Most of the phenomena associated with the Earth’s magnetic field are likely to be significantly modified or completely absent in and around Venus. Auroral displays and the trapping of charged particles in radiation belts such as our Van Allen would be missing. The field of the Earth keeps low- and moderate-energy cosmic rays away from the top of the atmosphere, except in the polar regions. The cosmic ray flux at the top of Venus’ atmosphere is likely to correspond everywhere to the high level found at the Earth’s poles.
As it encountered Venus, Mariner II made three scans of the planet.
SUN DIRECTION OF SCAN DATA READINGS (18 TOTAL)
In contrast to Venus, Jupiter, which is ten times larger in mass and volume and rotates twice as fast as the Earth, has a field considerably stronger than the Earth’s. The Moon has a field on the sunlit side (according to Russian measurements) which, because of the Moon’s slow rotation rate, is less than ⅓ of 1% of the Earth’s at the Equator. Thus, a planet’s rotation, if at a less rapid rate than the Earth’s, seems to produce smaller magnetic fields. This theory is consistent with the idea of a planetary magnetic field resulting from the dynamo action inside the molten core of a rotating planet.
The Sun, on the whole, has a fairly regular dipole field. Superimposed on this are some very large fields associated with disturbed regions such as spots or flares, which produce fields of very great intensities.
These solar fields are drawn out into space by plasma flow. Although relatively small in magnitude, these fields are an important influence on the propagation of particles. And the areas in question are very large—something on the order of an astronomical unit.
Mariner II seemed to show that, in space, a generally quiet magnetic-field condition was found to exist, measuring something less than 10 gamma and fluctuating over periods of 1 second to 1 minute.
As Mariner made its closest approach to Venus, the magnetometer saw no significant change, a condition also noted by the radiation and solar plasma detectors. The magnetic field data looked essentially as they had in interplanetary space, without either fluctuations or smooth changes.
The encounter produced no slow changes, nor was there a continuous fluctuation as in the interplanetary regions. There was no indication of trapped particles or near-Venus modification in the flow of solar plasma.