The vector B is the magnetic flux density and the vector A is the magnetic vector potential.
From the en.wikipedia.org
Gibson and her coworkers used coronameter images to observe dark, lower-density areas, known as cavities, that can be formed by magnetic flux ropes.
From the sciencedaily.com
To conduct her study, Gibson used Mark-IV images to observe dark, lower-density areas, known as cavities, that can be formed by the strong, sheared magnetic fields of magnetic flux ropes.