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Helioseismology: Results

Helioseismology: Results. Rachel Howe. Synopsis. Rotation profile Rotation changes over the solar cycle The torsional oscillation Tachocline fluctuations Frequency and parameter changes Global frequency shifts Local frequency shifts Looking for interior changes. Solar Rotation Profile.

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Helioseismology: Results

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  1. Helioseismology: Results Rachel Howe

  2. Synopsis • Rotation profile • Rotation changes over the solar cycle • The torsional oscillation • Tachocline fluctuations • Frequency and parameter changes • Global frequency shifts • Local frequency shifts • Looking for interior changes

  3. Solar Rotation Profile

  4. Convection-Zone Dynamics • So-called ‘torsional oscillation’ is a pattern of weak slower and faster zonal flows migrating from mid-latitudes to the equator and poles over the solar cycle. • First observed by Howard and Labonte (1980) in surface observations • Surface Doppler measurements from Mt Wilson go back to 1986. (Ulrich 2001).

  5. Helioseismic Detection of the Torsional Oscillation • Seen in 4 years of GONG and 3 years of MDI data by Toomre et al. (2000), Howe, Komm & Hill (2000), Howe et al. (2000) • Penetration depth at least 0.92R.

  6. Torsional Oscillation • Vorontsov et al (2003) showed that the phenomenon involves much of convection zone, and analyzed the signal in terms of 11-year sinusoidal variations.

  7. Zonal Flow Patterns (Time-Radius) 15 30 0 45 60 MDI OLA MDI RLS GONG RLS Howe et al 2005

  8. Comparison of near-surface flow observations MDI GLOBAL DOPPLER MDI LOCAL Howe et al, 2006

  9. Slow start to Cycle 24 Howe et al. 2009

  10. Delayed ending of Cycle 23

  11. High-latitude Residuals

  12. Zonal Flows -- Summary • Torsional oscillation pattern involves most of convection zone • Related to timing of solar cycle • Strength may not be related to cycle strength • High-latitude flows show this cycle is not like the last one (or maybe the one before that.)

  13. Variations at the Tachocline See Howe et al. (2000; Science 287, 2456)

  14. Basu & Antia (2001; MNRAS 324, 498) Tachocline oscillations

  15. Frequency Shifts • Rule 1: Everything varies with everything else • Rule 2: It’s always more complicated • Well established that p-mode frequency increases with solar activity • Response to activity increases with activity (until it starts decreasing again.)

  16. Sound-Speed Inversion • Differences from model are tiny. • Anomaly at base of convection zone – heavy element settling? • But things look worse with more recent opacity values.

  17. Chaplin et al. 2007 Frequency shifts with solar cycle • ACRIM (Woodard & Noyes 1985, 1988, Gelly, Fossat & Grec 1988) • BiSON, Mark I (Palle et al. 1989, Elsworth et al. 1990)

  18. Frequency shifts with solar cycle

  19. l=50, n=10 variation Slight systematic shift between MDI and GONG

  20. Frequency shift sensitivity to B Howe et al. 2002

  21. Even splitting coefficients follow magnetic activity distribution in latitude (Antia et al 2001, Howe et al 2002)

  22. Localized Global Frequency Shifts (Howe, Komm & Hill 2002)

  23. High-degree Frequency Shifts • Mode frequencies are higher in active regions • (Hindman et al, 2000). • Also, amplitude decreases and linewidth increases.

  24. Ring-diagram frequency sensitivity (HMI) • Method-dependent • Also depends on position on disk (or detector)

  25. Time-Distance – Results • Sound speed and flow patterns below a sunspot

  26. Structure inversions under active regions – ring diagrams • Bogart et al. 2008, Sol. Phys.

  27. Frequency shifts correlate with surface flux in time and space, at a wide range of scales • BUT remember Rule 2: It’s always more complicated • Is there interesting information in the deviations from the trends?

  28. Different short-term sensitivity in different years? From Tripathy et al., 2007 Solar Phys. 243, 105

  29. Fluctuations in Frequency Residuals Broomhall et al. 2011

  30. Search for Subsurface Structure • Frequency shifts are strongly correlated with surface activity, but such changes are mostly shallow. • Finding subsurface changes in structure (sound speed/density) requires careful removal of surface effects

  31. Subsurface Structure Changes? • Basu & Mandel (2004), Verner, Chaplin & Elsworth (2006) – evidence for solar-cycle change in amplitude of He ionization zone signature (0.98R) from GONG, MDI, BiSON data.

  32. Search for Structural Change • Eff-Darwich et al 2002 – upper limit of 3e-5 on stratification change at base of convection zone

  33. Subsurface Structure Changes • Chou & Serebryanskiy 2005, Serbryanskiy & Chou 2005 – possible wave speed change near bottom of convection zone.

  34. Sound speed changes at the base of the convection zone • Baldner & Basu, 2008 • l ≤ 176, 2 ≤ n ≤16 • Principal Component Analysis • Interior changes at latitudes below 45 deg.

  35. Conclusions • Helioseismology reveals changes in dynamics deep in the convection zone. • Improved knowledge of convection-zone dynamics may help predict future cycles. • Solar activity at the surface influences mode parameters. • Detection of interior structural change is still difficult.

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