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

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At line 19 changed one line
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".
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". This is illustrated in the plot below.
At line 21 changed 9 lines
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
[psf.png]
A point source imaged by EIS will yield an elliptical spot on the detector whose axes lie at an angle to the detector's axes. The spot will spread over a number of pixels on the detector and if Gaussian fits are performed at each pixel there will appear to be a blueshift on the north side of the spot and a redshift on the south side of the spot.
[Tian et al. (2010)|http://adsabs.harvard.edu/abs/2010ApJ...709L..88T] studied an EIS raster at the north coronal hole and they noted two features in the velocity maps that are likely due to the tilted point spread function. The Fe XII and Fe XIII velocity maps from their Figure 1 show distinctive ridges of redshift along the limb. For these coronal lines the region just above the limb is significantly more intense than the region just below the limb, and so there is a decreasing intensity gradient from north to south. From the statement above this means there is expected to be a redshift in this region, as observed.
The second feature noted by Tian et al. (2010) is that all the bright points found in the coronal hole have redshifts on one side and blueshifts on the other. Inspection of the images shows that the bright points are blueshifted on the north side and redshifted on the south side. This is again explained by the tilted point spread function.
If a velocity map is created at the south pole, then it is found that there is ridge of blueshift along the limb as shown in the thumbnail velocity map below, obtained from the Oslo Hinode Science Center.
[eis_l0_20070319_113354_6_0_vel.jpg]
This page (revision-16) was last changed on 11-Dec-2017 16:06 by Ignacio Ugarte-UrraTop