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Future Prospects for Helioseismology

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Future Prospects for Helioseismology Future Prospects for Helioseismology Michael J. Thompson High Altitude Observatory National Center for Atmospheric Research Boulder, Colorado, USA Abstract Helioseismology has been spectacularly successful at revealing many aspects of the Sun s internal structure and dynamics. "hat then are the new challenges and opportunities for the $eld? &n this talk & #ill briefly review the current state of helioseismology, and #ill then discuss science opportunities and future directions for observations from the ground and space. Solar oscillations: sound waves in the Sun Resonant cavities are of Refraction of #aves provides lower di*ering e+tent boundary of resonant cavity Resonant modes the sun l=-,m,. l=-,m,/ l=-,m,- 1+amples of spherical harmonics Spherical harmonic degree l, order m Schematic of a resonant mode of the sun l=/,m,0 Helioseismology Measure mode fre3uencies and other properties 1igenfunctions 4 spherical harmonics 5re3uencies νnlm(t7 depend on conditions in solar interior determining #ave propagation &nversion provides maps such as of sound speed, density and rotation Spherical harmonics Ring-Analysis Helioseismology · Observe line-of9sight velocity · 8ata used in this talk are 68opplergrams7! from SOHO/MD& · Make plane-wave decomposition of patches 6tiles) of the solar disk y t + :omography · 2ake surface observations at end-points of ray paths of waves propagating through the solar interior · Cross9correlating the observational signal at the two end-points reveals the wave travel time between the two points · &nversions of those travel times then reveal the subsurface structure and dynamics 6and magnetic field) Temperature Sound-speed inferences Convection zone Build-up of helium* Gravitational settling 2 ∝ Γ µ Opacity c 1T / Helium ionization zone Base of convection zone (Sound speed is in units of hundreds of km/s) ;&ndirect inference from small fre3uency separations Solar Rotation and the Tachocline Solar rotation Solar tachocline Near-surface Tachocline shear layer Tachocline lies substantially<. beneath Tachocline the base of the C= at lo# latitudes) covers stable convective overshoot layer Temporal variations in the rotation rate Limitations of helioseismology from a single vantage point · Global helioseismology has been remarkably successful at revealing the Sun’s internal radial structure and its internal rotation as a function of radius and latitudeA but global helioseismology can give no information about longitudinal variation of the solar interior! · Local helioseismic techni3ues such as time9distance helioseismology give longitudinal information, but are restricted to probing the upper third of the convection zone when the Sun is vie#ed from a single vantage point. Challenges & pportunities · &mproving on resolution 4 noise of inversions 6in particular global9mode inversions) · "e are dominated by systematic errors, not random errors C inadequate modeling of mode physics, mode leakage, etc. · S. Je*eries and S. Eorontsov proposed in the early 0...s an approach to simultaneously $t the modes and the internal structure dynamics all at once C but this has not been pursued to completion Challenges & pportunities · 8etermine flow/structure under active regions · 8ifferent local helioseismic techniques give contradictory resultsF Moradi et al. !"#" · One problem is that we are not modeling mode conversion caused by presence of magnetic $eld Challenges & pportunities · :hings that are “just” out of reach · CharacteriBation of structure/(ow at base of convection Bone · >imits of p9modes in the core Challenges & pportunities · Convective velocities in convection Bone C disagreement bet#een models and seismic inferences · 2aybe our models have the background structure of the convection zone #rong? Can we determine that better? Hanasoge et al. !"#! Challenges & pportunities · >atitudinal4longitudinal structures in or near the tachocline · Such structures are predicted 4 invo'ed by some models of the solar dynamo · 2ay also be seat of active longitudes 6IhotspotsI7 Importance of imaging the tachocline and its longitudinal variations · Understanding the solar dynamo is one of the outstanding challenges of solar physics. :he tachocline shear layer at the bottom of the Sun convective envelope is believed to be the seat of the solar dynamo. Direct detection of the magnetic $eld in tachocline is e+tremely challengingA but associated thermal anomalies and bul' (ows are accessible to helioseismology, #ith the right observational datasets! · Calculations of magnetoshear instabilities of the tachocline region, by ?ilman, Cally, Dikpati and collaborators, predict longitudinal variations as signatures of the magnetic $eld present there! · Active longitudes and hotspots of persistent surface magnetic activity of many rotations or even over more than one solar cycle are also likely associated #ith hotspots in the tachocline region #hich #ill only be revealed once #ith longitudinal resolution of this region. Challenges & pportunities · 2eridional circulation · Jey ingredient of flux-transport dynamo models, but challenging to determine beneath the outer K.!.L C .!< Rsun %hao et al. !"#& Future 'servational Platforms · Next-generation ground-based net#ork6s7 · Multi-height measurements in the solar atmosphere Solar Mhysics Research &ntegrated Net#ork ?roup 6SMR&N?7 Successor to GON? Future 'servational Platforms · Space mission6s7 to make helioseismic observations away from the Sun91arth line · Farside, Polar, or >N4>L Mrevious inner-heliosphere missions that have explored away from the Sun-Earth line Science Path Forward Beating do#n systematic errors in global-mode inversions for structure and evolutionO better spatial resolution, smaller error bars, larger domain of reliable inversions · &mproved analysis of global mode observations 6Defferies4Eorontsov7 · Simultaneous observations made at 1arth and a#ay from the 1arth9Sun line (better coverage of #hole Sun7 8etermining structure4(o#s/magnetic field in Active Regions · Multi9height observations (SMRING7 · Better modeling of oscillations and mode conversion in strongly magnetiBed regions (ARs7 · &terative approaches to inversions in ARs Science Path Forward 8etecting longitudinal structures in and near the tachocline · Observations Gin 3uadratureI 6e!g. from >L point7 and from far-side Improved determination of high latitudes and latitudinal structures in and belo# the convection zone · Observations from over the poles Better determination of structure and rotation of core · g mode detection 6maybe recent brea'through) Role of an L5 mission · Simultaneous helioseismic observations from the Earth and from the L5 point enable both ends of long, deep wave raypaths to be seen. Such rays penetrate to the tachocline! By measuring wave travel times along such raypaths, the thermal anomalies and flows in the tachocline will be resolved in longitude for the $rst time. :he resolution will be about .!< solar radii, resolving azimuthal variations up to about wavenumber m,/.! Science Path Forward 8etecting longitudinal structures in and near the tachocline · Observations Gin 3uadratureI 6e!g. from >L point7 and from far-side Improved determination of high latitudes and latitudinal structures in and belo# the convection zone · Observations from over the poles Better determination of structure and rotation of core · g mode detection 6maybe recent brea'through) Su'mitted to Frontiers in Stellar & Solar Physics +a section of Frontiers in ,stronomy & Space Science-.
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