Interplanetary Magnetic Field (IMF)

The magnetic field carried with the solar wind . Once the solar wind has passed the radius where its kinetic energy density is equal to that of the solar magnetic field, its particles will move out into the interplanetary medium  carrying the solar magnetic field with them. Beyond that critical radius, the solar wind particles move in a straight line with constant velocity to a good approximation, but because the Sun is rotating and with it the footprints of the magnetic field lines, the magnetic field takes up a spiral structure known as an Archimedean spiral.