A History of Solar Activity over Millennia

TL;DR

This review summarizes how indirect proxies like cosmogenic isotopes are used to reconstruct and understand the long-term history of solar activity over millennia, highlighting methods, findings, and implications for solar dynamo theory.

Contribution

It provides a comprehensive overview of proxy-based methods for millennial solar activity reconstruction, including verification, calibration, and implications for solar and stellar dynamo models.

Findings

Cosmogenic isotopes reliably reconstruct past solar activity.

Strong solar energetic-particle events are rare in the long-term record.

The occurrence of grand minima and maxima influences solar dynamo understanding.

Abstract

Presented here is a review of present knowledge of the long-term behavior of solar activity on a multi-millennial timescale, as reconstructed using the indirect proxy method. The concept of solar activity is discussed along with an overview of the special indices used to quantify different aspects of variable solar activity, with special emphasis upon sunspot number. Over long timescales, quantitative information about past solar activity can only be obtained using a method based upon indirect proxies, such as the cosmogenic isotopes \super{14}C and \super{10}Be in natural stratified archives (e.g., tree rings or ice cores). We give an historical overview of the development of the proxy-based method for past solar-activity reconstruction over millennia, as well as a description of the modern state. Special attention is paid to the verification and cross-calibration of reconstructions. It is argued that this method of cosmogenic isotopes makes a solid basis for studies of solar variability in the past on a long timescale (centuries to millennia) during the Holocene. A separate section is devoted to reconstructions of strong solar energetic-particle (SEP) events in the past, that suggest that the present-day average SEP flux is broadly consistent with estimates on longer timescales, and that the occurrence of extra-strong events is unlikely. Finally, the main features of the long-term evolution of solar magnetic activity, including the statistics of grand minima and maxima occurrence, are summarized and their possible implications, especially for solar/stellar dynamo theory, are discussed.