We made observations of sodium atoms to investigate the spatial distribution of Iogenic gas. From the observational results, the shapes of emission distribution are classified into three types. They are (i) brighter (∼kilorayleighs) cloud around Io as far as ∼ 50R Io (Io radii), (ii) cloud which is directed away from Jupiter and concentrated along the equatorial plane of Jupiter outside Io's orbit, and (iii) diffuse and faint cloud spreading outside Io's orbit. We also detected that the D-line emission distribution shows intensity asymmetry between the north and south sides of the equatorial plane. We conclude that this asymmetry results from the temperature anisotropy of the torus plasma which makes charge exchange with neutral atoms around Io. The value of the anisotropy is determined to be A = T ⊥ /T ∥ =8 and T ⊥ =60[eV]. From a comparison of the observational results with the results obtained from model calculation, we estimate that the sodium atom source for charge exchange is 6.0 × 10 25 atoms/sec.
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