Weekly Report 6May2011
From RHESSI Wiki
RHESSI Albedo Image Test
RHESSI Clean Imaging techniques were used to image a large number of flares. For each flare three different detector combinations and two different energy bands were used. The following parameters were used: The flares used were decided upon by the RHESSI response in the 25keV-50keV bands.
- Time: Decided by the peak of the 25keV-50keV energy band for each flare
- Energy: 12keV to 25keV, or 25keV to 50keV
- Detectors: 1 to 9, or 1 to 5,or 6 to 9
- Pixel Size: 2 arcsec by 2 arcsec
- Image Size: 128 pixels by 128 pixels
- Clean Beam Width Factor = 2
Clean images were made with each for each energy band with each detector combination. For each image the centroid of the source was measured, the total flux in the image was measured, and the standard deviation in the x and y direction for each source was measured. Using these measurements the plots below were made.
The plots were adjusted for the change in solar radius as observed from the earth at the time of the flare. All plots as a function of distance were made with an x axis as a ratio of the distance to limb at the time. An average value of 960 arcsec was added to the ratios for scaling purposes.
The plots below also contain models for the shift in distance due to the height of the extended source in the plots showing the difference in location between the extended source and the compact source. The plots showing the ratio of the ratio of the standard deviation in y over the standard deviation in x for the compact source contain the model for the foreshortening due to the distance away from the center of the sun.
For the plots showing the difference in location the model used was :
diff in location = height * sin(theta), where the height was the distance from the photosphere of the extended source and theta was the angle from the center of the sun. In the cases below a nominal value of 2 arcsec was used for the height of source in the model.
The model for the foreshortening used was:'
i: the standard deviation in y is unchanged with distance from the center ii: the standard deviation in x changes as a factor of 1/(cos (theta)) where theta is the angle from the sun center.'
The plots showing the difference in location do not have many points lying along the model curve. The 12keV - 25keV plots show a few more points than the 25keV - 50keV curve. This may be because the extended source in coronal and not due to the albedo. It may also be that the nominal height of 2 arcsec used in the model is incorrect. New plots will be made using other values of the height.
There are not many data points lying along the curve for the model showing the foreshortening in the standard deviation of x. For the 12keV - 25keV plot there is a 'cup' shape in the data around disk center that looks to follow the same trend as the model but the bottom is flatter. For the 25keV - 50keV data the trend is less apparent. In both cases the model assumes that the source is circular at the center so the ratio is 1, however if this is not the case and the standard deviation in x is larger than the standard in y at the center the model would be flatter and possibility follow the general shape of the data for the compact source, however I'm not sure there is a good physical reason to make this assumption.
Another issue is with the 25keV - 50keV data. The threshold requirements for the minimum number of counts was changed because using the original guidelines more than half of the flares used in the 12keV - 25keV band were excluded. For the points that are outliers the images need to be examined more closely to make sure they are correct.
Goals
- For the foreshortening model, change to model slightly to account for different ratios of sigma y to sigma x at the center to see if it fits the data especially in the 12keV - 25keV band.
- Check difference in position data versus the model for different heights
- Make images from simulated data at different location on the disk and see if it follows the foreshortening and difference in position due to height models.
