Dr David Hathaway is NASA’s solar cycle guru. In 2010, he published in Solar Physics a review of the methods used to forecast solar cycle activity. The review, titled “TheSolar Cycle”, is worth reading. He discusses many of the techniques currently in use that purport to be the method for solar cycle forecasting.
This post will only look at the key features that Hathaway says must be explained by any viable theory or model in order to provide a reliable forecast.
The Abstract for “The Solar Cycle” follows:
The Solar Cycle is reviewed. The 11-year cycle of solar activity is characterized by the rise and fall in the numbers and surface area of sunspots. We examine a number of other solar activity indicators including the 10.7 cm radio flux, the total solar irradiance, the magnetic field, flares and coronal mass ejections, geomagnetic activity, galactic cosmic ray fluxes, and radioisotopes in tree rings and ice cores that vary in association with the sunspots. We examine the characteristics of individual solar cycles including their maxima and minima, cycle periods and amplitudes, cycle shape, and the nature of active latitudes, hemispheres, and longitudes. We examine long-term variability including the Maunder Minimum, the Gleissberg Cycle, and the Gnevyshev–Ohl Rule. Short-term variability includes the 154-day periodicity, quasi-biennial variations, and double peaked maxima. We conclude with an examination of prediction techniques for the solar cycle.
Hathaway lists the critical features for making an accurate forecast:
Understanding the solar cycle remains as one of the biggest problems in solar physics. It is also one of the oldest. Several key features of the solar cycle have been reviewed here and must be explained by any viable theory or model. (I am adding several charts to aid in visualize his thinking.)
- The solar cycle has a period of about 11 years but varies in length with a standard deviation of about 14 months.
- Each cycle appears as an outburst of activity that overlaps with both the preceding and following cycles by about 18 months.
- Solar cycles are asymmetric with respect to their maxima – the rise to maximum is shorter than the decline to minimum and the rise time is shorter for larger amplitude cycles.
- Big cycles usually start early and leave behind a short preceding cycle and a high minimum of activity.
- The activity bands widen during the rise to maximum and narrow during the decline to minimum.
- Sunspots erupt in low latitude bands on either side of the equator and these bands drift toward the equator as each cycle progresses.
- At any time one hemisphere may dominate over the other but the northern and southern hemispheres never get completely out of phase.
- Sunspots erupt in groups extended in longitude but more constrained in latitude with one magnetic polarity associated with the leading (in the direction of rotation) spots and the opposite polarity associated with the following spots.
- The leading spots in a group are positioned slightly equatorward of the following spots and this tilt increases with latitude.
- The polar fields reverse polarity during each cycle at about the time of cycle maximum.
- Cycle amplitudes exhibit weak quasi-periodicities like the 7 to 8-cycle Gleissberg Cycle.
The Gleissberg Cycle is a period of about 80 to 90 years that overlays the well established 11 year cycle. The theory is that solar maxima and solar minima are forced by the gravitational pull of the major planets. The specific alignment, particularily Jupiter and also Saturn Neptune and Uranus have a major effect on the Sun’s activity. To see graphics of the alignment of these major planets, click here.
- Cycle amplitudes exhibit extended periods of inactivity like the Maunder Minimum.
- Solar activity exhibits quasi-periodicities at time scales shorter than 11 years.
- Predicting the level of solar activity for the remainder of a cycle is reliable 2 – 3 years after cycle minimum.
Hathaway tells us that theory must be able to predict the preceding. Until then, people will continue to predict the features of the next solar cycle but it may be just luck if they get it right.