I have a question which puzzled me when I was reading up about auroras. When talking about the interaction of the solar wind and the Earth's magnetic field, the book said that "particles are accelerated along magnetic field lines towards the earth" That didn’t sound quite right, as I was taught that the force on a charged particle in a magnetic field is always perpendicular to the motion, so the path would be bent into a circle. If there was a component of the velocity parallel to the magnetic field, it would move that way in a helical path, spiralling along the magnetic field lines. That is pretty much what the book said, but this would imply there is no acceleration. The speed of the particles is unchanged, they are just directed to the poles of the Earth. So I at first assumed that there is no acceleration, and this was just a sloppy choice of words by the author. But I have now read similar statements in several reputable articles, so I start to wonder if there is more to it. I know that magnetohydrodynamics is complicated because the current of all the moving particles will contribute to the magnetic field, but I can’t see how there can be an acceleration mechanism if the force is always perpendicular to the motion. Are the particles picking up extra kinetic energy from some other source?