Today, October 6, 2011, is German-American Day, when we celebrate the arrival of German immigrants in Philadelphia in 1683. Germans have worked in physics for many years. Joseph Fraunhofer discovered the spectral lines in the Sun we call Fraunhofer lines. We measure the brightness, Doppler shift, and magnetic field effects of those spectral lines on SDO to study solar activity. At left we show the spectrum near the iron line at 6173 Å used by HMI to measure the velocity and magnetic field at the surface of the Sun. The slight motion to the left and right is what happens as the surface moves back and forth.
Another German, Heinrich Schwabe, wrote down the number of black specks he saw on the Sun for almost 20 years. When plotted we see the number of spots rising and falling with about a 10 year cycle. Schwabe had discovered the solar cycle. Here is a plot of Schwabe's data from his article Sonnen-Beobachtungen im Jahre 1843. The rise and fall can be seen in the crosses that show the number of spots in each year. I drew a sine curve with a 10 year period so you can see what Schwabe announced. Also called the 11-year sunspot cycle, we still measure the number of spots and produce the Sunspot Number. Along the way we have invented many more ways to measure spectra and active regions.
Happy German-American Day!
