Spatial offset of velocity features relative to intensity features#

A striking feature of velocity maps obtained from EIS is that regions of strongest redshift or blueshift are often spatially offset from regions with the highest intensity.

This illustrated in the example below showing a pair of coronal loop footpoints observed in the Fe VIII 185.21 line. The velocity map shows structures that clearly correspond to the intensity structures, suggesting the loop footpoints are redshifted. Three crosses have been placed on the intensity map at where the intensity peaks in the Y-direction. It is seen, however, that the positions of the crosses on the velocity map do not correspond to where the redshift is largest.

Three crosses have been placed on the intensity image where the intensity peaks in Y-direction of the image. The velocity map obtained from the same line it can be seen

This effect is found in the present work for the loop footpoints and is shown in Fig. 7. The slice through the data at X-pixel 57 shows two distinct intensity peaks at Y-pixels 164 and 172, but the velocity peaks occur at Y-pixels 161 and 169, respectively. Although this feature could be explained by plasma rotating around the axes of the loops, a survey of several loop footpoints shows that, no matter whether the footpoints are on east or west side of the active region or whether the active region is in the north or south hemisphere, the south sides of the footpoints are always found to be redshifted. In addition, it is clear from inspection of high resolution TRACE images that loops that are apparently monolithic actually comprise multiple, narrow features thus a large scale twisting flow is difficult to interpret within this type of physical structure.

To state the observed effect simply, wherever there is a decreasing intensity gradient from north to south, the centroid of the emission line will be artifically shifted to longer wavelengths (redshift); and wherever there is a increasing intensity gradient from north to south, the centroid of the emission line will be artifically shifted to shorter wavelengths (blueshift). Observations of polar coronal holes provide another illustration of the effect that is apparent due to limb brightening in coronal lines. Tian et al. (2010) presented velocity maps of the north polar hole obtained with EIS where a distinctive ridge of redshifts is found along the limb in the Fe xii λ195.12 and Fe xiii λ202.04 emission lines. This arises because there is a decreasing intensity gradient from north to south at the limb. Inspection