  Magnetic flux ropes are bundles of twisted magnetic field enveloping a
central axis. They harbor free magnetic energy and can be progenitors of
coronal mass ejections (CMEs), but identifying flux ropes on the Sun can be
challenging. One of the key coronal observables that has been shown to indicate
the presence of a flux rope is a peculiar bright coronal structure called a
sigmoid. In this work, we show Hinode EUV Imaging Spectrometer (EIS)
observations of sigmoidal active region 10977. We analyze the coronal plasma
composition in the active region and its evolution as the sigmoid (flux rope)
forms and erupts as a CME. Plasma with photospheric composition was observed in
coronal loops close to the main polarity inversion line during episodes of
significant flux cancellation, suggestive of the injection of photospheric
plasma into these loops driven by photospheric flux cancellation. Concurrently,
the increasingly sheared core field contained plasma with coronal composition.
As flux cancellation decreased and the sigmoid/flux rope formed, the plasma
evolved to an intermediate composition in between photospheric and typical
active region coronal compositions. Finally, the flux rope contained
predominantly photospheric plasma during and after a failed eruption preceding
the CME. The Hence, plasma composition observations of active region 10977
strongly support models of flux rope formation by photospheric flux
cancellation forcing magnetic reconnection first at the photospheric level then
at the coronal level.
