Sunday, July 14, 2013

Which Way is North?

Compasses tell us which way is North on the Earth. It's easy, the needle with the big N points toward the North Pole of the Earth. Other directions can be read off the dial, like a primitive GPS!
Compasses work because the Earth has a simple magnetic field with the magnetic poles in almost the same place as the rotation poles. And the compass needle points along only the part of the magnetic field that is horizontal.
The Sun also rotates and has North and South poles. But compasses aren't very useful on the Sun. The solar magnetic field is very complicated and is usually vertical rather than horizontal. Now North and South refer to magnetic fields pointing out of and into the surface.
In an HMI magnetogram (such as one from July 15, 2013 on the left) we color the outward pointing fields as white and inward as black. The weak fields are grey.
So it is probably better to think of the solar field as outward (+) and inward (-) rather than North and South. That way you won't use the magnetic field to give directions on the surface of the Sun.
On the Sun it isn’t which way is North, but which way is up!