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:
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:
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:
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:
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
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:
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:
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.
Comment: Solar minimum: The sun is getting quieter, displaying some weird behavior