Scientists have traditionally used “solar nadir,” or periods of reduced solar activity, to indicate the start of each cycle. However, according to R Robert Leamon, a research scientist at the University of Maryland-Baltimore County’s Partnership for Heliophysics and Space Environment Research (PaSER) in collaboration with NASA, the “solar nadir” framework is rather arbitrary. and inaccurate.
A recent study by Limon showed that many major changes in the solar cycle can be accurately described and predicted with a “solar clock” based on the sun’s magnetic field rather than the presence or absence of sunspots. The new method improves on classic sunspot techniques to predict surges in dangerous solar flares or shifts in weather patterns years in advance.
The new study, published in Frontiers in Astronomy and Space Science, specifically demonstrates that the solar cycle functions as an independent sequence of events. At one-fifth of a cycle, noticeable and sometimes sudden changes occur. This is true regardless of how long a particular cycle is, which can vary from a few months to a year. Limon and his colleagues pay tribute to music lovers by calling it the “circle of fifths.”
The sun’s magnetic field changes direction during each solar cycle, but there is overlap between successive cycles. The sun’s magnetic field is also called the polar magnetic field because it points either toward one of the sun’s poles or the other. The terminator marks the complete disappearance of the polar magnetic field from the previous cycle from the Sun’s surface, and a dramatic rise in solar activity will soon follow.
The new paper points to other landmarks throughout the solar cycle from terminator to terminator. These landmarks are clearer and more consistent than using sunspots as a guide for cycle length. For example, “The maximum number of sunspots does not coincide exactly with the timing of the polar reversal, but the polar reversal occurs exactly one-fifth of the period from endpoint to endpoint.
About two-fifths into a cycle, dark regions known as “polar coronal holes” re-form at the sun’s poles. Three-fifths into a cycle, the last X-flare, a very large and potentially dangerous class of solar flares, occurs. At four-fifths of the time, the sunspots are at their lowest point, before the sun passes another end point, after which solar activity quickly picks up again. Other phenomena, such as UV emission, also rank well in the fifth.