Intensity and Linear Polarization of Fe XIII 10747 and ... · Intensity and Linear Polarization of Fe XIII 10747 and 10798 in the Corona Joseph Plowman December 4, 2014 Joseph Plowman

Intensity and Linear Polarization of Fe XIII 10747

and 10798 in the Corona

Joseph Plowman

December 4, 2014

Joseph Plowman Fe XIII 10747 and 10798 in the Corona

Introduction: The Corona

I Tenuous, optically thin

atmosphere of sun

I Brightness is ∼ 10 millionths

that of photosphere

I Magnetic field also weaker

(∼ 10 Gauss), but dominates

plasma

I Magnetic structuring clearly

visible

I How to measure magnetic field?


Coronal Polarimetry at 1 Micron

I 10747 and 10798 coronal lines sensitive to magnetic field

I Stokes I brightness is 10 millionths of solar diskI Requires coronagraph (can’t look on disk)I Relatively easy to measure...

I Stokes Q and U are 10% of Stokes II Still not too hard to measureI Sensitive to geometry of fieldI Not sensitive to field strength (‘Saturated Hanle Effect’

regime)

I Zeeman is effect is sensitive to field strength, but shift is <<line width

I Stokes V is 0.1% of Stokes I (10 billionths of solar disk)I Very difficult to measure, but not impossible...I Still a work in progress...


The Coronal Multichannel Polarimeter (CoMP)

I Designed to measure Fe XIII10747 & 10798

I Dual-beam (FG/BG)

CoronagraphI Tunable Lyot filterI Measures full Stokes

vectorI 20cm refracting telescopeI Installed at Mauna Loa


Stokes Vector

I Stokes vector for 10747 A line is:

I1 = I01 +

√9

8I01σ

201

(cos2 ΘB −

1

3

), (1)

Q1 = −√

9

8I01σ

201 sin2 ΘB cos(2ΦB), (2)

U1 = −√

9

8I01σ

201 sin2 ΘB sin(2ΦB), (3)

V1 = −√

9

8I01ωB cos ΘB(

√2 + σ201). (4)

I 10798 A has different coefficients, I0, and σ20.


Single-point Inversion of Stokes Vector

I Stokes Vectors can be analytically inverted to find parameters(Plowman ApJ 2014):

I Requires single-point assumptionI Noise introduces bias in analytic inversion of cos ΘB

I Can be reduced by applying priors (Bayesian/MCMC)I Errors in cos ΘB still largeI More detailed modeling may help; TBD/Work in progress...

I ΦB , BLOS straightforward to invert, free of (formal) biasI ΦB maps routinely available from CoMPI BLOS is work in progress; SNR is marginal (10−3), systematics,

bias...

I This talk will concentrate instead on magnitude of linear

polarization (L ≡√Q2 + U2), alignment.


Magnitude of Linear Polarization

I L/I determined by alignment, σ20, and sin2(ΘB):

L/I ∝ σ20 sin2 ΘB (5)

I For purely radiative excitation (i.e., by the photosphere)

σ20 =3 cos2 ΘB − 1

2√

2w(ω), w ≈ 1 + cos θM

2cos θM (6)

I For purely (isotropically) collisional excitation, σ20 = 0I No linear polarization; Collisions efface alignment signature

I 10747 alignment is at most ∼ ±0.42 in solar case.

I Models predict that collisions will dominate 10798 to greater

extent than 10747; L/I will be much smaller for 10798.


Motivation

I Why look at these data?I More directly related to magnetic field geometry, easier to

compare with magnetic field modelsI Haven’t been studied before (especially 10798); don’t know

what new insights may be presentI Alignments and intensities can shed light on atomic physics;

models predict specific relationship between density (from

intensity ratio, for instance) and alignment (cf. Arnaud &

Newkirk)

I CaveatsI Significant systematic effects present in 10798 data (no one

has looked closely at it before).I More of an advertisement and status report.


10747 Azimuth


10747 Intensity Data vs. Model


10798 L/I Data vs. Model


10798 L/I Radial Plot

Figure: 10798 Data L/I is much larger than for model. However, much of

this is likely due to systematics and calibration!


Q/I & U/I (Thresholded to ±1)

Figure: Top row: 10747 Q/I (left), U/I (right). Bottom row; same for

10798. Each should be roughly equal positive and negative...


Discussion

I Spectropolarimetry of these and similar lines largely untapped

for insight on coronal magnetic field and plasma

I Calibration and removal of systematic effects for this kind ofdata is not trivial

I Flat-fielding, dual-beam background, etc all carried outI 10798 Stokes Q and U (and 10747 Stokes V) still not

adequately correctedI Will be refined (much easier to check and update in solar case)

I PSIMAS model shows similarity to data for L/I in 10747 line,but details are quite different

I Understandable, as this kind of coronal modeling not a mature

field!I Considerable room for improvement nonetheless...


Documents

Intensity and Linear Polarization of Fe XIII 10747 and ... · Intensity and Linear Polarization of Fe XIII 10747 and 10798 in the Corona Joseph Plowman December 4, 2014 Joseph Plowman