X rays in the energy band 0.2–3.0 keV have been detected coming from both polar regions of Jupiter. The observations were made in 1979 and 1981 by using the imaging proportional counter and high resolution imaging detectors on the Einstein X ray astronomy satellite. The measured flux density of ∼6×10−4cm−2s−1 at earth corresponds to an X ray luminosity of ∼4×109 W in the 0.2‐ to 3.0‐keV energy band. The energy spectrum of the X rays is extremely soft and can be characterized by a power law with an exponent of ∼2.3. Detector energy resolution is insufficient to distinguish a soft line spectrum from a continuum. However, the shape of the response and the observed X ray power indicate that the source of this auroral emission is not electron bremsstrahlung as on the earth, but is most probably line emission from O and S ions with energies between 0.03 and 4.0 MeV/nucleon precipitating from the outer boundary of the Io plasma torus at L ∼ 8.

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