Four witnesses to a single crossing. On November 10, 2017, the four MMS spacecraft flew through an invisible boundary in the solar wind where magnetic field lines reconfigure and release energy. This event is called magnetic reconnection, and it occurs in a thin layer whose thickness is usually estimated from single-spacecraft data. But this time the satellites flew in formation just 16 km apart, less than the length of a small town. Such a tight cluster allowed, for the first time, a direct measurement of the boundary thickness rather than an indirect estimate.

When a single spacecraft crosses the boundary, it records the change in the magnetic field over time. To convert this time into a distance, one needs to know the boundary speed. Usually it is taken from the plasma or solar wind speed, but that is only an estimate. Four spacecraft offer a different approach: they register the same signal with a delay, like four microphones recording the same sound. By measuring the delays between pairs of spacecraft with millisecond precision, both the speed and the orientation of the boundary can be reconstructed. 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 sheets.

The results were encouraging for the standard method. For the passive exhaust edges, all three single-spacecraft speed estimates agreed with the direct measurement within 4–15%. The thickness was recovered to within tens of percent if the normal, that is, the perpendicular to the boundary, was chosen correctly. But at the actively reconnecting layer, the boundary speed exceeded the local plasma speed by 29±10%. This means that in such places the single-spacecraft method can give a noticeable error, because the boundary moves faster than the surrounding material.

Another finding concerned the geometry of the exhaust. The normals of its two edges differed by 22 degrees, with a confidence interval of 19–26 degrees. The exhaust edges are not parallel but diverge, like the walls of an expanding pipe. This suggests that the reconnection exhaust has a non-planar shape, and its thickness varies along the flow. The edge thicknesses were 30.5 and 29.2 ion inertial lengths, which in kilometers equals 3140 and 3010 km. For comparison, the diameter of Earth is about 12,700 km, so each exhaust edge is four times thinner than our planet.

The study for the first time tested the standard thickness estimation method on active reconnection using four spacecraft. Until now, it was unknown how accurate single-spacecraft estimates are, because a direct comparison on the same crossings was lacking. Now it is clear: for passive boundaries the error does not exceed 15% in speed, but at an active layer it can reach 29%. The cross-correlation method can be applied to archival Cluster data and future missions such as HelioSwarm to gather statistics under different solar wind conditions. Four witnesses to a single crossing showed that even a thin boundary can tell a lot if you listen carefully.