Variations in solar activity have been proposed to play an important role in recent and past climate change. To study this link on longer timescales, it is essential to know how the Sun has varied over the past millennia. Direct observations of solar variability based on sunspot numbers are limited to the past 400 years, and beyond this we rely on records of cosmogenic radionuclides, such as ¹⁴C and ¹⁰Be in tree rings and ice cores. Their atmospheric production rates depend on the flux of incoming galactic cosmic rays, which is modulated by Earth’s and the Sun’s magnetic fields, the latter being linked to solar variability. Here we show that accounting for differences in hemispherical production rates, related to geomagnetic field asymmetries, helps resolve so far unexplained differences in Holocene solar activity reconstructions. We find no compelling evidence for long-term variations in solar activity and show that variations in cosmogenic radionuclide production rates on millennial timescales and longer, including the 2,400-year Hallstatt cycle, are explained by variations in the geomagnetic field. Our results also suggest an on-average stronger dipole moment during the Holocene, associated with higher field intensities in the Southern Hemisphere.

人们认为太阳活动的变化在最近和过去的气候变化中发挥着重要作用。为了在更长的时间尺度上研究这种联系，有必要了解太阳在过去几千年中如何变化。基于太阳黑子数量对太阳变率的直接观测仅限于过去 400 年，除此之外，我们还依赖于宇宙成因放射性核素的记录，例如树木年轮和冰芯中的¹⁴ C 和¹⁰ Be。它们的大气产生速率取决于入射银河宇宙射线的通量，该通量受到地球和太阳磁场的调制，后者与太阳的变化有关。在这里，我们表明，考虑与地磁场不对称性相关的半球生产率差异，有助于解决全新世太阳活动重建中迄今为止无法解释的差异。我们没有发现太阳活动长期变化的令人信服的证据，并表明宇宙放射性核素产生速率在千年或更长时间尺度（包括 2,400 年哈尔施塔特周期）的变化可以通过地磁场的变化来解释。我们的结果还表明，全新世期间偶极矩平均较强，与南半球较高的场强相关。