In 2007, Nuno Loureiro, then a postdoctoral researcher at PPPL and a participant of the DOE Center for Multiscale Plasma Dynamics at the University of Maryland, developed what might be the final piece of the map. It was a new theory that became known as the plasmoid instability. It connects the Sweet-Parker and two-fluid models into a single theory. Later work at PPPL has expanded on his initial idea.
Like Sweet-Parker, the plasmoid instability model starts with a stretched-out, thin sheet of electrical current with an accompanying magnetic field. Like the two-fluid model, it assumes the electrons and ions that flow along the magnetic field lines break away at different times.
What makes this model different is that it is based on the fact that the sheets of electrical current are extremely unstable. As the sheets stretch, they break and form new ones, each thinner than the original. As these sheets separate, chains of magnetic bubbles (plasmoids) form between them. While previous theories had described these bubbles, no one had provided a good explanation of how and why they form.
The theory proposes that as more bubbles form and sheets break up, the magnetic lines crash into each other and break. The lines disconnect from the ions first, then the electrons. The breaking feeds magnetic energy into the particles, heating them up and accelerating them. As time goes on, the whole process becomes faster and faster. It creates a runaway effect — fast reconnection.
Unlike Sweet-Parker, this model provides the "oomph" to give fast reconnection its speed. Unlike the two-fluid theory, it illustrates why and how the process starts after that initial sheet of strong electric current forms.
