Change of solar oscillation eigenfrequencies with the solar cycle

Solar acoustic eigenfrequencies depend on the internal structure of the Sun, which may change during the 11-yr cycle of magnetic activity as a result of various effects associated with the solar dynamo. Observations of low-degree acoustic frequencies were made, using the ACRIM instrument on the Solar Maximum Mission (SMM) satellite, in 1980 (near solar maximum) and 1984 (near solar minimum). The analysis of these data, presented here, indicates that the frequencies of l = 0 and l = 1 acoustic modes in the 5-min band have decreased from 1980 to 1984, by ∼0.42 µHz or 1.3 parts in 104. This finding may have important implications for our understanding of the mechanism of the solar activity cycle.