Guest essay by David Archibald
Recently, a number of newspaper articles spoke of the potential of cycle 25 to be “Weakest Solar Cycle In Almost 200 Years”. “We’re in a new age of solar physics,” said David Hathaway of NASA’s Marshall Space Flight Center in Huntsville, Alabama.
Here is a collection of solar measurements that illustrate the current state of cycle 24, as well as provide insight into cycle 25.
Figure 1: Oulu Neutron Count 1964 – 2013
This graph suggests that it may be a further six months or more to solar cycle maximum. Neutron count tends to follow the solar cycle with up to a one year lag so it may be another 18 months before we get to the minimum neutron count for Solar Cycle 24.
Figure 2: Oulu Neutron Count for Solar Cycles 20 to 24 aligned on month of minimum
In terms of neutron count, Solar Cycle 24 isn’t much weaker than the previous four cycles at a similar stage of development.
Figure 3: Ap Index 1932 – 2013
The Ap Planetary Magnetic Index has now spent the last couple of years below the levels of previous solar cycle minima, including an all-time record low for the data set.
Figure 4: Heliospheric Current Sheet Tilt Angle
Solar minimum is marked by the flattening of the heliospheric current sheet tilt angle. This tends to be quite sharp. Solar maxima are a lot broader with the current maximum the broadest of the instrument record. There is no indication yet from this measure that solar maximum is over.
Figure 5: Monthly F10.7 Flux 1948 – 2013
The F10.7 flux shows that Solar Cycle 24 is quite a weak cycle relative to the ones that have preceded it in the instrumental record.
Figure 6: F10.7 Flux of Solar Cycles 19 to 24 aligned on month of minimum
In terms of F10.7 flux, Solar Cycle 24 peaked two years ago. The relationship between F10.7 flux and sea level rise indicates that a flux of 100 is the break-over between climate warming and cooling. The flux level has been at about that value for the last three years.
Figure 7: Interplanetary Magnetic Field 1966 – 2013
The 1970s cooling period had a weak and flat interplanetary magnetic field over Solar Cycle 20. Solar Cycle 24 could produce a similar result with a slightly lower average value over the cycle.
Figure 8: Solar Cycle 24 sunspot count relative to the Dalton Minimum
All things considered, the current solar cycle is tracking Solar Cycle 5, the first half of the Dalton Minimum, fairly closely.
Figure 9: Predicting the year of maximum of Solar Cycle 25
Just over two years ago, Richard Altrock of the National Solar Observatory at Sacramento Peak published the latest version of his green corona emissions diagram.
He stated at the time that the progression of the Solar Cycle 24 was 40% slower than the average of the previous two cycles. That would make it 15.5 years long. Given that the cycle started in December 2008 and solar maximum is in 2013, that makes the Solar Cycle 24 fall time 11.5 years.
Figure 9 shows the strong relationship between fall time and the time from maximum to maximum. Based on that relationship, the Solar Cycle 24 fall time derives a period of 17 years from the Solar Cycle 24 maximum to the Solar Cycle 25 maximum – putting it in 2030.