An Introduction to Helioseismology(Local)
2008 Solar Physics Summer SchoolJune 16-20, Sacramento Peak Observatory, Sunspot, NM
Local Helioseismology
•What is Local helioseismology?
–Comparison with Global Helioseismology
–History
–Why Local Helioseismology
•Observations for Local Helioseismology
•The three main methods
–Ring diagram analysis
–Time Distance
–Seismic Holography (seismic imaging)
•The Space Weather connection
Global Modes
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are needed to see this picture.
• A p mode is a standing acoustic wave.
• Each mode can be described by a spherical harmonic.
• Quantum numbers n (radial order), l (degree), and m (azimuthal order) identify the mode.
• -l m l
Global vs LocalGlobal1. Horizontal interference
selects integer values of ℓ 2. Entire sun is sampled3. Spherical harmonics
describe waves4. Cannot get structure as
function of longitude5. Cannot get non-
symmetric latitudinal structure
6. One basic technique7. Valid for ℓ < ~180
Local
1. No horizontal interference, can have any wavelength
2. Localized volume is sampled3. Sinusoids or Hankel
functions describe waves4. Longitudinal structure can
be determined5. Non-symmetric latitudinal
structure can be determined6. Several techniques7. Valid for ℓ > ~180
History
• 1987: Sunspots are observed to absorb p-modes
• 1988: Ring diagram method is invented
• 1990: Acoustic holography is invented
• 1993: Time-distance method is invented
Why local helioseismology ?
• Infer dynamics and structural parameters of localized areas of the Sun
• Difference between North and South hemisphere
• Longitudinal discrimination• Meridional circulation• Structure and dynamics of active regions
and filaments
Observations
• High spatial resolution velocity or intensity images
– Current Instruments:• GONG • MDI on board of SoHO • SOT on board of Hinode
– Future:• HMI on board of SDO • PHI on board of Solar Orbiter
Networks
• Six stations around the world for continual coverage.
• 256x256 pixels 1995-2001
• 1024 pixels since 2001
• Run from NSO Tucson.
Ring-Diagrams analysis
No tracking, solar rotation rate of 2000 m/s
With tracking to remove rotation
Ring-Diagrams analysisSolar Rotation
Confirmation of the solar
rotation shear layer from
local helioseismolog
y
Time distance
• Sound travels from a particular location, down, and comes back up at some time later and some distance away from the source.
• The time and distance is influenced by the conditions of the plasma that the wave travels through.
• More resolution that ring-diagrams.
• Reach deeper layers
Time distance
Observed time-distance
diagram
Cross-correlation function is well described by a Gabor wave packet
EARTH
Seismic holography (seismic imaging)
∫∫ ′′′′′′= ±± PtttzGdtdtzH ),(),,,,(),,( 2 rrrrr ψ
Egression/Ingression
Pupil
Seismic holography (seismic imaging)
Phase of the Correlation
),,(),,(),,( ττ += −+∫ tzHtzHdtzC rrr
Phase-difference due to:
• Reduction of gas pressure in magnetic region --> displacement of density variation
• Sound speed enhancement (evidence from acoustic moats) due to thermal anomalies
• Inclination of the magnetic field
Seismic holographyPrediction capability
Carrington Longitude
sin
(lati
tud
e)
Active Region NOAA-10808
Aug 29 Sep 9 2005 (GONG)
AR10808
Sept 7 2005
Source: MLSO K-coronameter
Calibrating the far-side maps
Carrington Longitude
sin
(lati
tud
e)
Active Region NOAA-10808
Aug 29 -> Sep 9 2005
AR10808
Active Region NOAA-10923
Oct 31 -> Nov 10 2006
AR10923
sin
(lati
tud
e)
• Rings: easy and intuitive; low spatial resolution and shallow depth range
• TD: harder; higher spatial resolution and greater depth range, but inversions….
• Holography: Similar to TD, but needs a good understanding of the Green’s functions.
• All methods have a trade-off between depth range and spatial resolution
Comparison of methods
• Images of activity in the far side of the Sun
• Flare prediction
• Emerging active regions
Local helioseismology & Space Weather
To Do
• Huge list!!!!
http://www.hao.ucar.edu/summerschool/program.html
•Has links to very detailed lecture notes on helioseismology and solar internal dynamics
For more information
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