Empirical Mode Decomposition studies of quasi-biennial variations in helioseismic frequencies
Natural Sciences (Astrophysics and Astrononmy)
It is well-known that the Sun’s magnetic activity varies primarily on a time scale of 11yrs. It is
also well known that the frequencies of the Sun’s natural acoustic oscillations vary in-phase
with surface and atmospheric measures the Sun’s activity. Additionally, both atmospheric
activity proxies and helioseimic frequencies exhibit variations on timescales much shorter than
11yrs, including the ‘quasi-biennial’ periodicity. Solar periodicities, including the 11yr cycle, are
hard to characterise, as they vary in both length and amplitude. We have used empirical mode
decomposition techniques to separate time variations in acoustic mode frequencies, the
10.7cm flux and sunspot areas into a set of intrinsic modes. We then use the Huang-Hilbert
transform to characterise the periodicities of these intrinsic modes. The periodicities obtained
are consistent between the different activity proxies, including the quasi-biennial periodicity.
This indicates that the quasi-biennial behaviour observed at the solar surface and in the solar
atmosphere is also symptomatic of the solar interior.
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