The Collaborative International Dictionary
Isochasm
Isochasm \I"so*chasm\, n. [Iso- + chasm.] (Phys. Geog.) A line connecting places on the earth's surface at which there is the same mean frequency of auroras.
Wiktionary
isochasm
n. (context geography English) A line connecting places on the Earth's surface that have the same mean frequency of auroras.