1. In comparing the occultation light curves at various latitudes, we used an equilibrium figure for the planet that was calculated assuming GM= 5,793,920 km^3 sec^(-2) (G, universal gravitational constant; M mass), a rotation period of 17.3 hours and J_2 = 3.346e-6 and J_4 = 3.21e-5.

2. In the analysis of these observations, we followed the procedure established by G. R. Smith et al., J. Geophys. Res. 88, 8667 (1983).

3. B. D. Savage et al., Astrophys. J. 216, 291 (1977).

4. Th. Encrenaz et al., Astron. Astrophys., in press.

5. S. K. Atreya, in Uranus and Neptune, J. T. Bergstralh, Ed. (NASA Conf Publ. 2330, 1984), p. 55.

6. R. G. French et al., Icarus 53, 399 (1983).

7. A. L. Broadfoot et al., Science 204, 979 (1979).

8. B. R. Sandel et al., ibid. 215, 548 (1982).

9. G. R. Smith et al., J. Geophys. Res. 88, 8667 (1983).

10. D. E. Shemansky, ibid. 90, 2673 (1985).

11. ______ and G. R. Smith, Geophys. Res. Lett. 13, 2 (1986).

12. D. M. Hunten et al., in Saturn, T. Gehrels and M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1984), p. 671.

13. Several aspects of the H_2 models depend on recent unpublished work by A. J. Ajello, D. E. Shemansky Y. L. Yung, A. Dalgarno, S. Guberman, T. L. Kwok, and A. Posen (1986).

14. D. E. Shemansky and J. M. Ajello, J. Geophys. Res. 88, 459 (1983).

15. The energy distribution of the exciting electrons should be nearly Maxwellian because electron-electron relaxation is rapid, this distribution was used for our calculated spectra. The threshold for excitation of the H_2 emissions is about 10 eV, so that the excitation in this model is caused by electrons in the high energy tail of the Maxwellian distribution. These electrons also dissociate H_2 and heat the atmosphere. The ratio of the H production rate to the ionization rate, which depends on the electron temperature, is ten times higher at Uranus than at Saturn. The total rates of production of escaping H are comparable at Uranus and Saturn.

16. The H_2 column abundance is inferred from self-absorption in the spectrum.

17. D. E. Shemansky, D. F. Strobel, Y. L. Yung, in preparation.

18. J. H. Waite et al., J. Geophys. Res. 88, 6143 (1983).

19. R. V. Yelle, B. R. Sandel, D. E. Shemansky, S. Kumar, J. Geophys. Res., in press.

20. K. M. Hagenbuch and R. E. Hartle, Phys. Fluids 12, 1551 (1969).

21. B. A. Smith et al., Science 233, 43 (1986).

22. R. G. French, J. L. Elliott, S. E. Levine, Icarus, in press.

23. N. F. Ness et al., Science 233, 85 (1986).

24. J. T. Clarke et al., J. Geophys. Res., in press.

25. R. V. Yelle and B. R. Sandel, Geophys. Res. Lett. 13, 89 (1986).

26. We thank R. S. Polidan, J. T. Clarke, and J. H. Waite for helpful discussions and the Voyager Project personnel at the Jet Propulsion Laboratory for the enthusiastic efforts that have made this mission successful. Supported by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract NAS 7-100. Additional support was provided by the Planetary Sciences Discipline of NASA's Office of Space Sciences under contracts NAGW-610, NSG-7404, and NAGW-649.

28 March 1986; accepted 5 May, 1986

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