Temporal variation of ~7Be concentrations in atmosphere for 8 y from 2000 at Yamagata, Japan: solar influence on the ~7Be time series

摘要

We have been continuously observing the daily ~7Be concentrations in surface air at Yamagata, Japan (38.25° N, 140.35° E) since 2000. The yearly profile of the ~7Be concentration indicates the variation in galactic cosmic rays owing to solar modulation. Over 8 y, the ~7Be concentration, cosmic neutrons, and number of sunspots varied by 37.4%, 12.2%, and 92.8%, respectively. The influence of precipitation on the ~7Be variability was approximately 5%. Hence, the yearly ~7Be concentration was mainly varied by the solar modulation of the ~7Be production rates. Based on the production rates found in an EXPACS simulation, the observed variability indicates ~7Be transport from high latitudes. The daily ~7Be concentrations have two significant periodic components of 19 d and 36 d. The 36-d component implies a relationship between the sun's rotation and the vertical transport of air masses under quiet solar activity.