  The origins and generation mechanisms of the slow solar wind are still
unclear. Part of the slow solar wind is populated by "number density
structures", discrete patches of increased number density that are frozen in to
and move with the bulk solar wind. In this paper we aimed to provide the first
in-situ statistical study of number density structures in the inner
heliosphere.
  We reprocessed in-situ ion distribution functions measured by Helios in the
inner heliosphere to provide a new reliable set of proton plasma moments for
the entire mission. From this new data set we looked for number density
structures measured within 0.5 AU of the Sun and studied their properties.
  We identified 140 discrete areas of enhanced number density. The structures
occurred exclusively in the slow solar wind and spanned a wide range of length
scales from 50 Mm to 2000 Mm, which includes smaller scales than have been
previously observed. They were also consistently denser and hotter that the
surrounding plasma, but had lower magnetic field strengths, and therefore
remained in pressure balance.
  Our observations show that these structures are present in the slow solar
wind at a wide range of scales, some of which are too small to be detected by
remote sensing instruments. These structures are rare, accounting for only 1\%
of the slow solar wind measured by Helios, and are not a significant
contribution to the mass flux of the solar wind.
