A hydromagnetic theory is presented which explains the average characteristics of geomagnetic storms. The magnetic storm is caused by a sudden increase in the intensity of the solar wind. Stresses are then set up in the geomagnetic field by the solar plasma impinging upon the geomagnetic field and becoming trapped in it. These stresses, which are propagated to the earth as hydromagnetic waves, account for the observed average magnetic storm variations. The sudden commencement of the magnetic storm is due to a hydromagnetic wave generated by the impact of the solar plasma on the geomagnetic field. The initial phase of the magnetic storm, during which the magnetic field is above average intensity, is due to the increased solar wind pressure. During the initial phase, instability causes small plasma clouds to become imbedded in the magnetic field. They break up and diffuse into the magnetic field to form a belt of trapped particles from the sun (principally protons and electrons). The trapped protons set up stresses, mainly due to centrifugal force, which account for the main phase of the magnetic storm. The recovery from the main phase is attributed to the relief of the stress on the geomagnetic field by the transfer of the energy of the trapped protons to neutral hydrogen by means of ion‐atom charge exchange. The correct recovery time for the magnetic storm is predicted from the measured cross section of the ion‐atom charge‐exchange process and the hydrogen density values around the earth deduced from the scattering of solar Lyman‐α radiation.

Results from the occultation of the sun by Neptune imply a temperature of 750 +/- 150 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane, acetylene, and ethane at lower levels. The ultraviolet spectrum of the sunlit atmosphere of Neptune resembles the spectra of the Jupiter, Saturn, and Uranus atmospheres in that it is dominated by the emissions of H Lyman alpha (340 +/- 20 rayleighs) and molecular hydrogen. The extreme ultraviolet emissions in the range from 800 to 1100 angstroms at the four planets visited by Voyager scale approximately as the inverse square of their heliocentric distances. Weak auroral emissions have been tentatively identified on the night side of Neptune. Airglow and occultation observations of Triton's atmosphere show that it is composed mainly of molecular nitrogen, with a trace of methane near the surface. The temperature of Triton's upper atmosphere is 95 +/- 5 kelvins, and the surface pressure is roughly 14 microbars.

Although Mercury and Mars appear to have magnetospheres of comparable size, Mercury's magnetosphere accelerates charged particles, whereas Mars' magnetosphere apparently does not. We propose that this difference results from the fact that rapid steady‐state convection, and the associated particle acceleration, cannot occur in a Martian magnetosphere because of its connection to a highly conducting ionosphere. Mercury, which has no conducting ionosphere and probably an insufficiently conducting surface, can exhibit rapid solar‐wind‐induced convection and hence particle acceleration in its magnetospheric tail.

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We present a model in which the Jovian magnetosphere is severely inflated by the centrifugal stress of partially‐corotating plasma streaming out along field lines from the ionosphere. The model is consistent with observations reported from the Pioneer 10 encounter, including the disc‐like field configuration, the diurnal modulation of trapped‐particle fluxes, and the inferred departure from rigid corotation in the outer magnetosphere. The field configuration is closed on the dayside, but on the night side the plasma can force the magnetic field open to form a planetary wind flowing in the antisolar direction.