A Tale of Two Hemispheres

About a year ago, NASA released a story describing what had happened to solar max. Solar scientists had gotten together years earlier to forecast when this Solar Cycle 24 would reach its maximum intensity, which registers in sunspot number, magnetic field strength, and radio frequency emission. They decided the Sun would reach max around May 2013. When May arrived, it appeared that the Sun was in the downturn.

“This is solar maximum,” [heliophysicist Dean Pesnell] suggests. “But it looks different from what we expected because it is double peaked.”

Well, we’ve arrived at the second peak, and NASA is now saying that this is the solar maximum, not the one from before. So, what’s going on? I interviewed solar scientist Richard Altrock back at the end of 2011 on a related story, the weakness of Cycle 24, who pointed out something interesting he saw in his data. Altrock studies iron spectral lines from the Sun’s corona as a proxy for locations of magnetic field intensity. He said his data suggested that the Sun had already reached solar maximum around November 2011. But, it was a solar maximum of the northern hemisphere only. It appeared to him that the southern hemisphere was lagging the north. Pesnell also hints at such a situation in the 2013 NASA press release. I thought it was just a little misleading to say this solar cycle is double peaked, and then leave it at that. It gives an image of a Sun that throbs twice, then goes quiet. Instead, something more interesting appears to be happening inside the Sun. I went and grabbed the international sunspot number counts from the Royal Observatory of Belgium, here, and plotted the sunspot number for each hemisphere independently. Guess what I found.

Plot showing relative sunspot number contributions from the two solar hemispheres.  (Data is from WDC-SILSO, Royal Observatory of Belgium, Brussels)

Indeed the total sunspot number is double peaked, but only because the two hemispheres are no longer in sync with each other.  Just to get an image of what this means, I also threw in images of the Sun from the two peak periods.  You can see where the sunspots are.  Now, why are the two hemispheres seemingly de-synchronized?  I don’t know, but this is certainly a story I will be following here.

Here is the Sun on November 9, 2011. Note the density of sunspots in the northern hemisphere. Image courtesy of NASA/SDO

And, here’s the Sun about two years later. Again, note the predominance of spots in the southern hemisphere. Image courtesy of NASA/SDO