  The Swarm satellites fly at altitudes that at polar latitudes are generally
assumed to onlycontain currents that are aligned with the local magnetic field.
Therefore, disturbances along the main field direction are mainly signatures of
auroral electrojet currents, with a relatively smooth structure due to the
distance from the currents. Here we show that superimposed on this smooth
signal is an irregular pattern of small perturbations, which are anticorrelated
with the plasma density measured by the Langmuir probe. We show that the
perturbations can be remarkably well reproduced by assuming they represent a j
x B force, which balances the plasma pressure gradient implied by the density
variations. The associated diamagnetic current, previously reported to be most
important near the equator, appears to be a ubiquitous phenomenon also at polar
latitudes. A spectral analysis indicates that this effect dominates magnetic
field intensity variations at small-scale sizes of a few tens of kilometers.
