  It is well established that elemental abundances vary in the solar atmosphere
and that this variation is organized by first ionization potential (FIP).
Previous studies have shown that in the solar corona low-FIP elements, such as
Fe, Si, Mg, and Ca, are generally enriched relative to high-FIP elements, such
as C, N, O, Ar, and Ne. In this paper we report on measurements of plasma
composition made during impulsive heating events observed at transition region
temperatures with the Extreme Ultraviolet Imaging Spectrometer (EIS) on Hinode.
During these events the intensities of O IV, V, and VI emission lines are
enhanced relative to emission lines from Mg V, VI, and VII and Si VI and VII
and indicate a composition close to that of the photosphere. Long-lived coronal
fan structures, in contrast, show an enrichment of low-FIP elements. We
conjecture that the plasma composition is an important signature of the coronal
heating process, with impulsive heating leading to the evaporation of
unfractionated material from the lower layers of the solar atmosphere and
higher frequency heating leading to long-lived structures and the accumulation
of low-FIP elements in the corona.
