Last Update 29-Jun-2020, Kim Tolbert (kim.tolbert at nasa.gov)
Images and related products were generated for >75,000 RHESSI flares using the procedures described in
RHESSI Image Archive Strategy. Quick access to the flare images is provided through the
RHESSI Flare Image Archive monthly listings. To view the available information for a given flare, first select the year and month
via the pulldown options, and click "Load". Then scan the line-per-flare listing for the flare of interest and select the link to the full web page for
that flare, or to a quick look at various plots. The figure below shows the beginning of the list for July 2002.
Some things to note regarding the list:
The link on the left showing the date/time of the flare leads to a single-flare web page with figures and links
to all products created for that flare. To highlight the larger flares, the peak rate may be listed to the right of the time in
orange (for peak rate > 1,000 c/s) or magenta (for peak rate > 10,000 c/s).
The second column shows the number of time/energy
bins, e.g. 9tx2e means there are images in nine time bins and two energy bins for this flare.
All of the plots linked to on the
rest of the line are also available in the single-flare web page (link on left). Hovering over the blue shaded items will pop up a menu to choose images made with different reconstruction methods.
The 'Panels' link shows the 'scaled' panel display of all images (for each image in one energy band, the
image intensity is scaled to the maximum value of ALL the images in that energy band).
The energy bands (3-6, 6-12, 12-25, 25-50, 50-100, and 100-300 keV)
link to the image movies in that band for the selected algorithm. The energy columns show a '-' when that energy band could not be imaged, so
this display also provides a quick view of which flares extend to higher energies.
Caveat
Some of the images in this archive are invalid. Usually this is obvious from the lack of coherent structure in a single image, or by inconsistency between successive times or energies. There are several reasons for this.
Problems with the aspect solution led to either an incorrect solar flare location for all images for a flare, or, even with the correct map center, isolated invalid images. We expect to have improved aspect software and database available in mid-2020, when images for those flares will be remade.
Multiple flares occurring at the same time but different locations can cause problems with the standardized image reconstruction methods used to make this archive. Customized analysis can usually overcome these problems.
Occasionally, the counts or livetime data contain unexplained glitches that we are unable to correct.
If you encounter problems with images in the archive, please let me know (kim.tolbert at nasa.gov).
Below, we discuss the items on each flare web page, explain how to use the FITS files in the archive, and describe each type of plot included in the image archive. We used the flare of
20-Aug-2002 08:21 - 08:36
(flare number 20820140) as an example.
Each single-flare web page shows some general information about the flare (start, peak, and end times, peak rate, and more from the RHESSI event list) and two plots - the aspect solution plot during the full flare interval and GOES and RHESSI time profile plots showing the time/energy binning selected for this flare (both plots are discussed in detail below). Links to the following items are located to the right of these plots:
Sections below in the flare web page with images and movies for each reconstruction algorithm
RHESSI Browser information for this time, with preselected plot choices
SDO/AIA movies at all available wavelengths for this flare. AIA data are not available before May 2010. Up until October 2013, all flares seen above 12 keV have AIA movies. After that time, all flares seen above 6 keV have AIA movies. The AIA link has a strikethrough if no movies are available.
The directory containing all of the metadata for this flare
Imagecube FITS file for each algorithm
Visibility FITS file used to make these image cubes
Eventlist FITS file used to make these image cubes
Scripts for each algorithm that could be used to regenerate the images in the archive. Two scripts are available for each algorithm - one providing only the parameters that are different from the default values, and one providing all image object parameter settings.
Below this general area, the images, image movies, and other plots generated for each image reconstruction algorithm (Back Projection, Clean, Clean_59, MEM_GE, VIS_CS, and VIS_FWDFIT) are displayed or links are provided.
Using the FITS files from the Image Archive
You can download the imagecube, visibility, and eventlist FITS files for a particular flare by right-clicking the link on the flare web page, or by going directly to the archive (links below). In the commands below, we assume the relevant file name is in the variable 'fitsfilename'.
In SSWIDL, use the command
hsi_fits2map, fitsfilename, map, /sep
This will return an array of map structures ordered by [nenergy, ntime] in the variable 'map'. The map structures can be used with the 'map' object, or plot_map routine.
The FITS file can be used as input to the RHESSI image object in SSWIDL. In the hessi GUI, select input of 'Image FITS file' and browse to your file. At the command line, if obj is a RHESSI image object:
obj->set,im_input_fits=fitsfilename
The image object will be populated with the control and info parameters as well as the images from the file. Start the hessi GUI with that object via
hessi, obj
or send all the images in obj to a plotman instance via
obj->plotman
Read the FITS file with any standard FITS reader. The images are stored in the primary record as an array dimensioned [nx,ny,nenergy,ntime], and the control and info parameters are stored in the subsequent records.
The visibility FITS files can be used in the SSWIDL RHESSI software as input to the image object when you plan to make images using any of the visibility-based algorithms (MEM_GE, MEM_NJIT, VIS_FWDFIT, UV_SMOOTH, VIS_CS, and VIS_WV). Using these prepared visibility data files as a starting point speeds up the image processing compared to using the Level-0 or eventlist files as input. When making images, you are limited to the range of time and energy in the FITS file, but you can specify time and energy bins that are combinations of the bins in the file. In the hessi GUI, select input of 'Visibility FITS file' and browse to your file. At the command line, if obj is a RHESSI image object:
obj->set,vis_input_fits=fitsfilename
Read the FITS file with any standard FITS reader. The control and info parameters are stored in records 1 and 2, and the array of visibility structures is stored in record 3.
The eventlist FITS files are only useful if used within the SSWIDL RHESSI software. They contain unpacked detector event data (time and energy of each detected photon) and can be used as input to the image object instead of the Level-0 files, saving time and the need to download the large Level-0 files. In the hessi GUI, select input of 'Eventlist FITS file' and browse to your file. At the command line, if obj is a RHESSI image object:
obj->set,ev_filename=fitsfilename
Aspect Solution Plots
The direction of the imaging axis is determined on sub-second time scales with sub-arcsecond accuracy from the data provided by the RHESSI Aspect System made up of the Solar Aspect System and the Roll Angle System. The aspect solution plot shows this direction of the imaging axis on the solar disk (solar limb shown in dark blue) as it moves in circles (black) about the spin axis (red +), during the full flare time interval. Also shown is the flare location (green X).
Each black circle in the plot represents one rotation of the spacecraft (4 seconds). The spin axis is plotted simply at the center of each 4s circle. The diameter of the circle changes with time as the image axis precesses about the spin axis. Three main factors control this precession - (1) the contraction/expansion of the spacecraft components with changes in temperature resulting from the day/night cycle, (2) the physical motion of the attenuators moving into and out of the detector fields of view, and (3) the actions of the spacecraft attitude control system as the magnetic torquers keep the spin axis in approximately the same location on the solar disk (near disk center in the southwest quadrant). Once a month or so, the torquers are used to increase the spin rate back to the nominal 15 rpm.
The aspect solution plot on the flare web page covers the entire 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.
Occasionally, errors in the aspect solution 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. Such a bad aspect solution makes it impossible to make an image for those time intervals. 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 for the last one as can be seen in this movie.
RHESSI image reconstruction relies on modulation by the grids, but note that there is no modulation at all for a source located on the spin axis if the image axis is also coincident with the spin axis. Thus, image reconstruction becomes more problematic if the source is close to the spin axis, especially if the diameter of imaging axis circles is small. In those cases, there may not be a full modulation period for the coarser grids and only compact sources can be imaged from the modulation with the finer grids.
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 (RHESSI flare time +- 1 hour) for context. The gray shading (and the dashed lines angling down to the next panel)
indicates 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 below the selected limit, an image will not be
generated for that time/energy bin.
The bottom panel shows the count rate time profile (counts s-1 detector-1) 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 detector front segments, while the Browser plots use the combined front plus rear segment 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 are blank if the signal-to-noise (SNR) ratio was too low in that time/energy bin. The SNR requirement was 4.0 for MEM_GE and 2.0 for the other algorithms. It was determined empirically with the goal of producing only scientifically meaningful images (unfortunately some nonsense images are still made).
There are two panel display plots:
Scaled - The scaled panel display is shown by default in the web page. The color scheme for 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 flux 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, the color scheme of each image is scaled to the maximum flux in that image, without regard to the other images in the panel. 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.
Scaled Panel Display Unscaled Panel Display
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 visibility comparison plot.
Above each image are links for the following:
"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 contours at levels relative to the peak flux dependent on the SNR of the image - SNR > 10: 5,10,30,50,70,90%, SNR = 5 to 10: 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 contains 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 > 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 segment),
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 normal because some visibilities cannot be made because there is incomplete phase coverage at some rotation angles.
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 tick values are the detector number plus the position roll 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 roll 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.
The plot symbols are
white Xs - measured visibility amplitudes,
red diamonds - predicted visibility amplitudes from the reconstructed image,
blue vertical lines - error bars on the measured visibilities, and
green triangles - vector difference between the measured and predicted visibilities.
The closer the predicted amplitudes are to the measured values, quantified by the chi-square value, the more reliable the image. In this example, measured and predicted visibilities for detectors 5, 6, and 7 are well matched but significant deviations exist for detectors 3 and 4 reflecting the limited ability of the clean algorithm to accurately reproduce the finer source structure. The deviations for detectors 8 and 9 are an indication of the systematic uncertainties in the relative sensitivities of the different detectors.
Profile Comparison Plots
Visibility (described above) and profile comparison plots have been generated for all events to provide two convenient ways to quantitatively evaluate how well the reconstructed images would reproduce the measured count rates in each of the detectors.
For each image generated using the CLEAN algorithm, we have generated a plot like the one to the left, comparing the measured count rates to the count rates predicted by the reconstructed 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 imagecube-generating software after making the visibilities.)
The plots show the individual detector count rates plotted as a function of the sum of the regularized roll angle (spacecraft roll angle corrected for the offset between the spin axis and the mean subcollimator optical axis) and the phase of the modulation in each roll-angle bin. The measured rates are in red and the rates predicted from the reconstructed image are in white. The data displayed have been phase-stacked into rotation angle/phase bins for the duration of the image time interval. There are typically between 6 and 64 roll-angle bins and 12 phase bins within each roll-angle bin. The number of roll-angle bins is larger for the finer grids and is equal to the number of peaks present in the plot for each detector. Thus, the X-axis corresponds to the combination of roll-angle bins with 12 phase bins (0-2π) within each roll-angle bin. The scale is marked with continuous angles for convenience.
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.
Each single-flare web page shows some general information about the flare (start, peak, and end times, peak rate, and more from the RHESSI event list) and two plots - the aspect solution plot during the full flare interval and GOES and RHESSI time profile plots showing the time/energy binning selected for this flare (both plots are discussed in detail below). Links to the following items are located to the right of these plots:
