sot time distance helioseismology in and around active regions
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SOT time-distance helioseismology in and around active regions. Takashi Sekii 1 Junwei Zhao 2 & Alexander Kosovichev 2 1 NAOJ 2 Stanford University. Solar-B and local helioseismology. SOT provides Dopplergrams FOV narrow but high spatial resolution Not suited for probing deep layers

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sot time distance helioseismology in and around active regions

SOT time-distance helioseismology in and around active regions

Takashi Sekii1

Junwei Zhao2 & Alexander Kosovichev2

1 NAOJ

2 Stanford University

solar b and local helioseismology
Solar-B and local helioseismology
  • SOT provides Dopplergrams
    • FOV narrow but high spatial resolution
      • Not suited for probing deep layers
      • (Horizontally) high-resolution view of the solar interior

SBSM6, Kyoto

high resolution 1 4
High resolution (1/4)
  • There are two implications
    • Observation of small-scale wave field that has never been observed consistently
    • High-resolution observation of medium-scale wavefield that would contribute to better inversion
  • Is there any wavefield power at such a small scale?

SBSM6, Kyoto

high resolution 2 4
High resolution (2/4)
  • MDI high-resolution power spectrum
    • No resonant p modes above l≈2000
    • The f-mode frequency ∝ sqrt(l)

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high resolution 3 4
High resolution (3/4)
  • Sekii et al 2001: MDI(left) versus La Palma SVST G-band (right, Berger et al 1998)

SBSM6, Kyoto

high resolution 4 4
High resolution (4/4)
  • La Palma result shows improvement in (mainly p-mode) time-distance S/N in <10,000 km range, down to <1,000km
  • Local helioseismology with high-degree f modes is an obvious thing to do, but p-mode seismology will also benefits from SOT high-resolution
    • Thermal & dynamical structure of ARs most important target

SBSM6, Kyoto

some other possibly interesting things to do 1 2
Some other possibly interesting things to do (1/2)
  • A high cadence observation of (~20 sec) for chromospheric waves
  • Switching between dopplergram and magnetogram for a more-or-less simultaneous observation
  • Study acoustic source property at very high degree

SBSM6, Kyoto

some other possibly interesting things to do 2 2
Some other possibly interesting things to do (2/2)
  • A bi-level observation by using both photospheric and chromospheric lines
    • Note that the Chromospheric Mg line is magnetic
  • Deliberately using a magnetic line even for photospheric Dopplergrams?

SBSM6, Kyoto

time distance helioseismology in and around active regions 1 2
Time-distance helioseismology in and around active regions (1/2)
  • What are the issues?
    • MDI & HMI use magnetic lines
    • Algorithm for deriving V from filtergrams is optimized for quiet regions
    • In active regions, magnetic field has measurement effect as well as (real) physical effect on travel times

SBSM6, Kyoto

time distance helioseismology in and around active regions 2 2
Time-distance helioseismology in and around active regions (2/2)
  • SOT can use non-magnetic line in photosphere
    • Can decouple measurement effect and physical effect on V of magnetic field
    • Is a non-magnetic line really safe?
      • Absorption (or reduced excitation) effect removed by re-normalizing…is this correct?
    • …but not in lower chromosphere

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mdi v i comparison
MDI V/I comparison
  • It is a standard practice to use MDI Doppler velocity data (V). What if we use intensity data (I)?
    • Inoisier than V (cf. S/N correlation)
    • How does I compare to V in active region?
  • V/I comparison using MDI coeval sets

SBSM6, Kyoto

mdi coeval datasets
MDI coeval Datasets
  • 512-min coeval V&I (tracked)
    • High-resolution mode
      • 17.4 deg×17.4deg(heliographic)
      • 512×512 pixels rebinned to 256×256
    • QR:17 May 1997
    • AR:19 June 1998
      • NOAA AR8243

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analysis
Analysis
  • Start from V/I wavefield time series (data cubes)
  • Apply phasespeed filter and average signals in annuli/segments around each bin
  • Compute cross-covariance functions of the averaged signals
  • Measure travel times by wavelet fitting for EW,NS,OI
  • Invert travel times for 3d flow, using ray-approximated sensitivity kernels

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v i travel time maps 1 4
V/I travel time maps (1/4)
  • Quiet, small annulus(0.306-0.714 deg)

N→S

S→N

diff

V

I

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v i travel time maps 2 4
V/I travel time maps (2/4)
  • Quiet, larger annulus(0.714-1.19 deg)

N→S

S→N

diff

V

I

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v i travel time maps 3 4
V/I travel time maps (3/4)
  • Quiet, small annulus (Low-pass filter applied for I)

N→S

S→N

diff

V

I

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v i travel time maps 4 4
V/I travel time maps (4/4)
  • Active region, larger annulus (0.714-1.19 deg)

N→S

S→N

diff

V

I

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v i travel times summary
V/I travel times summary
  • Some difference between travel time from intensity (τI) and that from velocity (τV) in small spatial scales
    • Noise level: τI noisier than τV
    • Systematically τI> τV
    • The difference increases in active region
  • Apply low-pass filter before inversion?

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inversions 1 2
Inversions (1/2)
  • Flow maps

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inversions 2 2
Inversions (2/2)
  • In spite of the V/I difference, large-scale structures are captured by both
    • converging flow & down flow in upper layers
    • diverging flow in deeper layers
  • V/I difference is somewhat cancelled because only differentials e.g.(N→S)-τ(S→N) are used. Not so for soundspeed anomaly

SBSM6, Kyoto

what causes the v i differences 1 2
What causes the V/I differences ?(1/2)
  • Because of different noise statistics, high-frequency components are stronger in V, weaker in I(shows up e.g. in p2/p1 amplitude ratio)
    • For the same phasespeed filter, then, I-phasespeed must be smaller (yes it is)
    • And group velocity larger (yes it is too)
  • Phasespeed filter can be tuned to reduce the difference

SBSM6, Kyoto

what causes the v i differences 2 2
What causes the V/I differences ?(2/2)
  • But this is the difference that should be cancelled out for flow inversion
  • k-ω powers &

t-d powers

SBSM6, Kyoto

implications on sot time distance
Implications on SOT time-distance
  • Will be interesting to do t-d analysis not just with photospheric non-magnetic line but also with
    • a photospheric magnetic line
    • white light
  • For a better understanding of
    • behaviour of V travel time in ARs
      • for SOT( chromospheric line) as well as for HMI

SBSM6, Kyoto

how do we do t d analysis in ars
How do we do t-d analysis in ARs?
  • Calibration of V measurement
  • Use I instead?
  • Masking out AR signal
  • Double-skip (Zhao & Kosovichev 2005 for soundspeed) time-distance
  • All these can be tested with SOT

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summary 1 2
Summary (1/2)
  • From MDI data V/I comparison, we found that
    • Time-distance analysis generally agree but there are small-scale differences

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summary 2 2
Summary (2/2)
  • For time-distance analysis around ARs
    • It is important to understand how Doppler travel-time measurement is affected by magnetic field
    • At the same time we are searching for alternative means to avoid complications
    • SOT provides an ideal set of tools for these tasks as well

SBSM6, Kyoto

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