Using EIS, we wish to measure the variation of line widths at coronal temperatures. Recent CDS/SoHO studies indicate that line widths decrease at high altitudes off-limb in coronal lines, suggesting the damping of upwardly propagating waves and the heating of the corona. EIS will allow us, not only to measure the decrease in line widths, but to accurately measure the related nonthermal velocities, thus allowing us the ability to estimate energy flux densities of the propagating and dissipating waves. The reported decrease in line width above 1150 arcsec coincides with the location where the observed line ratio of Mg X 624/609 indicates a change from a collisionally to a radiatively dominant excitation regime. This has implications for far off-limb electron density measurements, as all density diagnostics calculations uses theoretical line ratios which only take collisional excitation into consideration. The present results suggest that line ratio calculations of electron densities, using solely collision excitation calculations, may be incorrect in regions far off-limb where radiative excitation becomes stronger than collisional excitation (~1150 arcsec). Further investigation into this is required. Data at both poles off-limb and at the equatorial off-limb is required. |
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