1. Scientific rationale for vector polarimetry aboard SDO Or “Why do we need to determine photospheric vector fields?” Hector Socas-Navarro

2. Some caveats = HVMI vs HVPI = Observed quantity is polarization, not magnetic field or flux = The magnetic field is inferred by indirect means (inversion, magnetograph formulae, etc.) •Difficult •Time consuming •Uncertain

3. Quiet sun magnetic flux Socas-Navarro & Sanchez Almeida (2002) Sanchez Almeida & Lites (2000) ' '

4. Comparison with spectropolarimetric instruments (ASP, SolarB, etc) = Advantages: • Full disk coverage (no spatial scanning) • High temporal cadence • Uninterrupted synoptic observations (no nights, clouds, etc.) = Drawbacks: • Very limited spectral information (magnetometry accuracy? Graham et al 2002) • Sensitivity (?)

5. Vector polarimetry with HVMI = More accurate velocities in the presence of magnetic fields (sunspot umbrae) • Remove crosstalk of Q, U & V into I • Sunspot seismology. Combine information in I, Q, U & V for inversion = More accurate flux determinations • Total flux (as opposed to longitudinal) • Mixed polarities and flux cancellation (Socas-Navarro & Sanchez Almeida 2002) = True full disk magnetometry • Limb fields (polar fields) '

6. Contamination by molecular lines NSO Observations

7. Science with HVMI = Connection between magnetic fields and solar irradiance: • Plages and network are associated with enhanced irradiance (Vrsnak, Ruzdjak & Placko 1991; Foukal, Harvey & Hill 1991) • Combine magnetic and irradiance measurements. Physical origin of the surface structures responsible for irradiance variability. (Harvey & White 1999; Fligge, Solanki & Unruh 2000)  Combine network flux measurements with H  = Provide magnetic indices with uninterrupted coverage and high spatial and temporal resolution • Vector polarimetry implies more accurate indices

8. Science with HVMI = Magnetic helicity in active regions (Pevtsov et al 2001) = Hemispheric chirality rule (Zirker et al 1997) = Global dynamo models: • Testing the axisymmetric approximation (Durney 1995; 1996; 1997; Dikpati & Charbonneau 1999) • Migration of magnetic flux. Polar fields (Wang, Nash & Sheeley 1989a; 1989b; Dikpati & Charbonneau 1999) • Solar dipole moment (Ulrich et al 2002) • Twist and writhe in sunspots and active regions. Number of turns predicted by dynamo models • Temporal variations in sunspot twist (toroidal oscillations?)

9. Science with HVMI = Magnetic structure of sunspots and active regions (Lites et al 1995; Westendorp Plaza et al 2001a; 2001b) • Large field of view • High temporal cadence = Magnetic shear. Energy accumulations and impulsive release (Schmieder et al 1994; Wang 1997) • What configurations lead to flares? (synoptic observations) • Which energy build-up mechanism is more important? – Interior stress of the field (Piddington 1974; Leka et al 1996) – Surface distortions induced by plasma flows (Tanaka & Nakagawa 1973; Parker 1979; Rust et al 1994)

10. Science with HVMI = Role of the magnetic field in coronal heating and solar wind • Moore et al 1999; Falconer et al 2000 = Field extrapolation from the photosphere to the corona • Chiu & Hilton 1977; Sakurai 1989; McClymont, Jiao & Mikic 1997

11. Conclusions = Lower accuracy and sensitivity than full spectro- polarimeters = Improvement over longitudinal magnetograms: • Magnetic flux (accuracy & coverage) • Velocities (removing x-talk, magnetic elements) = Suitable for synoptic studies involving: • Irradiance variations • Solar dynamo • Coronal heating • Eruptive events • Active region large-scale structure & evolution