The sparkling light of the northern, and southern, polar light (called Aurora by radio amateurs) is a reflector for radio signals allowing distances up to around 2000 kms to be covered. For station located in northern Europe this is a good propagation mode to work the stations on distances below 800 kilometers, which otherwise can be hard to work due to the short distance. Due to the fact that the aurora curtain is moving and not homogenous the CW tone is no more a tone when reflected but more a buzz. For that reason it is hard to read very weak signals as well as SSB signals. During strong auroras the tone gets better and so does the readability on SSB. Strong auroras can also convert into a propagation mode called aurora E, where the tone is clean and distances up to 5000 kms or more can be covered.
Aurora (northern polar light) in SM2 (near Kiruna)