In the solar wind, the stream of charged particles from the Sun, there are thin layers where magnetic field lines break and reconnect. This is like a rubber band snapping and being glued back together, releasing energy. This process is called magnetic reconnection, and it happens in a very thin layer. But how thin? Usually, it is measured by a single satellite that flies through the layer and records how the magnetic field changes. The problem is that a single satellite cannot tell whether the boundary is moving or just thick.

The four MMS satellites solved this problem. They flew in formation only 16 km apart, less than the length of a small town. When the boundary passed through the formation, each satellite recorded the signal with a slight delay, like four microphones recording the same sound. From these delays, one can accurately calculate the speed and orientation of the boundary. The authors applied this method to four crossings: two were edges of the reconnection exhaust, that is, the region of accelerated plasma, and two were adjacent current layers.

The results showed that for quiet boundaries the standard method works well: the error in speed does not exceed 15%. But on an active layer, where reconnection is happening right now, the boundary moves faster than the plasma by 29%, and then the single-spacecraft method gives a noticeable error. The thickness of the exhaust edges turned out to be about 3000 km, which is four times smaller than the diameter of Earth. It also turned out that the exhaust edges are not parallel but diverge by 22 degrees, like the walls of an expanding pipe. This means that the reconnection exhaust has a non-planar shape. Now scientists have a way to check data from single spacecraft and better understand how much energy is released during reconnection in the solar wind.