Was the solar magnetic cycle fundamentally different in the past?

The Sun is an active magnetic star, varying on every observable time scale, from seconds to millennia. Reconstructing the Sun’s past activity is critical to understanding both the Sun’s influence on terrestrial climate and how space weather may behave in the future. Solar reconstructions also provide much-needed information about how the churning motions inside the Sun act as a dynamo to generate its magnetic field. This dynamo gives rise to the approximately 11-year solar cycle, as well as much longer patterns of variability, over hundreds and thousands of years. Understanding these processes at the Sun provides our best opportunity for deciphering magnetic cycles on other stars. Unfortunately, the very long timescales involved in solar variability mean that direct solar observations are too short; even sunspot records – which provide only qualitative information about the magnetic field -- extend back only a few hundred years. Crucially, they provide little insight about the solar cycle during the last “grand minima” of activity around 1650-1700: sunspots vanished for much of this interval, but computer simulations and auroral reports hint that the solar cycle continued throughout. Fortunately, carbon-14 (14C) in tree trunks contains information about the Sun’s magnetic field back thousands of years. This is because 14C only exists on Earth due to nuclear reactions between atmospheric molecules and “cosmic rays” – energetic particles from outside the solar system. The stronger the Sun’s magnetic field, the fewer cosmic rays reach Earth and the less 14C is produced. On this basis, variations in the solar magnetic field have been successfully reconstructed, but only at low time resolution (every 10-50 years) which is incapable of identifying individual solar cycles. Recently, new techniques have allowed 14C to be measured at annual resolution, potentially allowing the solar cycle to be reconstructed back more than a thousand years for the first time. However, the current reconstruction methods are not suitable for use with these new data. We are proposing a completely new approach to converting 14C measurements to the physical properties of the Sun’s magnetic field, using physics-based models rather than statistical relationships that are invalid for the new data. In this way, we will determine whether the Sun behaved differently in the past to the last few hundred years. We will also assess whether such behaviour would be visible on other stars and determine what this means for our own star over the coming decades.