The IMF is represented by the blue arcs in the picture above. This picture shows the spiral nature of the IMF. Click on image for full size (105K JPEG) Courtesy of NASA

IMF stands for Interplanetary Magnetic Field. It is another name for the Sun's magnetic field.

The Sun's magnetic field is enormous and is carried by the solar wind. The solar wind and magnetic field are twisted into a spiral by the Sun's rotation. All the planets reside within this field, which is why the Sun's magnetic field can be called interplanetary.

Eventually the solar wind and IMF encounter the interstellar magnetic field and plasma of interstellar space. The boundary between space dominated by the Sun (or the heliosphere) and interstellar space is called the heliopause.