Geomagnetic Indexes, Overlapping Magnetic Cycles, and Prediction: What is Solar Minimum?

McIntosh et al. (2014) proposed a model for the 22-year magnetic cycle, based on a pattern of overlapping magnetic polarity bands. Typical predictors of solar activity such as sunspot number are shown to align with the model. However, patterns of geomagnetic activity were not compared in that original work.

Legrand and Simon (1981) investigated the solar cycle by classifying geomagnetic activity into three main categories:(1) quiet day activity when aa< 20γ, and active days of either (2) recurrent activity, when the activity level changes smoothly and the pattern that persists for longer than four solar rotations, (3) fluctuating activity, each of which they linked to a different solar feature. Through their analysis they found evidence for an “extended” solar cycle of 17 years with a 6 year overlap between 2 successive cycles. Utilizing their classification scheme, 143 years of aa data spanning from 1868-2011 was analyzed, and is compared to the 22-year model for the solar magnetic cycle proposed by McIntosh et al. (2014).

Solar wind data from the OMNI database is used to provide information about the possible solar sources of the geomagnetic activity, their relationship with this "extended solar cycle," and the global-scale evolution of solar magnetism. Further, we see that geomagnetic activity in the declining phase of one cycle (N) is a robust predictor of the sunspot activity to come (cycle N+1), but why? What does the present state of geomagnetism say about sunspot cycle 25?

In very quiet times, the structure of the solar wind, and hence geomagnetic forcing, will become very complex when multiple sources of fast solar wind (equatorial coronal holes) are intertwined in interplanetary space to drive the magnetosphere and our atmosphere.

As an aside, we will discuss how the overlapping 22-year cycle can produce that last hurrah of many sunspot cycles.