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Persistent structures in near-surface flows from ring diagrams

Persistent structures in near-surface flows from ring diagrams. Rachel Howe, University of Birmingham D. Baker, MSSL R. KomM , NSO R. Bogart, Stanford. Introduction. The Sun has flows at many scales, from global rotation to flows around active regions and down to granules.

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Persistent structures in near-surface flows from ring diagrams

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  1. Persistent structures in near-surface flows from ring diagrams Rachel Howe, University of Birmingham D. Baker, MSSL R. KomM, NSO R. Bogart, Stanford

  2. Introduction • The Sun has flows at many scales, from global rotation to flows around active regions and down to granules. • Many of these show variations with solar cycle. • Here we are looking for persistent structures in local flow features over an extended period.

  3. Differential rotation • 2D rotation profile, based on RLS inversions • Faster near equator, slower at poles

  4. Analysis • Use synoptic maps of zonal/meridional flows from ring diagrams (GONG and HMI) • Average over all depths • At each latitude, we subtract mean over all times/longitudes to remove biases and first-order temporal variations • Then plot residuals as longitude—time maps • Differential rotation causes features to drift in longitude from one rotation to the next. • Features (e.g., flows into active regions/remnants) persisting for more than one rotation will show up as diagonal streaks.

  5. GONG zonal velocity Note diagonal ‘grain’ Paired light-dark streaks suggest flows in or out of feature that drifts with differential rotation. Slope up == faster than Carrington rate Slope down == slower than C.R. Shallower slope means more difference.

  6. GONG Meridional velocity Not much to see here – quieter at minimum?

  7. MDI/HMI Magnetograms (Timescale of GONG)

  8. GONG zonal velocity On HMI timescale.

  9. HMI zonal velocity Note strong stripes at high latitudes.

  10. HMI meridional velocity Some grain visible here too.

  11. HMI Magnetograms

  12. Differential rotation Use cross-correlation analysis to translate stripes slope to rotation rate Green – Magnetograms Black – Global inversions Red – HMI ring vx correlation Blue – GONG ring vx correlation

  13. High-latitude vx rewrapped with estimated velocity

  14. High-latitude vx rewrapped with est. velocity -5 nHZ

  15. Rewrapped HMI velocity Use inferred rotation rate at each latitude instead of Carrington rate. ‘l=1’ structure at high latitudes

  16. Discussion • Near-surface flows –especially zonal – show signatures of features persisting from a few to many rotations, migrating as would be expected from differential rotation. • Low latitudes traceable to active region/plage? • Rotation rate from magnetic features not quite the same as from flows – anchoring depth? • High latitudes – Hathaway giant cells? Looks like an l=1 structure, anticorrelated n/s. • Correlation analysis not good enough to pick up changes in differential rotation.

  17. HMI V sun-as-a-star Daily variations

  18. HMI IC sun-as-a-star Daily variations

  19. Sun-as-a-Star • Use keywords (DATAMEAN) from JSOC database to form time series. • Line Core= Continuum-Line Depth • Compare HMI Velocity with BiSON. • All data interpolated to 45s cadence HMI Continuum HMI Line Core AIA 1600 AIA 1700 BiSON Velocity HMI Velocity

  20. Coherence with HMI V

  21. Phase relative to HMI V

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