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TheextendedatmosphereoftheMsupergiantBetelgeuseiscomplex withcoolplasmadominating thestructure by mass and small amounts of embedded hotter chromospheric plasma. A major challenge is to understand the inter- relationship and juxtaposition of these different components, which in turn may provide clues to the nature of the process of nonradiative heating and the mechanisms that drive mass loss. We examine the chromospheric C ii)k2325 multiplet emission line electron density diagnostic using spatially scanned HST STIS echelle spectra. Escape prob- ability modelsfor the electron density-sensitive line ratios reveal that the mean electron density decreases by 0.6 dex as the sight line goes from disk center to  75 mas. Radiative transfer simulations using spherical model atmospheres show that this trend can be explained if the electron density declines with radius by nearly 2 dex acrossR  2R . Theemissionprofilesindicatethatthechromosphericmaterialcorotateswiththestar andthenbecomesdecoupledby  75 mas from disk center. We find no evidence for radial outflow in the chromospheric plasma. We find that the strongest C ii) k2325 emission lines are opacity broadened and that the gradient of atmospheric turbulence is surprisingly small. Using empirical constraints, we derive a relation between the relative C ii column densities in the cool and chromospheric atmospheric components and the excitation temperature. These UV chromospheric results and previous radio analyses suggest that the chromosphere is pervasive but has a small filling factor at  3R , suggestive of confinement and heating in magnetic structures. Subject headinggs: stars: atmospheres — stars: individual ( Ori, BD +75 325, Betelgeuse) — ultraviolet: stars

Electron Density and Turbulence Gradients within the Extended Atmosphere of the M Supergiant Betelgeuse (α Orionis)