1 GONG Magnetogram pipeline

2 The GONG Data Processing Pipeline

3 Field Tape Reader (FTR) Receive the (approximate) weekly DLT cartridge mailed to the DMAC from each of the observing stations and make a copy. Extract from each image on the cartridge copy a subset of header parameters for the header database. Extract from each image on the cartridge copy the complete FITS header for the instrument header database. Extract the daily sequence of calibration images from the cartridge copy. Make the header parameter plots from the subset of header parameters. Ship the original cartridge to the offsite storage facility.

4 FTR Parameter Plots

5 Process Calibration Images (PROCCAL) Process each daily sequence of calibration images, identify and reject unacceptable sequences. Temporally average (60 days) the acceptable white-light modulation (wlm) and narrow-band correction (nbc) images Replicate dark and flat images to fill gaps so that there is a set of calibration correction images for each site day. Manually inspect the temporally-averaged calibration correction images and the parked solar images calibrated with the temporally-averaged correction images.

6 Diffuser for flat field Solar image Dark CurrentPinhole for image rotator center Eastern solar quadrant Western solar quadrant - = Rotation Raw Calibration images

7 Flat Field Dark Current White Light Modulation – corrects for spurious modulation Narrow Band Correction – fixes up field effects Processed Calibration Images

8 Calibrate Raw Data (VMBICAL) Apply the camera corrections Crop the images Apply the optical calibration Convert to velocity, intensity, modulation, magnetic field Compute the image centroid to locate the center Compute the solar limb parameters Compute image statistics Check image statistics, discard unacceptable images Add ephemeris data to image headers Apply intensity flat field image to all images Recompute the solar limb parameters Compute the Hankel geometry and the modulation transfer function (MTF)

9 Angular Registration (COPIPE) Instruments not exactly aligned north-south. Perform cross-correlation between Doppler images to get relative orientation at overlapping sites. Perform network-wide optimization to find best set of relative angles on daily basis. Use various measurements (all-day and noon drift scans, MDI images, planetary transits, Ronchi rulings) to set absolute angle.

10 Magnetogram pipeline Calibrated magnetograms Polarity check Zero point correction Image merge + restoration? Remapping Synoptic map generation These two steps can be reversed

11 The Image Merge GONG's image merge is quite simple: 1.All of the site images for a given date are staged to disk. 2.Each image then has the OFFSET keyword updated using our latest, best estimate for the direction to Solar North. This keyword is both site and time dependent. 3.For magnetograms, the polarity (+1 or -1) for each site is found from our polarity table and it is applied to the site images as they are processed. 4.One can (optionally) apply a correction for the estimated MTF (Modulation Transfer Function), i.e., perform a deconvolution using a circularly symmetric point-spread function. 5.The site images are registered to a fixed size and shape. Choices are: A.Sky coordinates (i.e., circular) with Solar North at the top and Solar East on the left or B.Heliographic coordinates: either latitude-longitude or sin(latitude)- longitude. 6.The final merged image is computed by simply summing the contributing registered images for that minute then and dividing by the number of images in the sum.

12 Sample merged magnetograms no restoration (left) and restored (right)

13 GONG+ Merged Magnetograms Non-filled calibrated magnetograms from all six GONG sites are used to create a merged magneotgram per 10 minute time period Processing performed on calibrated magnetograms: –Corrected to flux assuming a radial field and apodized to remove the solar limb –EMPHEMINTERP and CAMOFFSET are run on each magnetogram to update the current ephemeris values and add the camera offset angle in the image headers –Re-mapped into heliographic coordinates in a 900 x 900 pixel grid using HGREMAP –Shifted by the Carrington longitude of the disk center, L 0, into a 1800 x 900 pixel grid –A 49-point Gaussian filter is applied –10-minuted segments of individual magnetograms are summed and normalized by the number of images contributing to each pixel of the merged magnetogram –The final product is a FITS image that consists of a 1800 x 900 pixel grid with 16-bit integer pixels containing magnetic field strength measured in units of gauss Currently polar regions are handled as any other surface region

14 Sample Merged Magnetogram For additional information on processing of GONG+ magnetograms feel free to browse the provided online documentation

15 GONG+ Synoptic Maps Non-filled calibrated magnetograms from all six GONG sites are used to create a synoptic map per Carrington rotation Processing performed on calibrated magnetograms: –Corrected to flux assuming a radial field and apodized to remove the solar limb –EMPHEMINTERP and CAMOFFSET are run on each magnetogram to update the current ephemeris values and add the camera offset angle in the image headers –Re-mapped into heliographic coordinates in a 180 x 180 pixel grid using HGREMAP –Shifted by the Carrington longitude of the disk center, L 0, into a 360 x 180 pixel grid –A cos 4 (L- L 0 ) filter is applied –Individual magnetograms are summed and normalized by the number of images contributing to each pixel of the synoptic map –The final product is a FITS image that consists of a 360 x 180 pixel grid with 16- bit integer pixels containing magnetic field strength measured in units of gauss Currently polar regions are handled as any other surface region

16 Sample GONG+ Synoptic Map For additional information on processing of GONG+ magnetograms feel free to browse the provided online documentation