Magnetic double helix*

Magnetic flux ropes, the fundamental building block of magnetohydrodynamic plasma configurations, have often been observed to wrap around each other to form a helical braided structure with net axial current. Braiding has been observed in astrophysical jets, solar coronal loops, and laboratory experiments. The equilibrium of braided flux ropes is more complicated than familiar axisymmetric systems because it requires balancing forces between the individual braids. We present here a method for constructing these equilibria. This method generates a double helix equilibrium with net axial current which is characteristic of observed solar loops and of laboratory-produced braided magnetic flux ropes. To the best of our knowledge, no previous model has been able to describe braided structures with net axial current and instead have only described braided structures with no net axial current; these no-net current structures had equal-magnitude positive and negative axial currents. The net-axial-current equilibrium presented here reproduces the observed braided structure of the double helix nebula and is expected to be a powerful tool in other contexts.