OVERESTIMATING THE CORONAL DENSITY AND ITS EFFECT ON THE VELOCITIES DERIVED FROM L-ALPHA DOPPLER DIMMING

It is shown that relatively small errors of the order of 10% in the electron densities, for example derived from polarization brightness measurements, might lead to large errors in the velocities inferred from intensity measurements of the Lalpha H I lambda 1216 spectral line in the inner corona. It is demonstrated that in some cases this effect can result in very high velocities close to the coronal base with subsequent small acceleration with heliocentric distance. It is also pointed out that the errors in the deduced velocities can be reduced if simultaneous observations that place constraints on the mass flux are available, and by ensuring that the mass flux computed from the derived velocities and densities is constant.