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.

• Many of these show variations with solar cycle.

• Here we are looking for persistent structures in local flow features over an extended period.

DIFFERENTIAL ROTATION

• 2D rotation profile, based on RLS inversions

• Faster near equator, slower at poles

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.

GONG ZONAL VELOCITYNote 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.

GONG MERIDIONAL VELOCITYNot much to see here – quieter at minimum?

MDI/HMI MAGNETOGRAMS(Timescale of GONG)

GONG ZONAL VELOCITYOn HMI timescale.

HMI ZONAL VELOCITYNote strong stripes at high latitudes.

HMI MERIDIONAL VELOCITYSome grain visible here too.

HMI MAGNETOGRAMS

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

HIGH-LATITUDE VX REWRAPPED WITH ESTIMATED VELOCITY

HIGH-LATITUDE VX REWRAPPED WITH EST. VELOCITY -5 NHZ

REWRAPPED HMI VELOCITYUse inferred rotation rate at each latitude instead of Carrington rate.

‘l=1’ structure at high latitudes

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.

HMI V SUN-AS-A-STAR Daily variations

HMI IC SUN-AS-A-STARDaily variations

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 ContinuumHMI Line CoreAIA 1600AIA 1700

BiSON VelocityHMI Velocity

COHERENCE WITH HMI V

PHASE RELATIVE TO HMI V