A history of early work on the heliospheric magnetic field

The idea of a magnetic field in space around Earth began 400 years ago with Gilbert's recognition that the magnetic field of Earth extends outward int o space to form what we now call the magnetosphere. The concept of the sola r wind and the heliosphere had its first glimmerings with the recognition, about 270 years ago, that geomagnetic activity is correlated with solar act ivity. It was suggested about a 100 years ago that the connection is throug h solar corpuscular radiation with velocities of the order of 10(3) km/s. T he observed acceleration of comet tails indicated the universal nature of s olar corpuscular radiation, giving rise to the first speculation on the exi stence of the heliosphere. The hydrodynamic expansion of the million degree solar corona was then shown to provide the solar wind (solar corpuscular r adiation), stretching out the magnetic fields of the Sun to fill the helios phere with a spiral magnetic field. The advent of the space age soon verifi ed the solar wind and magnetic field with direct measurements.