  Active subspace analysis is a useful computational tool to identify and
exploit the most important linear combinations in the space of a model's input
parameters. These directions depend inherently on a quantity of interest, which
can be represented as a function from input parameters to model outputs. As the
dynamics of many plasma models are driven by potentially uncertain parameter
values, the utilization of active subspaces to perform global sensitivity
analysis represents an important tool to understand how certain physical
phenomena depend upon fluctuations in the values of these parameters. In the
current paper, we construct and implement new computational methods to quantify
the induced uncertainty within the growth rate generated by perturbations in a
collisionless plasma modeled by the one-dimensional Vlasov-Poisson system near
an unstable, spatially-homogeneous equilibrium in the linear regime.
