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)

SBSM6, Kyoto


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

SBSM6, Kyoto


Mdi v i comparison
MDI V/I comparison (2/2)

  • 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 (2/2)

  • 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

SBSM6, Kyoto


Analysis
Analysis (2/2)

  • 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

SBSM6, Kyoto


V i travel time maps 1 4
V/I travel time maps (1/4) (2/2)

  • Quiet, small annulus(0.306-0.714 deg)

N→S

S→N

diff

V

I

SBSM6, Kyoto


V i travel time maps 2 4
V/I travel time maps (2/4) (2/2)

  • Quiet, larger annulus(0.714-1.19 deg)

N→S

S→N

diff

V

I

SBSM6, Kyoto


V i travel time maps 3 4
V/I travel time maps (3/4) (2/2)

  • Quiet, small annulus (Low-pass filter applied for I)

N→S

S→N

diff

V

I

SBSM6, Kyoto


V i travel time maps 4 4
V/I travel time maps (4/4) (2/2)

  • Active region, larger annulus (0.714-1.19 deg)

N→S

S→N

diff

V

I

SBSM6, Kyoto


V i travel times summary
V/I travel times summary (2/2)

  • 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?

SBSM6, Kyoto


Inversions 1 2
Inversions (1/2) (2/2)

  • Flow maps

SBSM6, Kyoto


Inversions 2 2
Inversions (2/2) (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) (2/2)

  • 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) (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 (2/2)

  • 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? (2/2)

  • 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

SBSM6, Kyoto


Summary 1 2
Summary (1/2) (2/2)

  • From MDI data V/I comparison, we found that

    • Time-distance analysis generally agree but there are small-scale differences

SBSM6, Kyoto


Summary 2 2
Summary (2/2) (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