... or, how to have an equation named after you.

It started as a joke borne of laziness. A PhD student had taken over my half-finished project after I graduated. She was fed up of writing "Equation 2.3 in Beveridge and Longcope (2005)" over and over again, and of her boss saying it cluttered up her paper. I suggested we give it a name, like we did the Longcope-Klapper equations (don't ask).

"What, like the Beveridge-Longcope Equation?"

I liked the sound of that. The referee (an expert who checks papers to make sure they're right) didn't question it, so it stuck.

Since then, it's been cited, ooh, handfuls of times - it's really useful if you're trying to do a topological analysis of the Sun's atmosphere, but otherwise not really. And since there are only a few people with that particular research interest, it's not used all that often.

So what is it? I had to look it up, since I no longer have it written on my whiteboard. It says: D = S + X + N_c - 1 , where D is the number of flux domains, S the number of source regions (including infinity), X is the number of separators and N_c the number of coronal null points. Simple, right?

Don't worry about what it all means. Remember in primary school when you did a project investigating vertices, edges and faces? This is pretty much the same thing, only with volumes (the flux domains) thrown into the mix.

What is it used for? Like I said, it's useful if you're analysing the Sun's atmosphere. Topologists always want to know how many different connections (D) there are between magnetic sources (S) such as sunspots, and the number of separator field lines (X) that exist. Separators are particularly important because they lie on the boundary of four distinct regions - this is where the Sun can release colossal amounts of magnetic energy, potentially leading to a solar flare or a coronal mass ejection (the kind of giant eruption that causes the aurora).

The Beveridge-Longcope Equation is just a check to make sure you haven't - or rather, your computer hasn't - miscounted and that everything adds up.