A Global Correction to PPMXL Proper Motions
  In this paper we notice that extragalactic sources seem to have non-zero
proper motions in the PPMXL proper motion catalog. We collect a large, all-sky
sample of extragalactic objects and fit their reported PPMXL proper motions to
an ensemble of spherical harmonics in magnitude shells. A magnitude dependent
proper motion correction is thus constructed.
  This correction is applied to a set of fundamental radio sources, quasars,
and is compared to similar corrections to assess its utility. We publish, along
with this paper, code which may be used to correct proper motions in the PPMXL
catalog over the full sky which have 2 Micron All Sky Survey photometry.
