SOT time-distance helioseismology in and around active regions

1. SOT time-distance helioseismology in and around active regions Takashi Sekii1 Junwei Zhao2 & Alexander Kosovichev2 1 NAOJ 2 Stanford University

2. 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

3. 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

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

5. High resolution (3/4) • Sekii et al 2001: MDI(left) versus La Palma SVST G-band (right, Berger et al 1998) SBSM6, Kyoto

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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 SBSM6, Kyoto

13. 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 SBSM6, Kyoto

14. V/I travel time maps (1/4) • Quiet, small annulus（0.306-0.714 deg） N→S S→N diff V I SBSM6, Kyoto

15. V/I travel time maps (2/4) • Quiet, larger annulus（0.714-1.19 deg） N→S S→N diff V I SBSM6, Kyoto

16. V/I travel time maps (3/4) • Quiet, small annulus (Low-pass filter applied for I) N→S S→N diff V I SBSM6, Kyoto

17. V/I travel time maps (4/4) • Active region, larger annulus （0.714-1.19 deg） N→S S→N diff V I SBSM6, Kyoto

18. 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? SBSM6, Kyoto

19. Inversions (1/2) • Flow maps SBSM6, Kyoto

20. 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

21. 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

22. 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

23. 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

24. 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 SBSM6, Kyoto

25. Summary (1/2) • From MDI data V/I comparison, we found that • Time-distance analysis generally agree but there are small-scale differences SBSM6, Kyoto

26. 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