sol actividad
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This recent post was on the fact that the Sun's EUV emissions had fallen to solar minimum-like levels well ahead of solar minimum. The implication was that the Solar Cycle 24/25 minimum was either going to be very deep and prolonged, or that Solar Cycle 24 would be very short, which in turn would be strange for a weak cycle.

The indicator of the EUV flux is the Lyman alpha index. To recap, this chart shows the index over the last three cycles, starting from solar minimum:
lyman alpha solar cycles 22-24

Figure 1: Lyman alpha index Solar Cycles 22,23,24

Figure 1 shows that Solar Cycle 24 has reached solar minimum-like levels three years ahead of minimum, if Solar Cycle was going to be 12 years long. What happens at solar minimum is that the proportion of EUV as part of Total Solar Irradiance falls. For the 23/24 minimum, the extent of the fall was a surprise, with the density of the thermosphere shrinking 30%. The following figure plots up the ratio of the F10.7 flux, less its activity floor at 64, and the Lyman alpha index, less a presumed average floor of activity of 3.5:
F10.7 Flux/Lyman alpha ratio 1980 – 2017

Figure 2: F10.7 Flux/Lyman alpha ratio 1980 – 2017
The peak associated with the 23/24 minimum that surprised atmospheric researchers is quite evident. Also evident is a smaller peak associated with the 22/23 minimum. Nothing much seemed to happen prior to that. How that plots up with the F10.7 flux, and thus the solar cycles, is shown in the following figure:
F10.7 Flux/Lyman alpha ratio 1980 – 2017

Figure 3: F10.7 Flux/Lyman alpha ratio 1980 – 2017
As Figure 1 showed, the departure of the Lyman alpha index to minimum-like levels seemed early. But just how early is it if everything else is normal? That is shown in the following graphic:
F10.7 Flux/Lyman alpha ratio aligned on solar maximum

Figure 4: F10.7 Flux/Lyman alpha ratio aligned on solar maximum
Figure 4 aligns the F10.7 Flux./Lyman alpha ratio on solar maximum for solar cycles 21 to 24 to two years beyond solar minimum, with the maxima being:
  • Solar Cycle 21 December 1979
  • Solar Cycle 22 November 1989
  • Solar Cycle 23 November 2001
  • Solar Cycle 24 April 2014
Based on the normal cycle tail from solar maximum, Solar Cycle 24 might have another three and a half years to go. So what is going to be: a monstrous minimum with a shrunken thermosphere and all the climatic effects associated with that, or a strangely short cycle?

We know when a solar cycle is over when the heliospheric current sheet flattens. The current state of the heliospheric current sheet is shown in the following figure:
Heliospheric Current Sheet Tilt Angle 1976 – 2017

Figure 5: Heliospheric Current Sheet Tilt Angle 1976 – 2017
The heliospheric current sheet tilt angle is 10° off the apparent floor of 3° but, based on the prior solar cycles, could still take a few years to get there. If Solar Cycle 24 does turn out to be short, then there is one person who predicted that: Ed Fix. Ed Fix, a retired B52 pilot in Ohio, sent me his planet-based solar model in 2009. He was inspired to created the model because the oscillation of the solar cycle reminded him of the ideal spring in mechanics. This is how the model plots up (red) and the historic sunspot record in green:
Ed Fix’s solar activity model

Figure 6: Ed Fix’s solar activity model
The model has the Solar Cycle 24/25 minimum in 2017. Solar Cycle 25 is predicted to be weak and short also. If events of the next year or so prove Ed Fix's model to be correct, then it will be as significant as the results of any of the expeditions to observe solar phenomena over the last three centuries, but we get to watch in real time.