used to estimate the geometry and density of the flare. The x-ray image, built up over 13 sec by rastering the 15’ (FWHM) field-of-view of the Flat Crystal Spectrometer (FCS), suggested the occurrence of at least 3 small (<7’) hot (>10 MK) kernels connected by a larger (approx 28” x 14”), cooler (<10 MK) structure which increased in volume with time. Although they note discrepancies and the possibility of alternate interpretations Linford and Wolfson conclude that the simplest interpretation of their observations yields a single long thin loop ofquite high
52
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Hard X-ray (25
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Ca XIX (10-20 MK)
- -.
Mg Xl (5-10 MK)
~IT: i:
-
407 keV)
Fe XXV kern&sT~,
Minutes past 0900 UT
Fig. 5. X-ray light curves of an impulsive compact flare. Arbitrary intensity scale. SXT will change to flare mode at flare trigger. Thick (H) and thin (C) SXT filters will respond to Ca XIX and Mg XI intensities while HXT wifi image hard X-ray emission. Different instruments of the SOLAR-A wide band spectrometer will generate light curves ofthe total event similar to these. The SOLAR-A bent crystal spectrometer will measure dominant line profiles and shifts without spatial resolution in X-ray emission lines of S XV, Ca XIX, Fe XXV and Fe XXVI. 3) which is heated at the two footpoints by non-thermal particles. They derive a loop cross density (>10 12 cm section diameter of 150 km from the length (2 x iø~km), emission measure (3 x i048 cm3), and measured electron density. This flare, even though of short duration, appeared to require post-impulsive-phase heating to maintain the soft x-ray source. Although the SMM results, especially the density measurements from Mg XI spectra, are an important contribution to the physical description of a flare ofthis type they leave a number ofquestions open. For example: 1. Questions concerning the topology of the flare: a. Does the single uniform loop adequately describe the flare? The SMM data are not adequate to be defmite on this point. Many lines of evidence, e.g., chromospheric brightenings, indicate that even compact flares may be asymmetric and complex. b. Do individual hardx-ray spikes typically come from different locations? Time correlations between hard Xray bursts and spatially-resolved UV bursts lead Cheng, et al. /6/ to infer that this may often be the case. c. Are hard X-ray burst locations and high temperature soft x-ray locations coincident, do these sites serve as
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the source regions for all of the soft x-ray plasma, and how are they connected? d. How are the chromospheric flare brightenings which ~n~i correspond to hard X-ray bursts heated and how do they connect into the overall flare topology? 2. Questions concerning the time development of the flare: a. Are the photospheric, magnetic, and chromospheric changes comprising the build-up to the impulsive phase uniquely identifiable? At what phase do the first soft X-ray signatures appear? Recent high resolution movies with a CCD camera and tunable filter by Title, et al. [1/ indicate that great progress can be expected in this area when the highest quality visible data are combined with co-aligned X-ray movies and analyzed with powerful image processing and display techniques. b. Does the coronal topology of acompact flare change during the flare? If so, are changes limited to the impulsive phase? Do compact flares affect their surroundings? Can we observe changes in the connections between the brightest x-ray emitting volume(s) and surrounding structures? Do changes reflect concurrent modifications of the magnetic field configuration? c. Where does post-impulsive-phase heating take place? 3. Questions concerning physical conditions: a. Are there unique relationships between pre-flare coronal conditions oftemperature and density and flare production? b. Can we infer an upper limit to the density at primary energy release sites? c. How does the thermal structure (spatial distribution of differential emission measure) evolve as compared to the predictions of loop models? These are not fundamentally new questions and yet they remain important today. Table 4 is intended to illustrate an SXT experiment designed to observe aflare similar to the one discussed by Linford and Wolfson /5/. All ofthe physical parameters given are derived from their paper. In the period before the flare trigger the very small emission measure of the preflare loop require long exposures to effectively measure temperatures. Therefore,an image size of 4 PH by 4 PFI is chosen, yielding a frame frequency of 32 sec. [Note: This small emission measure per pixel seems inconsistent with other data on active regions./8/ We expect typical preflareexposures to be ofthe orderof I sec.] Following the trigger the SXT field of view is changed to a single PH centered on this small flare. In order to follow the thermal evolution of the event we have chosen to cycle through all four filters every 8 sec. For an experiment tailored to observe moving x-ray emission fronts we would choose to stay with a single filter, probably Cds, for a time resolution of 2 sec. Approximately once perminute a 4308 A picture for aspect and monitoring of magnetic changes in the active region would be inserted. Please note that the actual exposure times chosen here are primarily for illustration. The exposures will normally be chosen by the SXT automatic exposure control. CONCLUSION The SXT provides great scientific potential for solar studies although it has considerable operationalcomplexity in the choice of experiment sequences. This paper has attempted to summarize the key scientific considerations for its use. Along with the other instruments on SOLAR-A and modern ground based observations it should provide a rich harvest of advances in understanding of the active sun. ACKNOWLEDGEMENTS The authors are grateful to the Institute for Astronautical and Space Sciences and to NASA for initiating the SXT program. The program is supported at Lockheed under contract NAS8-37334 with NASA Marshall Space Flight Center and by the Lockheed Independent Research Program.
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TABLE 4 Observing Parameters for Impulsive Compact Flare Pre-Flare
3)
EmMeas (cm T(MK) Density (cm3) Vol (cm-3) Projectedsize(”) No. pixels EM/pix (cm-3)
3.5E+43 2.8 1E+i0 3.5E+23 18 7.4 4.8E+42
Flare Fe XXV kernel 1 Fe XXV kernel 2 2.4E+47 1.5E+47 23 29 4E+12 4E+12 1.5E+22 9.6E-i-21 0.28x0.28 0.28x0.28 1 1 2.4E+47 1.5E+47
Electrons/pixel for exPosures given below Cal 18285 35890 Cds 9488 26080 Hbe x 122727 Hal x 70644
21908 15978 80699 43686
Exposure (sec) for chosen filters Pre-Flare Cal 30 Cds 30 Hbe x Hal x
Flare 0.001 0.001 0.02 0.02
Initial Flare 3E+48 8 4E+l2 3.5E÷23 18 7.4 4.1E+47
99561 66165 81634 157024
Decay phase 3E+48 5 1E÷12 3.5E+23 18 7.4 4.1E+47
127787 75957 27371 108147
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