GONG Magnetograms -Raw to Synoptic Map

J. Harvey

GONG Mags 101

• GONG has 6 sites around the world, 85% duty• Started in 1995, ~10” pixels, 1 image/min• Full-disk, filter-based system using 6768 Å• Built for good velocity data, not magnetograms• 1995-2002 -- LOS mags done one per hour at

each site, staggered by 20 min• 2002-2009 – 2.5” pixels, better S/N and 1/min

mags (3 G noise level)• 2006 – new modulators installed for STEREO

GONG Classic Synoptic Map(CRs 1896-1942)

Modulator upgrade

• Large, varying zero point error

• Traced to asymmetric switching of LCD between ±¼ wave retardation, and temperature variations of retardation

• This was fixed by driver waveform shaping and temperature stabilization of LCDs

• An expensive scheme for correcting pre-fix data has been developed

Current GONG Recipe (CR2047+)

1. Average 10 minutes of FD mags• Reject bad images in making the average• Multiply by sec θ

2. Map to Carrington coordinates (sin B, CMD)• 1 resolution: 180 x 180• 2 maps: sum and weight• Near limb pixels are quality weighted• Flux conserved

Recipe – con’t.

3. Weighted merge of images– Cos4 CMD– Build sum and weight merged images– Divide merged sum by weight

4. Pole correction identical to SOLIS

5. Publish– Once per hour, and– End of Carrington rotation

Sample CM map

Sample ‘Janus’ map

http://gong.nso.edu/data/magmap

Sample extrapolation

Synoptic Map Status

• Being produced hourly in near real time

• Classic maps and “Janus” versions

• Potential field extrapolations hourly

• Available on GONG web site

• Noise level << 1 G

• Zero point better than 0.1 G

Improvements

• Make snapshot maps

• Merge data from various sites rather than cut and paste

• Better calibration taking into account modulator efficiency

• Higher resolution maps could be made if useful

Problems

• Solar and instrument noise near limb• Does not represent the full solar surface at

any specific time• Dubious polar correction (SOLIS recipe)• Strong umbral field signals can wrap• Leaves a small monopole• Radial correction is bad in AR• One GONG site currently has high scatter