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

Abstract

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.