NOAA/SEC

About SOLAR2000 and E10.7

SET/SpaceWx

Introduction

• SOLAR2000 is an empirical solar irradiance specification tool for characterizing solar irradiance variability across the solar spectrum.

• The overarching scientific goal behind SOLAR2000 is to understand how the Sun varies spectrally and through time from X-ray through infrared wavelengths. This will contribute to answering key scientific questions and will aid national programmatic goals related to solar irradiance specifications.

• SOLAR2000 captures the essence of historically measured solar irradiances and this expands our knowledge about the quiet and variable Sun, including its historical envelope of variability.

• A new image- and full-disk proxy model, SOLAR2000, is continually being improved to specify the solar irradiance over the spectral range of 1-1,000,000 nm for historical modeling and forecasting throughout the solar system.

• SOLAR2000 is designed to be a fundamental model of the energy input into planetary atmosphere models, a comparative model with numerical first-principles solar models, and a tool to model or predict the solar radiation component of the space environment. It is compliant with the developing International Standards Organization (ISO) solar irradiance standard.

• The implementation of the SOLAR2000 model includes several new EUV proxies such as E10.7, which has the same units as the commonly used F10.7. E10.7 can be used in existing models where F10.7 is traditionally used but it offers significant improvement as an index of the energy input to the thermosphere and ionosphere. Rsn, a derived Sunspot number, Qeuv, the thermospheric heating rate, and Tinf, the exospheric temperature, are additional proxies provided by SOLAR2000.

• SOLAR2000 also provides an operational forecasting and global specification capability for solar irradiances.

E10.7 can be used nearly anywhere the traditional F10.7 (2800 Mhz) index is used to improve space-physics models. This is especially important to operational models that predict satellite orbits, as shown below. Mission planning, debris avoidance, and many other orbital applications can show dramatic improvement using E10.7.

Orbital Predictions

In a series of E10.7 validations that was performed by Tobiska (2000,2001), and an improvement in thermospheric density modeling for satellite operators was demonstrated using the daily E10.7 compared to F10.7. In those studies, the daily altitude decay for the Solar Mesosphere Explorer (SME) satellite was modeled using both E10.7 and F10.7 daily proxies and the results were compared with the actual mean equatorial altitude of SME during the decline of solar cycle 21. Those studies indicated that the F10.7 overestimated the daily EUV energy input into the atmosphere by up to 60% or underestimated it by as much as 50% during active solar conditions. Conversely, E10.7 was able to capture nearly all the solar variability that affected atmospheric densities over a 16-month period of time. The figure below shows this improvement in SME's orbit specification using E10.7.

Comparison of Orbital Altitudes Derived from E10.7 and F10.7

The top panel (a) shows the comparison between E10.7 and F10.7 for the period of April 1, 1982 through August 9, 1983. F10.7 varies much more than E10.7 and produces an over-estimate of the EUV heating of the atmosphere. The bottom panel (b) demonstrates that highly-variable F10.7 causes the Jacchia (J71) model and the orbit propagator to overestimate the drag on SME. This results in unrecoverable orbit altitude error compared to the SME ephemeris data. On the other hand, E10.7 used in the J71 atmosphere and orbit propagator captures nearly all the solar variability.

Other Effects

As society becomes more dependent upon technology, we find that our systems have an increased vulnerability. Complex systems are susceptible to solar variability in unexpected ways. From data provided by new ground and satellite sensors, our understanding of solar-terrestrial physics grows. This has enabled the transition of space physics models into operational models and this is one primary method by which the negative effects of solar variability are being mitigated. SOLAR2000 is but one of these new operational models and it provides a substantial improvement over a wide range of previous models and data because it spans previously unmeasured time and spectral domains as well as providing operational forecasts of irradiances and proxies. Solar-terrestrial effects that are now being addressed by these operational proxies include:

• Satellite drag caused by atmospheric density changes.
• HF radio communications, in conjunction with geomagnetic measurements and other models, as applied to radio communications over polar regions.
• GPS signal availability where the largest error is ionospheric variability; SOLAR2000, coupled with other data and models, can significantly improve GPS uncertainty estimates as well providing a system for reducing location errors.

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