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Persistent structures in near-surface flows from ring diagrams
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Transcript of 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.
• 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