- Email: [email protected]
Solar maximum mission experiment: Ultraviolet spectroscopy and polarimetry on the solar maximum mission
75-283. © COSPAR, 1981. Printed in Great Britain.
Adv. Space Res. Vol.1, pp.2
0273—1177/81/0401—0275$05.OO/O
SOLAR MAXIMUM MISSION EXPERIMENT: ULTRAVIOLET SPECTROSCOPY AND POLARIMETRY ON THE SOLAR MAXIMUM MISSION The UVSP Team: E. Tandberg-Hanssen,’ B.E. Woodgate,2 R.G. Athay,3 J.M. Beckers,4 J.C. Brandt,2 E.C. Bruner,5 R.D. Chapman,2 C.C. Cheng,’ J.B. Gurman,6 C. L. Hyder,7 P.J. Kenney,2 A.G. Michalitsianos,2 R.A. Rehse,5 S.A. Schoolman,5 R.A. Shine2 and W. Henze8 ‘NASA-Marshall Space Flight Center, Greenbelt, Maryland, USA; 2 NASA-Goddard SpaceFlight Center; ~High Altitude Observatory, Boulder Colorado; ~University ofArizona; 5 Palo Alto Research Laboratory, Palo Alto, California; 6Applied Research and Systems, Annapolis, Maryland; ~University ofAlabama in Huntsville; 8 Teledyne Brown Engineering, Hunstsville, Alabama, USA ABSTRACT
We describe the Ultraviolet Spectrometer and Polarimeter (UVSP) on the Solar Maximum z4ission (SMM) spacecraft. The instrument, which operates in the wavelength range 1150 — 36~0 ~, has a spatial resolution of 2-3 arc sec and a spectral resolution of 0.02 A FWHM in second order. A Gregorian telescope, focal length 1.8 n, feeds a 1 m Ebert-Fastie spectrometer. A polarimeter comprising rotating Mg F 2 waveplates can be inserted behind the spectrometer entrance slit and allows all four Stokes parameters to be determined. The observing modes include rasters, spectral scans, velocity measurements, and polarimetry. Finally, we present examples of initial observations made since launch. SCIENTIFIC OBJECTIVES The main scientific objective of this experiment is the study of solar activity manifestations, viz., active regions, prominences, sunspots and, in particular, flares. The quiet Sun also will be studied to improve our knowledge of the normal solar atmosphere and to provide a basis against which solar activity can be measured. Another objective is to conduct an aeronomy program to measure various constituents of the Earth’s atmosphere. The main objectives can be classified in the following way which also shows the versatility of the instrument: 1.
Dynamics of solar activity manifestations (measurements of intensity and velocity in flares, prominences, sunspots, and active regions, using selected emission lines).
2.
Plasma diagnostics (determination of density and temperature in solar atmospheric plasmas, using selected emission—line profiles 275
E. Tandberg—Hanssen et al.
276
or line—intensity ratios). 3.
Polarization measurements Circular polarization
-
effect of magnetic fields.
Linear polarization - effect of anistropic excitation (impact or resonance scattering). 4.
Aeronomy Distribution of major absorbers with height. Detection of trace constituents.
INSTRUMENT DESCRIPTION The UVSP instrument has been described in some detail by Woodgate et al. (1]; only a brief summary will be given here. The layout of the principal optical and mechanical components showing the light path is illustrated in Figure 1. OREGOnIAN TELESCOPE RASTERED SECONDARY MIRROR
//
POLARIMETER
/
//
EBERT-FASTIE SPECTROMETER
PRIMARY
EBERT MIRROR
GRATING
~J DETECTORS IS)
Fig. 1
SLIT CHANGER
WAVELENGTH DRIVE
Layout of Instrument
The instrument consists of ~ telescope, spectrometer, polarimeter, detectors, and control system electronics. The basic structure was the engineering model (flight spare unit) of the University of Colorado ultraviolet spectrometer flown on OSO-8 (2]. The primary modifications to the OSO-8 instrument were the addition of entrance and exit slit combinations, and the addition of more detectors. In addition, the instrument was mounted in a set of gimbals, known as the coalignment adjustment system, which could be used to coalign the UVSP with the other pointed instruments on SMM, if necessary. The coalignment system has not been used during the first half year of the mission. Telescope The Gregorian telescope design allows a field stop to be placed at the prime focus to limit the solar irradiance on the secondary mirror. The ultraviolet component can polymerize any hydrocarbons on the mirror surface; this effect is believed to be the cause of the large reduction in sensitivity experienced by the OSO—8 instrument. Further protection against contamination was provided during the first few days after launch by using heaters on the mirrors to reduce
si~ni~u.v. TABLE 1
Spectrometer/polarineter
277
UVSP Instrument Properties
Telescope Effective Focal Length
1.8 m
Telescope Collecting Area
66.4 cm
2
Telescope Resolution
‘\~
2 to 3 arc sec
Telescope Pointing Range and Maximum Baster Size
256 x 256 arc sec
Minimum Raster Step Size
1 arc sec
Maximum Raster Rate
1 pixel/64 ma
Ebert Mirror Focal Length
1 m
Grating Size
75 x 75 mm
Grating Groove Spacing
3600 grooves/mm
Spectral Resolution
2nd Order 1st Order
0.02 FWHM 0.04 ~ FWHM
Spectral
2nd Order 1st Order
1150-1800 1750-3600 A
2nd Order
0.005
1st Order
0A 0.01 A
0
Range
Wavelength Drive Step Size
0
Recommended Spectral Scanning Rate
2 step/s
Maximum Wavelength Drive Slewing Rate
500 steps/s
Entrance Slit Sizes*
1 x 1, 3 x 3, 4 x 4, 10 x 10, 30 x 30, 1 x 10, 15 x 286, 1 x 180, all in arc sec
Exit Slit Widths*
0.01,
0.03, 0.05, 0.1~, 0.2, 0.3, 0.5,
0.8, 2.3, 3.0, all in A Waveplate Rotation Step Size
22.5 degrees
Maximum Waveplate Rotation Rate
1 step/128 ms
*~ly certain combinations of entrance slit and exit slits are available.
E. Tandberg—Hanssen et al.
278
The polarimeter can be used with any of the slit sets. For example, use with the spectroscopy slits allows the complete Stokes profile of individual spectral lines to be observed. Use with the intensity slits allows the net polarization of the total line to be measured. When the polarimeter is combined with the velocity slits which split a line profile into two halves, it is possible to measure the longitudinal magnetic field. Detectors The five detectors are photomultipliers operating ~n the pulse-counting mode. Four have CsI photocathodes for wavelengths below 1800 A. The f~,fth detector has a CsTe photocathode for wavelengths below approximately 3600 A; it can be used with only three of the slit sets. All of the detectors have LiF windows. There are only two counters and data channels; therefore, only two detectors can operate simultaneously. It is possible for two detectors to share a channel, with signals alternating from the two detectors; in this way up to four detectors may be used in a nearly simultaneous manner. The maximum rate at which data can be sent to the telemetry is twice every 64 milliseconds. The actual rate will depend on the gate time for an individual measurement. Control System Electronics Most UVSP operations are controlled by the instrument computer known as Junior. The spacecraft computer is normally used only for certain tasks, such as changing slits, inserting the polarimeter waveplates, and moving the telescope door, and for commanding Junior to start an observational sequence. Junior then controls the raster mechanism, the wavelength drive, the rotation of the waveplates, the turning on of the desired detectors, the duration of an individual measurement, and the number of times a sequence is repeated. Additional command channels from the spacecraft computer are provided for many of the instrument functions to allow operation in a degraded mode in the event of a failure of Junior. Performance For the most part, the UV5P has performed extremely well during the first months after launch. An exception is one of the detectors which apparently sometimes sends transient electrical signals into the instrument and causes all or part of the instrument to turn off. The remedy has been to discontinue use of that detector and leave it turned off. This can limit certain observations which relied on that particular detector but has not really caused any significant problems. The sensitivity for the first month following launch was the same as that which had been measured in the laboratory before launch. However, ~he sensitivity at the shorter wavelengths has since begun to decrease. At 1216 A the sensitivity decreased by about two orders of magnitude until approximately 2.5 months after launch and has 0rexnained constant since that time. At longer wavelengths, e.g., 1300 and 1400 A, the de8rease in sensitivity has been much smaller. At the important wavelength of 1548 A (CIV line), there has been no evidence of any decrease. In comparison with the earlier OSO—8 experience, the decrease in UVSP sensitivity at the short wavelengths has not been as large and has occurred more slowly. EXAMPLES OF INITIAL OBSERVATIONS The UVSP is capable of making a variety of different types of observations. The observing modes range from the simple, where only one mechanism is used, to the complex, where the raster, wavelength drive, and polarimeter mechanisms are all used. A few of the more important nodes or “experiment types” are listed in
SMM:
U.V. Spectrometer/polari,neter
279
conden~ation and opening the telescope door only during orbital night to prevent solar radiation from reaching the mirrors while the spacecraft and instrument outgassed. The secondary mirror is mounted on gimbals and can be tilted to provide an internal pointing and rastering capability within a 256 x 256 arc sec field of view. The location of the center of the raster, the number of pixels and spacing between pixels in each direction, and the gate time per pixel can all be chosen by the experimenter. All of the optics are coated with aluminum and overcoated The effectiv~ focal length is 1.8 m.
with magnesium fluoride.
Spectrometer The spectrometer is of the Ebert—Fastie design. The focal length of the Ebert mirror is 1 m. The 0holographic photoresist grating is used in second order below approximately 1800 A and in first order at longer wavelengths. The wavelength is changed by rotating the grating with the wavelength drive. The wavelength region to be observed, the number of wavelength points in a spectral scan, and the spacing between wavelength points can all be chosen by the investigator. Twenty—two entrance and exit slit combinations have been provided. The entrance and exit slits are mounted on a single plate that can be rotated to bring a desired slit set into the light path. All of the slit sets have multiple exit slits so that more than one detector can be used at the same time. There are several types of slit sets, the main ones being the intensity, velocity, and spectroscopy sets. The intensity slits have wide exit slits to allow the entire line to be observed. The velocity slits have wide exit slits that are split into two halves by deflectors. When a spectral line is properly centered, the long and short wavelength halves of the line are sent to different detectors. A Doppler shift will move the line, and the resulting imbalance in the signals fron the two detectors can be used to measure the velocity. Both the intensity and velocity slit sets provide a selection of entrance slit sizes that allows a choice between higher spatial resolution and higher count rates which allow higher time resolution. The spectroscopy slits have narrow entrance and exit slits for high spectral resolution. Polarimeter The polarimeter includes two retarders or waveplates, one of which is followed by a four—mirror linear polarizer, and drive mechanisms to insert either waveplate into the light path and to rotate the waveplate to allow the polarization to be measured. Because one of the waveplates is not followed by a special polarizer, the spectrometer grating which is a partial linear polarizer serves as the analyzer for that waveplate. The waveplates are each made of two disks of magnesium flour— ide with their fast axes crosses. The difference in thickness of the two disks determines the retardance for the waveplate. The two waveplates have retardances of approximately one—qua~ter wave and three—quarters wave in the important spectral region around 1500 A. Polarization is measured by observing the variation in detected signal as the waveplate is rotated. A maximum of 16 waveplate positions per ccmplete rotation can be used. For measurement of the circular polarization alone, it is possible to use only four waveplate positions.
E. Tandberg—Hanssen et al.
280
Table 2. In addition, it is possible to use the results of one experiment as a parameter for the following experiment. For example, a spectral line can be scanned, and the wavelength drive position of the peak intensity in the profile can be stored in the instrument computer; a raster can then be made at that wavelength drive position. TABLE 2
Examples of UVSP Experiment Types
Name
Explanation
Spectrohe liogram
Raster
Dopplergram
Raster with velocity slits, periodic recentering of lines, and wavelengths offsets to calibrate
Polargram
Raster using polarimeter
Magnetograin
Raster with velocity slits and polari— meter, periodic recentering of lines, and wavelength offsets to calibrate
Spectrogram
Spectral scan
Profile
Matrix
Raster of spectrograms over a single line profile with change to new pixel after complete profile is observed
Raster through the line
Set of rasters at different wavelengths in line profile; after raster is completed at one wavelength, then wavelength drive steps to new wavelengths
Polarized profile matrix
Raster of spectral scans, including polarization neasurements, with change to new wavelength after waveplate rotation is completed and change to new pixel after complete profile is observed
Spectroscopy The use of a narrow exit slit when scanning the spectrum results in high spectral resolution line profiles. Figure 2 shows the ~yman alpha line of HI at 1215 A and the 01 lines at 1302.2, 1304.8 and 1306.0 A, obtained early in the mission. The p~ofiles show the geocoronal absorption lines, from which can be deduced the 0.02 A spectral resolution. Images Figure 3 shows an image typical of those obtained in the spectroheliogram and dopplergram modes in the C IV 1548 A line, a transition region line formed at 10 K. The figure shows loops at the southwest limb and structures on the disk. When repeated many times, such observations show transverse motions and other changes in the structure of the transition region.
SMM: 4050.
-
U.V. Spectrometer/polarjmeter 056
281
-
~jY~ LLIe Fig. 2 HI Lyman alpha — 1215 (left), and 01 — 1302.2, 1304.8, and 1306.0 (right), line profiles. The ordinate gives the counts, while the abscissa is proportional to the wavelength.
0
Fig. 3 Image of the solar limb in the C IV line at 1548 A, obtained in March 1980. North is to the left, and west is up. The pixel size is 3 arc sec square, and the image size is 4 arc minutes square.
JASA 1:13-S
E. Tandberg—Hanssen et al.
282 Dynamical Studies
In another paper [31, we have reported observations of oscillations in the transition region above sunspots. The observations were made in the dopplergram mode (using the velocity slits), which allows the velocity of the emitting plasma to be obtained from the difference in the intensities in the two halves of the line. The periods measured for the oscillations above spots in two different active regions in April 1980 were 170 and 130 seconds. Plasma Diagnostics Figure 4 illustrates observations which can be used to determine densities and motions on the Sun. The top part of the figure shows spectroheliogr~msof active region ~363 (N12W13) obtained on April 2, 1980, in the Si IV 1402.8 A and 0 IV 1401.2 A lines. The slit size was 4 arc sec square, and the image size was 1 arc minute square. The intensity ratio of the intersystem 0 IV line to that of the allowed Si IV line is used to determine the density distribution in the transition region, as shown in the figure. Since the observations were made in the rasterthrough-the—line mode with five wavelength positions, we can also use the first moment of the intensity to derive the distribution of the line-of-sight velocity, as shown in the figure.
Fig. 4 Active region 2363 on April 2, l9~0, observed in raster-through-theline mode. Top: Images in Si IV 1402.8 A and 0 IV 1401.2 A lines obtained from the sum over the profiles of the five individual rasters. The pixel size is 4 arc sec square, and the image size is 1 arc sec square. Bottom left: Density distribution inferred from ratio of top images. Bottom right: Velocity distribution inferred from the first moment of the intensity at the five wavelength points.
.
SMM:
u.v.
Spectrojneter/polarjmeter
283
As can be seen from the figure, the emission from the active region is concentrated in loop-like structure. The density in the loop is approximately 1 - 3 x 10” cm , whereas in the surrounding area, the density is much less. In the density map, bright features are of high density; in the velocity map, bright areas are of upward motion, while dark areas are of downward motion. The highest upward or downward velocity is approximately 10 km/s. There is no obvious correlation between the density and velocity distributions. The spatial distributions of density and velocity in the an active region are, in general, inhomogeneous and show common to find a change of density of more than a factor 4 arc sec apart. A detailed description of the analysis results will be forthcoming.
transition zone plasma in time variations. It is of 2 for adjacent pixels, and the observational
Polarimetry We have done polarimetry in several modes. For example, we have measured all of the Stokes parameters across a line profile in a sunspot [3]. We have also searched for linear polarization due to anistropic excitation during flares. In general, the polarization is small and requires long integration time for the observations. We have also measured a longitudinal magnetic field of 1100 gauss above a sunspot [3]. Such observations are made in the magnetogram mode and are based only on the circular polarization due to the Zeeman effect. ACENOWLEJ~EMENTS The original 060-8 spectrometer was built by the Laboratory for Atmospheric and Space Physics of the University of Colorado. Most of the modifications to the OSO-8 hardware and the integration of the instrument were performed by the General Electric Company, Valley Forge, Pennsylvania, with overall systems design and engineering by the Goddard Space Flight Center. The optical design and optical parts fabrication were also provided by the Goddard Space Flight Center. The polarimeter was designed and built by the Marshall Space Flight Center. The electronics, including the computer, were built by SCI Systems, Huntsville, Alabama. Software for the computer was provided by the Lockheed Palo Alto Research Laboratory, Palo Alto, California. We acknowledge the efforts of our operations team at Goddard - R. P.ndrieux, C. Condor, L. Fesq, C. J. Hughes, P. Pritchard, and S. Turnier - in obtaining the observations and thank E. J. Reichmann for his assistance at Marshall. REFERENCES 1.
B. E. Woodgate, E. A. Tandberg-Hanssen, E. C. Bruner, 1. H. Beckers, J. C. Brandt, W. Henze, C. L. Ryder, M. W. Kalet, P. J. Kenny, E. D. Knox, A. G. Michalitsianos, R. Rehse, R. A. Shine, and H. D. Tinsley, Solar Physics 65, 73 (1980).
2.
E. C. Bruner, Space Science Instrumentation
3.
E. Tandberg-Hanssen, R. G. Athay, J. M. Beckers, .1. C. Brandt, E. C. Bruner, R. D. ~1açznan,C. C. cheng, .1. B. Gurman, W. Henze, C. L. Ryder, A. G. Micha’litsianos, R. A. Shine, S. A. Schoo].man, and B. H. Woodgate, Astrophysical J (Lett.) (.198O~, in press.
3, 369 (1977).
Adv. Space Res. Vol.1, pp.2
0273—1177/81/0401—0275$05.OO/O
SOLAR MAXIMUM MISSION EXPERIMENT: ULTRAVIOLET SPECTROSCOPY AND POLARIMETRY ON THE SOLAR MAXIMUM MISSION The UVSP Team: E. Tandberg-Hanssen,’ B.E. Woodgate,2 R.G. Athay,3 J.M. Beckers,4 J.C. Brandt,2 E.C. Bruner,5 R.D. Chapman,2 C.C. Cheng,’ J.B. Gurman,6 C. L. Hyder,7 P.J. Kenney,2 A.G. Michalitsianos,2 R.A. Rehse,5 S.A. Schoolman,5 R.A. Shine2 and W. Henze8 ‘NASA-Marshall Space Flight Center, Greenbelt, Maryland, USA; 2 NASA-Goddard SpaceFlight Center; ~High Altitude Observatory, Boulder Colorado; ~University ofArizona; 5 Palo Alto Research Laboratory, Palo Alto, California; 6Applied Research and Systems, Annapolis, Maryland; ~University ofAlabama in Huntsville; 8 Teledyne Brown Engineering, Hunstsville, Alabama, USA ABSTRACT
We describe the Ultraviolet Spectrometer and Polarimeter (UVSP) on the Solar Maximum z4ission (SMM) spacecraft. The instrument, which operates in the wavelength range 1150 — 36~0 ~, has a spatial resolution of 2-3 arc sec and a spectral resolution of 0.02 A FWHM in second order. A Gregorian telescope, focal length 1.8 n, feeds a 1 m Ebert-Fastie spectrometer. A polarimeter comprising rotating Mg F 2 waveplates can be inserted behind the spectrometer entrance slit and allows all four Stokes parameters to be determined. The observing modes include rasters, spectral scans, velocity measurements, and polarimetry. Finally, we present examples of initial observations made since launch. SCIENTIFIC OBJECTIVES The main scientific objective of this experiment is the study of solar activity manifestations, viz., active regions, prominences, sunspots and, in particular, flares. The quiet Sun also will be studied to improve our knowledge of the normal solar atmosphere and to provide a basis against which solar activity can be measured. Another objective is to conduct an aeronomy program to measure various constituents of the Earth’s atmosphere. The main objectives can be classified in the following way which also shows the versatility of the instrument: 1.
Dynamics of solar activity manifestations (measurements of intensity and velocity in flares, prominences, sunspots, and active regions, using selected emission lines).
2.
Plasma diagnostics (determination of density and temperature in solar atmospheric plasmas, using selected emission—line profiles 275
E. Tandberg—Hanssen et al.
276
or line—intensity ratios). 3.
Polarization measurements Circular polarization
-
effect of magnetic fields.
Linear polarization - effect of anistropic excitation (impact or resonance scattering). 4.
Aeronomy Distribution of major absorbers with height. Detection of trace constituents.
INSTRUMENT DESCRIPTION The UVSP instrument has been described in some detail by Woodgate et al. (1]; only a brief summary will be given here. The layout of the principal optical and mechanical components showing the light path is illustrated in Figure 1. OREGOnIAN TELESCOPE RASTERED SECONDARY MIRROR
//
POLARIMETER
/
//
EBERT-FASTIE SPECTROMETER
PRIMARY
EBERT MIRROR
GRATING
~J DETECTORS IS)
Fig. 1
SLIT CHANGER
WAVELENGTH DRIVE
Layout of Instrument
The instrument consists of ~ telescope, spectrometer, polarimeter, detectors, and control system electronics. The basic structure was the engineering model (flight spare unit) of the University of Colorado ultraviolet spectrometer flown on OSO-8 (2]. The primary modifications to the OSO-8 instrument were the addition of entrance and exit slit combinations, and the addition of more detectors. In addition, the instrument was mounted in a set of gimbals, known as the coalignment adjustment system, which could be used to coalign the UVSP with the other pointed instruments on SMM, if necessary. The coalignment system has not been used during the first half year of the mission. Telescope The Gregorian telescope design allows a field stop to be placed at the prime focus to limit the solar irradiance on the secondary mirror. The ultraviolet component can polymerize any hydrocarbons on the mirror surface; this effect is believed to be the cause of the large reduction in sensitivity experienced by the OSO—8 instrument. Further protection against contamination was provided during the first few days after launch by using heaters on the mirrors to reduce
si~ni~u.v. TABLE 1
Spectrometer/polarineter
277
UVSP Instrument Properties
Telescope Effective Focal Length
1.8 m
Telescope Collecting Area
66.4 cm
2
Telescope Resolution
‘\~
2 to 3 arc sec
Telescope Pointing Range and Maximum Baster Size
256 x 256 arc sec
Minimum Raster Step Size
1 arc sec
Maximum Raster Rate
1 pixel/64 ma
Ebert Mirror Focal Length
1 m
Grating Size
75 x 75 mm
Grating Groove Spacing
3600 grooves/mm
Spectral Resolution
2nd Order 1st Order
0.02 FWHM 0.04 ~ FWHM
Spectral
2nd Order 1st Order
1150-1800 1750-3600 A
2nd Order
0.005
1st Order
0A 0.01 A
0
Range
Wavelength Drive Step Size
0
Recommended Spectral Scanning Rate
2 step/s
Maximum Wavelength Drive Slewing Rate
500 steps/s
Entrance Slit Sizes*
1 x 1, 3 x 3, 4 x 4, 10 x 10, 30 x 30, 1 x 10, 15 x 286, 1 x 180, all in arc sec
Exit Slit Widths*
0.01,
0.03, 0.05, 0.1~, 0.2, 0.3, 0.5,
0.8, 2.3, 3.0, all in A Waveplate Rotation Step Size
22.5 degrees
Maximum Waveplate Rotation Rate
1 step/128 ms
*~ly certain combinations of entrance slit and exit slits are available.
E. Tandberg—Hanssen et al.
278
The polarimeter can be used with any of the slit sets. For example, use with the spectroscopy slits allows the complete Stokes profile of individual spectral lines to be observed. Use with the intensity slits allows the net polarization of the total line to be measured. When the polarimeter is combined with the velocity slits which split a line profile into two halves, it is possible to measure the longitudinal magnetic field. Detectors The five detectors are photomultipliers operating ~n the pulse-counting mode. Four have CsI photocathodes for wavelengths below 1800 A. The f~,fth detector has a CsTe photocathode for wavelengths below approximately 3600 A; it can be used with only three of the slit sets. All of the detectors have LiF windows. There are only two counters and data channels; therefore, only two detectors can operate simultaneously. It is possible for two detectors to share a channel, with signals alternating from the two detectors; in this way up to four detectors may be used in a nearly simultaneous manner. The maximum rate at which data can be sent to the telemetry is twice every 64 milliseconds. The actual rate will depend on the gate time for an individual measurement. Control System Electronics Most UVSP operations are controlled by the instrument computer known as Junior. The spacecraft computer is normally used only for certain tasks, such as changing slits, inserting the polarimeter waveplates, and moving the telescope door, and for commanding Junior to start an observational sequence. Junior then controls the raster mechanism, the wavelength drive, the rotation of the waveplates, the turning on of the desired detectors, the duration of an individual measurement, and the number of times a sequence is repeated. Additional command channels from the spacecraft computer are provided for many of the instrument functions to allow operation in a degraded mode in the event of a failure of Junior. Performance For the most part, the UV5P has performed extremely well during the first months after launch. An exception is one of the detectors which apparently sometimes sends transient electrical signals into the instrument and causes all or part of the instrument to turn off. The remedy has been to discontinue use of that detector and leave it turned off. This can limit certain observations which relied on that particular detector but has not really caused any significant problems. The sensitivity for the first month following launch was the same as that which had been measured in the laboratory before launch. However, ~he sensitivity at the shorter wavelengths has since begun to decrease. At 1216 A the sensitivity decreased by about two orders of magnitude until approximately 2.5 months after launch and has 0rexnained constant since that time. At longer wavelengths, e.g., 1300 and 1400 A, the de8rease in sensitivity has been much smaller. At the important wavelength of 1548 A (CIV line), there has been no evidence of any decrease. In comparison with the earlier OSO—8 experience, the decrease in UVSP sensitivity at the short wavelengths has not been as large and has occurred more slowly. EXAMPLES OF INITIAL OBSERVATIONS The UVSP is capable of making a variety of different types of observations. The observing modes range from the simple, where only one mechanism is used, to the complex, where the raster, wavelength drive, and polarimeter mechanisms are all used. A few of the more important nodes or “experiment types” are listed in
SMM:
U.V. Spectrometer/polari,neter
279
conden~ation and opening the telescope door only during orbital night to prevent solar radiation from reaching the mirrors while the spacecraft and instrument outgassed. The secondary mirror is mounted on gimbals and can be tilted to provide an internal pointing and rastering capability within a 256 x 256 arc sec field of view. The location of the center of the raster, the number of pixels and spacing between pixels in each direction, and the gate time per pixel can all be chosen by the experimenter. All of the optics are coated with aluminum and overcoated The effectiv~ focal length is 1.8 m.
with magnesium fluoride.
Spectrometer The spectrometer is of the Ebert—Fastie design. The focal length of the Ebert mirror is 1 m. The 0holographic photoresist grating is used in second order below approximately 1800 A and in first order at longer wavelengths. The wavelength is changed by rotating the grating with the wavelength drive. The wavelength region to be observed, the number of wavelength points in a spectral scan, and the spacing between wavelength points can all be chosen by the investigator. Twenty—two entrance and exit slit combinations have been provided. The entrance and exit slits are mounted on a single plate that can be rotated to bring a desired slit set into the light path. All of the slit sets have multiple exit slits so that more than one detector can be used at the same time. There are several types of slit sets, the main ones being the intensity, velocity, and spectroscopy sets. The intensity slits have wide exit slits to allow the entire line to be observed. The velocity slits have wide exit slits that are split into two halves by deflectors. When a spectral line is properly centered, the long and short wavelength halves of the line are sent to different detectors. A Doppler shift will move the line, and the resulting imbalance in the signals fron the two detectors can be used to measure the velocity. Both the intensity and velocity slit sets provide a selection of entrance slit sizes that allows a choice between higher spatial resolution and higher count rates which allow higher time resolution. The spectroscopy slits have narrow entrance and exit slits for high spectral resolution. Polarimeter The polarimeter includes two retarders or waveplates, one of which is followed by a four—mirror linear polarizer, and drive mechanisms to insert either waveplate into the light path and to rotate the waveplate to allow the polarization to be measured. Because one of the waveplates is not followed by a special polarizer, the spectrometer grating which is a partial linear polarizer serves as the analyzer for that waveplate. The waveplates are each made of two disks of magnesium flour— ide with their fast axes crosses. The difference in thickness of the two disks determines the retardance for the waveplate. The two waveplates have retardances of approximately one—qua~ter wave and three—quarters wave in the important spectral region around 1500 A. Polarization is measured by observing the variation in detected signal as the waveplate is rotated. A maximum of 16 waveplate positions per ccmplete rotation can be used. For measurement of the circular polarization alone, it is possible to use only four waveplate positions.
E. Tandberg—Hanssen et al.
280
Table 2. In addition, it is possible to use the results of one experiment as a parameter for the following experiment. For example, a spectral line can be scanned, and the wavelength drive position of the peak intensity in the profile can be stored in the instrument computer; a raster can then be made at that wavelength drive position. TABLE 2
Examples of UVSP Experiment Types
Name
Explanation
Spectrohe liogram
Raster
Dopplergram
Raster with velocity slits, periodic recentering of lines, and wavelengths offsets to calibrate
Polargram
Raster using polarimeter
Magnetograin
Raster with velocity slits and polari— meter, periodic recentering of lines, and wavelength offsets to calibrate
Spectrogram
Spectral scan
Profile
Matrix
Raster of spectrograms over a single line profile with change to new pixel after complete profile is observed
Raster through the line
Set of rasters at different wavelengths in line profile; after raster is completed at one wavelength, then wavelength drive steps to new wavelengths
Polarized profile matrix
Raster of spectral scans, including polarization neasurements, with change to new wavelength after waveplate rotation is completed and change to new pixel after complete profile is observed
Spectroscopy The use of a narrow exit slit when scanning the spectrum results in high spectral resolution line profiles. Figure 2 shows the ~yman alpha line of HI at 1215 A and the 01 lines at 1302.2, 1304.8 and 1306.0 A, obtained early in the mission. The p~ofiles show the geocoronal absorption lines, from which can be deduced the 0.02 A spectral resolution. Images Figure 3 shows an image typical of those obtained in the spectroheliogram and dopplergram modes in the C IV 1548 A line, a transition region line formed at 10 K. The figure shows loops at the southwest limb and structures on the disk. When repeated many times, such observations show transverse motions and other changes in the structure of the transition region.
SMM: 4050.
-
U.V. Spectrometer/polarjmeter 056
281
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~jY~ LLIe Fig. 2 HI Lyman alpha — 1215 (left), and 01 — 1302.2, 1304.8, and 1306.0 (right), line profiles. The ordinate gives the counts, while the abscissa is proportional to the wavelength.
0
Fig. 3 Image of the solar limb in the C IV line at 1548 A, obtained in March 1980. North is to the left, and west is up. The pixel size is 3 arc sec square, and the image size is 4 arc minutes square.
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282 Dynamical Studies
In another paper [31, we have reported observations of oscillations in the transition region above sunspots. The observations were made in the dopplergram mode (using the velocity slits), which allows the velocity of the emitting plasma to be obtained from the difference in the intensities in the two halves of the line. The periods measured for the oscillations above spots in two different active regions in April 1980 were 170 and 130 seconds. Plasma Diagnostics Figure 4 illustrates observations which can be used to determine densities and motions on the Sun. The top part of the figure shows spectroheliogr~msof active region ~363 (N12W13) obtained on April 2, 1980, in the Si IV 1402.8 A and 0 IV 1401.2 A lines. The slit size was 4 arc sec square, and the image size was 1 arc minute square. The intensity ratio of the intersystem 0 IV line to that of the allowed Si IV line is used to determine the density distribution in the transition region, as shown in the figure. Since the observations were made in the rasterthrough-the—line mode with five wavelength positions, we can also use the first moment of the intensity to derive the distribution of the line-of-sight velocity, as shown in the figure.
Fig. 4 Active region 2363 on April 2, l9~0, observed in raster-through-theline mode. Top: Images in Si IV 1402.8 A and 0 IV 1401.2 A lines obtained from the sum over the profiles of the five individual rasters. The pixel size is 4 arc sec square, and the image size is 1 arc sec square. Bottom left: Density distribution inferred from ratio of top images. Bottom right: Velocity distribution inferred from the first moment of the intensity at the five wavelength points.
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As can be seen from the figure, the emission from the active region is concentrated in loop-like structure. The density in the loop is approximately 1 - 3 x 10” cm , whereas in the surrounding area, the density is much less. In the density map, bright features are of high density; in the velocity map, bright areas are of upward motion, while dark areas are of downward motion. The highest upward or downward velocity is approximately 10 km/s. There is no obvious correlation between the density and velocity distributions. The spatial distributions of density and velocity in the an active region are, in general, inhomogeneous and show common to find a change of density of more than a factor 4 arc sec apart. A detailed description of the analysis results will be forthcoming.
transition zone plasma in time variations. It is of 2 for adjacent pixels, and the observational
Polarimetry We have done polarimetry in several modes. For example, we have measured all of the Stokes parameters across a line profile in a sunspot [3]. We have also searched for linear polarization due to anistropic excitation during flares. In general, the polarization is small and requires long integration time for the observations. We have also measured a longitudinal magnetic field of 1100 gauss above a sunspot [3]. Such observations are made in the magnetogram mode and are based only on the circular polarization due to the Zeeman effect. ACENOWLEJ~EMENTS The original 060-8 spectrometer was built by the Laboratory for Atmospheric and Space Physics of the University of Colorado. Most of the modifications to the OSO-8 hardware and the integration of the instrument were performed by the General Electric Company, Valley Forge, Pennsylvania, with overall systems design and engineering by the Goddard Space Flight Center. The optical design and optical parts fabrication were also provided by the Goddard Space Flight Center. The polarimeter was designed and built by the Marshall Space Flight Center. The electronics, including the computer, were built by SCI Systems, Huntsville, Alabama. Software for the computer was provided by the Lockheed Palo Alto Research Laboratory, Palo Alto, California. We acknowledge the efforts of our operations team at Goddard - R. P.ndrieux, C. Condor, L. Fesq, C. J. Hughes, P. Pritchard, and S. Turnier - in obtaining the observations and thank E. J. Reichmann for his assistance at Marshall. REFERENCES 1.
B. E. Woodgate, E. A. Tandberg-Hanssen, E. C. Bruner, 1. H. Beckers, J. C. Brandt, W. Henze, C. L. Ryder, M. W. Kalet, P. J. Kenny, E. D. Knox, A. G. Michalitsianos, R. Rehse, R. A. Shine, and H. D. Tinsley, Solar Physics 65, 73 (1980).
2.
E. C. Bruner, Space Science Instrumentation
3.
E. Tandberg-Hanssen, R. G. Athay, J. M. Beckers, .1. C. Brandt, E. C. Bruner, R. D. ~1açznan,C. C. cheng, .1. B. Gurman, W. Henze, C. L. Ryder, A. G. Micha’litsianos, R. A. Shine, S. A. Schoo].man, and B. H. Woodgate, Astrophysical J (Lett.) (.198O~, in press.
3, 369 (1977).