line connecting points of equal frequency of aurorae
• (n.) A line connecting places on the earth`s surface at which there is the same mean frequency of auroras.
I'so·chasm noun [ Iso- + chasm .] (Physics Geology) A line connecting places on the earth's surface at which there is the same mean frequency of auroras. 
isochasm A line connecting points on the earth’s surface at which the aurora is observed with equal frequency.
a line on a map or chart connecting points where auroras are observed with equal frequency.