Callisto's ionosphere was first detected during Galileo flybys; its high electron density of 7–17 × 104 cm−3 cannot be explained by the photoionization of the atmospheric carbon dioxide alone.
Ionosfer Callisto pertama kali dilacak oleh wahana Galileo; massa jenis elektronnya yang besar (7–17 × 104 cm−3) tidak dapat dijelaskan melalui fotoionisasi karbon dioksida atmosfer saja.