Magnetic reconnection — or magnetic merging — is a fundamental physical process that allows field lines to break and form complex new geometries. To date, however, no comprehensive explanation of how this occurs has been found. The magnetic reconnection experiment (MRX) at Princeton University was built to resolve this problem, but the rate of magnetic reconnection within the experiment greatly exceeded the expectations of classical theory. Writing in Physics of Plasmas, Russell Kulsrud1 and co-workers argue that this discrepancy could be explained by electromagnetic fluctuations observed2 within the core of the MRX.

When plasmas are subject to magnetic fields, the physics governing their behaviour becomes much more complex. When regions of oppositely directed magnetic polarity are forced together within a magnetized plasma, magnetic reconnection can occur, linking previously unconnected field lines (see Fig. 1). During reconnection, the energy held within a magnetic field can be transferred to the plasma, heating it as it flows outwards from the point of reconnection. Although poorly understood, such phenomena are believed to be responsible for accelerating particles within the Sun's corona, and are predominant in the flow of energy within solar flares.

Figure 1: Animation of the process of magnetic reconnection
figure 1

As two oppositely directed field lines (represented by the blue and red lines) are squeezed together by the immense field gradients that can exists in a magnetized plasma, they can merge to cause an abrupt change in the local field structure.

Animation reproduced with permission. Created by Center for Visual Computing in collaboration with Dr Gary Zank from the University of California, Riverside.

An important parameter in controlling the rate of reconnection is the local electrical resistivity of the plasma. Previous work has shown that the resistivity within the reconnection region of MRX greatly exceeds that expected due to Coulomb collisions alone. Kulsrud et al. show that this increase in the local resistivity can be linked to electromagnetic fluctuations observed within the MRX. These fluctuations are caused by plasma instabilities occurring within the reconnection region — the presence of which leads to local heating and hence increased resistivity.

Although the classical picture of magnetic reconnection remains unchanged, these results clearly show that effects due to waves and instabilities must be considered in order to account for the reconnection rates observed. Similar analyses may shed light on the discrepancy between theory and observation in other regions where reconnection plays a significant role — from the study of stellar atmospheres to fusion research.