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GONG10, June 2010

Solar Irradiance, Diameter, Shape, and Activity J.R. Kuhn, Institute for Astronomy, University of Hawaii Rock Bush Marcelo Emilio Isabelle Scholl Phil Scherrer. GONG10, June 2010. What can we learn about the solar cycle from precise “global” measurements?. …since 2002.

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GONG10, June 2010

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  1. Solar Irradiance, Diameter, Shape, and ActivityJ.R. Kuhn, Institute for Astronomy, University of HawaiiRock BushMarcelo EmilioIsabelle SchollPhil Scherrer GONG10, June 2010

  2. What can we learn about thesolar cycle from precise“global” measurements?

  3. …since 2002 • A solar cycle of MDI; HMI debuts • More than a solar cycle of helioseismic measurements • COROT, “night-time solar physics”

  4. Global solar properties Luminosity and irradiance Luminosity, radius, temp Frequency, magnetic field, temperature ‘Even’ m-dependent frequency splittings

  5. Is solar the irradiance change primarily luminosity change?

  6. Frequencies and F10.7 Broomhall et al. 2009

  7. Even coefficient frequency splittings Splitting coefficient temporal variability qualitatively describes surface magnetism changes

  8. Its hard to change the solar surface temperature by changing solar luminosity

  9. The solar limb is largely fixed by rapid opacity decline “few km” thick transition from opaque to transparent

  10. Solar radius, past results from under the atmosphere….

  11. A fluctuating solar radius is seen from the ground • 76 yr fluctuation with 0.2 arcsec half-amplitude • 11 yr fluctuation, smallest sun at peak in sunspot number with 0.1 arcsec half-amplitude 76 yrs

  12. Solar astrometry: Is the Sun shrinking? • 0.05 – 0.2 arcsec/century Gilliland, 1981

  13. Limb astrometry from Space NB: Telescope diffraction limit has very little to do with astrometric accuracy dr Angle of arrival fluctuations define dr dI Photometric gain uncertainty (flatfielding) defines dr In practice limb isn’t knife edge, spacecraft pointing jitter is about 0.01 pixel (and correlated!), long term stability limitations are due to optics thermal drifts [(MDI) 1px=2”]

  14. Limb Astrometry Systematic Errors • Spacecraft pointing jitter (not limiting) • “coherent” • MDI, 0.02 arcsec • Optical errors (limiting) • Temporal stability • Thermal changes, dimensional stability, index changes • Spatial changes • Field focus variations • Two orders of magnitude larger than solar signals (MDI, 0.5arcsec) • “Roll” calibration essential • MDI approach • Measure and calibrate all aspects of instrument • PROVEN: Shape measurements essentially achieved photometric precision (i.e. oblateness/hexadecapole uncertainty 0.5 mas in 12 images)

  15. HMI Solar Limb Astrometry • What Limb Astrometry from HMI? • The solar radius • The solar radius variations with time (and oscillations) • The solar radius variations with central angle (shape, and oscillations) • Why Do This With HMI? • Can’t be done on the ground with HMI accuracy (in some cases by two orders of magnitude) • HMI will surpass MDI astrometric accuracy by at least one order of magnitude • These are difficult measurements, no other space experiment addresses the same technical issues and no other space experiment reproduces the HMI astrometric approach • What are the pressing questions? • Does the solar radius change (at all) with solar cycle? • Knowledge of radius changes and irradiance or luminosity changes constrains the solar cycle mechanisms… a long debated problem • What is the Sun’s shape and is this consistent with solar system limits on its gravitational potential and the internal rotation rate? • Limb Oscillations (p-modes, g-modes, r-modes) dispersion relation information has yet to be carefully measured and interpreted

  16. Satellite limb profiles

  17. MDI Raw Radius Data

  18. Calibrated MDI astrometry systematics Front window: 6C gradient  1.5km focal length  0.84” Primary lens: 10C temperature focal shift  -0.2” OSS expansion: 10C temperature change expansion  0.75”

  19. Instrument changes

  20. The solar radius change…

  21. The solar radius over time km

  22. No solar cycle radius changes! • W = dr/r / dL/L < 2 x 10-2 • Solar cycle luminosity is much smaller than irradiance change • Solar asphericity and 2D atmosphere structure dominates dR and dL • Solar cycle frequency changes not due primarily to changing geometry (s) • Some models can predict small W, c.f. Mullan et al. 2007 (although H- opacity effects on ‘radius’ ignored? )

  23. Asphericity and solar shape • Are solar cycle irradiance variations due to redistribution of emergent solar luminosity? • Latitudinal variation, dR(μ)/R • MDI and HMI solar shape measurements Modern ground-based solar shape measurements

  24. Limb astrometry, MDI 6-50 pixel annulus 480pix MDI: 1.96” pixel HMI: 0.5” pix

  25. HMI raw shape and limb photometry pole equator See GONG10 Bush et al. poster

  26. Rolling HMI separates solar shape from optical distortion Satellite roll angle  cos2θ cos3θ cos4θ cos5θ

  27. MDI and HMI sun during some rolls has no magnetic activity MDI: March 1997 HMI: April 9 2010 HMI: April 16 2010 MDI roll in 2001 available, but active sun HMI roll available every 6 months MDI: Nov. 2009

  28. Oblateness from 1997-2010 MDI and HMI observations without magnetic corrections 1997 MDI 2009 MDI 2010a HMI 2010b HMI

  29. MDI Solar minimum (1997) and maximum (2001) roll data

  30. MDI limb shape analysis, magnetic contamination – e.g. 2001 • Magnetic contamination increases limb brightness, decreases limb radius • Note scale: 40mas radius decrease, 0.01 intensity increase

  31. After accounting for magnetic activity, the limb shape is still variable Active latitutes: If we missed magnetic contributions, oblateness would be even larger!

  32. Solar oblateness isn’t constant MDI and HMI Solar shape data But note: Fivian et al. 2007 from RHESSI claim 2006 oblateness is surface value

  33. RHESSI photometry technique Fivian, Hudson, Lin, 2007

  34. Oblateness coefficient variability from RHESSI

  35. Helioseismic splittings also sample solar shape • These are tiny shape variations, 2001 to 2010 Req-Rpole change is about 2.5km, smaller than our limits on the solar cycle mean radius variation • Helioseismic “oblateness” (the “even” frequency splitting coefficients) are anticorrelated with geometric oblateness • Acoustic (interior) atmosphere non-homologously expanding with respect to “surface” (Kosovichev, Lefebvre 1995, 1996) • Oblateness changes are too small to account for even coefficient variations (and opposite in sign)

  36. The solar brightness, ground, MDI, HMI Ground Oblateness Measurements HMI MDI

  37. Solar cycle acoustic changes • Primarily NOT geometric effects (in mean frequencies or splittings) • The solar atmosphere change with cycle is not well described by any 1-dimensional model (either magnetic or thermal) • Diffuse, unresolved, magnetic flux and surface brightness is needed

  38. “Superficial” vs. “seeing the tachocline” • Tough problem: “everything” is correlated with possibly complex causal connections (cf. Basu et al. 2009 “hints of tachocline” visible in helioseismic time dependence) • Magnetic vs. “thermal”

  39. Deep origins of magnetized plasma must carry excess entropy to surface Magnetized fluid is “hotter” Photosphere Thermal “antishadows” Solar cycle magnetic fields Convection Zone Temperature gradient enhanced stable stratification becomes unstable Tachocline region Radiative Zone Radiative flux through magnetized fluid sees lower opacity and increased entropy relative to non-magnetized fluid Over a solar cycle magnetized fluid over 11yr increases entropy by 0.1% at base of SCZ

  40. Alternatively, vertical surface B fields decrease vertical “irradiance” Continuum contrast vs. vertical orientation and CaK contrast The integrated disk brightness change due to bright faculae is 38% of the faint faculae “Bright” faculae are dark, at any wavelength near disk center Data from the Precision Photometric Solar Telescope NB: cf. Ken Topka facular contrast results

  41. Magnetic fields and irradiance

  42. Fast and slow B vs. irradiance Fast variations: B increases “I” Slow variations: B decreases I

  43. Frequency variations are not determined simply by solar activity (from Broomhall et al. 2009)

  44. Global photometric timeseries analysis • Solar and stellar observations converge  studies of resolved stellar magnetic atmospheres are happening: Night-time solar physics

  45. Spots and faculae may produce only a tiny luminosity pertubation (flux redistribution) T/4 dI time Use solar rotation to describe angular variation in active region or spot “irradiance” … luminosity

  46. Full-disk observations show flux redistribution (data high-pass filtered with 60d moving-mean) Regardless of phase of the solar cycle (min-to-max) the irradiance autocorrelation shows clear evidence that active regions (faculaea nd sunspots) redistribute flux. Low temporal frequency signal shows evidence of additional luminosity signal

  47. CoRoT Photometry – stay tuned

  48. Conclusions • Very precise global solar measurements are important for understanding the solar cycle • Solar cycle helioseismic effects are primarily thermal or magnetic sound speed effects (not geometry) • One-dimensional models don’t convincingly account for cycle variations heterogenous, unresolved (mixed) magnetic field effects are required

  49. Magnetized plasma from RZ is hotter At the top of the radiative zone... Tachocline region l BP6MG, lP3E5cm Tachocline shear layer unresolved helioseismically, lO 0.018R (Schatzman et al. 2000)

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