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.