Guide to RHESSI Image Archive

Aspect Solution Plots

The aspect solution plot shows the location of the source (green X), the imaging axis (black), and the spin axis (red +) on the solar disk (solar limb shown in dark blue) during the full flare time interval. Movies showing the aspect solution during each image time bin are available through the 'Movie of Aspect Solution in Image Time Bins' link.

Each black circle represents one rotation of the spacecraft (4 seconds). The diameter of the image axis indicates the amount of wobble in the rotation. The amount of wobble varies for a number of reasons, the two main ones being contraction/expansion with changes in temperature from the day/night cycle and changes in the attenuators (the physical motion of moving the attenuator 'bumps' the spacecraft ???). The spin axis shown is simply the center of each 4s rotation. The spin axis moves frequently because the on-board magnetic torquers are constantly trying to keep the spin axis in approximately the same location on the solar disk (near the center in the southwest quadrant) as well as trying to keep the spin rate at ~15 rpm.

Occasionally, errors in the aspect solution will show up as wild departures from the circular motion of the imaging axis. An example is shown in the figure on the right for 01-Mar-2002 16:24 - 16:27. In those time intervals with a bad aspect solution, an image can't be made. Looking at the aspect solution plot movies reveals when the aspect solution failed. In the case on the right, all image time intervals had a good aspect solution except the last as can be seen in this movie.

Because RHESSI image reconstruction relies on modulation by the grids, it is nearly impossible to image a source that is very near the spin axis when the image axis circles are small. In cases where the source lies on the image axis circle, but the circle is large, there may be enough modulation from parts of the rotation to make an image.???

Time Profile Plots

The time profile plot has 3 panels.

• The top panel shows the profile of the two GOES channels (1.0-8.0 Å in red, 0.5-4.0 Å in blue) in watts / m². The time interval is expanded (flare time +- 1 hour) for context. The gray shading (and the dashed lines angling down to the next panel) indicates the flare time interval and the time range of the two lower panels.
• The middle panel shows the time and energy bins selected for making images. The colors correspond to the energy band colors in the bottom panel, i.e.

  3 - 6 keV - black

  6 - 12 keV - magenta

  12 - 25 keV - green

  25 - 50 keV - cyan

  50 - 100 keV - yellow

  100 - 300 keV - red

  Note: these are the potential bins - if the signal-to-noise ratio is deemed too low to produce a meaningful image an image will not be generated for that time/energy bin.
• The bottom panel shows the count rate time profile (counts / second / detector segment) in six energy bands for the flare. These profiles are generated from the quicklook data in the 'full_rate' observing summary files. They are different from the observing summary plots shown in the RHESSI Browser because they show only the rates from the front detectors, while the Browser plots use the combined front/rear rates.

  The colored bars across the top of the plot indicate the flare (red), night (cyan), SAA (orange), and attenuator state (magenta). The rates are not corrected for changes in attenuator state or decimation level. The attenuator changes are seen as large steps in the count rate as the shutters moved in and out of the detector fields of view. This example includes all three attenuator states used by RHESSI, A0 (no attenuators), A1 (thin attenuator), and A3 (thin and thick attenuator).

Image Panel Display

For each algorithm, all of the images that were generated are displayed in a grid of time (horizontal) and energy (vertical) bins. Some of the cells may be blank if the signal-to-noise (SNR) ratio was too low in that time/energy bin. The SNR thresholds were determined empirically with the goal of producing only scientifically meaningful images (unfortunately some nonsense images are still made). The threshold is 4.0 for MEM_GE, and 2.0 for the other algorithms. There are two panel display plots:

• Scaled - The scaled panel display is shown by default in the web page. Each image in one energy band is scaled to the maximum value of ALL the images in that energy band. At the top of each image, two numbers are displayed: 'SNR' and 'Rel Flux'. The SNR is the signal-to-noise ratio for that time/energy bin. The Rel Flux, relative flux, is the ratio of the maximum of that image to the maximum of the peak image in that energy band. The dashed line around an image indicates which image the others are scaled to.
• Unscaled - In the unscaled panel display, each image is scaled to itself, without regard to the other images in the panel. I.e. for each image, the image's maximum value corresponds to the brightest color in that image. The SNR for each image is displayed at the top of the image.

Here (and similarly in the flare image web page), clicking the plot opens the plot file, and clicking again in the plot enlarges the plot, centered on the location you clicked, with a scroll bar along the bottom if needed.

The example shown here is for the CLEAN algorithm.

Images

For each image reconstruction algorithm, the image frame at the peak time interval for each energy band that could be imaged is displayed and can be enlarged by clicking the image. Below the image is a figure showing the corresponding visiblity comparison plot. Above each image are links for:

• "Image Movie" - movie of the images for each time interval in that energy band.
• "Image Movie with Contours" - movie of the images for each time interval in that band overlaid by self contours at levels dependent on the SNR of the image - SNR > 10: 5,10,30,50,70,90%, SNR > 5: 10,30,50,70,90%, and SNR < 5: 30,50,70,90%.
• "Visibility Comparison Movie" - movie of the visibility comparison plots for each time interval in that energy band.
• "Profile Comparison Movie" - movie of the count rate profile comparison plots for each time interval in that energy band (available only for CLEAN and BACK PROJECTION).

For some flares, the left-most column will contain an image and link for a movie of the low-energy images overlaid by contours at 50,70,90% of the high-energy images. These are available when images were made for more than one energy band and there is a high-energy band at 12-25 keV or above with an SNR value of at least 5.

Each movie frame (an example of the 12-25 keV CLEAN image at peak time is on the left) shows the following:

• The quicklook time profile in six energy bands for the entire flare, with the time interval of the image identified by a box. The color of the energy traces and the box are the same as for the Time Profile Plot described above. The attenuator state changes are shown in the magenta bar along the top.
• The image for the time/energy band in units of photons cm^-2 s^-1 asec^-2 (except for back projection images which are unitless) using IDL color table 5. The dotted lines are the latitude/longitude grid every five degrees. The solid line is the solar limb (not shown in example since this flare is not near the limb).
• The legend in the image shows
  • the time interval and energy band used,
  • the detectors used (F means front),
  • the attenuator state during this time interval,
  • the reconstruction algorithm used,
  • the Signal-to-Noise Ratio (SNR),
  • and the total number of counts in the time/energy bin used to make the image. (You may wonder why the total number of counts is smaller in the visibility-based images (MEM_GE, VIS_CS, and VIS_FWDFIT). That is to be expected because some counts are lost when converting the count rate profiles to visibilities ????? Richard?)

Visibility Comparison Plots

For each image generated (each time/energy/algorithm) we have generated a plot like the one to the right, comparing the observed visibilities to the visibilities predicted by the image. The x axis is the detector number plus the position angle (spatial direction of each grid response referenced to solar north) fraction, so e.g. the points between ticks 3 and 4 show the data for Detector 3 as a function of angle. The red outlines show which detectors were used to make the image. The y axis is the visibility amplitude in photons cm^-2 s^-1.

The legend includes the time/energy/algorithm used and the reduced chi-square values computed from the measured and predicted visibility vectors weighted by the statistical uncertainties for

• the detectors used to make the image,
• all detectors, and
• each detector separately.

• white Xs - observed visibility amplitudes,
• red diamonds - predicted visibility amplitudes,
• blue vertical lines - error bars on observed, and
• green triangles - vector difference between observed and predicted.

The closer the predicted amplitudes are to the observed, quantified by the chi-square value, the more reliable the image. In this example, Detectors 5,6, and 7 are well matched, while ...????? Brian?

Profile Comparison Plots

For each image generated using the CLEAN algorithm, we have generated a plot like the one to the left, comparing the measured (or observed) count rate profiles to the profiles predicted by the image. (These plots are not available for back projection because the image is unitless, or for visibility-based algorithms because the count data is no longer available in the software after making the visibilities.)

The plots show the count rate profiles as a function of regularized roll angle (spacecraft roll angle corrected for the offset between the spin axis and the mean subcollimator optical axis) for each detector used to make the image, with the measured profile in red and the profile predicted by the image in white. The count rates are summed modulo the spacecraft spin period (~4 s) for the time interval used for the image (known as phase stacking).

The agreement between the measured and predicted rates is an indication of how well the reconstructed image matches the data. This agreement is quantified with the Cash or C-statistic, given as a separate value for each detector and as an overall value for all detectors together.

More discussion ????? Brian?