Speaker
Description
The development of our understanding of long-term change in the magnetic field of the inner heliosphere will be reviewed. The basic technique is to use modern understanding from space age observations and numerical models, with available proxy indicators, such as historic observations of sunspots, geomagnetic activity, aurorae, eclipses, polar faculae, and polar crown filaments. In addition, cosmogenic isotope abundances in various terrestrial reservoirs can be used to constrain variations and extend them back to times before observations were routinely recorded. The various techniques show a considerable convergence in annual means but evaluation of major space weather events remains difficult and depends on the metrics used to quantify them.