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This page was created on 16-Jun-2010 22:23 by PeterYoung

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

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[{ALLOW edit EISMainUsers}]
[{ALLOW view Anonymous}]
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The effect as shown in this loop feature was presented and discussed in [Young et al. (2012)|http://adsabs.harvard.edu/abs/2012ApJ...744...14Y].
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[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.
[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 can be explained by the tilted PSF model. 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.
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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 (PSF).
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 can again be explained by a tilted PSF.
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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. This is again consistent with the tilted PSF.
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. This is consistent with a tilted PSF.
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A key point to note is that the tilted PSF only affects features where there is a significant intensity gradient, thus the main result of Tian et al. (2010) - the measurement of blueshifts in a large area of largely uniform intensity coronal hole - is not affected.
A key point to note is that a tilted PSF only affects features with significant intensity gradients, thus the main result of Tian et al. (2010) - the measurement of blueshifts in a large area of largely uniform intensity coronal hole - is not affected.
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There is currently no software or prescription available to correct velocity maps for the tilted PSF. The following is a general observation:
There is currently no software or prescription available to correct velocity maps for a tilted PSF. The following is a general observation:
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In general users should be cautious about showing velocity maps in publications, as a reader's eye tends to be drawn to the features that are most redshifted or most blueshifted. These features could be artificially enhanced by the tilted PSF and give a misleading impression of the velocity field of the structure.
In general users should be cautious about showing velocity maps in publications, as a reader's eye tends to be drawn to the features that are most redshifted or most blueshifted. These features could be artificially enhanced by a tilted PSF and give a misleading impression of the velocity field of the structure.
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The EIS team is investigating the PSF function and whether a software correction can be performed. Users are encouraged to look for the effect in their data and report their findings to their EIS team.
The EIS team is investigating the PSF function and whether this is responsible for the observed effect. Users are encouraged to look for the effect in their data and report their findings to their EIS team.
__Update__: There is now a [Software Note 8|ftp://sohoftp.nascom.nasa.gov/solarsoft/hinode/eis/doc/eis_notes/08_COMA/eis_swnote_08.pdf] that describes this effect. Also, [Warren et al. (2017)|http://adsabs.harvard.edu/abs/2012ApJ...744...14Y] show in their Appendix that the velocity pattern observed in a flare current sheet, a bright elongated structure with strong gradients perpendicular to its axis, is consistent with the effect of an inclined PSF.
This page (revision-16) was last changed on 11-Dec-2017 16:06 by Ignacio Ugarte-UrraTop