IMF stands for Interplanetary Magnetic Field. It is another name for the Sun's magnetic field.
The Sun's magnetic field is huge! It goes beyond any of the planets. The Sun's magnetic field got its name because of that. We call the Sun's magnetic field the Interplanetary Magnetic Field meaning it has all of the planets within it.
The magnetic field of the Sun is carried by the solar wind which comes out from the Sun. The solar wind and magnetic field are twisted into a spiral by the Sun's rotation.
Eventually the solar wind and IMF encounter interstellar space. The boundary between space dominated by the Sun (or the heliosphere) and interstellar space is called the heliopause.
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