On posting the story “The Sun Bursts to Life: Sunspots, Flares and CMEs” on the Universe Today, something was strange about my source material. Although the Sun had started Solar Cycle 24 back in January of this year, the new sunspots recently observed were the “leftovers” from the previous cycle and not new ones from this cycle. Something is strange. Surely one cycle ends and another begins? Think again…
Happily typing away, thinking I had solar physics down to a fine art, I spoke about how three new sunspots had rotated into view, blasting space with coronal mass ejections (CMEs) just after unleashing an M2-class solar flare. This was exciting stuff, as the Sun has pretty much done nothing for the last few months. Its disk had been void of any sunspot activity of any description, and then three come at once, galloping into view like valkyries unleashing hot magnetic fury into space.
But according to SpaceWeather.com, these “brand new spots” are actually leftovers from the previous solar cycle and don’t belong to Solar Cycle 24. Looking at data from the Michelson Doppler Imager (MDI), the polarity of the sunspot pairs were in fact the same polarity as sunspots from the previous cycle, and not the polarity expected from this cycle (the magnetic polarity switches after each cycle). That’s not to say sunspots from the new cycle haven’t been observed. On January 10th, a small sunspot pair had been observed at high latitudes on the solar disk (note high latitudes) demonstrating a switch in polarity, marking the beginning of Solar Cycle 24. But today, we have three active sunspots from the old cycle obviously alive and well.
This is where Spörer’s law comes in. In 1861, this law was derived by English astronomer Richard Christopher Carrington and then refined by Gustav Spörer. This law states that during any given solar cycle, sunspots will be observed at 30° to 45° latitude on the solar body as they come to life. As the cycle progresses toward solar maximum, the spots will be observed to drift toward a latitude of 15°. This law is evident in the “butterfly diagram” (pictured). Recent physics theory suggests that this is directly related to the winding of the magnetic field around the solar body. The more stressed the magnetic field becomes, the more flux that is collected around the equatorial regions, more sunspots are therefore produced by the up welling of magnetic flux.
This is why sunspots at the start of the solar cycle are found in high latitudes, and sunspots later on in the cycle are found at the equator.
So at what latitudes were the sunspots recently observed? You guessed it, at lower latitudes (pictured top), consistent with sunspots later in the solar cycle. Therefore we have two pieces of evidence that points to these recent spots being leftovers of Cycle 23: the magnetic polarity and the low latitude observations.
It therefore seems that a solar cycle can continue to “wind down” after a new cycle has begun. This explains why sunspots with characteristics of Solar Cycle 23 and Solar Cycle 24 can be observed at the same time.
But to me, there is something strange about these new findings. Surely when the Sun undergoes the transition from one cycle to the next, magnetic energy is lost as the Sun swaps polarity. If this is so, why are these old sunspots so energetic? Why are they blasting M2-class flares and CMEs into space? If I were to predict the nature of leftover sunspots, I’d guess they would be very quiet and short-lived events. Perhaps there is a lag between the magnetic energy being lost from the magnetic flux and the time from polarity switch, making these old spots as active as ever… I’ll be sure to keep my eye open for any research about this.