D. Judge, A. Jones, D. McMullin, J. Pap1, L. Floyd2
Space Science Center
University of Southern California
Los Angeles, CA 90089
14 June 1999
This document outlines some ideas for a Solar Physics Space Weather Palette for the International Space Station. The Space Station could provide a platform for a collection of instruments designed to aid our understanding of solar variability, and to help to predict space weather. Such a platform would be able to support both scientific experiments and allow technology and engineering tests to be performed on instruments destined for future flight opportunities. A palette would consist of several instruments from various groups.
Radiometers:
Roger Helizon of JPL has developed a new state of the art radiometer called the Differentially Balanced Solar Irradiance Monitor (DBSIM). It weights 4.5 Kg and consumes 5 watts of power. It is a volume of a cylinder 10 cm in diameter 25 cm long.
Very important experiments can be accomplished by flying on Space Station Palette. Issues of Total Solar Irradiance (TSI) instrument degradation and performance characteristics still need to be resolved between sensor types and surface coatings. Comparisons between the DBSIM and a version of the older ACRIM-type sensors which have been used for the past twenty years, will be necessary to maintain a consistent TSI database.
Spectrometers:
Work at USC is continuing on several UV/EUV spectrometers:
The Solar EUV Monitor (SEM) is a small transmission grating spectrometer that continuously measures the full disk absolute solar flux in the He
The Optics Free Spectrometers (OFS) are a new type of spectrometer under development at USC. In the OFS the energy spectrum of incoming EUV photons is transformed directly into an electron energy spectrum by taking advantage of the photoelectric effect in one of several rare gases at low pressure (1 to 10 mTorr). This instrument has no degradable optical elements, a problem common to conventional optical spectrometers, and it also eliminates the problem of multiple order separation. A prototype OFS has flown successfully on a sounding rocket flight. Development of much smaller OFSs is underway now at USC, as well as the addition of a Grazing Incidence Non Imaging Collector (GINIC), developed under the NASA Planetary Instrument Definition & Development program, to increase the instrument sensitivity.
NRL is developing UV / IR Spectrometer, covering 200 - 300 nm in the UV. This instrument would allow intercomparison with the SOLSTICE and TSIM instruments.
Solar Position Monitor:
A precise knowledge of the pointing of the palette instruments would be required for the scientific data reduction. Several schemes have been tested in the past that can provide pointing information of sufficient precision, and we would propose to use one of those instruments.
Wide Field of View UV/EUV/X Photodiode Photometers:
A characteristic signature of a CME might be seen in a simple ratio measurement with filtered photodiodes. These observations could be used as a warning of incoming solar particles, and allow suitable precautions to be taken before their arrival. Such diode photometers have been successfully flown on the Space Shuttle as part of the IEH-SEH package, and on sounding rocket flights.
Solar Radius/Shape Measurements:
Although the ultimate source of the solar energy is the nuclear reactions taking place near the center of the Sun, the immediate source of energy for the Earth is radiation from the solar surface. While the nuclear reactions are almost certainly steady on time scales shorter than millions of years, the mechanisms which carry the energy to the solar surface may not be. Indeed, observations of the solar radiative output integrated over the entire solar spectrum - hence total irradiance -, helioseismic and precise photometric measurements have all shown variability during the course of an 11-year solar cycle. If the central energy source remains constant while the rate of energy emission from the surface varies, there must be an intermediate reservoir, where the energy can be stored or released depending on the variable rate of energy transport. The gravitational field is one such reservoir, which if used, will result in a change of the solar radius. Thus, a careful determination of the solar radius can provide a constraint on models of total irradiance variations. For example, a radius change 0.06" would be sufficient to explain the long-term 0.1% variation in total irradiance.
Initial studies led by Andrew Jones (USC), Jeff Kuhn (U. Hawaii) and Judit Pap (UCLA) of an instrument to measure the solar diameter and shape has identifies the principal design goals. These are to achieve a dimensionally stable, photometrically precise optical telescope and detector which allows very accurate differential photometric calibration (``flat-fielding'') with good sensitivity to optical distortion measurement and control. In practice these requirements have led to the design of a monolithic telescope/detector assembly with few optical surfaces. This design harmonizes nicely with the natural advantages of a simple, light-weight experiment package. Currently the outline looks like a cylinder about 75cm long and 25 cm diameter. Projected mass is less than 20 Kg, and power consumption less than 50 W.
Critical design requirements are:
1 Division of Astronomy and Astrophysics, UCLA, Los Angeles CA 90095-1562
2 N.R.L
An electronic version of this paper is available on the WWW at:
http://www-rcf.usc.edu/~arjones/SpaceStation/PS-SW.html
arj 6/14/1999