As many readers know, I follow the Average Magnetic Planetary Index (Ap) fairly closely as it is a proxy indicator of the magnetic activity of our sun. Here is the latest Ap Graph:
I’ve pointed out several times the incident of the abrupt and sustained lowering of the Ap Index which occurred in October 2005.
David Archibald thinks it may not yet have hit bottom. Here is his most recent take on it.
The low in the Ap Index has come up to a year after the month of solar cycle minimum, as shown in the graph above of 37 month windows of the Ap Index aligned on the month of solar minimum. For the Solar Cycle 23 to 24 transition, the month of minimum is assumed to be Ocotber 2008. The minimum of the Ap Index can be a year later than the month of solar cycle minimum, and the period of weakness can last eighteen months after solar cycle minimum.
The graph also shows how weak this minimum is relative to all the minima since the Ap Index started being measured in 1932. For the last year, the Ap Index has been plotting along parallel to the Solar Cycles 16 – 17 minimum, but about four points weaker. Assuming that it has a character similar to the 16 – 17 minimum, then the month of minimum for the Ap Index is likely to be October 2009 with a value of 3.
The shape of the Ap Index minima is similar to, but inverted, the peaks in neutron flux, which are usually one year after the month of solar minimum.