When I think about solar maximum it isn’t just sunspot number. I think about how bright the Sun has gotten in the wavelengths of light that cause space weather. Satellite drag and radio outages are two space weather effects that are caused by light at extreme ultraviolet (EUV) wavelengths, roughly 10-125 nm. This light is so completely absorbed by the atmosphere that it never reaches the surface of the Earth. In space we must be to see the EUV.
Here is a squiggly line chart of the Sun's EUV brightness in six wavelengths, averaged over each day. The data come from EVE on SDO. The squiggles start in May 2010 and continue until February 2014. I plotted the logarithm of the brightness to make all of the lines visible. As the Sun emits more light at one wavelength the squiggle goes higher on the chart. Each line has a different color and they are labeled to the right with the element and wavelength of the line. The other number is we will explain in a minute. He II 304 is the brightest, followed by C III 977 and H I 1026. The three iron wavelengths are next.
You can make similar plots of each wavelength with the EVE data browser on the SDO website.
Since SDO was launched each of these wavelengths has gotten brighter in its own way. The peak He II 304 irradiance was in February 2014 and a horizontal line is drawn at that value. The smallest He II 304 value was in June 2010 and another horizontal line is drawn at that value. The ratio of the brightest to the dimmest is 2.1. The same max and min lines are drawn for the H I 1026 irradiance, with a ratio of 1.7. The Fe XVI 335 wavelength is 12 times brighter, the biggest ratio of these wavelengths. We now see that the other number in the label is the ratio of the brightest to the dimmest the Sun has been in the EVE data.
The biggest monthly sunspot number so far in Solar Cycle 24 is 97 in November 2011. I drew a vertical line at that date. The sunspot number was 91 for January 2014, a little less than November 2011. The iron wavelengths are all about the same brightness now as they were then, but He II 304 is distinctly brighter now. H I 1026 is as bright now as it was in November 2011. These differences in brightness during solar maximum show how complicated the sunspot cycle is.
The different wavelengths look the same at other times. A “wave” appears during the second half of 2012 in all six wavelengths. Each wave is a turn of the Sun. During this time the Sun was brighter and dimmer in a pattern that turned with the Sun. We see the rotations most of the time but 2012 is a great episode of “As the Sun Turns.”
These data show why it so difficult to say when solar maximum happened. Each wavelength has a different timing of it’s peak brightness. Even as we use sunspot number to say when solar maximum was, we should keep in mind that solar maximum is not a point in time but extends over several years.