Haosheng LinInstitute for Astronomy, University of Hawaii

Current Status of Solar Magnetism ResearchLarge Off-Axis Reflecting Coronagraph

and Advanced Solar Instrumentation

Outline of This Talk

Overview of Solar Magnetism and Unresolved Problems—solar dynamo• Sunspot magnetic fields• Quiet Sun magnetic fields• Coronal magnetic fields• Why do we need large aperturSe solar telescopeLarge Aperture Coronagraph• Why do we need large aperture?• Why do we need a coronagraph?• Why Off-Axis coronagraph?• Ongoing and Expected Off-Axis Coronagraph ProjectsAdvanced Instrumentation• Clever multiplexing to achieve high spatial, spectral, and temporal resolution and

sensitivity with simultaneous 2-D spatial coverageWhat can we do with a coronagraph during the night?

Solar Magnetism—An OverviewSunspots and active regions are manifestation of a global-scale solar dynamo operating at the base of the solar convection zone. The existence of magnetic fields makes the Sun an interesting star to study…

‘Quiet’ Sun ‘Quiet’ Sun

Line-of-sight Magnetic Flux Continuum Intensity

Solar Cycle

Sunspot Cycle The number of sunspots on the surface of the Sun follows a 11-year cycle. Butterfly diagram Sunspots appear at higher latitude at the beginning of the solar cycle, and migrate toward the equator, as the cycle evolve

Magnetic Butterfly Diagram

Magnetic Field Configuration of the Sun During Different Phase of the Solar Cycle

Solar Minimum • Dipole Magnetic Field • No Sunspot

Solar Maximum • Toroidal Magnetic Field • Many Sunspots

~5 years later…

The magnetic field configuration of the Sun evolves with a 22 year cycle.

S

N

11 years later…

22 years later…

The solar dynamo is believed to be generated at the base of the convection, where the rotation rate changes abruptly.

‘Quiet’ Sun

MHD instability causes the flux tubes to raise to the surface—sunspots.

Continuum Intensity

Current Solar Magnetism Research Emphasis—ATST Science Goals

2.2 Magnetic Field Generation and the Solar Cycle 72.3 Magnetic Flux Emergence: Active Region Emergence and Evolution 9 2.3.1 Surface Manifestation of Subphotospheric Processes 102.4 Small-Scale Magnetic Flux Concentrations 112.5 Magnetoconvection and Flux Transport 15 2.5.1 Sunspots: An Example of Magnetoconvection 16 2.5.2 Formation of sunspots 19 2.5.3 Radiative Flux balance in sunspots. 19 2.5.4 Filamentary Structure of Sunspot Penumbrae 202.6 Structure and Dynamics of the Chromosphere 212.7 Coronal Structure and Heating 23

2.7.1 Coronal Structures in Three-Dimensions 252.7.2 Electric Current Systems 262.7.3 Coronal Magnetic Fields: The Need for Ground-Based Coronal Studies 272.7.4 New Spectral Diagnostics 302.7.5 Wide-Field Coronal Photometry 31

2.8 Flux Expulsion: Flares and Mass Ejections 312.8.1 High Resolution, High Cadence Studies of Solar Flares 332.8.2 Structure and Evolution of Magnetic Fields Associated with Flares and CMEs 342.8.3 CMEs: The need for prominence studies 35

Sunspot: The Oldest Mystery of Solar Physics

1. Why are sunspots dark against the photosphere?

2. Why is there a sharp boundary between the photosphere and the penumbra?

3. Why is there a sharp boundary between the penumbra umbra?

4. Why is there an Evershed flow?5. …6. …

Ancient Chinese noted the existence of sunspots more than two thousand years ago. Galileo observed sunspots through the first telescope he made…but sunspot phenomenon remains one of the oldest mystery of solar physics. We know a lot about sunspots observationally, but we still do not have a self consistent model of sunspot today.

Swedish Vacuum Tower Telescope Image

Dynamic of Sunspot

Swedish Vacuum Tower Telescope Movie

What is a Sunspot?

G. E. Hale (1908) first demonstrated that sunspots are regions of strong (2,000 ~ 3,000 Gauss) magnetic fields…

Zeeman Effect in Sunspot

Zeeman splitting B is proportional to 2. Therefore, it is preferable to perform magnetic field observation in the infrared wavelength:

B = 4.67 10-7 g 2 B

where g is the Lande factor

The FeI 1564.8 nm (g =3) and FeI 1564.3 (g=1.53) nm line pair are the most sensitive magnetic field diagnostics in the visible and near-IR wavelengths. Visible lines (such as FeI 630.2 nm line (g=2.5) offers lower magnetic sensitivity, but higher spatial resolution.

Why is Sunspot Dark?

Zero-th Order Explanation:

Given the fact that there sunspots are regions with strong magnetic fields, and that sunspots are long-lived, stable features, magnetohydrostatic equilibrium must exist between the magnetized plasma of the sunspots, and the non (or weakly)-magnetized plasma outside of the sunspot. The presence of the magnetic pressure (B2/8) requires the sunspot plasma to be cooler in temperature…

Tspot ~ 3,500 KB ~ 3,000 Gauss

Pphoto (z) = Pspot (r,z) + Bz2

(r,z)/8 + Fc(r,z)/8

Where,

Fc(r,z) = 2 ∫ Bz(r’, z) (Br(r’, z) /z) dr’Pphoto(z) = n(z) k Tphoto(z)Pspot(z) = n(z) k Tspot(z)

A carton model of sunspot

So, sunspots are dark because of the presence of strong magnetic fields.

But why is there a penumbra, and an umbra?

Tphoto ~ 6,000 K

Bphoto ~ 0 Gauss

Is Sunspot in MHS Equilibrium?

Martinez et al (1993) found a linear relation between B2 and dT with magnetic field measurements in the visible wavelength (FeI 630.2 nm line).

Kopp & Rabin found different slope of the B2 vs. dT curve for different part of sunspot with IR FeI 1564.8 nm line measurements.

Although the idea about magnetohydrostatic equilibrium between the magnetized sunspot atmosphere and its surrounding is plausible, observational verification (or dismissal) has not been demonstrated yet. Furthermore, given the complicated structures of the sunspots, we should not expect a simple relationship between B and T. Magnetic field measurements have not achieved the same spatial resolution as those demonstrated from the new Swedish Vacuum Tower Solar Telescope yet. Obviously, better magnetic field data are needed…

‘Good’ resolution visible vector magnetic field measurement

Polarized Spectra of Sunspot

Solar radiation in general is not polarized, except when observing near or off the solar limb where scattering polarizations are significant. However, magnetic fields modify the absorption and emission coefficients of Zeeman sensitive spectral lines and produce polarized spectral radiation. Therefore, the polarization states of solar spectral radiation carry information about the vector magnetic field configuration of solar magnetic fields.

Measurement of Flux vs. Field Strength

B1

B3

B2

• Longitudinal magnetograph (SOHO MDI, BBSO magnetogram) measures the magnetic flux within the resolution element along the line-of-sight…

B = ∫s B · da

• Magnetic field strength |B| can only be measured by direct (spectroscopic) measurement of the Zeeman splitting…

• Minimum measurable |B| depends on the doppler width of the spectral line.

• Structures within the resolution elements are ignored.

‘pixel’ of observation

New High-Resolution IR Measurements of Sunspot Magnetic Fields

Magnetic Field Strength B

OH Equivalent Width

OH equivalent Width vs temperature is independent of the sunspot size The formation of OH molecules depends only on the temperature of the sunspot, not magnetic fields?

B vs. OH

• OH molecules form only in the umbrae of the sunspots…

• Dissociation energy 4.3 eV• Slopes of B-OH curves are

different for different size sunspots…

B vs Tc

B-T curve is a strong function of sunspot size…

Phase transition in sunspot? • Theoretical calculation predicted

that about 20% of Hydrogen are in molecular form (H2)…

• Molecules have internal degree of freedom (rotation and vibration).

• The formation of molecules introduce a discontinuous change in the heat capacity of the sunspot plasma first-order phase transition?

• What’s the thermodynamic effect of molecules in sunspot atmosphere?

Quiet Sun Magnetic Fields• ‘Quiet’ Sun is not really magnetic-field-free. Weak field (<1,000 G) features can

be seen everywhere…• Quiet Sun magnetic fields may be generated by a small-scale surface dynamo…

‘Quiet’ Sun ‘Quiet’ Sun

SOHO MDI Magnetogram SOHO MDI ‘White Light’ Image

Example of Weak Field Stokes V Spectra

• Precision IR spectropolarimetry allows for determination of |B| down to ~ 150 G.

• Sensitivity to B ~ 5 x 1015 G·cm2

• Sensitivity to B is limited only by photon noise!

Quiet Sun Magnetic Fields

Small-scale, weak magnetic fields with mixed polarities can be found every where on the surface of the Sun.• These fields are mostly cospatial with the

intergranular lane, and evolve with a time scale similar to that of solar granulation

• <B> ≈ 500 G, equipartition with turbulent motion of the solar granulation

Intensity

MagnetogramKilo-gauss component in equipartition with the gas pressure of the photosphere

Pending Questions about the Weak Fields

How are the weak field component generated? •Local dynamo associated with the convective motions? •Recycling of the magnetic fluxes generated by the global scale dynamo? •Does ‘flux tube’ exist?

If the quiet Sun magnetic fields are generated by a local solar dynamo, then we expect the magnetic fields to exist in every spatial scale and in equipartition with the turbulent gas pressure. This ‘turbulent magnetic fields’ will evolve with the same time scale as the turbulent flows.

In order to answer the pending questions concerning the quiet Sun magnetic fields, we need to observe the quiet Sun magnetic field with the highest spatial resolution and magnetic field sensitivity attainable, and with the highest temporal resolution possible and simultaneous 2-dimensional coverage—this statement is in fact true for almost every aspect of solar magnetism research now…

Coronal Magnetic Fields

Coronal magnetic field is something of a dark energy problem for solar physics in that we know it permeates the corona and controls its static and dynamic behavior, yet we are unable to usefully measure it...Because of the high temperature (T ~ 106 K) and low field strength (B ~ 10 G) condition of the solar corona, measurement of coronal magnetic field is one of the most challenging task of observational solar astronomy.

Without direct measurement, coronal intensity loop structures are thought to be proxies of coronal magnetic field. But, observational verification is need!

This talk is about how we can measure coronal magnetic field today…

SOHO/EIT Fe XVI 284 Å

Coronal Magnetic Fields and Coronal Mass Ejections and Flares

TRACE image of post-flare loopsSOHO/LASCO images of coronal mass ejection

The energy released in coronal mass ejects (CMEs) and flares are thought to be stored in the coronal magnetic fields…

Tools for Coronal Magnetic Field Diagnostics

1. Faraday rotation of astronomical radio source (Patzold et al., 1987)2. Faraday rotation of polarized solar radio ration (Alissandrakis and Drago, 1995)3. Gyrosynchrotron radiation magnetometry (Gary and Hurford, 1994, coronal B at the

‘base’ of the corona, on the solar disk)4. UV (E1) Hanle effect of O VI 103.2 nm (Sahal-Brechot et al., 1986)

— depolarization and rotation of linear polarization…5. Extrapolation from photospheric magnetic field measurements

6. M1 (magnetic dipole) Hanle and Zeeman effect polarimetry• Stokes V gives strength of B• Stokes Q and U yield orientation of B

projected in the plane of the sky with a 90 ambiguity

MHD simulation of pre-CME coronal magnetic field structure, Roussev et al. 2003

Difficulties of Coronal Magnetometry

Due to the high temperature (106 K) and low magnetic field strength (B ~ 10 G) of the corona:

• V ~ few × 10-4 IL Need 108 photons per measurements!• Low photon flux (10–5 of disk center intensity) from the solar

corona• Large scattered background• Q, U ~ 1 × 10-1 IL >> V

Just a very small linear-to-circular polarization crosstalk is sufficient to mask the weak Stokes V profiles.

History of CEL Coronal B ObservationsEarly Attempt

Harvey, 1969: Fe XIV 530 nm Stokes V magnetometry No detection.

Linear Polarization MapsMickey, 1973: Fe XIV 530 nmQuerfeld, and Smartt, 1984: Fe XIII 1075 nmArnaud & Newkird, 1983: Fe XIII 1075 nm

Successfully obtained maps of the orientation of coronal magnetic fields.

Recent EffortsKuhn, 1995: Fe XIII 1075 nm Stokes V

spectropolarimetry No detection.

Lin, Penn, & Tomczyk, 2000: Fe XIII 1075 nm Stokes V spectropolarimetry First Definitive detection of line-of-sight coronal magnetic field!

Linear polarization map of the Fe XIII 1075 nm line. Habbal et al. 2001, Arnauld 1983

CEL Polarimetry—Incoherent Magnetic Resonant Scattering

Physical Process:Resonant scattering of anisotropicphotospheric radiation by atoms and ions in the corona in the presence of a magnetic field.

Linear Polarization– Orientation of CEL linear polarization maps the orientation of magnetic field projected in the plane-of-sky (POS)

– Magnetic field orientation subject to 90 degree ambiguity (Van Vleck Effect).

Circular Polarization – Circular polarization of CEL is proportional to the strength of line-of-sight magnetic field – The magnetograph formula is modified by an alignment factor that depends on the inclination angle between B and the local vertical direction, and the anisotropy of the incident radiation field.

Physics of Resonance Scattering Polarization

Permitted Lines(E1 Transition)

e.g., He I 1083.0 nm, O VI 103.2 nm

• Weak field regime:B <= A (≈ 106 to 108 sec-1),

• Strong interference between the magnetic substates,

• Coherent superposition of the magnetic sub-states,

|E|2 = |E –1 + E 0 + E+1|2

Forbidden Lines(M1 Transition)

e.g. , Fe XIII 1074.7 nm, Si IX 3934.6 nm

• Strong field regime:B >> A (≈ 101 to 102 sec-1),

• Weak interference between the magnetic substates,

• Incoherent superposition of the magnetic sub-states,

|E|2 = |E -1|2 + |E 0|2 + |E+1|2

B

A

B

A

Stokes Parameters of CEL

,cos1cos1cos324

1),( 2

MMMf

Diagnostics of Coronal B

U/Q = tan 2

V ~ cos vB (dI/d)

P = (Q2+U2)-1/2/I ~ f (,M ) sin2

• Q and U vanish at van Vleck angle vv = 54.7º• Q and U change sign for > vv

,θ,θfγva,vFκCV

χ,γθθfa,vHκCU

χ,γθθfa,vHκCQ

,γθθfa,vHκCI

MBν

Mν

Mν

Mν

)(2

11cos)(

2

1

2sinsin22

3),()(

2

1

2cossin22

3),()(

2

1

)1cos3)(,(22

11)(

2

1

1

21

21

21

Example of Fe XIII 1075 nm Coronal Emission Line Spectropolarimetry

Weak Stokes V signal in the FeXIII 1074.7 nm line can be detected!

Target of Observation

Magnetogram of target active region observed on disk

1999/10/25

First Definitive Coronal Stokes V Measurement

• NSO/SP Evans Solar Facilities 40 cm coronagraph

• 240 arcsec2 FOV (summed over the entire length of the slit).

• 2560 seconds (44 minutes) integration time (Q & V).

• Careful telescope and instrumental polarization cross-talk control

Coronal magnetic field can be measured with Zeeman effect diagnostics!

Can We Make 2-D Coronal Magnetic Field Maps? IfA Coronal B Initiatives

While Lin et al. (2000) demonstrated the feasibility of using CEL polarimetry to measure the strength of coronal magnetic fields, useful measurements require 2-dimensional spatial coverage.

Given the long integration time required to obtain one measurement, can we make 2-D coronal magnetic field maps?

To this goal, we initiated a new effort to establish the capability to make regular 2-dimensional maps of both longitudinal magnetic field strength and the orientation of the magnetic field projected in the plane of the sky.

IfA effort includes:

1. Construction of a 50 cm aperture off-axis mirror coronagraph—SOLARC

2. Construction of an Optical Fiber-bundle Imaging Spectropolarimeter (OFIS) : The polarized spectra of a extended 2-D target can be obtained simultaneously without scanning of the spectrograph slit.

SOLARC: Off-Axis Mirror Coronagraph

LCVR Polarimeter

Input array of fiber optics bundle

Re-imaging lens

Prime focus inverse occulter/field stop

Secondary mirror

Primary mirror

SOLARC and its dome on the summit of Haleakala, Maui.

Optical Configuration of SOLARC and OFIS

PI—Jeff Kuhn (IfA) • 50 cm aperture off-axis gregorian

telescope• No secondary mirror and spider

structure in the optical path for coronagraphic performance

OFIS: A True Imaging Spectropolarimeter

The coherent optical fiber-bundle rearrange the 2-dimensional image sampled by the 16 × 8 input array to two linear array (2 × 64 ). The two linear arrays act as the slits of the spectrograph, thus allowing for the simultaneous recording of the spectra from all the field points in the 2-D image plane.

Fiber Bundle

Collimator

Echelle Grating

Camera Lens

NICMOS3 IR camera

• NICMOS3 IR Camera• 16 8 => 2 64 optical fiber-bundle• 160 308 mm, 79 lines/mm echelle

grating with 63.5 blaze angle• f = 800 mm, = 150 mm (F/5.3)

collimator and camera lens

Sample CEL Spectra from OFIS

One 64-fiber column illuminated16 × 4 pixels area coverage

Two 64-fiber columns illuminated16 × 8 pixels area coverage

2004/04/06 ObservationFull Stokes vector observations were obtained on April 6, 2004 on active region NOAA 0581 during its west limb transit.

Corona activity is low compared with the 1999 observations!

Stokes I, Q, U, & V Observation:• 20arcsec/pixel resolution• Telescope pointing @

Radius Vector 0.25 R Position Angle (Geocentric): 260°.

• 70 minutes integration on V• 15 minutes integration on Q & U

Stokes I, Q & U Scan:• RV = 0.25 R • From PAG 250° to 270°• Five 5° steps

Fe X 171Å image of the solar corona at approximately the time of SOLARC/OFIS observation from EIT 195 A.

Full Stokes Spectra of CEL

Full Stokes pectra obtained above NOAA 0581. The display ranges for I, Q, U, and V are -0.5 IC to 0.5 IC, -0.05 IC to 0.05 IC, -0.05 IC to 0.05 IC, and -0.005 IC to 0.005 IC, respectively.

• CEL intensity falls off as a function of height h,

• Linear polarization increases with h, As expected from theory

• Spectral characteristics of Stokes V similar to I (and Q, U),

• Spatial variation of V resembles that of U, Strong linear to circular

polarization crosstalk!!

Polarization Crosstalk Correction

The crosstalk contaminated circular polarization V’ () can be expressed by

V’ = V + a ·Q + b ·U = V + ·I,

where V () is the uncontaminated circular polarization signal, a and b are the Q-V and U-V crosstalk coefficients, respectively, and is an ‘apparent’ I-V crosstalk coefficient.

Using a least squares algorithm minimizing

2 = (V’ – V - ·I),

we can derived assuming I ·V=0 due to the antisymmetric property of V,

= I ·V’ / I 2.

Also, since in weak-field approximation,

V = B·dI/d,

Row Stokes V and crosstalk-corrected V. The image is rearranged such that the each 8-fiber strip in the vertical direction corresponds to a 8-fiber column in the north-south direction. The first 8 rows (0-7) correspond the column closest to the solar limb. The weak antisymmetric V profiles can be seen in the first two north-south columns (fiber 0 to 16) in the crosstalk-corrected V image.

the observed circular polarization can be written as

V’ () = ·I () + B ·dI () /d = ·I (+ B/),

Thus, B can be directly measured by comparison with the shift of V with respect to I in the spectral direction.

Line-of-Sight Magnetic Fields

B

BSamples of measured and fitted Stokes I and V spectra of the 10 4 (200” 80”) pixel region closest to the solar limb. The errors of the magnetic fields are 1 sigma error. Geocentric north is up, and east is left.

The longitudinal field reverses sign around h=0.17 R!

Radial Variations of B and Comparison with Model Calculations

• Solid line with error bar IR data• The dotted line Abbett et al. (2003)

near-limb magnetic model scaled to an active region with 1000G longitudinal field strength at the photosphere.

• * with error bars Global Ledvina et al. (2004) B model (rms field evaluated along averaged horizontal sight path). The upper error bars show the maximum field at given horizontal level, the lower error flag shows the standard deviation of the model B and the plotted symbols show the mean rms B at the given horizontal level.

What Light’s Up Some Field Lines? – – Is CEL Intensity Correlated with Magnetic Field Strength?

Magnetic fields fill the entire volume of the corona. However, intensity images of coronal emission lines always show highly distinctive loop structures.

• Why some of the magnetic field lines are filled with high density highly ionized atoms, while the adjacent dark regions are not?

• Why does the corona appear different in different CEL images?

• Are bright coronal loops actually representative of higher magnetic field strength regions?

• Do they actually trace the magnetic field lines?

We found NO correlation between |B| and Iline

bright CEL emission does not necessary imply stronger magnetic fields.

Transverse Magnetic Field Orientation

Does intensity loops track magnetic field lines?

Yes, and No?See boxes 1 and 2

1. Degree of polarization decreases as a function of height=> higher anisotropy and less collisional depolarization.

2. Van Vleck effect in box 2?

Need More Work!

1

2

‘Vector’ Coronal Magnetogram

Contour plot of the line-of-sight magnetogram over-plotted on the EIT FeXVI 284 A image. The contours are 5G, 3G, and 1G.

Transverse field orientation Longitudinal Field Strength

Summary• We have successfully obtained the first coronal magnetogram, with

measurements of both the longitudinal magnetic field strength and orientation of the magnetic field projected in the plane of the sky. The magnetic field sensitivity is ~ 1 G near the limb with approximately 1 hr integration with a 20” 20” spatial resolution.

• We observed a radial fall-off of B qualitatively similar to that predicted by some numerical models.

• We observed a non-radial magnetic field configuration similar to that implied by the EIT image. However, it is still not clear if the loop structures in the EIT image actually follow the magnetic field lines we measure in these FeXIII data. More studies are needed.

• We find no correlation between the brightest emission structures and the strongest longitudinal magnetic fields

Need vary large coronagraph to improve on sensitivity and Need vary large coronagraph to improve on sensitivity and resolution!resolution!

Current Status of Coronal B Experiment…

B

BThe Sun is now in the bottom of the solar minimum…there isn’t much to observed, especially with the 50 cm aperture of SOLARC…

Are we actually seeing the reversal of B because we are looking at the loop structure edge-on in the 2004 data? We (Lin and Petrie, 2005) have started an effort to model the coronal magnetic field structure by extrapolation of the observed photospheric magnetic field…If we can match the height of the longitudinal field reversal point, then it will be a triumph of this effort.

Why do we need new large aperture solar telescopes

• The aperture of the new Swedish Vacuum Solar Telescope (1 meter) and the Italian-French THEMIS solar telescope (90 cm) are both smaller than the MacMath-Pierce Telescope…

• Solar Physics is currently in a state where development in theoretical and numerical modelings have out paced observations. New theoretical and modeling results now demand observational verification that cannot be achieved with existing telescopes and instrumentations.

• New observational diagnostics (e.g., coronal magnetic fields) require large aperture.• Both MacMath-Pierece Solar Telescope, the existing solar telescope with the largest

aperture (1.5 meter), and Dunn Solar Telescope, the best equipped solar telescope (AO, visible and IR spetropolarimeters, 75 cm aperture), of the National Solar Observatory (NSO) were built more than 30 years ago.

Evolution of solar telescope diameter. Until the 1980’s, the resolution of solar telescopes was better than the resolution of theoretical models, which in many cases had no spatial resolution at all. With the availability of fast computers, numerical simulations have now have a spatial resolution as good as 10 km (2D). With adaptive optics the diffraction- limited resolution can now can be achieved from ground- based telescopes. The ATST will achieve resolution comparable to that achieved in numerical simulations.

Current and Future Large Aperture Off-Axis Coronagraphs

Big Bear Solar Observatory• 1.6 meter aperture• NJIT and IfA Collaboration

New Solar Telescope (NST)

Advanced Technology Solar Telescope

What’s Next? – The Advanced Technology Solar Telescope

ATST is a 4-m off-axis coronagraph designed from the ground up for precision polarimetry with wavelength coverage from UV to Far-IR…

• High order AO for high-resolution imaging.• Large aperture for high precision polarimetry• No obscuration in the optical path yields

coronagraphic performance

Project Status• NSF $10M funding for design and development

study• Construction proposal ($160 M) was submitted to

NSF MRE ~January 2004• Designated Haleakala on Maui as the primary site

—December, 2004• Received excellent review in 2005. Awaiting NSF

funding decision.

What is Coronagraph?

A coronagraph is a special telescope that employs special scattered light reduction optical arrangement to facilitate the observation of a faint light source immediately adjacent to a bright light source. • Every optical surface and mechanical

structure in a telescope produce scattered light. If the amplitude of scattered light from a bright light source overwhelm the brightness of an adjacent faint light source, then the faint source cannot be observed easily.

• For example, we cannot see stars during the day—the amplitude of sun light scattered off Earth’s atmosphere is much larger than the brightness of the brightest stars.

• Dirty car windshield produces scattered light when illuminated by sunlight, or head light of incoming traffic, makes seeing the road difficult…

Classical Lyot Coronagraph

Internal Occulter block light of the bright light source and prevent it from further scattering by the rest of the optical system.

Primary Focus

Super-polished Objective Reimaging optics

Lyot Stop block bright diffraction from the edge of the objective.

The primary scattering source is the surface of the objective. The surface of the objective (or primary mirror for reflecting coronagraph) needs to be kept very clean…

Externally Occulted Coronagraph

External Occulter block light of the bright light source from reaching the objective, and prevent it from further scattering by the rest of the optical system.

Primary Focus

Lyot Stop block bright diffraction from the edge of the objective.

Super-polished Objective Reimaging optics

Internal Occulter block bright diffracted light off the edge of the external occulter, and prevent it from further scattering by the rest of the optical system.

Comparison between Externally and Internally Occulted Coronagraphs

Internally Occulted Coronagraph• Lower scattered light performance (than the

externally occulted coronagraph)• Can get very close to the bright light source• Compact optical configuration

Externally Occulted Coronagraph• Very good scattered light performance

(because non of the optical surfaces are illuminated by the bright light source.

• Difficult to observe close to the bright light source

• Requires very ‘long’ optical system—the distance between the external occulter and the objective and the relative size of these components determine how close we can observe to the bright light source.

The Moon is a perfect natural external occulter during a solar eclipse…

The K corona is a million times fainter than the solar disk, and cannot be seen with the sky illuminated by Sun light, except during total eclipse, or with a coronagraph

Why Do We Need Large Aperture?

Photon Statistics• Photon noise = N 1/2

• S/N = / N = N -1/2

Why Off-Axis Gregorian Coronagraph

1. Gregorian configuration allows for implementation of primary focus occulter—better scattered light performance.

2. No spider and secondary mirror structure in the light path—better scattered light performance.

3. Un-obscured aperture allow for complete wave front distortion measurement—better AO performance

4. More space for heat stop—easier heat removal for solar application

5. True coronagraphic performance—ideal for imaging search of extra-solar planets.

ATST Optical Configuration

In coronagraphic application, what matters is not the size, but the scattered performance of telescope. A 4-meter coronagraph can easily outperform an 8-meter ‘regular’ telescope with 10 times worst scattered light performance…

Advanced Solar InstrumentationAll observational study of solar magnetism call for spectropolarimetric measurement with high spatial, spectral, and temporal resolution and high magnetic field sensitivity with simultaneous 2-D spatial coverage—Seven-dimensional data ‘cube’:

I [x,y,I(),Q(),U(),V(),t]

None of the conventional ‘imaging’ spectropolarimeters, either with a scanning long-slit spectrograph (scanning in spatial direction), or a Fabry-Perot Interferometer (scanning in wavelength direction), or an imaging Fourier Transform spectrograph (scanning in ‘spatial frequency domain’) can deliver data with the desired quality.

With multiple slits—either with Coherent Fiber Optics Bundle or mirror image slicer, true imaging spectroscopic/spectropolarimetric capability can be achieved—e.g., the Optical Fiber Imaging Spectropolarimeter on SOLARC…

OFIS: A True Imaging Spectropolarimeter

The coherent optical fiber-bundle rearrange the 2-dimensional image sampled by the 16 × 8 input array to two linear array (2 × 64 ). The two linear arrays act as the slits of the spectrograph, thus allowing for the simultaneous recording of the spectra from all the field points in the 2-D image plane.

Fiber Bundle

Collimator

Echelle Grating

Camera Lens

NICMOS3 IR camera

• NICMOS3 IR Camera• 16 8 => 2 64 optical fiber-bundle• 160 308 mm, 79 lines/mm echelle

grating with 63.5 blaze angle• f = 800 mm, = 150 mm (F/5.3)

collimator and camera lens

Sample CEL Spectra from OFIS

One 64-fiber column illuminated16 × 4 pixels area coverage

Two 64-fiber columns illuminated16 × 8 pixels area coverage

Line-of-Sight Magnetic Fields

B

BSamples of measured and fitted Stokes I and V spectra of the 10 4 (200” 80”) pixel region closest to the solar limb. The errors of the magnetic fields are 1 sigma error. Geocentric north is up, and east is left.

The longitudinal field reverses sign around h=0.17 R!

• With new large format (2048 x 2048) IR array detectors, large format (~ 100 x 100 fibers x 400 spectral pixels ) coherent fiber optics bundle can be constructed, providing high-resolution and moderate area coverage…

• Each spectrograph/camera system can only support a limited number of fibers, but with fiber optics bundle, massive parallelism design utilizing multiple spectrograph/camera systems can be employed to achieve large area coverage

Spectrograph 1

Spectrograph 2

Spectrograph 3

Spectrograph 4

200 x 200 Bundle

100 x 100 pigtail

100 x 100 pigtail

100 x 100 pigtail

100 x 100 pigtail

Polarimeter

What’s Next?

• Coronal magnetometry is photon starved – Larger aperture coronagraph and larger format OFIS will significantly improve the spatial resolution and coverage, time resolution, and magnetic field sensitivity of the observations.

WE NEED A LARGER CORONAGRAPH – ATST?

• Data, lot’s of data!

• Vector Magnetogram?

• Lot’s of data coordinated with other instruments!

• Resolving line-of-sight integration problem--Tomography?

• Comparison with model calculations.

The Sun as a Magnetic StarAlmost all the solar surface and coronal features (except for solar granulation) we talked about so fare are related to magnetic fields…• Sunspots• Plages• Filaments• Prominences• Coronal loops

The numbers of these features on the Sun change over time, follow a 11 year cycle…

Solar CycleSunspot Numbers and Butterfly Diagram

Solar Cycle The number of sunspots on the surface of the Sun follows a 11-year cycle.

Butterfly diagram Sunspots appear at higher latitude at the beginning of the solar cycle, and migrate toward the equator, as the cycle evolve

No. of sunspots as a function of time since 1900.

No. of sunspots as a function of time and solar latitude since 1900.

Solar Magnetic Field and Solar Cycle

Since sunspots are strong magnetic field regions on the surface of the Sun, the sunspot cycle suggest the Sun has a magnetic cycle.

400 years of solar activities….

The Sun in H (hydrogen absorption line)The bright patches in these images are associated with strong magnetic field regions

Maunder Minimum Reduced solar activities caused climate changes?

High magnetic field activities

Low magnetic field activities

Solar Cycle in X-Ray• The temperature of the solar corona a few million degrees (no explanation yet). • The high temperature causes it to emit photons mostly in the UV and X-ray

wavelengths (high energy photons). • The activities in the solar corona also follow the solar cycle. • In fact, the level of almost every aspect of solar activities (flares, coronal

mass ejections, etc.) follow the solar cycle.

Magnetic Field Configuration of the Sun During Different Phase of the Solar Cycle

Solar Minimum • Dipole Magnetic Field • No Sunspot

Solar Maximum • Toroidal Magnetic Field • Many Sunspots

But, this is only half of the story!

S

N

11 years later…

~5 years later…

The magnetic field configuration of the Sun evolves with a 22 year cycle.

22 years later…

How Solar Cycle Works• Since the magnetic field of the Sun reverse its orientation every 11 years, the

solar cycle is really a 22 year cycle. • The number of sunspot only depends on the strength of the solar magnetic

activities, but not the orientation of the magnetic fields. Therefore, sunspot number cycle is half that of the magnetic field cycle.

• How does the Sun changes its magnetic field orientation every 22 years? In comparison, the Earth’s magnetic field direction is stable. There are a few clues. For example, we believe that the differential rotation of the Sun must play a role in changing the magnetic field configuration from that of a dipole (like a bar magnet) to that of a torus (shaped like a doughnut). But,

•

The exact mechanism of the solar cycle is still unknown!