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)
6/29/2019
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?
6/29/2019
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
6/29/2019
LoHCo, Tucson, AZ

5. ν-variations as evidence for surface effects
ν-variations as evidence for surface effects? (Braun & Birch 2006, ApJ, 647, L187)
2 mHz
3 mHz
4 mHz
5 mHz
6/29/2019
LoHCo, Tucson, AZ

6. the basic travel-time shifts
surface-focus HH applied to MDI obs. of AR 9787 (HELAS website)
mean
mHz
S-N
E-W
out-in
6/29/2019
LoHCo, Tucson, AZ

7. variations with ν and w what’s going on with filter a at 4mHz?
6/29/2019
LoHCo, Tucson, AZ

8. ridge filters
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
6/29/2019
LoHCo, Tucson, AZ

9. ridge vs. phase-speed: what’s the difference?
mean
out-in
(Braun & Birch 2007, Sol Phys.
submitted)
6/29/2019
LoHCo, Tucson, AZ

10. so let’s bisect the ridge filter…
strong differences (including sign-changes) between different wings of ridges!
6/29/2019
LoHCo, Tucson, AZ

11. what about n=2?
note: Thompson & Zharkov (2008; preprint) see positive mean travel-time shifts between p1 and p2 ridges
6/29/2019
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!
6/29/2019
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
support NASA (NNH05CC76, NNG07EI5IC) NSF (AST ) NASA/Stanford/HMI project
6/29/2019
LoHCo, Tucson, AZ

14. phase-speed filters
6/29/2019
LoHCo, Tucson, AZ