Scientific Objectives for Flight 1 (SBI-1)
Two 11-year cycles of space-borne radiometry have demonstrated that the total solar irradiance (and luminosity) increases by about 0.1% around activity maximum (Fröhlich and Lean, 1998). Analyses of this total irradiance record have shown that almost 90% of its variation is well correlated with the changing of the projected areas of dark sunspots and bright faculae and network (Fröhlich and Lean, 1998; Chapman et al., 1996). This strongly suggests that the measured luminosity variation can be attributed to the net effect on photospheric heat flow of the compensating contributions of these bright and dark magnetic structures (Foukal and Lean, 1988). However, it is still not clear whether the photometric effect of sunspots, faculae and network is actually equal or simply proportional to the measured radiometric fluctuations. Uncertain broad-band photometric contrasts of spots, and especially faculae and network, currently present the main obstacle to improved modeling of total irradiance fluctuations. The bolometric contribution of faculae is currently uncertain by as much as a factor of two. Until this uncertainty is removed it cannot be considered proven that the compensating effects of bright and dark photospheric magnetic structures account entirely for measured solar luminosity variation. Demonstration of this equality is critical in determining whether the thermal blocking model (Foukal el al., 1983) provides an adequate physical explanation of solar irradiance variation, or whether more complex processes such as magnetic storage or enthalpy advection (Chapman,1984; Schatten and Mayr, 1985) play a significant role. In addition, the possible existence of global changes that might dominate solar luminosity variation over climatologically important time scales is the most important unsolved problem in studies of solar luminosity variation (Shindell et al., Science 284, p. 305, 1999).
Here, we present a balloon-borne solar telescope equipped with an innovative bolometric detector (Foukal and Libonate 2001), capable of recording images with an angular resolution of about 5 arcsec in essentially total photospheric light. The Solar Bolometric Imager (SBI) provides the first opportunity to bolometrically image brightness variations at the solar photosphere. Its flat spectral response from the ultraviolet to the infrared (like that of ACRIM) directly provides the facular and network contribution to the total irradiance, and complements the non-imaging space-borne radiometer measurements.
The 3 main objectives of the balloon-borne SBI are:
- To accurately measure (better than 10% per pixel) the bolometric contribution to the total solar irradiance of sunspots, faculae and enhanced network. This will help determine whether these structures can account for the rotational and 11-yr variability of the total irradiance, or whether other mechanisms highly correlated with their area variation might contribute significantly. We note that the ±10% precision in photometry and the comparable accuracy in photometric contrast refer to the errors relative to the amplitude of the fluctuations in total irradiance caused by spots, faculae and network. Since these fluctuations are themselves of 0.1% amplitude relative to the total irradiance signal, our ±10% photometric goal represents ±0.01% precision relative to the total irradiance. This is similar to the precision achieved by the space borne radiometers.
- To search for other lower level inhomogeneities in photospheric heat flux uncorrelated with the photospheric magnetic structures themselves, and possibly associated with large-scale convective cells, meridional circulations, etc. Such inhomogeneities might prove more important over time scales longer than the 11-yr cycle.
- To provide important engineering data to validate the space flight-reliability of the novel gold-blackened thermal array detector and to verify the thermal performance of the SBI uncoated optics in a vacuum environment.
Scientific Objectives for Flights 2 & 3 (SBI-2 & SBI-3)
Bolometric Imaging at Solar Minimum
SummarySolar irradiance variations affect Earth's climate, but the magnitude of the sun's intrinsic variation is uncertain. Current observations cannot reject the possibility that intrinsic variations played a major role in the climate changes recorded over the past few millennia. Physical understanding, based on images of the sources of irradiance variation, will clarify the sun's role in global climate change. From space-borne bolometric radiometers, we know that during the 11-year sunspot cycle the total solar irradiance (TSI) varies in proportion to local magnetic fields. Why do we propose to study irradiance at the upcoming sunspot minimum, when the local fields will be weakest? Only then can observations detect other possible sources of TSI variation with the least confusion by the large amplitude signals from local magnetic fields. This is the best observational approach to physical understanding of the possible long-term TSI variations. For the second flight we propose to operate the Solar Bolometric Imager (SBI-2) above Antarctica, where near-space conditions can be attained for 10-20 days. SBI will provide bolometric (wavelength-integrated light) and color temperature images from which we may assess both the irradiance signals and their underlying physical causes. Images are necessary to characterize irradiance variations associated with subtle magnetic structures, acoustic oscillations, pole-equator temperature differences, and rotational-convective cells. The investigation builds on the successful flight of the SBI-1 on September 1, 2003. Scientific Goals and Objectives
Solar variability is the main external driver in climate change. Solar driving of climate may occur on timescales from decades to eons through a variety of mechanisms (reviewed by Lean 1997; Rind 2002). Heating of the troposphere by the total solar irradiance (TSI) is the most direct effect of the sun on the Earth. Driving of some Ice Age cycles is well explained by the orbital changes described by the Milankovitch mechanism. A question at the research frontier is: Does luminosity variation intrinsic to the sun drive climate change on the multi-decadal timescales relevant to the global warming problem?
Space-borne full-disk bolometry (Hickey et al. 1980; Willson et al. 1981) permits quantitative investigation of this crucial question. The irradiance variation on solar rotational and 11-yr timescales has been correlated successfully with photospheric magnetic structures such as spots, faculae, and network (reviewed by Hudson 1988). The irradiance variation on longer, multi-decadal timescales is not well characterized. The observed TSI variation over the past 25 years is small and uncertain. Correlative studies overlook mechanisms, as described below, that could dominate long-term variation. Simple monitoring of TSI variations has yielded controversial results on long-term irradiance variations. Description of long-term irradiance impacts on past and future climate depends on improved empirical and physical insight into TSI variation (Foukal 2003).
For the observational part of our program, we will characterize the bolometric signals from the known and predicted spatial structures on the sun. The observations will provide a critical measurement of the sources of irradiance variation at solar minimum. They will also be a baseline for understanding variation over the sunspot cycle and for comparison with future sunspot minima.
We will critically challenge current models of solar intensity structure and irradiance variation. These include: the standard model with its emphasis on surface magnetic field contrasts; numerical models of convective and large-scale structure; and analytic models of disturbance of heat flow by magnetic fields. Validation of the models would provide confidence in our understanding of the critical physical processes of irradiance variation. Discrepancies between the observations and models would indicate areas requiring further physical understanding or the identification of new physical processes.
In the following sections we detail those goals and objectives.
For the past two decades, radiometric observations have been available from many, overlapping TSI instruments. Today the suite includes SOHO/VIRGO, ACRIMSAT/ACRIM3, and SORCE/TIM. We have learned that much of the TSI variation is correlated with surface magnetic fields (e.g., Chapman 1986; Walton et al. 2003). In the standard empirical model, all irradiance variation is caused by changes in the projected areas and contrasts of photospheric magnetic structures - sunspots, faculae, and network. Other known contributors to TSI variation such as acoustic oscillations and convection simply produce irradiance noise of constant low level. Global flows may have thermal structures with this property also. The sun's surprising brightening around activity maximum is caused by the dominance of bright magnetic faculae over spot darkening, in determining the 11-yr irradiance cycle. The disappearance of solar magnetic activity at sunspot minimum causes a return to a constant baseline value of TSI. No secular variation of TSI from other sources is expected, according to the standard model.
The standard model can be tested with time series of TSI observations and measurements of the areas and contrasts of the magnetic structures. On the rotational time scale, testing of the model is limited by current uncertainties in the broadband photometric contribution (area and contrast) of faculae. One goal of this proposal is to improve this accuracy for comparison with radiometry. The comparison can teach us whether photospheric magnetic structures account for essentially all irradiance variation on the rotational time scale, or whether, even on this short time scale, there is evidence for other influences from e.g. "convective stirring" (Parker, 1994). This distinction would have fundamental implications for the "standard" model.
The standard model is incomplete, as it excludes nonmagnetic sources of irradiance variation. These other sources are best studied at activity minimum, when noise from magnetic structures is least. Nonmagnetic sources include known photospheric structures such as convective cells and acoustic oscillations and inferred structures such as pole-equator temperature gradients and global variations of effective temperature.
We expect that all measurable sources of irradiance variation will have distinct spatial patterns over the solar disk, just as surface magnetic fields and convection cells have. Variation of TSI will occur from an unbalanced summation of the local contrasts. For example, the correlation of TSI with the disk-integrated magnetic flux is nil, while the correlation with the sum of the local magnetic signals is high. Therefore we propose to use bolometric images to understand the localized sources of irradiance variation.
The above figure, generated from a SOHO/MDI magnetic and intensity image pair, illustrates the variety of structures to be studied. From left to right the structures range from the observed limb-darkening with an amplitude of ±1000K centered at 5250K; through surface magnetic fields in form of spots and faculae signals, scaled -50±150K; convection, and acoustic oscillations signals, scaled ±30K; predicted global convective and flow structures like giant cells, pole-equator gradient, and torsional waves, with predicted amplitudes of ±3K (far right).
The extraordinary precision of TSI observations permits the detection of very weak signals from a variety of known thermal structures. Power spectra quickly showed 3 mHz (5-minute) acoustic oscillations at a few parts-per-million (ppm) per mode (Woodard and Hudson 1983). At lower frequencies a continuous spectrum of irradiance fluctuations is seen, identified with convective noise and active region signals (Pelletier 1996; Vazquez Ramio 2002). Jimenez et al. (1999) made the correspondence between the TSI signal and the spatially resolved acoustic patterns. No equivalent correspondence between TSI and convective patterns exists, leaving an obvious gap in characterizing irradiance variation, which we propose to fill.
Physical modeling of energy and momentum flow inside the sun predicts several large-scale thermal structures. The highest amplitude is a pole-equator temperature difference (DeRosa 2002). The extant observations are limited (Altrock and Canfield 1972) and do not address time variations. Helioseismic observations are beginning to measure the form (Braun and Fan 1998) and variation (Chou and Dai 2001) of the meridional flow that is coupled to a latitudinal temperature variation. We will search for this pattern to test physical understanding of the solar interior and our techniques for probing that volume.
The radial temperature gradient of the photosphere produces the largest intensity signal − the limb-darkening. Limb-darkening is normally treated as a constant, but the dynamic photosphere alters its profile locally. Petro, Foukal, and Kurucz (1985) modeled the range of variations expected, given changes in the source function, effective temperature, and convection in the photosphere, and showed that irradiance variations might be detectable with precise photometry. Separation of the contributions to limb-darkening variation requires multispectral photometry (Foukal and Duvall 1985). Typical fractional limits on the stability of limb-darkening are 10-2 (Livingston and Wallace 2003); only Petro et al. (1984) were able to approach 10-3, which corresponds to realistic TSI signals. This figure shows the first bolometric limb-darkening observation a full-disk mosaic made during the SBI-1 flight. The general agreement with model values indicates that data calibration is accurate. When the full flight data are reduced, the precision of the bolometric curve will exceed that of the models. On the proposed next flight we will achieve more precise photometry, to understand the stability of limb-darkening and its correlation to TSI variation. Multi-spectral images will diagnose the photospheric temperature structure.
The high thermal conductivity of the solar interior redistributes much of the heat flow disturbed by surface magnetic fields, but the redistribution is not perfect. Some of the heat blocked by sunspots should leak out in surrounding bright rings with amplitudes of 1 to 5K (Spruit 1977). Ground-based observations have limits (Fowler, Foukal, Duvall 1983; Rast et al. 2001) near the upper end of the predicted range. Large-scale thermal structures are also predicted to form near large concentrations of surface magnetic fields (Spruit 2003). The corresponding pressure gradients accelerate mass flows that couple with the Coriolis force to generate the flow pattern seen in the torsional wave (Vorontsov et al. 2002). The predicted amplitude, ~0.3K, is too low to be detected from ground-based photometry in the presence of seeing and transparency fluctuations. The model also fails to explain the presence of the torsional wave during sunspot minimum, when surface magnetism is weak. We propose to search for thermal structures, to test our understanding of heat flow in the solar convection zone.
The success of the standard model in correlating TSI variation with surface magnetic fields derives from the large feature contrasts. Large areas of magnetic field in sunspots block convective flow, permitting the surface to cool and appear dark. Smaller areas of magnetic field in faculae reach horizontal pressure balance at lower gas density and opacity than the nonmagnetic surroundings, enabling deeper, hotter layers to radiate excess flux and appear bright. Despite its success, correlative modeling uses constant, typical values for many parameters such as spot temperatures, umbra/penumbra area ratio, center-to-limb contrast function, and so on. To improve precision in understanding irradiance variation, we propose to replace these values by direct measurement, in multispectral and bolometric images.
The standard model segments the solar disk into sunspots, faculae, and network, to assess their irradiance signals. Those structures are not mutually exclusive, but may overlap. Some small sunspots and penumbral regions appear dark in the continuum and bright in facular images. The standard model counts those areas twice with both positive and negative signals. Those areas are actually dominated by horizontal or rapidly diverging magnetic fields and have an unusual temperature gradient in the photosphere. Observations at any resolution will necessarily smooth over such structures, but our proposed observations will do so in a bolometrically correct manner. Our multispectral images will simultaneously identify magnetic structures.
Further, the parameters used in the standard model may vary through the sunspot cycle (Albregtssen, Joras, and Maltby 1984; Ermolli, Berilli, and Florio 2003) in response to variations of the magnetic field vector orientation, scale length (ephemeral regions vs active regions), flux density, and so on. Even with detailed observations of magnetic structure, our proposed bolometric images are needed to understand the impact on irradiance variation.
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