Three Dimensional Magnetic Reconnection at Null Points and Separators

Abstract

Three-dimensional reconnection is much more diverse than two-dimensional reconnection. In deed, the characteristics of these two types of reconnection are very different. For instance, three-dimensional reconnection can occur both in the vicinity of null points, as well as in the absence of null points, whereas two-dimensional reconnection must occur at null points. Since, in two-dimensions reconnection only occurs at a single point, at most one pair of field lines can be reconnected to form a new pair of field lines at any instant in time. In three dimensions, however, there is a finite diffusion volume in which many field lines can be processed simultaneously. For the entire time that a portion of a field line is in the diffusion volume it will reconnect continuously and continually with all the other field lines it meets. This means that in three-dimensions field lines do not reconnect in pairs of lines making understanding three-dimensional reconnection difficult. In this chapter, we review the characteristics of two-dimensional and three-dimensional reconnection in more detail and also discuss some of the consequences of these characteristics. It is well know that the magnetic fields that thread the Sun's surface cover a wide range of scales from tiny intranetwork features of just 10(16) Mx up to sunspots with 10(23) Mx of flux. There is an extensive mix of features of opposite polarities and scales which carpet the Sun. However, unlike a carpet the pattern is not static, and these features are highly dynamic. This inevitably leads to a complex and highly structured magnetic field in the solar atmosphere. It has been thought that null points rarely occurred in the solar atmosphere's complex magnetic field, however, we show here that null points can readily occur. Furthermore, by studying the magnetic topology of a flux emergence experiment we reveal that reconnection occurring at separators linking two clusters of nulls is the mechanism by which the newly emerged flux interacts with the pre-existing flux in the solar atmosphere. Furthermore, we show that separator reconnection occurs along the length of the separator and not at the nulls at the ends of the separator. Thus, if a separator reaches up into the corona, even if its null points reside below the corona, it can be an important site for reconnection and thus a site of coronal heating.