Second solar spectrum

The second solar spectrum is an electromagnetic spectrum of the Sun that shows the degree of linear polarization. The term was coined by V. V. Ivanov in 1991. The polarization is at a maximum close to the limb (edge) of the Sun, thus the best place to observe such a spectrum is from just inside the limb.^[1] It is also possible to get polarized light from outside the limb, but since this is much dimmer compared to the disk of the Sun, it is very easily polluted by scattered light.

The second solar spectrum differs significantly from the solar spectrum determined by the intensity of light.^[1] Large effects come around the Ca II K and H line. These have broad effects 200 Å wide and show a sign reversal at their centers.^[1] Molecular lines with stronger polarization than the background due to MgH and C₂ are common.^[1] Rare-earth elements stand out far more than expected from the intensity spectrum.^[1]

Other odd lines include Li I at 6708 Å which has 0.005% more polarization at its peak, but is almost unobservable in the intensity spectrum. The Ba II 4554 Å appears as a triplet in the second solar spectrum. This is due to differing isotopes and hyperfine structure.^[1]

Two lines at 5896 Å 4934 Å being the D₁ lines of sodium and barium were predicted not to be polarized, but nevertheless are present in this spectrum.^[1]