It is best to understand a few definitions given that there are several different types of Polar sunsets, namely:
The line: Very strong horizon refraction causes the sun sphere to compress almost into a straight line. Higher refraction at the extreme lower part of the sun disk in space is pushed upwards within the disk seen from the ground on Earth. Being definitely an optical effect, it is the folding of space not only within the sun disk but at the same elevation throughout the sky .
Phenomenon first classification attempt by Willard J. Fisher, he called it a Type B sunset in the 1920s. J. F. Chappell photograhed it somewhat later (ref. apparent distortions of the setting Sun, Pub. Astr. Soc Pacific. 45 281-282 (1933):
"The final singular long line, which oddly enough is substituted for the small tip of light that could reasonably be expected as the final glimpse of a bright descending sphere. JF Chappell
Novaya Zemlya effect: A superior mirage of the line caused by a shallow inversion layer. Without Zooming by some visual aids, Novaya Zemlya effects give the appearance of a rectangular sun, upon closer inspection the sun images are more like a flattened hour glass. Novaya Zemlya effects are called mirages, which may vary from the preceding definition, observation of the whole sunset sequence may help identify mirage segments.
Multiple lines: Things get complicated by various atmospheric features, the simplest way of putting it, is that multiple lines are caused by again very strong refraction, mirror effects by the ducting of light through one or several inversion layers. Multiple lines are not featured here (because they need high quality equipment to be filmed).
The Green Flash: Best described by Doctor Young (see the link at the bottom), the Green Flash is mostly Green due to Ozone and the scattering of light low on the horizon , the Green Flash looks similar to the line, and is confused quite easily with it.