Validating and Cross-Calibrating Long-term Solar Cycle Data for Driving Solar Cycle Models

The Sun is the main driver of variability in the interplanetary environment and Earth's upper atmosphere. This influence is felt across a multiplicity of spatial and temporal scales ranging from seconds to decades. Long-term variability requires homogeneous observational surveys covering long periods of time, which are incompatible with modern funding cycles and are seriously undervalued by governmental agencies, especially in the United States. For this reason, it is often necessary to piece multiple heterogeneous instruments and surveys with different experimental design, characteristics, and systematics. Here we discuss an array of historical data sets (magnetic, optical and in the form of reduced time series) that give us direct insight on the long-term evolution of solar activity and the efforts that are being made to piece them together into homogenous composites that can be used to constrain and drive models of solar activity. We highlight the importance of ensuring that historical surveys are properly preserved and modernized for future generations, and discuss important aspects of documenting them so that future users can take better advantage of the insight they provide.