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Butterfly diagram.This shows the distribution of sunspots in solar latitude as a function of time in the sunspot cycle. Early in a cycle, spots appear at higher latitudes; late in a cycle, they appear close to the equator. Note that a new cycle starts before the old cycle completely ends. [Roger Ulrich, UCLA, Mt Wilson.This study includes data from the synoptic program at the 150 ft Solar Tower of the Mt Wilson Observatory.The Mt Wilson 150 ft Solar Tower is operated by UCLA, with funding from NASA, ONR and NSF, under agreement with the Mt Wilson Institute]

on a charge moving parallel to the magnetic field lines. The force is maximum when the velocity is perpendicular to the magnetic field. The force is perpendicular to both the direction of motion and to the magnetic field, and its direction is given by the so-called "right hand rule". This means that the component of motion parallel to the field lines is not altered, but the component perpendicular to the field lines is. So, charged particles will move so that they spiral around magnetic field lines.

Through the forces that it exerts on moving charges, a magnetic field will exert a torque on a current loop. We can think of a current loop as having a magnetic dipole moment (as in Fig. 6.22), and the torque will cause the dipole to line up with the field lines. In this way, the dipole moment of a compass needle lines up with the Earth's magnetic field.

How do we measure the Sun's magnetic field? For certain atoms placed in a magnetic field, energy

Field lines for a dipole magnetic field.

Field lines for a dipole magnetic field.

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