The lost sunspot cycle: New support from ¹⁰Be measurements

¹ Department of GeoscienceAarhus University, Høegh-Guldbergs Gade 2, 8000 Aarhus C, Denmark
e-mail: karoff@phys.au.dk
² Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
³ AMS, 14C Dating Centre, Department of Physics, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark

Received: 5 September 2014
Accepted: 1 December 2014

Abstract

It has been suggested that the shortage in the number of spots on the surface of the Sun between 1790 and 1830, known as the Dalton minimum, contained an extra cycle that was not identified in the original sunspot record by Wolf. Though this cycle was shorter and weaker than the average solar cycle, it shifted the magnetic parity of the solar magnetic field of the earlier cycles. This extra cycle is sometimes referred to as the “lost solar cycle” or “cycle 4b”. Here we reanalyse ¹⁰Be measurements with annual resolution from the North Greenland Ice Core Project in order to investigate if the hypothesis regarding a lost sunspot cycle is supported by these measurements. Specifically, we make use of the fact that the Galactic cosmic rays, responsible for forming ¹⁰Be in the Earth’s atmosphere, are affected differently by the open solar magnetic field during even and odd solar cycles. This enables us to evaluate if the numbering of cycles earlier than cycle 5 is correct. For the evaluation, we use Bayesian analysis, which reveals that the lost sunspot cycle hypothesis is likely to be correct. We also discuss whether this cycle 4b is a real cycle or a phase catastrophe, and what implications this has for our understanding of stellar activity cycles in general.