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Spatial offset of intensity and velocity features
Spatial offset of intensity and velocity features

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This is illustrated in the example below showing a pair of coronal loop footpoints observed in the Fe VIII 185.21 line.
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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.
[fe8_footpoints.png]
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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.
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While this may seem to be an interesting physics result, evidence from velocity maps of a range of different solar structures suggests that this intensity-velocity spatial offset is actually an instrumental effect. To state the effect simply:
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''"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)."''
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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
A similar phenomenon was observed in SOHO/CDS data and was explained by [Haugan (1999)|http://adsabs.harvard.edu/abs/1999SoPh..185..275H] as being due to an "elliptical, tilted point spread function".
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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.
This page (revision-16) was last changed on 11-Dec-2017 16:06 by Ignacio Ugarte-UrraTop