1. Three-Dimensional Power Spectra of GONG++ High- Cadence Magnetograms F. Hill, J. Bolding, R. Clark, K. Donaldson-Hanna, J. Harvey, G. Petrie, C. Toner & T. Wentzel

2. GONG Magnetograms Obtain one per minute, 24-7, average duty cycle of 0.87 Noise level: 3G per pixel per minute Recent installation of new modulators reduced zero-point background to 0.1 G Hourly synoptic products Here look at 1-min merged magnetograms

3. Merging Obtain as many as four simultaneous images for a given minute – need to merge Tested “histogram equating” method (H. Jones) –Take two magnetograms –Form cumulative histogram of field on disk –Find linear transformation to produce nearly identical histograms –Apply to images Found that this does not work when considering a time series of images with variable seeing Currently using a very simple scheme: –Register and circularize simultaneous images –Simply average

4. The movie Sequence of 3200 images/minutes starting on July 1, 2006 at 0 UT Fill factor of 0.95 (148 missing images) Missing images are not interpolated across, simply skipped 32 ° square patch tracked and remapped using GONG ring pipeline Patch tracked symmetrically across central meridian Tracking at photospheric rate, not magnetic rate, so some drift

5. One frame Contains quiet area, sunspot, and network/decaying active region

6. The movie Highly time compressed: 53 hours in about 10 sec (20,000:1 compression) Things to look for: –Evershed flow from spot –Flickering field in spot (probably seeing and line blending in strong fields)

7. 3-D Power Spectra Analyze field in three 16 ° areas –Spot region (lower right) –Network/decaying region (lower left) –Quiet region (upper left) Compute 3-D FFT, then power

8. Slices at constant frequency of 31.25 μHz (Period of 8.9 hr) Power offset due to drift from tracking rate. Once corrected, should be able to measure outflow velocities.

9. Slices at constant frequency of 3333.33 μHz (5-min band) Spot not circular5-min oscillations detectable in quiet region – but anisotropic!?

10. Closer look at anisotropic oscillations 2917 µHz3125 µHz3333 µHz 3542 µHz 3750 µHz Use entire 32° area for analysis – mixes different field characteristics, increases k resolution.

11. Causes? Instrumental –Velocity undoubtedly leaks into B –Polarization measurements taken at different times? –Spatial offset of polarization measurements? Processing –No problems in spectrum computation –Merging? –Interpolation? Solar –Magnetic field selects a propagation direction? –??

12. Conclusions GONG continual high-cadence merged magnetograms available shortly. Good for rapid changes (e.g. flares, flows, oscillations). 3-D power spectra useful for magnetic field studies. Strange anisotropic behavior of oscillations?

13. Postcsript While at Melbourne, R. Bogart managed to process some MDI 1-min magnetograms He found similar results – incomplete rings in a quiet area Need to get larger sample in many different situations of activity to understand