IBEX

One of those things you learn in E&M is that you don’t have static electric fields inside a conductor. That is easy enough to understand when you’re dealing with chunks of copper (though the exercise of figuring out the surface charge densities required for that were sometimes a bit painful). What isn’t quite so obvious is what happens when you move a conductor into a magnetic field: as it goes in, induced currents in the conductor push on the existing field. But you can try the experiment and feel for yourself that it works, so all’s well.

But out there in the vacuum of space, where do you find conductors?

Turns out that vacuum does have some stuff in it: not much, and mostly not ionized, but enough. Enough that on really big (astronomical) scales of space and time, that extremely dilute gas is enough of a conducting plasma to move magnetic fields around.

Some little time back there was a supernova in our neck of the woods, with the usual blast of hot gas following “slowly” behind the xrays and other zippy energy release. That expanding cloud of gas is still partly ionized, and acted like that copper plate you held in the lab demonstration to push the existing magnetic fields away. In fact, the existing galactic fields were sort of stretched around the expanding bubble of gas. The fields at the edge are pretty much tangent to the bubble; at right angles to the direction of the old supernova.

Our sun recently moved into the bubble.

Now if you look really closely at that copper plate you see that the electric fields don’t stop exactly at the edge—they fall off quickly though. When you’re talking about the rarified plasma that makes up the edge of the bubble around the old supernova, that fall-off isn’t small the way we think of small. So if you were floating all by your lonesome here without a star nearby, you’d still see some magnetic field—and it would be roughly at right angles to the direction of the old supernova.

Of course you’re on the Earth, which has a magnetic field of its own far more intense than the local galactic one. And the Earth is inside the heliosphere—the Sun blows its own bubble of charge particles. So we have a bubble inside a bubble in the galaxy, and we’re sitting on a planet with an even more intense field. How can we possibly measure the galactic field?

You can launch a probe or two and wait 40-50 years. That works.

You can also launch a probe to look at energetic neutral particles. Paulo Desiati had worked on IBEX earlier, and he explained the paper to the IceCube journal club Wednesday (he’s on IceCube now). I’ll pass over some of the gorier details, but the general approach is this:

A neutral particle doesn’t really notice magnetic fields much. If it is a dipole the field will twist its orientation, but it won’t change the direction. A charged particle does notice the fields, and its motion is a trifle complicated.

• It doesn’t slow down or speed up
  
• The component of its velocity parallel to the magnetic field doesn’t change. If it happens to be traveling completely parallel to the magnetic field, nothing happens.
  
• The component of its velocity perpendicular to the magnetic field is shifted by a change which is perpendicular to both the magnetic field and the current direction of the particle. If the particle happens to be traveling completely perpendicular to a uniform magnetic field, it goes in circles.
  

In the general case, in a uniform magnetic field, charged particles move in spirals. If the fields are bent a little (as with the Earth’s fields), the spirals follow the bend of the field (not true mathematical spirals, but close enough).

So at the “edge” of the heliosphere, the bubble our Sun blew, charged particles are spiraling around along the field lines of the magnetic fields in the bubble the supernova blew. As they go on their merry ways, every now and then one of the protons will catch up with an electron and form hydrogen. The hydrogen is neutral: the magnetic field doesn’t bother it any more. Whichever direction it was heading at the time, it keeps on going: it breaks out of the spiral like a stone from a sling. If it hits IBEX, we can tell the direction it came from. The chances that any one particle will do this is very small, but the distances are so large that the number of particles available is very large.

And IBEX sees extra neutral particles coming from a band in the sky. That band is therefore like a kind of equator that tells us where the bent galactic magnetic field is most tangent to a sphere around us, and therefore mostly in its original direction. (Modulo distortions from the supernova bubble, of course. The original original field would have been somewhere in the plane defined by the current field and the direction to the old supernova.)

The abstract of their paper says that their result is consistent with anisotropies at higher energies, which is kind of odd when I think about it. They are indirectly measuring quite small fields using low energy neutral particles, but the sky map differences are most noticeable at very high energies, where these fields wouldn’t matter so much.

Very nice work. (Theirs, not my scribble-scrabble artwork)