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Magnetic flux ropes are bundles of twisted magnetic field enveloping acentral axis. They harbor free magnetic energy and can be progenitors ofcoronal mass ejections (CMEs), but identifying flux ropes on the Sun can bechallenging. One of the key coronal observables that has been shown to indicatethe presence of a flux rope is a peculiar bright coronal structure called asigmoid. In this work, we show Hinode EUV Imaging Spectrometer (EIS)observations of sigmoidal active region 10977. We analyze the coronal plasmacomposition in the active region and its evolution as the sigmoid (flux rope)forms and erupts as a CME. Plasma with photospheric composition was observed incoronal loops close to the main polarity inversion line during episodes ofsignificant flux cancellation, suggestive of the injection of photosphericplasma 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 plasmaevolved to an intermediate composition in between photospheric and typicalactive region coronal compositions. Finally, the flux rope containedpredominantly photospheric plasma during and after a failed eruption precedingthe CME. The Hence, plasma composition observations of active region 10977strongly support models of flux rope formation by photospheric fluxcancellation forcing magnetic reconnection first at the photospheric level thenat the coronal level.

Evolution of Plasma Composition in an Eruptive Flux Rope