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Persistent structures in near-surface flows from ring diagramsPowerPoint Presentation

Persistent structures in near-surface flows from ring diagrams

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### Persistent structures in near-surface flows from ring diagrams

Rachel Howe, University of Birmingham

D. Baker, MSSL

R. KomM, NSO

R. Bogart, Stanford

Introduction diagrams

- 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 diagrams

- 2D rotation profile, based on RLS inversions
- Faster near equator, slower at poles

Analysis diagrams

- 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 velocity diagrams

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.

GONG diagramsMeridional velocity

Not much to see here – quieter at minimum?

MDI/HMI diagramsMagnetograms

(Timescale of GONG)

GONG zonal velocity diagrams

On HMI timescale.

HMI zonal velocity diagrams

Note strong stripes at high latitudes.

HMI diagramsmeridional velocity

Some grain visible here too.

HMI diagramsMagnetograms

Differential rotation diagrams

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 diagramsvx rewrapped with estimated velocity

High-latitude diagramsvx rewrapped with est. velocity -5 nHZ

Rewrapped HMI velocity diagrams

Use inferred rotation rate at each latitude instead of Carrington rate.

‘l=1’ structure at high latitudes

Discussion diagrams

- 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 diagrams

Daily variations

HMI IC sun-as-a-star diagrams

Daily variations

Sun-as-a-Star diagrams

- 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

Coherence with HMI V diagrams

Phase relative to HMI V diagrams

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