Timing Terminators: Forecasting Sunspot Cycle 25 Onset


Recent research has demonstrated the existence of a new type of solar event, the “terminator.” Unlike the Sun’s signature events, flares and coronal mass ejections, the terminator most likely originates in the solar interior, at or near the tachocline. The terminator signals the end of a magnetic activity cycle at the Sun’s equator and the start of a sunspot cycle at mid-latitudes. Observations indicate that the time difference between these events is very short, less than a solar rotation, in the context of the sunspot cycle. As the (definitive) start and end point of solar activity cycles the precise timing of terminators should permit new investigations into the meteorology of our star’s atmosphere. In this article we use a standard method in signal processing, the Hilbert transform, to identify a mathematically robust signature of terminators in sunspot records and in radiative proxies. Using a linear extrapolation of the Hilbert phase of the sunspot number and F10.7 cm solar radio flux time series we can achieve higher fidelity historical terminator timing than previous estimates have permitted. Further, this method presents a unique opportunity to project, from analysis of sunspot data, when the next terminator will occur, May 2020 (\(+4\), −1.5 months), and trigger the growth of Sunspot Cycle 25.

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    And his Hale Prize lecture, St. Louis American Astronomical Society - Solar Physics Division meeting.

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    The present consecutive active region numbering system only started in January 1972.


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This material is based upon work supported by the National Center for Atmospheric Research, which is a major facility sponsored by the National Science Foundation under Cooperative Agreement No. 1852977. RJL acknowledges support from NASA’s Living With a Star Program.

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Leamon, R.J., McIntosh, S.W., Chapman, S.C. et al. Timing Terminators: Forecasting Sunspot Cycle 25 Onset. Sol Phys 295, 36 (2020). https://doi.org/10.1007/s11207-020-1595-3

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  • Solar cycle
  • Observations