Key science by 1.5m telescope

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The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research

under sponsorship of the National Science Foundation. An Equal Opportunity/Affirmative Action Employer.


Science Opportunities with a 1.5 m Space Solar
Telescope

Bruce Lites


High Altitude Observatory

Earth and Sun Systems Laboratory

National Center for Atmospheric Research

Boulder, CO



10 March 2010

ISAS

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:


灲散e獩潮s灯污物浥瑲t

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. 11

Magnetic Flux (
-
2000 to +500 Mx cm
-
2

)

Fully Compressible 3
-
D Simulations of Magneto
-
Convection

Sch
üssler & Vögler 2008,
A&A

481, L5

630nm Continuum

Vertical Field,
τ

= 10
-
2

Horiz. Field,
τ

= 10
-
2

Log (B
Horiz
)

Log (B
Horiz
)

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


…………



H
α

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


汯湧 楮瑥杲it楯渠瑩浥猠⠵
-
㄰1獥挬s
潲 浯m攩e


image degradation due to residual seeing, blurring

2.
High polarimetric accuracy


桩杨h楮獴牵i敮瑡e 瑨t潵o桰畴
(
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

Shock

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 Shocks

Example: 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
(b
1
)

g
(b
2
)


Low formation ht

Ca II IRT

8498

8542

800
-
1300


Middle chromosphere


Two lines, differing opacities


Photospheric contrib


Lines have similar
g

H
α

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

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 B
T
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