RHESSI Image Archive Strategy

The nominal energy bands used are the RHESSI quicklook bands 3-6, 6-12, 12-25, 25-50, 50-100, 100-300 keV. Determination of which of these bands to use for a flare is described below in the time/energy binning description. We only use the 3-6 keV band if the entire flare was in the A0 attenuator state (no shutters in front of detectors).

The xyoffset (offset of the map center from Sun center) used for a flare was determined by making a Back Projection image centered at the flare position from the RHESSI flare catalog, and using a weighted average in x and y of the pixels within 90% of the peak to get a better FOV center. This xyoffset is fixed for the imagecube even if the flare centroid moves during the flare time period.

Each imagecube consists of images in a rectangular grid of time/energy bins. The time intervals are based on the flux in the highest energy band observed in the flare. Any time/energy bin in which an image could not be made is zero-filled. If no time/energy bins meet the criteria below, an imagecube is not made for the flare.

Time / Energy bins are determined as follows:

• Avoid attenuator changes by .2 s
• Don't image during attenuator 'glimpse' intervals (defined as being in a lower attenuator state for < 80 s)
• Maximum time bin is 120 s
• Minimum time bin is [90, 90, 60, 16, 16, 16] s for the six energy bands (3-6, 6-12, 12-25, 25-50, 50-100, 100-300 keV)
• Minimum total counts in each time bin is [3000, 1600, 1600, 1600, 1600, 1600] for the six energy bands
• The broader minimum time and counts requirement at lower energies ensures the balance of not too many bins in the thermal part of the spectrum but enough resolution in the non-thermal region to capture evolving solar features of interest
• Within each attenuator state time period, start at beginning of period, and go forward in time until reach minimum number of counts. If reach maximum bin size before reaching minimum number of counts, slide start time forward and try again (this results in some gaps)
• Do the above for the highest energy band observed in the flare and use those bins for all lower energies.
• For times where there are no high energy bins that meet the criteria listed above, repeat for the next lower energy band.
• The end result is that all time and energy bins are optimized and aligned to form a regular grid

If the signal-to-noise ratio (SNR) of visibilities in a time/energy bin is < a threshold, we don't even attempt to make an image (the image will be zero-filled). SNR is determined as follows:

• Use visibilities from Detectors 6,7,8. If there aren't at least 2 visibilities (after prep, edit,combine) then try Detectors 5,6,7,8. If there still aren't enough visibilities then don't make image.
• Compute mean of SNR (obsvis amplitude/sigamp) for those visibilities (obsvis is the semicalibrated complex visibility in photons/cm^2/s, the amplitude is the magnitude of the complex number, and sigamp is the sigma in the amplitude). Use resistant_mean routine to reject all values > 3 standard deviations from median, and return mean of remaining values.
• If SNR is < threshold, don't make image.
• Threshold was set to 2.0 for image archive (except for MEM_GE, threshold=4.0).
• The SNR is displayed on the image plots and the image panel displays.