Secular Change in the Sun-Earth System

Judith Lean [], Naval Research Laboratory, Washington, DC.

            Changing fluxes of solar photons, particles and plasma define a dynamic external environment to which the Earth continually responds and adjusts. Most relevant for climate change is the least variable part of the Sun’s photon energy output – the visible and near-IR radiation that heats the Earth’s surface directly. More variable UV photons, may influence climate indirectly by altering the stratosphere. The growing empirical evidence for solar-induced atmospheric variability on decadal time scales indicates complex geographical and altitudinal patterns which model simulations suggest are driven by changing solar irradiance. On centennial-millennial time scales, a multitude of correlations exist between proxies of climate change and solar activity. Cosmogenic isotopes in tree-rings and ice-cores provide information about solar-driven changes in the plasma environment of the heliosphere, where the complexity of embedded magnetic fields affects the ease with which galactic cosmic rays penetrate the Earth’s lower atmosphere. Since solar modulation of galactic cosmic ray energy is one millionth that of total solar irradiance, the paleo connections are, like the decadal associations, assumed to reflect secular changes in photon fluxes. But the physical associations of irradiancc and cosmogenic isotopes are not well specified. Surface magnetic fields, driven by the sub-surface solar dynamo, are the source of both the closed magnetic flux in active regions that alter irradiance and of the open flux regions that extends into the heliosphere, altering galactic cosmic rays. This talk will illustrate sun-climate correlations, and explore the association between secular change in the solar sources of irradiance versus cosmic ray variations, using recent simulations of secular change in total and open magnetic flux made with NRL’s flux transport model.