Sweet–Parker model and the “too-slow” reconnection paradox

19:06>> Before he came along there was something called the Sweet Parker model. This was in the late 1950s. And what they wanted to do was model how these current sheets work. And what ended up happening was if you've got, you know, a magnetic field going in one direction and a magnetic field going in the other direction, you've got this massive magnetic field gradient. >> Yes. >> What's going to end up happening is there's going to be current that goes into that interface >> and then it's going to get shot out because of the continuity equation. Continuity equation being >> stuff that comes in needs to go out, right? And so if the stuff is coming in in this direction, it needs to shoot out in that direction, right? And

19:46the the idea was that this was a kind of way of figuring out how plasma works at all of the scales. >> Okay, it it was something that they they made a bunch of assumptions, but they created an analytic solution of it in the 19 late 1950s. and that reconnection of the magnetic field going in one direction and the other direction the lines are going to crisscross and they could actually calculate a time scale. >> Okay, [clears throat] >> that that would happen. >> Uh yeah, >> so now we've got an observable. You've done a theory but a theory is only good as long as you've got an observable >> cuz then you can go into experimental into experiments create experimental designs that then would show that that

20:27theory is reproducible. >> Exactly. >> Uh you know in an observable context. >> Exactly. Right. And so we've got we've got these observables, right? And these sheets, these Sweet Parker sheets, what they're saying is, you know, at something like the sun, near the sun. >> Yes. >> Where you've got a magnetic field line going in one direction and then it comes back in the other direction. So you do have these crisscrosses. I can see that happening. >> Yes, >> those sheets are happening at a certain Lquist number. And so I can calculate how long it would take for that sheet to stay alive in some sense. Okay. >> Mhm. >> And what they calculate is it should stay alive for a few months.

21:07>> Okay. >> Maybe a few years. >> Okay. >> That is very wrong. [laughter] Okay. Observations show that it doesn't take months or years. It takes hours,