Planetary magnetospheres

The study of extraterrestrial planetary magnetospheres during the past 4 years has experienced a dramatic growth in the availability of observational data and a concomitant growth in the level of scientific interest comparable to the experience of the early 1960's in the study of earth's magnetosphere. Owing in large part to the success of space probes to our four nearest neighbor planets, the domain of magnetospheric physics has grown from the study of ‘the magnetosphere’ into the study of ‘planetary magnetospheres.’ It has now become possible to identify three rather different categories of planetary magnetospheres: (1) intrinsic slowly rotating magnetospheres, (2) intrinsic rapidly rotating magnetospheres, and (3) induced magnetospheres. By ‘intrinsic’ we mean that the planet possesses a sufficiently strong magnetic field to hold the solar wind off its surface; by ‘induced’ we mean that the planetary field exists mainly by virtue of the solar wind interaction with the planet. The distinction between ‘rapidly rotating’ and ‘slowly rotating’ is a quantitative one, indicating whether or not the structure and dynamics of the magnetosphere are influenced strongly by the rotational energy of the magnetospheric plasma. Thus although corotation in earth's magnetosphere is a well‐documented fact, this rotation is sufficiently slow that it can be safely ignored in most considerations of the overall dynamics of the magnetosphere, so that earth's magnetosphere is slowly rotating by the above definition.