Inconsistent initialization of the magnetic dipole

[mfournier01]

The magnetic dipole is incorrectly defined in cluster.cpp. This line:

athenapk/src/pgen/cluster.cpp

Line 598 in bffa62a

const Real m_cross_r_z = mx * y - mx * y;

should rather be:

const Real m_cross_r_z = mx * y - my * x;

[BenWibking]

Cluster dipole vector potential has zero z component

Summary

The cluster dipole initialization computes m_cross_r_z as mx * y - mx * y, which is
identically zero.

Why this is a bug

The vector potential for a dipole uses m x r. Its z component is mx*y - my*x.
The current expression removes the z component for every dipole orientation, producing
the wrong magnetic field whenever init_dipole_b_field is enabled.

Patch

diff --git a/src/pgen/cluster.cpp b/src/pgen/cluster.cpp
--- a/src/pgen/cluster.cpp
+++ b/src/pgen/cluster.cpp
@@
-              const Real m_cross_r_z = mx * y - mx * y;
+              const Real m_cross_r_z = mx * y - my * x;