The frequency of major solar eruptions, and their space weather impacts at earth vary with the cycle of solar activity but large amplitude events can occur at any time. Each solar cycle has a distinct amplitude and duration so that the solar cycle dependent frequency of rare, extreme space weather events is challenging to quantify. By constructing the analytic signal of daily sunspot numbers since 1818 we construct a new solar cycle phase clock which maps each of the last 18 solar cycles onto a single time-base. This clock orders solar coronal activity and extremes of the aa index, which tracks geomagnetic storms at the earth’s surface over the last 14 solar cycles. We identify and quantify the occurrence times of a geomagnetically quiet solar cycle interval of ~4.4 years (~2 pi/5 phase or 40% of the cycle) in extent centered on solar minimum within which only two severe (aa>300nT) and one extreme (aa>500nT) geomagnetic storms occurred since 1868. The solar cycle modulation of activity is such that 1-3% of severe (aa>300nT) geomagnetic storms and 4-6% of C, M and X class solar flares occurred in the solar cycle quiet phase. Terminators of solar EUV bright point activity indicate the end of this quiet interval and the ‘switch on’ of increased frequency of solar flares and geomagnetic storms. This provides quantitative support to planning resilience against space weather impacts since only a few percent of all severe storms occur in this quiet interval and its start and end are forecast-able.

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 takes place in the solar interior. 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 letter 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 this technique we can achieve higher fidelity terminator timing than previous estimates have permitted. Further, this method presents a unique opportunity to project when the next terminator will occur, 2020.33(±0.16), and trigger the growth of sunspot cycle 25. We also will use this method to show why Cycle 23 was unusually long, why the Cycle 23-24 minimum was unusually quiet, and why neither of these occurrences will happen with the end of Cycle 24. Ignoring the wealth of observational evidence and viewing the solar activity cycle as merely the growth and decay of sunspot number is one “social constraint that hampers progress” to be overcome.