Phase of solar activity affects response of solar proxy 10Be

• Solar proxy Be-10 lags solar activity. We show that lag depends on phase of Sun.
• Fraction of stratospheric to tropospheric production varies with phase of Sun.
• Form of the reconstructed 11-yr cycle will be biased of no correction is applied.

The solar proxy 10Be, measured in natural archives, has previously been assumed to lag solar activity by 1–2 yr. This reflects its residence time in the stratosphere where its main source lies. This study, based on a 30-yr global model simulation of the atmospheric transport of 10Be, shows that the stratospheric fraction of production varies significantly with the phase of solar activity. This leads to a lag between production change and deposition response of ca. 1 yr during the decreasing phase of solar activity but nearly no lag when solar activity increases. No lag is found for 7Be. The fraction of 10Be production in the stratosphere varies from ca. 65% to 69% between solar minimum (ϕ=400 MVϕ=400 MV) and maximum (ϕ=1200 MVϕ=1200 MV), being highest during low solar activity when production is highest. When solar activity starts to decrease and production rate increase, the change is largest in polar stratosphere where residence time is long. This leads to a delayed response in 10Be deposition. When production rate decreases, the relative production change is larger in the troposphere, leading to a quicker deposition response. The difference between phases is larger in southern hemisphere. The form of reconstructed 11-yr cycles based on high-resolution 10Be records could therefore be biased if no correction is applied.