On the Near-One-Year Variation of the Sun’s Mean Magnetic Field

The mean magnetic field (MMF) of the Sun-as-a-star was measured over the last 38 years by six observatories (about 17 000 MMF daily records, 1968 – 2005). The MMF power spectrum reveals the presence of an enigmatic 1.029(7) year periodicity whose origin requires explanation. We show that this quasi-annual variation is not produced by modulation of the MMF signal due to the annual change of the Earth’s helio-latitude (one-year change of visibility of the Sun’s polar regions) as commonly accepted. The nature of this new solar phenomenon is open for discussion.