  Plasmas in Earth's outer magnetosphere, magnetosheath, and solar wind are
essentially collisionless. This means particle distributions are not typically
in thermodynamic equilibrium and deviate significantly from Maxwellian
distributions. The deviations of these distributions can be further enhanced by
plasma processes, such as shocks, turbulence, and magnetic reconnection. Such
distributions can be unstable to a wide variety of kinetic plasma
instabilities, which in turn modify the electron distributions. In this paper
the deviations of the observed electron distributions from a bi-Maxwellian
distribution function is calculated and quantified using data from the
Magnetospheric Multiscale (MMS) spacecraft. A statistical study from tens of
millions of electron distributions shows that the primary source of the
observed non-Maxwellianity are electron distributions consisting of distinct
hot and cold components in Earth's low-density magnetosphere. This results in
large non-Maxwellianities in at low densities. However, after performing a
stastical study we find regions where large non-Maxwellianities are observed
for a given density. Highly non-Maxwellian distributions are routinely found
are Earth's bowshock, in Earth's outer magnetosphere, and in the electron
diffusion regions of magnetic reconnection. Enhanced non-Maxwellianities are
observed in the turbulent magnetosheath, but are intermittent and are not
correlated with local processes. The causes of enhanced non-Maxwellianities are
investigated.
