Evolution of the Sun's large-scale magnetic field since the Maunder minimum

The most striking feature of the Sun's magnetic field is its cyclic behavio ur. The number of sunspots, which are dark regions of strong magnetic field on the Sun's surface, varies with a period of about 11 years. Superposed o n this cycle are secular changes that occur on timescales of centuries and events like the Maunder minimum in the second half of the seventeenth centu ry, when there were very few sunspots(1,2). A part of the Sun's magnetic fi eld reaches out from the surface into interplanetary space, and it was rece ntly discovered(3) that the average strength of this interplanetary field h as doubled in the past 100 years. There has hitherto been no clear explanat ion for this doubling. Here we present a model describing the long-term evo lution of the Sun's large-scale magnetic field, which reproduces the doubli ng of the interplanetary field. The model indicates that there is a direct connection between the length of the sunspot cycle and the secular variatio ns.