A.R. Jones (USC), J.M. Pap (UCLA), J.R. Kuhn (U. Hawaii), J. Gwo (Stanford),
Very high accuracy astrophysical measurements of angular scale or size (``astrometric'' observations) are generally limited by low order optical aberrations, focus changes, and the dimensional stability of the telescope and detector. Recent satellite measurements from SoHO/MDI have proven the astrometric capabilities of an imaging array system from above the atmosphere. The possibility of combining this capability with the ultra-stable monolithic telescope technology developed by Gravity Probe-B (GP-B) opens the possibility for extremely interesting solar (and other) astrometric 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 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.
To understand the physical mechanisms of the magnetic cycle in the solar interior, we must learn how to measure the energy flow through the solar interior and the tiny changes it produces in the Sun's global properties like: radius, internal temperature distribution and surface luminosity. The total energy output of the Sun has been measured for two decades, revealing that total irradiance varies over the solar cycle, being higher during maximum activity conditions. Short-term changes on days to months are directly related to the evolution of active regions. On time scales of minutes to hours, the effect of granulation and supergranulation is seen in total irradiance, while in the 5-min range the p-modes cause rapid irradiance fluctuations.
Although considerable information exists on solar irradiance variations, the underlying physical mechanisms are not well-understood. It has been shown that empirical irradiance models, based solely on the surface manifestations of magnetic activity, cannot explain all the aspects of irradiance changes. Identification of the unexplained residual variability is a difficult problem, since global effects, such as photospheric temperature changes, large scale convective cells, large scale mixing flows and/or radius changes can all cause changes in total irradiance. An important key to understanding how surface magnetic features and solar interior mechanisms affect the solar irradiance and luminosity is contained in helioseismic data. Recent analyses show that Solar cycle variations in global p-modes cannot be explained as a consequence of surface faculae and sunspots (Fröhlich et al., 1997; Kuhn, 1998).
Simultaneous studies of total irradiance, p-mode oscillations and radius changes are essential to understand the underlying physical mechanisms of irradiance variations and predict the solar-induced climate changes. In order to measure the solar radius, one needs to establish the position of the solar limb with high accuracy. The solar limb is potentially a sharp spatial reference with which we can hope to detect the effect of solar oscillations (both p-mode and g-mode); the gravitational quadrupole moment (or the solar oblateness); and changes in the solar radius (e.g. as a diagnostic for interior changes due to the solar luminosity cycle). Ground-based measurements of the solar radius exist over the last 300 years (e.g. Ribes et al., 1991), however, the results are controversial. Historical radius data show that the Sun's radius may have been larger during the Maunder Minimum, which coincided with the extremely cold periods in Europe and Atlantic regions (e.g. Eddy, 1977). These results are confirmed by the French CERGA radius measurements, i.e., the solar radius has been found larger during solar minimum (e.g. Lacrare et al., 1996). In contrast, Ulrich and Bertello (1995) showed a positive correlation between the apparent radius changes and the solar activity cycle. While there are also hints of periodic solar radius variations over time scales of 1,000 days to 80 years, the measurements are generally neither consistent nor conclusive. These controversial results underscore the necessity of further efforts to measure the solar radius, using more sophisticated techniques which are also free from the atmospheric seeing effects.
An important issue in the field of climate change is the degree to which solar variability may affect climate. Although the Sun supplies most of the energy for the Earth's atmospheric and climate systems, the measured 0.1% level of long-term total irradiance variations is considered too small to cause changes in the Earth's climate above its intrinsic noise. However, the correlation between observed historical solar changes and that of climate implies that there is an unaccounted solar forcing. In addition, theoretical considerations and observations of the variability of solar-type stars have shown that we cannot rule out the possibility of long-term irradiance variations in the 0.4-0.7% range.
Recently measurements suggest that changes in the cosmic ray flux may be crucial for the cloud formation, and thus they may also play an important role in climate (Swensmark and Friis-Christensen, 1997). The form of the solar impact on this process is complex due to the fact that the solar and galactic cosmic rays are modulated differently by solar activity. While the correlation is positive between the changes of the solar-originated cosmic ray flux and solar activity, there is an anticorrelation between the changes in the more energetic galactic cosmic ray flux and solar activity.
APT provides an important new approach since we have reached an impasse, namely exhaustive efforts using only correlative modeling of irradiance proxies have not and will not generate answers to questions like ``Could the solar irradiance vary more than 0.1% over decades or longer (centuries to interglacial) time scales?" Although it is unquestionable that a better understanding of, for example, the cosmic ray - terrestrial cloud physics connection is important for climate, the major issue is that we must understand the mechanisms of solar variability. Even the correlative studies assure us that the solar variability is (at least) on the verge of what is required to produce significant terrestrial climate changes, even without amplification by cosmic ray-cloud interactions.
It must be underscored that this is not purely a solar physics problem, it is a Sun-Earth Connection problem. Global solar changes on decadal to interglacial time scales with small amplitudes and their effect on climate will only be understood if we combine our knowledge of the physics of the terrestrial climate system (as a detector) with astrophysical models which might account for such small, but important variations. Since solar variability together with the accumulation of anthropogenic pollutants determine the human milieu of the future, predictability is an extremely desirable feature of climate studies, which is impossible to accomplish with any monitoring scheme alone, regardless of its precision.
The ``Astrometric and Photometric Telescope (APT)'' will measure the relative solar limb position with an accuracy that exceeds (under some circumstances) 0.1 microarcseconds. Its sensitivity should surpass previous solar astrometric measurements by almost two orders of magnitude. Solar variations at this level are caused by physical changes in both the photosphere and in the interior, these data can provide a unique view of solar cycle changes that occur in the deep within the Sun. In addition, measurements of the radius and shape oscillations are needed to understand how the Sun's convective envelope responds to emergent energy fluctuations. The determination of this outer boundary condition is essential to understanding the yearly and historical variations of total irradiance.
Kuhn et al. (1997; 1998) reported results of solar limb measurements using the Michelson Doppler Imager (MDI) experiment onboard SoHO. The MDI experiment was principally designed as a helioseismic experiment to provide full disk Doppler images of the Sun with 1-min cadence (Scherrer et al., 1995) but soon after launch, analyses carried out with the MDI data revealed that exceedingly small solar shape oscillations (at the 10 µarcsec level) were directly measurable. Accurate as the MDI instrument is, the experiment was not designed for photometric / astrometric solar observations and most probably will not uncover solar radius or gravity mode variations. A carefully designed instrument could readily achieve several orders of magnitude improvement in the detection threshold of coherent oscillations. Such accurate radius and shape observations could enable a new set of helioseismic and photometric diagnostics of the radiative and convective interior. Concurrent measurements of the solar shape, shape oscillations, radius and total solar irradiance may reveal the mechanisms causing the changes in the solar energy flux which, until understood, will keep us ignorant of the possibility for larger solar-cycle-related irradiance variations than have been observed over the last two decades.
The principal design goals of the APT 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.
Critical design requirements for the APT are:
Figure 1: Schematic cutaway of APT concept
The figure shows a schematic cutaway drawing of the telescope concept. A small Ritchey-Chrétien telescope is constructed of monolithic bonded ULE or fused quartz. A similar optical structure provides the astrometric stellar pointing reference for the gyroscope on the GP-B experiment. An off axis aperture with a fixed filter at 1.6 µm illuminates a custom InGaAs detector through a dichroic beamsplitter behind the primary to provide error signals for an active guiding system using piezo-electric actuators to tip-tilt the secondary mirror. The telescope-detector assembly can be rotated through ±180° for calibration of the optical aberrations in flight.
This design yields a full disk image that is slightly less than 2 cm in diameter. With proper focus the solar image will not shift as the 4 off-center pupil masks are individually selected. Changes in the system focal length by a fractional amount, cause the image radius to change by the same factor. Any such focus or detector shift is distinguishable from a scale change in the solar images (e.g. a real solar radius change) because the solar image produced from opposite pupil masks will be shifted by a few millipixels - an amount easily measured in a single solar image. Thus, in practice, images are obtained from at least 2 pupil mask positions. Focus and low order optical aberration changes can be distinguished from true solar variations by measuring image shifts from 4 pupil masks. These aberrations and scale variations are minimized from the outset by using a monolithic, homogeneous, low coefficient of thermal expansion telescope. Redundant filters are needed to monitor passband drifts and possible geometric changes in these components. The filter wheel can position individual filters over each pupil mask.
All operations of the instrument will be predefined, and there will be no requirement for ground-station to spacecraft communication for instrument operation after the main instrument cover has been opened.
We will refine the algorithms necessary for the data processing and data analysis. To study the solar shape and shape oscillations, the algorithm to study MDI data uses many (approximately 20,000) pixels near the solar image to derive accurate limb position and brightness information. In this case, the ultimate astrometric limitation comes from flat-fielding uncertainty and optical distortion drift. Least-squares techniques are used in the basic algorithms for generating geometrically registered and photometrically calibrated data.
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The Hydroxide catalyzed bonding process was developed at Stanford University for the GP-B experiment and has been awarded the trade name OptoBond. The technique has the following primary advantages:
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A version of this poster is available on the WWW at:
http://www-rcf.usc.edu/~arjones/APT/sec-poster.html