It has been awhile since I’ve looked at the Ap Index. The last time was April of 2009.
From the data provided by NOAA’s Space Weather Prediction Center (SWPC) you can see just how little Ap magnetic activity there has been since. Here’s my graph from September 2009 SWPC Ap data:
For a longer perspective, David Archibald, has a graph of the Ap Index back to 1932. The solar average geomagnetic planetary index, in Dec 2008, Ap was at its lowest level in 75 years:
Click for a larger image – I’ve added some annotation to the graph provided by Archibald to point out areas of interest and to clarify some aspects of it for the novice reader.
The last time the Ap index was this low was 1933. The December 2008 Ap value of 2,, has never been this low. (Note: Leif Svalgaard contends this value is erroneous, and that 4.2 is the correct value – either way, it is still lower than 1933) Further, the trend from October 2005 continues to remain low, though some signs of a slight rebound are showing.
This Ap index is a proxy that tells us that the sun is now quite inactive, and the other indices of sunspot index and 10.7 radio flux also confirm this. The sun is in a full blown funk, and your guess is as good as mine as to when it might pull out of it. So far, predictions by NOAA’s SWPC and NASA’s Hathaway have not been near the reality that is being measured.
As Leif Svalgaard points out, Ap is just one of several indices that describe geomagnetic activity. There are several others [aa, am, IHV, …] that go much further back in time [to the 1840s]. You can get more info from:
For those that follow the sunspot number (SSN) I’ve graphed the Ap and SSN together. As you can see, we’ve been in a reduced state of solar activity now for quite some time. It has been almost 4 years since the prominent drop in Ap in October 2005. SSN mirrors the decline of the Ap index since then.
As many regular readers know, I’ve pointed out several times the incident of the abrupt and sustained lowering of the Ap Index which occurred in October 2005. The abrupt step change seemed (to me) to be out of place with the data, and since then the data seems less “active”, with reduced amplitudes. And then we have the fact that the sun seems to have reestablished at a lower plateau of the Ap index after that October 2005 step change and has not recovered now in almost 4 years. It seems to me to be a noteworthy event.
UPDATE: Thanks to Leif Svalgaard, we have a more extensive and “official” Ap dataset (NOAA’s SWPC has issues, see comments) that I’ve plotted below. The step change in October 2005 is still visible and the value of 3.9 that occurred in April of this year is the lowest for the entire dataset.
And I’ve also plotted the 1991 to present data from BGS/Svalgaard to compare against the NOAA SWPC data: