lo i• J11rro1111ded by a halo of arom.1 which radillll' in emission lineJ . ThiJ 11·as disc01·ered by gruundbased ohserl'lllions in 1973. a year bej(1re Pioneer 10 pa;sed thn1111d1 the Jo1·ia11 system. Earlier op1ical and radio obJen·ers had reported 01her t1111111wlie1 associllled 11·i1h lo. and 1he Pioneer Jpacecraft diJCOl'ered an 11l1ra1·iole1 e111fasio11 doud around the rn1e/li1e and a .111b.1w111ial ionosphere. rl 11e11· field of p/aneiary research is dedica1ed 10 i111egra1i11g the.\<! phenomena i1110 a model of Ju and <~(1he Jol"ian e111·iro11111e111 wi1h ll'hich it Hrongly i111erac1s . Sodium dominmes rhe op1ica/ emission cloud around lo. The producrio11 rate i1 1·ery· large and has not .'et been sa1isfac1orily explained. Sp1meri11g by charged particle bo111bard111e111 may play 1111 imporram role. The primary exc itlllio11 mec/11111ism i~ the reJ01w111 sca11eri11g of s1111lif1hl. While 1he sodium cloud is brif/hle.11 near lo. ii ex1e11ds uround 1he emire orbiI and is present a1 lo11· le1•els in the general Jol'iw1 e111·irc111me111. The ionospheric elecrron de11si1y 011 lo is comparable to 1'1111 011 J fars, a s11rprisi111: rernli since 1he solar flux is much 11 eaJ..er. H ere. again. l o" charged particle em·iro11111e111 111ay play an important part. The obsen·ed electron pro.files plus the cc111s1rni111.1 pru1·ided by 1he emission cloud are cu11sis1e111 wi1h at leaH 111·0 models of I o's 11e111ral 11r1110.1plu·re.
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