G.E. Brueckner, et al., A Proposal To The National Aeronautics And Space Administration To Measure The Solar Ultraviolet Irradiance With High Accuracy in the Wavelength Region 120-400 nm by means of A Solar Ultraviolet Spectral Irradiance Monitor[SUSIM] on-board UARS Missions, December, 1978.
Contributed by Dianne Prinz.1.0 Experiment Summary
1.1 Scientific Objective
The main objective is to improve the existing accuracy of solar flux measurements in the 120 - 400 nm region of the spectrum and to establish the variations of this flux over a solar cycle. The measurements are of great importance for modeling the earth's atmosphere in and below the lower thermosphere.
It is proposed to measure the full-sun spectral irradiance in the 120 - 400 nm region with two spectral resolutions, 0.15 nm and 5 nm, with an absolute accuracy of ±6-10% (wavelength dependent). The accuracy of the measurements below 210 nm relative to measurements of the more stable solar continuum above 210 nm is ±1-5% (wavelength dependent). Such measurements, using the instrument described below, will have been initiated on two shuttle missions - SS-5 (scheduled for Sept. 1980) and Spacelabe 2 (scheduled for Jan. 1982).
1.2 Instrument Description
The instrument proposed to accomplish this goal is the solar ultraviolet spectral irradiance monitor (SUSIM). It consists of two identical double-dispersion scanning spectrometers, seven detectors and a deuterium calibration lamp. The spectrometers and detectors are sealed in a canister filled with 1.1 atm of argon gas. One spectrometer is used more-or-less continuously; the second is used infrequently to track the stability of the first. The deuterium lamp serves as a secondary standard for inflight calibration. Pre-flight and post-flight calibrations are carried out through a cooperative program between NRL and the National Bureau of Standards.
Magnesium floride diffuser windows in front of the two interchangeable entrance slits for each spectrometer provide leniency in the required pointing accuracy (±30 arc min from sun center) and stability (±6 arc min). The clear field of view required at each entrance slit is ±1.50.
The instrument weighs 55 kg and occupies a volume of 24 x 77 x 86 cm with the 24 x 77 cm side facing the sun. It has an average power consumption of 45 watts. An interactive microprocessor system controls the sequencing of all instrument functions. The data bit rate is 160 bits per second.
1.3 Mission Options
Three options are offered for consideration. They differ in the number of SUSIMs to be built and consequently in the total time spanned by the observations and the number of cross-calibrations that can be performed. The scientific advantages and disadvantages of each is discussed in detail in Section 3.5. The options are:
(1) to refurbish the original SUSIM that will be aboard the shuttle missions SS-5 and Spacelab 2 for flight on UARS #1, (2) to build one new SUSIM for UARS #1 and refurbish the original SUSIM for UARS #2, and (3) to build two new SUSIMS, one for each UARS mission, reserving the original SUSIM for periodic Spacelab flight for cross-calibration. 1.4 Rationale for UARS Flights of SUSIM
Although SUSIM will fly on the two shuttle missions SS-5 and Spacelab 2, such missions, having a limited 7-9 day duration and spaced 6 months or more apart, represent a piece-wise approach toward achieving the main objective of the experiment. SUSIM was originally designed for a long duration mission (SMM). The UARS missions thus represent an opportunity for realizing the intent of the instrument, especially if they are continued passed UARS #2. The advantages of long-term observations over periodic shuttle flights are worth stating.
(1) Because of the superposition of variations having vastly different time scales, long duration (well-calibrated) measurements are essential if differentiation among the different types of variations are to be made with confidence. With long-term observations we will be able to distinguish among variations due to the birth and death of an active region (with a time scale of a few days), the 27 day solar rotation, and longer-term phenomena.
(2) Flare induced changes can be observed for a wide variety of different flare importances.
(3) Despite the anticipated deterioration of the instrument sensitivity over long time periods, high accuracy can be maintained because many inflight calibrations can be made. Furthermore, with option (3) which has three like instruments making simultaneous observations (from UARS #1, UARS #2, and an independent shuttle mission), intercomparison of the measurements enables the deterioration of the UARS instruments to be determined within the precision of the freshly calibrated shuttle instrument, which is 0.3%. The remaining error is the knowledge of the absolute radiation scales.
(4) All shuttle missions are configured differently. Although we do not anticipate any significant influence of the environmental differentials on the instrument, they can not be excluded.
There are also several advantages to be had by flying the same type of instrument that will be on periodic shuttle missions on the UARS missions.
(1) Continuity of the body of measurements is most important for detecting the long-term variability which is the #1 goal. (2) Consistency of the calibration methods is maintained. (3) The SUSIM calibration facility presently under developement at NBS is designed particularly to optimize the SUSIM calibration. Any different instrument would have to use a different calibration facility which would introduce errors into the intercomparison. (4) The design of the shuttle SUSIM cannot be improved at the present time. We believe it represents the best possible state of the art. (5) All of the principles that went into the original design of SUSIM for a long duration flight have been maintained - in particular the long-term stability tracking. The same instrument is thus suited to both types of missions. 2. Scientific Objectives and Background
2.1 Scientific Objectives
The main objective is to establish full-disk solar fluxes and their changes over a solar activity cycle in the wavelength region 120-400 nm. This task can be broken down into the following subtasks.
A. Improve the absolute accuracy of the solar continuum irradiance measurements in the 140-400 nm region.
Goal: ± 6 - 10%, wavelength dependent.B. Improve the absolute accuracy of solar emission line irradiance in the 120-400 nm region.
Goal: ± 6 - 10%, wavelength dependent.C. Measure with high accuracy the intensities of the continuum below 208 nm relative to the intensities of the continuum above 208 nm.
Goal: ± 1%D. Measure with high accuracy the intensities of solar emission lines relative to the stable solar continuum above 208 nm.
Goal: + 1 - 5 %, wavelength dependent.E. Measure the (wavelength dependent) degree of correlation of the solar fluxes in the 120-400 nm region with the following ground observables: the Zurich sunspot number, the solar 10.7 cm radio and t he Ca II plage index and the full-sun CAII H and K indicies. The long wavelength cutoff has been selected to include the Ca II lines in order to establish correlations between their variability and the variability at other wavelengths. It is also desirable to extend the measurements into the 300-400 nm region to compare these measurements with high accuracy ground-based measurements.
The short wavelength cutoff is imposed by the necessity of environmental control to retard degradation of the sensitivity of the instrument, as explained in Section 2.2.3. The method of control we have chosen requires a window. The shortest wavelength transmitted by any window material is 100 nm by LiF. But LiF is not stable with respect to exposure to uv radiation, x-rays, high-energy electrons, or moist air. We, therefore, have selected MgF2 which is a much more stable material. Although the short wavelength cutoff-for MgF2 is 120 nm, the 20 nm gain in wavelength coverage to be had by using LiF is-not worth the loss in stability.
2.2 Scientific Basis for the Measurements
2.2.1 Origin of the Variations and Anticipated Results
Solar emissions in the 120-400 nm region emanate from a wide altitude range in the solar atmosphere - the photosphere, the chromosphere, the transition zone and the corona. The variablity of the radiation depends on both the wavelength being studied and the time scale over which changes are sought. The following phenomena can cause variations in the solar output.
(1) Aperiodic flares, lasting several minutes to 1 hour. (2) The birth and death of an active region, with a time scale of a few days. (3) Solar rotation, with a 27 day period. (4) Changes in the number of active regions, with a period of about 11 years. (5) Possible long-term changes in the chromospheric and transition zone network or a varying degree of scattered, small activity not included in the classified active regions, with about an 11 year period. Table 2-1 lists the variations anticipated when some of these phenomena occur for full-disk fluxes in 4 emission lines and 2 continuum regions. Values in parentheses are estimates because reliable measurements of the variations are scarce. (See Section 2.2.3 and Sections 2.3.)
Table 2-1 Percent Variation Anticipated for Some Emissions Designation SiIII Ly C IV Cont. Cont. Ca II (nm) 120.6 121.6 155.0 175-208 210-400 396 Flares (100) 17 (100) (1) (0.1) (0.5) (Importance 3) Appearance of (5-10) 10-20 (5-10) (0.50) (5) 1 major active region Sunspot # change (60) 60-80 (60) 3 (0.1) (15) from 0 to 120 11-year cycle (100) (100) (100) (3) (0.1) (15)2.2.2 The Need for Improved Measurements and their Value
Although full-disk solar fluxes and variations thereof in the 120-400 nm region influence models of the solar atmosphere, current, increased-interest in the measurements stems from their importance in modeling the terrestrial atmosphere. An improvement in the existing accuracy of such measurements has been called for by every national and international group recently addressing the area of solar-terrestrial relations. For example COSPAR Decision No. 4/77 (IAU Information Bulletin No. 39, January, 1977) states "COSPAR, having reviewed the solar XUV/UV flux important for solar-terrestrial relations, and taking into account the strong temporal variation of this radiation, and the inadequate accuracy of the data collected so far, resolves that future experiments should measure the solar XUV/UV output with high accuracy-----". Similar recommendations without wavelength restrictions were reached by all four panels at the workshop, "Monitoring the Solar Constant and Solar Ultraviolet", held at Estes Park, CO, in August, 1977. (U.S. Govt. Printing Off. 1978-777-067/1263, Region 8). Furthermore, the final report of the working group convened by the Solar-Terrestrial Office of NASA to assess the UARS program (JPL Publication 78-54, July 15, 1978) lists as number one among the highest priority key scientific questions, "What is the solar spectrum and its temporal variation between the wavelengths of 120 and 400 nm?". This is precisely the question the solar ultraviolet spectral irradiance monitor (SUSIM) was designed to answer.
Solar radiation in the 120 - 400 nm region influences models of the terrestrial atmosphere principally in and/or below the lower thermosphere. This radiation penetrates to the lower thermosphere because of the small optical depths in this wavelength region for the intervening atmospheric species, except in narrow wavelength intervals around a few strong absorption lines for the atomic species 0 and H. Since the optical depth increases with increasing solar zenith angle, the relative importance of competing processes at a given altitude depends on the local time of day. Nevertheless, there are a number of areas in which the solar flux is currently considered to be the controlling factor. A few of these areas are given below.
(1) The dominant heat source in the thermosphere below about 170 km is absorption of the solar flux in the 130-175 nm region, the Schumann-Runge continuum of 02, which leads to the photodissociation of 02 (Chandra and Sinha (1973). (2) The major source of atomic oxygen in the 60 - 90 km region is predissociation of 02 by absorption of the solar flux in the 175-200 nm region, the Schumann-Runge bands of 02 (Banks and Kockarts (1973). (3) The principal loss mechanisms of H2 0 above about 70 km (for an overhead sun) are dissociation by the solar Lyman-alpha line of atomic hydrogen and dissociation by the solar flux in the 175-200 nm region. Above 100 km dissociation by the solar flux in the 121.7 - 175 nm region also contributes significantly to the loss (Anderson (1971). (4) The photodissociation of ozone, which has various thresholds beginning at 1184.3 nm and extending to 171.4 nm, produces excited states of 02 and 0. These species play fundamental roles in the chemical reactions determining the lifetimes of many other molecules in the lower atmosphere. For example, the 0 (ID) atoms react with N2 0 to produce two NO molecules (Anderson and Donahue (1975), with CH4 to produce CH3 + OH + heat in the stratosphere (Banks and Kockarts (1973), etc. (5) Photolysis by solar 174 - 220 nm radiation is the only known loss mechanism for the flurocarbons FC11 (CFC13) and FC12 (CF2Cl2) used as refrigerants and as propellants in spray-cans. (Robbins and Stolarski (1976). The Cl atoms resulting from the dissociation then participate in reactions that destroy ozone. (6) Above ~25 km the loss of methyl bromide (CH3Br) by photodissociation by the solar flux in the 174-262 nm region is more than an order of magnitude greater than the loss by any other process. For methyl chloride (CH3Cl) above ~25km, the loss by photodissociation in the same wavelength region is comparable to the losses by diffusion and by reaction with OH. CH3Br and CH3Cl are produced by marine algae; the halogen atoms produced when these molecules are dissociated catalytically destroy ozone (Robbins (1976). (7) Ionization of NO by the solar Lyman-alpha line of atomic hydrogen is an important source of NO+ in the D region of the ionosphere, where it is a major ion (Strobel (1972). Accurate measurements of the solar flux and its variations in the 120 - 400 nm region are thus very important for modeling the lower atmosphere, since the relative importance of competing processes and hence the subsequent chain of chemical reactions are determined by the magnitude of the solar flux.
2.2.3 Why the Variations are not Well Established
There are at least five reasons why full-disk variations in the 120 - 400 nm region have not been well established to date.
(1) No single instrument has been designed to observe the entire 120 - 400 nm region. (2) The many different experiments used to cover this wavelength have employed different calibration procedures. (3) For long-term variations in wavelengths not transmitted to the ground all of the instruments mounted in satellites have suffered severe degradation in sensitivity, which is believed to be due to changes in the chemical composition of the surfaces caused when far uv radiation and x-rays break down outgassing molecules. (4) Because of the above limitation on observing time for radiation not reaching the surface of the earth, much effort has been spent in seeking an observable that would accurately predict the solar flux in any given wavelength interval and could be monitored continuously from the ground. Two observables which have been tested for the role are the Zurich sunspot number (Z) and the solar 10.7 cm (radio) flux (F10.7) measured daily at Ottawa. Although both of these quantities are measures of solar activity, the linear correlation coefficient between either of them and any uv emission line flux thus far measured on a daily basis has not been convincingly large, e.g. Woodgate et al (1973) reported 0.66 ± 0.04 as the correlation coefficient between the H-Lyman-alpha emission and both Z and F10.7 . Indeed, as discussed by Timothy (1977), there are physical reasons why large correlation coefficients between the uv flux and either of these quantities should not be expected. Thus, there is as yet no substitute for direct measurements of the 120 - 300 nm flux. (5) Because of the divergence of interests between solar physicists and aeronomers, full-disk fluxes have not been measured as frequently as fluxes from specific solar features. Extrapolation from the latter to the former requires estimates of one or more of the following quantities: (a) for chromospheric and transition zone emissions, the fraction of the disk occupied by active regions, the intensity contributed to the whole by these, and the average quiet region intensity; (b) the amount of limb darkening or brightening; (c) in the case of ground-based spectral measurments, both (b) and the amount of scattering and/or absorption caused by the intervening air mass. 2.3 Present State of Knowledge
Table 2-2 list frequently compared observations of the solar output. Two published comparisons of this data are "Intercomparison/Compilation of Relevant Solar Flux Data Related to Aeronomy", the first report of Working Group IV of COSPAR prepared by Delaboundiniere et al (1976), and the book "The Solar Output and Its Variation", edited by 0. R. White (1977). The COSPAR comparison concentrates on "quiet sun" measurements, but even this limited sub-set of observations shows (wavelength dependent) differences that are often larger than the combined quoted absolute accuracies. Because none of the observations compared by the COSPAR group were simultaneous, nor were they made with instruments having similar characteristics, it is difficult to assess whether the differences represent real solar variations or are a product of reducing different types of observations to a common set of conditions.
In the Appendix we discuss both of these comparisons as well as our own assessment of the situation. Table 2-1 represents the conclusions reached therein for four emission lines and two continuum regions. As reflected in that table, the long term variation in all except perhaps H-Lyman-alpha remains an open question.
One of the most promising new types of ground-based observations are those reported recently by White and Livingston (1978). The observations are of the full-disk in the CaII H and K lines and are being made at the Kitt Peak National Laboratory. These lines are of chromospheric origin and as such should be good indicators of the emission in other chromospheric lines that cannot be observed from the ground such as the important H-Lyman-alpha line. A joint observing program between Kitt Peak and NRL (using the SUSIM aboard a UARS) would provide an additional cross calibration with regard to the relative variations in these lines and would enhance the confidence level of the degree of correlation among different chromospheric lines.
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