High Altitude Observatory (HAO) – National Center for Atmospheric Research (NCAR)

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Remote Sensing of Solar Magnetism via

Polarimetry

Bruce W. Lites

High Altitude Observatory

National Center for Atmospheric Research

HAO 75th Anniversary

1 September 2015

Why Observe Solar Magnetic Fields?

“The nation that controls magnetism will control

the universe” -- purportedly from Dick Tracy Comic Strip September 1962

in reference to the “space coupe”:

(…….I could not find the quote in a review of the Dick Tracy comics “space coupe”

series published in the Chicago Tribune in September 1962………)

"If the sun didn't have a magnetic field, then it would be

as boring a star as most astronomers think it is.” -- Attributed to Robert B. Leighton on various occasions in the

1960’s, quote above as recalled by R. Noyes

(most sources, but not all, attribute this quote to Leighton. Versions with varied wording

have been reported; there appears to be no written or standard version)

A more academic rationale:

More seriously……….

The variable solar magnetic field is the source of nearly all the

variability of Earth’s space environment.

1992 1999

Major Solar Influences On Earth’s Space Environment:

• Flares, Coronal Mass Ejections (CMEs)

• Impulsive disturbance of geomagnetism

• Impulsive particulate irradiation

• Impulsive ultraviolet enhancements

• Solar Magnetic cycle; Long-term variability of:

• Solar wind

• Topology of the interplanetary magnetic field

• Total solar irradiance

• Ultraviolet solar radiance

To understand these magnetic influences, must determine:

• How the emergence of active region magnetic flux leads to flares, CMEs

• The interplay between convection and magnetic fields

• Mechanisms by which magnetic fields energize the upper solar atmosphere leading

to heating and enhanced emission

• The interplay between the large-scale magnetic field structure and fields at the

smallest scales

How to “Measure” Solar Magnetic Fields?

With Polarization!

The “lab experiment” definition of the Stokes parameters:

I = I + I = I + I (light intensity)

Q = I - I (linear polarization)

U = I - I (orthogonal linear polarization)

V = I - I (net circular polarization)

Description of the state of polarization: the Stokes vector {I,Q,U,V}T

Atomic energy level diagram for “Normal Zeeman Triplet”:

Degenerate atomic M-states change their energy slightly

in presence of a magnetic field. Classical analogue is

precession of the atom about the field direction at the

Larmour frequency νL:

+hνL

-hνL

Anti-symmetric about

line center in net

circular polarization

Symmetric about line

center in linear

polarization

Details of shapes of spectral profiles in polarization contain

information on the strength and orientation of the magnetic field

vector in the solar atmosphere – SPECTRO-POLARIMETRY:

Polarization Measurements Inferred Vector Magnetic

Field

The Stokes 4-vector {I, Q, U, V}T describes the complete state of polarization of light

Zeeman

Effect

Adjust

model

parameters

Recompute

synthetic

profiles

from model

“Inversion”: Typically a Least-Squares Fitting Procedure

Early Magnetic Field Measurement at HAO

HAO Stokes I Observations of a Sunspot

At this time I entered the field of solar

polarimetry:

• The analysis procedures were either

too simplistic, or in the case of

“inversions” the results were not

stable

• R. Grant Athay (had been my thesis

advisor) suggested in the early 1980s

that I attempt to include magneto-

optical effects (Faraday rotation) into

the “inversion” procedure of Auer,

Heasley, and House (1977)

M.O. effect (also

in Stokes Q,U)

• Addition of magneto-optical effect,

and other improvements, resulted in

an “inversion” procedure that finally

yielded reliable values for the

magnetic field vector for Stokes II

data

Grant Athay

In the mid-1980s, the successful inversion methods fostered conviction

that a new instrument overcoming shortcomings of Stokes I and II would

deliver breakthrough science

The new instrument would:

• Have spatial resolution sufficient to begin to explore the fine-structure

of solar magnetic fields (~1 second of arc)

• Gather data much more rapidly by using modern (at that time) area

detectors

Improved Analysis Impetus for New Instrumentation

Toward Comprehensive Sensing of the Magnetic Field:

• The Advanced Stokes Polarimeter (1991 – 2007?)

• Hinode (2006 – Present)

A “Stokes Consortium” was formed of interested parties,

including: • HAO

• National Solar Observatory (NSO)

• University of Hawaii

• University of Sydney (Australia)

• Astrophysical Observatory of Arcetri (Italy)

Deliberations began on how to build the instrument:

• Would an existing facility be used, or would a new facility be built?

• Would wavelength discrimination be accomplished by a filtergraphic or

spectrographic means?

• What would be the focus science targets (active regions, scattering

polarization, ….)?

The Stokes Consortium:

I Q U V

V U Q I

λ

y

y

λ

λ

y

5-Dimensional Data Hyper Cube [x, y, λ, Polarization, time] • Polarization usually multiplexed in time: reduces the cube to 3 dimensions [x,y,λ]

• Limited by detectors that are 2-dimensional

• Spectrographic observations:

simultaneous imaging of [y, λ]

• Spectrograph slit steps

temporally across the solar

image in the x-direction

• Filtergraph observations: simultaneous

imaging of [x, y]

• Filter tunes in λ at separate times to build

up cube

Peter Gilman

A New Instrument for Solar Spectro-Polarimetry

The Stokes Consortium deliberated possible instrument

configurations

• Imaging instruments were in favor

• HAO recognized that only a spectrographic

instrument yielded uncompromised Stokes profiles

• A new facility of the scale needed was beyond the

possibility of funding

• HAO with NSO committed to hardware

development

• Decision to focus on solar active region science

• HAO provided the instrumentation for deployment

at the Sac Peak Vacuum Tower Telescope (later the

Dunn Solar Telescope)

• In the end, Peter Gilman, then director of HAO,

made some hard choices (i.e., cuts in other HAO

programs) to fully fund the ADVANCED STOKES

POLARIMETER (ASP)

Evans Facility:

“Big Dome”

Richard B. Dunn

Solar Telescope

Observing

Facilities at

Sacramento

Peak

Observatory

Sunspot,

New Mexico

The Advanced Stokes Polarimeter (ASP)

• First instrument that combined the following

attributes:

o Simultaneous, spectrally-resolved I, Q, U, V

profiles

o Spectral imaging with 2-D detectors

o High frame rate (at least for that era: late

1980’s): 60 Hz

o Dual-beam polarimetry

o Image-frame demodulation

o High resolution capability at a large solar

telescope (DST at Sac Peak, NSO)

o Rapid image motion compensation (“tip-tilt

mirror”)

Dunn Solar Telescope (DST), National Solar

Observatory, Sunspot, NM USA

The Advanced Stokes Polarimeter

Telescope configuration always

changing telescope calibration!

Internal polarization

calibration optics

Slit jaw imaging full

Stokes polarimetry Polarization

analysis and

dual beam

device just

behind the slit

Simultaneous

imaging of both

beams, two spectral

regions

David Elmore

Implementation of the ASP

• Very complex instrument:

o Some optical/mechanical systems integrated

to DST

o Set up and aligned for each observing run on

several optical tables, then taken apart after

the run

o Multiple computers, custom CCD cameras,

computer-controlled mechanical devices, data

storage systems, data displays………

o The architect of the ASP was David Elmore

o This complicated system functioned for

scientific observations, fully filling

expectations, from its first deployment in

December 1991!

Photospheric vector magnetic fields

suggest a rising, closed toroidal flux

system

ASP Science Example: Observed signatures of emerging flux

ropes in the solar photosphere

Most coronal mass ejections are

associated with erupting flux ropes

Animation of emergence of a theoretical closed flux

system of toroidal topology

Theoretical Model:

BC Low

Lites 2005, fig. 6

Active Region Filament

(prominence seen against the

disk), Advanced Stokes

Polarimeter

Magnetogram

Hα

Continuum

Filament in H seen along

AR polarity inversion line

Vector magnetogram

demonstrates inverse

configuration of field near

polarity inversion line

below prominence

Conclusion:

• At least some active region flux emerges from the solar interior with the twist

in the magnetic field already in place

• Such twist appears to be necessary to support a prominence above the solar

surface

• The amount of twist is linked to the stability (or instability) of the

prominence against upward forces leading to ejection (CME)

More questions remain:

• Do some prominences form twist via “footpoint” motions driven by

photospheric flows?

An unexpected discovery:

Small-scale horizontal

magnetic flux in the quiet

solar photosphere

Symmetric signature of

transverse Zeeman effect

No significant Stokes V:

field is transverse to the

line-of-sight (horizontal)

Emergence of small-scale

magnetic flux elements into

the quiet solar photosphere

• Small scale events – 1” or smaller

• Transient – last a few minutes only

• Occur independently of strong network

flux – not the result of buffeting of

small-scale vertical flux elements

• Hypothesized to be emergence of small

magnetic loops

• Magnetic flux emergence rate estimated

to be ~1000 times that of solar cycle

sunspot regions!

The quest for higher angular resolution, consistent time

coverage

Typical ASP Intensity Map of a Sunspot Region

Swedish Solar

Observatory

Higher angular resolution:

• Although imaging can be achieved from the ground at high resolution, it

is another matter altogether to do precision polarimetry at high

resolution through the turbulent Earth’s atmosphere

• Adaptive optics was still developing, and today it still is not capable of

correcting all the ill effects of observing through atmospheric turbulence

Continuity of observations:

• Atmospheric “seeing”, clouds, and the day/night cycle disrupt the

continuous observations needed to understand evolution of solar

phenomena

A space mission was the obvious first choice for advancement

In 1994, discussions began between HAO and Japanese scientists regarding a

spectro-polarimeter in orbit for the Japanese Solar-B mission as a follow-on to

the highly successful x-ray instrumentation on Yohkoh

Prof. Saku Tsuneta YohKoh X-ray Image of Sun

•First space mission with emphasis on precision measurements of magnetic fields

•First high resolution magnetic field measurements from space: ≤0.3 arcsec

•Two instruments (Jointly by Lockheed Martin + HAO)

•SOT/Narrowband Filtergraph Instrument (NFI)

•SOT/Spectro-Polarimeter (SP)

The Joint Japan/USA/UK Hinode Mission (October 2006 – Present)

Status almost 9 years after launch:

•SOT/SP operating nominally with slight throughput degradation

•SOT/NFI has limited capability due to degradation of optical elements

Hinode Science Example: The Weakest Magnetic Signals from

the Very Quiet Sun

Hinode Science Example: The Weakest Magnetic Signals from

the Very Quiet Sun

Lots More of Those Horizontal Flux Elements!

Vertical Field Horizontal Field

Hinode/SP data reveal that horizontal elements are indeed emerging loops

From Centeno et al. 2007

Hinode/SP clarified the behavior of these horizontal fields:

• Vertical fields mostly between granules, horizontal ones typically over

granules themselves

• Horizontal flux much larger than vertical flux

• Considerably more horizontal flux present relative to earlier ASP results

Are These Weak Fields Associated With the Global Solar Cycle?

Magnetic Butterfly Diagram

Solar Cycle Dependence of the Weakest Flux

Lites et al. 2014

•Hinode “Irradiance” monthly

observing program

•Isolate weak internetwork

(IN) flux elements above noise

•Examine behavior from 2008

– 2013 (solar minimum to past

maximum)

•Annual fluctuation due to

changing solar B0 angle

•No obvious signature of the

global solar cycle in the

weakest IN flux

•Results consistent with the

operation of a small-scale

solar dynamo independent of

the global solar dynamo

Continuous Observing of the Entire Solar Disk:

• The SDO/HMI instrument (February 2010 - Present)

HMI provides continuous vector magnetic field measurements of the entire solar disk

HMI: Helioseismic

and Magnetic

Imager

SDO: Solar Dynamics

Observatory

• As a result of innovations in instrumentation and analysis at HAO, spectro-

polarimetry has become a mainstay of observational solar physics

• ASP, Hinode, SDO/HMI continue as facilities freely available to the

international community

• HAO continues to maintain the Hinode/Spectro-Polarimeter database and

provides access to data reduction and analysis tools through the NCAR

CSAC program

Impact of HAO on Solar Spectro-Polarimetry

Present and Planned Research

What Do the Quiet Sun Fields Really Look Like?

Compare realistic numerical simulations of

quiet Sun solar magnetism (M. Rempel) with

observations from Hinode Observed continuum Intensity

Simulated continuum Intensity

Example: seek understanding

of the observed systematic

variation of the weak flux

from disk center to the poles

What Do the Quiet Sun Fields Really Look Like? Hinode Observations:

Horizontal Field (dark=strong) Vertical Field

What Do the Quiet Sun Fields Really Look Like? Rempel Simulations:

Horizontal Field (bright=strong) Vertical Field

Estimates of the magnetic energy of the magnetic fields in these small-scale

dynamo simulations is several orders of magnitude greater than that estimated for

fields involved in the global-scale dynamo

Which active region

filaments emerge from the

solar interior as flux ropes,

and which form their twist

within the solar

atmosphere?

Full-disk vector magnetic

field measurements from

SDO/HMI data are optimal

for this study

The Future of Solar Magnetic Field Measurement

Some aspects of photospheric magnetism not yet understood:

•Weak fields of the quiet Sun apparently do not arise from the global

solar cycle. Do they play an important role in the structure of the Sun?

•What is the distribution of field strength in the quiet internetwork?

•How (quantitatively) does the Sun remove active region fields from

photosphere?

•What is the evolution and dynamics of the chromosphere and corona in

response to changes of the field at and through the photosphere?

•However……

Personal View: Frontier of solar magnetic field studies lies higher in the

atmosphere – chromosphere and corona

Hα line center imaging

from the Swedish Solar

Telescope, Courtesy of G.

Scharmer, M. Carlsson

The Dynamic

Chromosphere

Chromospheric Field Measurements: Future Possibilities

Solar-C (Proposed)

Solar UV-IR-Visible Telescope

(SUVIT) will accomplish

photospheric field measurements to

complement its prime objective of

magnetism of the chromosphere

and corona

Daniel K. Inouye Solar

Telescope/Visible Spectro-

Polarimeter

Major leap for solar observing –

ideal for difficult problem of

measurement of chromospheric

magnetic fields

Images from the Finca LILO in Costa Rica

My deepest gratitude for an extremely

exciting and fulfilling career goes out to

HAO, NCAR, UCAR and the many

people who supported my scientific

adventures here, starting in 1968 as an

HAO graduate assistant. I would

particularly like to acknowledge just a

few of my outstanding HAO mentors

over the years:

Charles Hyder

Grant Athay

Dick White

Lew House

Dimitri Mihalas

Andy Skumanich

Walt Roberts

Gordon Newkirk

John Firor

Bob MacQueen

Peter Gilman

BC Low

Tom Holzer

Michael Knölker