Magnetic measurements near Venus

Mariner 2 magnetometer data are presented with a more thorough analysis and discussion than in a previously published preliminary report. No magnetic fields attributable to Venus were detected. An upper bound on the magnetic dipole moment of Venus Mv is estimated in several ways, subject to uncertainties regarding the dipole orientation and the nature of the interaction between the solar wind and the planetary field. On the basis of Mariner's not entering the Venus magnetosphere, the upper bound lies between 1/2 and 1/6 of ME, the earth's magnetic dipole moment. However, the absence of field fluctuations near Venus corresponding to the shell of disordered fields detected outside the earth's magnetosphere by satellites and space probes shows that Mv is less than ME by at least an order of magnitude and makes it probable that Mv is less than ME/20. This result is consistent with qualitative predictions based on the dynamo theory of planetary fields. The energy flux of cosmic radiation above the Venus atmosphere, except perhaps for cosmic rays of the lowest energies, should everywhere approximate the intensity in the earth's polar regions. Any radiation zone of trapped high-energy electrons is likely to be minor in comparison with the earth's.