Because the wind is a plasma rather than a simple gas, it is highly electrically-conductive, which means that vibrating magnetic waves propagated by the Sun are carried along with the wind, forming an interplanetary magnetic field (IMF). The interplanetary magnetic field lines are said to be ‘frozen in’ to the solar wind plasma.

Because of the rotation of the Sun (averaged out as being about once every 27 days as viewed from the earth) and the dynamic pressure of the wind, which dominates over the magnetic pressure through most of the solar system, the IMF is forced into an Archimedean spiral rather than being purely radial.

© Peter Dawkins