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?
Joseph Plowman Fe XIII 10747 and 10798 in the Corona
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...
Joseph Plowman Fe XIII 10747 and 10798 in the Corona
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
Joseph Plowman Fe XIII 10747 and 10798 in the Corona
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.
Joseph Plowman Fe XIII 10747 and 10798 in the Corona
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.
Joseph Plowman Fe XIII 10747 and 10798 in the Corona
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.
Joseph Plowman Fe XIII 10747 and 10798 in the Corona
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.
Joseph Plowman Fe XIII 10747 and 10798 in the Corona
10747 Azimuth
Joseph Plowman Fe XIII 10747 and 10798 in the Corona
10747 Intensity Data vs. Model
Joseph Plowman Fe XIII 10747 and 10798 in the Corona
10798 L/I Data vs. Model
Joseph Plowman Fe XIII 10747 and 10798 in the Corona
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!
Joseph Plowman Fe XIII 10747 and 10798 in the Corona
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...
Joseph Plowman Fe XIII 10747 and 10798 in the Corona
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...
Joseph Plowman Fe XIII 10747 and 10798 in the Corona