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
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).
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.
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.
Zonal Flow Patterns (Time-Radius)
MDI OLA
MDI RLS
GONG RLS
0 15 30 45 60
Howe et al 2005
Comparison of near-surface flow observations
MDI LOCAL
MDI GLOBALDOPPLER
Howe et al, 2006
Slow start to Cycle 24
Howe et al. 2009
Delayed ending of Cycle 23
High-latitude Residuals
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.)
Variations at the Tachocline
See Howe et al. (2000; Science 287, 2456)
Tachocline oscillations
Basu & Antia (2001; MNRAS 324, 498)
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.)
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.
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)
Chaplin et al. 2007
Frequency shifts with solar cycle
l=50, n=10 variationSlight systematic shift between MDI and GONG
Frequency shift sensitivity to B
Howe et al. 2002
Even splitting coefficients follow magnetic activity distribution in latitude
(Antia et al 2001, Howe et al 2002)
Localized Global Frequency Shifts
(Howe, Komm & Hill 2002)
High-degree Frequency Shifts
Mode frequencies are higher in active regions
(Hindman et al, 2000).
Also, amplitude decreases and linewidth increases.
Ring-diagram frequency sensitivity (HMI)
Method-dependent
Also depends on position on disk (or detector)
Time-Distance – Results
Sound speed and flow patterns below a sunspot
Structure inversions under active regions – ring diagrams
Bogart et al. 2008, Sol. Phys.
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?
Different short-term sensitivity in different years?
From Tripathy et al., 2007 Solar Phys. 243, 105
Fluctuations in Frequency Residuals
Broomhall et al. 2011
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
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.
Search for Structural Change Eff-Darwich
et al 2002 – upper limit of 3e-5 on stratification change at base of convection zone
Subsurface Structure Changes Chou &
Serebryanskiy 2005, Serbryanskiy & Chou 2005 – possible wave speed change near bottom of convection zone.
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.
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.