travel-time maps of sunspots: surface effects and artifacts?

1. travel-time maps of sunspots:surface effects and artifacts? D. Braun & A. Birch NWRA, CoRA Division Boulder CO USA

2. surface effects • unresolved structure which may affect inferences and models of deeper structures • vertical scales below vertical resolution at surface (~ 1 Mm; Couvidat et al. 2006) • surface terms are included in some ring-diagram structural inversions (Basu et al. 2004) • examples include (among other things) • shallow wave-speed structures (Fan, et al 1995) • phase-shifts due to mode conversion (Cally et al. 2003; Crouch et al. 2005) • upper-turning-point changes (e.g. Kosovichev & Duvall 1997; Braun & Lindsey 2000) • photospheric (line-of-sight) effects in penumbrae (Schunker et al. 2005) LoHCo, Tucson, AZ

3. surface effects (continued) • claims of the importance of surface effects in TD/HH models range from negligible to essentially explaining everything • e.g. Zhao & Kosovichev 2005; Lindsey & Braun 2005; Korzennik 2006; Braun & Birch 2006; Couvidat & Rajaguru 2007; others • how important are they? • the changes-in-sign of travel-time shifts (at smaller phase-speeds) in sunspots are difficult to understand in the context of known surface terms (e.g. global or ring analyses) • how confident are we of these sign-changes? LoHCo, Tucson, AZ

4. what is needed (imho) • to understand and include surface magnetic effects in seismic (TD/HH) models of ARs • we do not (and will not) know how important these effects are until they are included in models • measurements of frequency variations of travel-times are critical for this • other measurements may be important as well, e.g. line-of-sight variations, multiple-height observations, etc. • artificial data is essential for validation • to identify and understand possible artifacts in travel-time measurements • “artifact” = measurement which is strongly influenced by method of observation or analysis instead of the desired properties of target LoHCo, Tucson, AZ

5. ν-variations as evidence for surface effects?(Braun & Birch 2006, ApJ, 647, L187) 2 mHz 3 mHz 4 mHz 5 mHz LoHCo, Tucson, AZ

6. the basic travel-time shifts surface-focus HH applied to MDI obs. of AR 9787 (HELAS website) mean 2.5-5.5 mHz S-N E-W out-in LoHCo, Tucson, AZ

7. variations with ν and w what’s going on with filter a at 4mHz? LoHCo, Tucson, AZ

8. ridge filters 2.5-5.5 mHz ridge filtering consistently yields negative mean travel-time shifts and outflow/downflow-like signatures note: Jackiewicz et al. (2008; private comm.) see similar signatures LoHCo, Tucson, AZ

9. ridge vs. phase-speed:what’s the difference? mean out-in (Braun & Birch 2007, Sol Phys. submitted) LoHCo, Tucson, AZ

10. so let’s bisect the ridge filter… strong differences (including sign-changes) between different wings of ridges! LoHCo, Tucson, AZ

11. what about n=2? note: Thompson & Zharkov (2008; preprint) see positive mean travel-time shifts between p1 and p2 ridges LoHCo, Tucson, AZ

12. filter a at 4mHz produces a sign-switch artifact in quiet Sun E-W travel-time differences Note: P. Rajaguru (2007: private comm.) has seen similar effects differential rotation! LoHCo, Tucson, AZ

13. summary • large, systematic frequency-variations of travel-time decreases suggest strong near-surface origin (see Aaron’s talk coming up next…) • + travel-time shifts seem to be artifacts (or at least require explanation): • extremely sensitive to filter properties, e.g. contribution of low-ν wing of ridge • not observed with ridge filters • at least one filter shows spurious sign-changes in quiet Sun • ridge filters offer opportunity to measure and model influences as a function of mode properties (e.g. mode mass) • comparison with ring-diagram and global analyses may facilitate construction of surface-terms in modeling • validation by the use of numerical modeling is needed supportNASA (NNH05CC76, NNG07EI5IC)NSF (AST-0406225)NASA/Stanford/HMI project LoHCo, Tucson, AZ

14. phase-speed filters LoHCo, Tucson, AZ