Heliohistory experts are meeting at the High Altitude Observatory (HAO) this month to explore ways to recover the history of solar variability from indirect data. The organizing committee sought scientific participation from a variety of disciplines, including geophysics, geochemistry, and dendrochronology. The 40 selected participants will be looking at the indirect evidence of the sun's past behavior, especially during the Quaternary period, or since the last ice age (approximately 20,000 years ago).
Indirect, or ‘proxy,’ evidence includes auroral and geomagnetic records and evidence from isotopes found in meteorites. Isotopic abundances and flare particle tracks in moon rocks have also been useful. Other indirect evidence is provided by observations of comet tails and of changes in the brightness of the moon at eclipse, of the planets, and of zodiacal light. The best secondary evidence is provided by the radiocarbon abundances derived from tree rings. Other secondary evidence may be found in ice cores taken from the earth's Polar Regions. A knowledge of terrestrial paleoclimate establishes bounds on solar change. Finally, the history of the sun can be deduced from observations of other stars of solar type.