One of the most mysterious phenomena in all of space physics is magnetic reconnection. This happens when two magnetic fields come into contact and their field lines link up. One example is at the outer edge of the sphere of Earth’s magnetic influence, called the magnetosphere. Here Earth’s field lines reconnect with the Sun’s magnetic field.
Viewed schematically in its simplest form, reconnection takes place within a very wide rectangle; estimates put it at 10 times wider than it is tall. Two field lines belonging to opposing magnetic fields drift in—nearly parallel to the long sides of the rectangle but bulging slightly toward each other. When they finally meet at the middle, making an X, they reform as vertical field lines, and the energy that’s released when they snap together is like a rubber band rebounding—the newly linked field lines shoot out the sides of the rectangle.
However, the details of the theory remain fuzzy, and comparing predictions of the process to real-life measurements is important. According to theory, the rate at which these lines reconnect depends on the strength of the magnetic field and the density of the plasma flowing into the box from both sides. For Earth and the Sun, those two sides are Earth’s own magnetosphere and the surrounding region just outside of it known as the magnetosheath, the latter of which is mostly influenced by the Sun and the solar wind that rushes by.
Wang et al. tested this theory by comparing it to data from the European Space Agency’s Cluster satellites, a constellation of four satellites that fly in formation in and out of Earth’s magnetosphere. They found that the theory worked well when taking into account the conditions on both sides of the boundary, as intended. However, it gave even more accurate results when they ignored the conditions inside the magnetosphere and only focused on the conditions in the surrounding magnetosheath. This suggests the theory could be tweaked to take into account variations when the merging field lines aren’t quite symmetric or have uneven amounts of kinetic energy. (Journal of Geophysical Research: Space Physics, doi:10.1002/2015JA021524, 2015)
—Mark Zastrow, Freelance Writer
Citation: Zastrow, M. (2016), Satellites test theory of magnetic reconnection, Eos, 97, doi:10.1029/2016EO044821. Published on 2 February 2016.