Scientific Rationale For Assigning Priorities
A program of investigation of solar energy inputs into all parts of the Earth system is very much in concert with the goal and objectives of the U.S. Global Change Program. The USGCRP is motivated by the realization that global change can have tremendous impact on conditions essential to life on earth. This realization provides the basis for prioritization among the various components that can be expected to comprise a USGCRP scientific element on Solar Influences on Global Change. Of highest priority are those activities that will be most important for national and international policymaking.
One activity ranks above all others for determining solar influences on global change:
1. Monitor the total and spectral solar irradiance from an uninterrupted,overlapping series of spacecraft radiometers employing in-flightsensitivity tracking.
There is an urgent need to rapidly implement the necessary long term commitment for this monitoring because of the danger that the present monitoring sequence will be interrupted and the long term record invalidated as a result of lack of instrumental cross-calibration. This primary recommendation is particularly challenging and probably will not be achieved because of the dearth of ready access to space.
A series of small spacecraft dedicated to solar monitoring could provide the necessary data. Overlapping observations are required to cross-calibrate measurements by different instruments whose inaccuracies typically exceed the true solar variability. Simultaneous observations from different instruments provide important validation that real variability, rather than instrumental degradation, is being measured and provide the redundancy needed to preserve the long term data base in the case of instrument failure. Improved radiometric long term precision and calibration accuracies would contribute to a more reliable solar forcing record.
In lieu of a spacecraft series dedicated to solar monitoring, it may be possible to use the NOAA or DMSP operational satellites, for which overlapping is a feature of their design.
To augment the prime monitoring task, a suite of efforts from diverse geophysical research fields is needed to achieve the USGCRP objectives of monitoring, understanding, and predicting solar influences on global change. Pursuit of recommendations 2 to 6 is essential to the crossdisciplinary effort needed to reduce uncertainties in knowledge of solar forcing of global change in order to provide a sound scientific basis for policy-making on global change issues. The actions of recommendations 7 to 12 are essential to ensure that complete understanding is achieved of all potential coupling mechanisms.
2. Conduct exploratory modeling and observational studies to understand climate sensitivity to solar forcing.
Implied connections between the Sun and the paleoclimate record (Milankovitch orbital-induced variations and the Little Ice Age) should be fully investigated for the insights they might provide about the sensitivity of the climate system to solar forcing compared with increased greenhouse gases. New knowledge should be incorporated into existing GCMs utilized for climate prediction.
3. Understand and characterize, through analysis of solar images andsurrogates, the sources of solar spectral (and hence total) irradiancevariability.
The overall goal of this activity is to improve the ability of solar variability models to calculate solar radiative output variations and to provide reliable proxies to bolster the spaceborne monitoring effort. Toward this end, continue, without interruption, to monitor from ground based observatories the relevant proxy data, in particular certain relative spectral irradiances (such as the He I and Ca II indices, and the 10.7 cm flux) and solar images that display magnetic active regions (using, for example, full disk magnetograms, and He I, Ca II, and white light spectroheliograms). Use the improved solar variability models to extend the variability record into the past and to predict limits on future variability. Also important in this regard is connecting the variability sources to the physical solar processes that modulate the 14C and 10Be records.
4. Monitor, without interruption, the cycles exhibited by Sun-likestars, and understand the implications of these observations for long termsolar variability.
Tying the calculations of solar radiative output variations derived from solar observations (Recommendation 3) to the broader stellar context will help in this regard.
5. Monitor globally, over many solar cycles the middle atmosphere'sstructure, dynamics, and composition, especially ozone and temperature.
Long term records of ozone, temperature, and nitrogen oxides are especially important as they may allow the separation of solar from
anthropogenic forcing in the troposphere. Solar effects on this region will only be determined from the results of such monitoring.
6. Understand the radiative, chemical, and dynamical pathways thatcouple the middle atmosphere to the biosphere, as well as the middleatmosphere processes that affect these pathways.
Both modeling and observational studies are needed.
7. Monitor continuously, with improved accuracy and long termprecision, the ultraviolet radiation reaching the Earth's surface.
This effort is critical, not only for determining the dosage of UV radiation at Earth, but also because of the dependence of the UV dosage on ozone concentrations, which are affected by both anthropogenic and solar forcings.
8. Understand convection, turbulence, oscillations, and magneticfield evolution in the solar atmosphere so as to develop techniques forassessing solar activity levels in the past and to predict them in thefuture.
A reliable theory of the solar activity cycle, of longer term variability, and of stellar dynamos in general will require physical descriptions of the processes that successfully reproduce solar phenomena observed over a number of solar cycles. Reliable monitoring of solar diameter could help to understand solar variability processes. The goal is to understand why the Sun varies at all.
9. Monitor continuously the energetic particle inputs to theEarth's atmosphere.
Space based measurements should emphasize the higher energies(> 100 MeV) and relativistic electrons. Understand, through in situ measurements, the relationship of space based measurements to the energy spectrum and fluxes of both solar and galactic energetic particles reaching different altitudes in the Earth's atmosphere.
10. Monitor the solar extreme ultraviolet spectral irradiance (atwavelengths less than 120 nm) for sufficiently long periods to fully assessthe long term variations.
These measurements could be accommodated on the dedicated solar monitoring spacecraft identified in Recommendation 1.
11. Monitor globally over long periods the basic structure of thelower thermosphere and upper mesosphere so as to properly define the presentstructure and its response to solar forcing.
12. Conduct observational and modeling studies to understand thechemical, dynamical, radiative and electrical coupling of the upperatmosphere to the middle and lower atmospheres.
Analysis of solar soft X-ray forcing of nitric oxide levels, with possible inferences for nitrate deposits in ice cores, is an example of such a study. Ultimately, a global model of the Earth system is needed.