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Science Opportunities with a 1.5 m Space Solar Telescope

Bruce Lites

High Altitude ObservatoryEarth and Sun Systems Laboratory

National Center for Atmospheric ResearchBoulder, CO

10 March 2010ISAS

Personal Outlook: Where Are the Frontiers for Solar Physics in the Coming Decade?

•Solar dynamo: internal structure, rotation, models……

•Fundamentals of solar-terrestrial effects: CMEs, flare physics, global solar variability…..

•Energy and mass transport from the photosphere to the solar wind: chromospheric/coronal heating, momentum transfer,…..

Underlying much of this is the need to explore the physics of the solar chromosphere – The NEW FRONTIER for solar physics

Why Is the Chromosphere the New Frontier?•Chromosphere is conduit of kinetic and magnetic energy input from the photosphere to the corona

•Chromosphere remains relatively unexplored, while photospheric and coronal processes have seen a rapid expansion in our understanding

• Some MHD and plasma/kinetic processes unique to chromosphere/transition region (i.e. non-equilibrium ionization, lateral transport across field lines)

•Vast range in density, plasma β, ….

•NLFFF extrapolations of chromospheric fields have more validity:

“When applied to the chromospheric boundary data, the codes are able to recover the presence of the flux bundle and the field’s free energy, though some details of the field connectivity are lost. When the codes are applied to the forced photospheric boundary data, the reference model field is not well recovered, indicating that the combination of Lorentz forces and small spatial scale structure at the photosphere severely impact the extrapolation of the field.” (Metcalf, et al. 2008, Solar Phys. 247, p.269)

Non-Linear Force-Free Field Extrapolations

(From DeRosa et al. 2008)

•This goal requires complete coverage of a large region with high precision and good angular resolution!

To address these challenges, new tools are becoming available:

•A new generation of ground-based solar observing facilities

•Rapid development of numerical models and theory

The over-riding question surrounding Solar-C Option B is:

“What would be the optimal use of resources to address these major challenges of solar physics?”

Perspective:

NOT “What science from a 1.5 m space solar telescope?”,

RATHER “What UNIQUE science from a 1.5 m space solar telescope?”

What science themes drive new large solar telescopes?

•“Tiny Things”: fundamental solar processes at small spatial scales

•Magnetic fields: precision polarimetry

New Large Ground-based Solar Telescopes

•NJIT/BBSO New Solar Telescope: 1.6 m off-axis, 2010

•KIS GREGOR: 1.6 m Gregorian, 2010

•NSO Advanced Technology Solar Telescope, 4 m off-axis, 2017

What Photospheric Science Can We Expect From New Ground-Based Facilities?

•Photosphere: small-scale structure/dynamics and magnetism

•Sunspots: umbral, penumbral structure•Plage fields: flux tubes/sheets•Flux emergence•Flux interaction•Integranular fields: explore the as-yet unresolved fields in intergranular lanes

Umbral Fine Structure Not Revealed Clearly by Hinode

(Observations with Swedish 1 m Solar Telescope)

Pores and Dark Structures:

•Most pores and small darkenings show the “hot wall” effect on their limbward edge

•Feature “D” appears to have a swirled penumbral outer boundary

(to limb)

Hinode Spectro-Polarimeter 2007 Dec. 11

Continuum Intensity

Hinode Spectro-Polarimeter 2007 Dec. 11Magnetic Flux (-2000 to +500 Mx cm-2 )

Fully Compressible 3-D Simulations of Magneto-Convection

Schüssler & Vögler 2008, A&A 481, L5

630nm Continuum Vertical Field, τ = 10-2 Horiz. Field, τ = 10-2

Log (BHoriz) Log (BHoriz)

Personal View: Hinode observations and other recent ground-based observations, combined with simulations, have defined the essential physics of small-scale magnetism in the photosphere.

Solar-C Option B should not make the photospheric magnetism a primary goal.

What Chromospheric Science Can We Expect From New Ground-Based Facilities?

•Chromosphere:•Spicules (types I, II)•Reconnection jets•Filaments/prominence fine structure•Penumbral jets•…………

Hα line center imaging from the Swedish Solar Telescope, Courtesy of G. Scharmer, M. Carlsson

[ From De Pontieu, McIntosh, Hansteen, & Schrijver, ApJ 701, L1 (2009) ]

Spicule Dynamics

What Ground-Based Observations Will Likely Accomplish

•Fine structure of moderate-to-strong field structures of the photosphere

•Dynamics of small-scale chromospheric events

•Some chromospheric field measurements

•Ground-based facilities will excel at short time sequences of small-scale objects with modest polarimetric precision. employing:

•Rapid advances in image processing techniques, e.g.:• Multi-Frame Multi-Object Blind Deconvolution •Multi-Conjugate Adaptive Optics

HOWEVER, Ground-based facilities will be challenged by the following:

•Science goals requiring long time series (active region evolution, filament evolution)

•Science goals requiring low instrumental scattering (off-limb measurements of spicules, prominences)

•Chromospheric field measurement at high angular resolution, because:

1. High polarimetric accuracy long integration times (5-10 sec, or more) – image degradation due to residual seeing, blurring

2. High polarimetric accuracy high instrumental throughput (but MCAO leads to many reflections)

Challenges of Chromospheric Field Measurement

Observation:•Few spectral lines form in chromosphere

•Small sensitivity to the Zeeman effect

•Wider line profiles → smaller polarization from Zeeman effect

•Weaker fields in chromosphere → smaller polarization from Zeeman effect

Inversions:•Large optical thickness in many lines (but not HeI 10830)

•Non-LTE formation a necessity

•Hydrostatic equilibrium often invalid (highly dynamic, nonlinear, structured by field)

•Non-monotonic source functions (invalidates Milne-Eddington, for example)

Interpretation:•Large departures from planar surface where field is measured

Line-of-Sight

Shock

T=6000K

T=9000K

Photosphere

•LTE invalid (Even TE is invalid)

•HSE invalid, even along flux tube!

•Chromospheric “surface” highly non-planar

•Rapid, non-monotonic variations of source function along LOS

Challenges for Chromospheric Inversion Methods

Ample Evidence for Chromospheric ShocksExample: Sunspot Umbra

•Chromospheric He I 1083 nm

•High amplitude oscillations (10-20 km s-1)

•“Sawtooth” waveform characteristic of shocks noted in Stokes V profiles

(Centeno, Carlsson, & Trujillo Bueno 2005, ApJ 640, 1153)

Observed (Network) Simulation (Network)

(Pietarila, Socas-Navarro, & Bogdan 2007, ApJ 663, 1386 – SPINOR)

Shocks Visible in Ca II IRT Lines?

Chromospheric Zeeman Diagnostic Lines

Lines λ (Å) Height(km)

Advantages Disadvantages

Na I D 5890

5896

400 •Simulations indicate simple formation

•Low formation ht•“Enigmatic” scattering pol

Mg I b 5173

5184

400 •2 lines, g(b1) ≠ g(b2) •Low formation ht

Ca II IRT 8498

8542

800-1300 •Middle chromosphere•Two lines, differing opacities

•Photospheric contrib•Lines have similar g

Hα 6563 300->2000

•Familiar intensity diagnostic •Photospheric contrib•Large thermal width

Ca II H,K 3933

3969

1000-2000

•Mid-upper chromosphere •Small splitting, low polariz•Low intensity•Effectively-thick formation, branching to IRT

Mg II h,k 2796

2803

1500-2000

•Effectively thin formation•In emission in most locations

•Weak linear polariz•Low intensity•Small mag splitting

He I 10830 1500-? •Optically-thin formation (mostly)•Multiple components

•Scattering polarization•Weak absorption

The MgII h&k Lines

•The Mg II resonance lines have higher sensitivity and emission to the chromosphere than the Ca II resonance lines

•Only visible above the Earth’s atmosphere

•Diagnostic potential is not yet fully explored, but IRIS will produce Stokes I spectra at high resolution

•Some sensitivity to magnetic fields, but there are better diagnostics (polarimetry is more difficult in the ultraviolet)

Example: Hanle-modified Scattering Polarization in a Filament on the Disk

Q/I ~ +1.5 x 10-

4

Q/I ~ -4 x 10 -4

(Zero Field)

Scattering polarization is small

Example: Ca II 8542 Å 10 G horizontal field

2000 km above surface

On disk, normal incidence, polarization is Q/I ~ +1.5 x 10-4

In absence of field, scattering polarization at limb is Q/I ~ -4.0 x 10-4

Solar-C Option B must have optimized optical throughput (minimize number of reflections)!

Illustrating the Need for Continuous Measurement of Chromospheric Fields: Active Region Filaments

•Filaments are central to the CME phenomenon

•Magnetic topology is probably a flux rope

•Filaments are integral to larger-scale coronal field structures

Active Region Filament Chanel

Grey scale: Intrinsic field strength

Grey scale: transverse Apparent Field Strength BT

app

Active Region Filament Chanel

Doppler shift of magnetic component (Q/U/V) differs qualitatively from that from Stokes I profile

Fill fraction small in filament

Intrinsic field strength low (500G) in filament channel

Active Region Filament Chanel

Fill fraction, velocity pattern in magnetic component suggest filament resides above the photosphere

Prominence/Filament Field Structure

Ideal science target for 1.5 m space telescope:

•Weak fields – very high polarimetric sensitivity required (high S/N)

•Structure existing within photosphere, through chromosphere, into corona

•Relationship of fine structure to magnetic field?

•Range of time scales:

•Days – evolution of the large scale structure

•Hours – formation time

•Minutes – de-stabilization when associated with eruptive prominence/CME

•Hanle + Zeeman diagnostics required

What will Solar-C Option B Contribute?

•Solar dynamo: internal structure, rotation, models……Not addressed by this Option B

•Fundamentals of solar-terrestrial effects: CMEs, flare physics, global solar variability…..

Option B contributes uniquely through essential measurement of the chromospheric magnetic field vector, consistently over long time periods

•Energy and mass transport from the photosphere to the solar wind: chromospheric/coronal heating, momentum transfer,…..Option B is ideal instrument for small-scale processes, but this will also be addressed by ground-based instrumentation

Note:

For chromospheric fields, high instrumental throughput is more important than diffraction-limited performance!

•Field structure more uniform in low-beta plasma (current sheets will exist, but will be non-resolvable in any case)

But…..

Off-limb, low scattered light observations would benefit greatly from extremely high angular resolution, as these observations are very difficult from the ground

Summary

•Major thrust of observational solar physics: large-aperture observing facilities

•High angular resolution: many issues of small scale structure will be addressed effectively with ground-based observing

•Chromospheric magnetic field measurement, however, puts strong constraints on the polarimetric precision. The ability for ground-based facilities to address these issues is in question, even with advanced image correction

•Solar-C Option B should be effective in low-scattered light applications (above the solar limb)

•Ultraviolet spectroscopy (Mg II h&k) has potential, but IRIS data will reveal if larger aperture is needed to explore chromospheric dynamics

•Important problems (prominence/filament/CME) demand continuous observing of chromospheric field at rather high resolution – most practically done from space