Prospective mesure du champ magntique coronal

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Prospective mesure du champmagnétique coronal

• Jean ARNAUD
• Laboratoire dAstrophysique
• Observatoire Midi-Pyrénées
• Toulouse, France

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Coronal Emission Lines (CEL) give access to
coronal magnetic fields
The solar corona is a high temperature and low ß
plasma where the magnetic field controls or
influences everything from loops heating to
flares to coronal mass ejections. A comprehensive
understanding of many coronal phenomena
(including heating, particules acceleration,
stability or instability of coronal loops, ...)
requires B measurements. This field is an
important component of the solar-terrestrial
system, permanently observed from space missions
like Yohkoh, SoHO, TRACE and, soon, STEREO and
Solar B. However it remains hidden to most
coronal observations and no existing or planned
space mission is designed to measure coronal
magnetic field. the corona. CEL Stokes
polarimetry is the only direct way to access
magnetic fields in the low corona.
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Coronal magnetic field measurements in the inner
corona

• The Fe XIII 1074.7 nm line is sensitive to Zeeman
  and Hanle effects very few V measurements yet
  (Lin et al. 2000, Lin et al. 2004 found B values
  between 2 and 33 G)
• Other IR lines, like the Si IX 3.93 µm line may
  be of stong interest for B coronal measurements.
• In the UV domain Hanle effect dominates
  Advanced Solar Coronal Explorer Mission (ASCE) is
  a proposed NASA mission including a UV
  coronograph with polarimetry in Lyman lines of H
  and in OVI 103 nm emission line.
• LYOT mission ?

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The projects underway

• Their aim is Full Stokes Coronal Emission Lines
  polarimetry
• Linear polarization Compared to first
  observations (KELP, Pic du Midi Coronameter), it
  is now possible to built more realistic models
  able to resolve more often the 900 ambiguity due
  to the Van Vleck effect, this thanks to a much
  better spatial resolution and the availability of
  complementary observations from ground and space
  coronagraphs.
• Circular polarization Will give the Line Of
  Sight magnetic field magnitude.
• Ideal case the vector magnetic field may be
  determined.
• Observations The 1074.7 nm Fe XIII line (W2 1
  and g 1.5) magnetometry, complemented by the
  1079.8 nm Fe XIII line intensity to constrain
  the coronal model as the ratio of the two Fe XIII
  lines depends on Nion . The nearby 1083.0 nm
• He I line can nicely complement those data.

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SOLARC Off-Axis Mirror Coronagraph
SOLARC and its dome on the summit of Haleakala,
Maui. SOLARC is a 50 cm aperture off-axis mirror
coronagraph. A field stop located at its prime
focus serves as an inverse occulter to select the
coronal target region and reject the glaring
photospheric radiation. The FOV (field-of-view)
of the telescope is approximately 400 arcsec in
diameter. A LCVR-based (Liquid Crystal Variable
Retarder) polarimeter at the gregorian focus
analyzes the polarization of the CEL before the
fiber optics bundle.
Secondary mirror
Prime focus inverse occulter/field stop
Re-imaging lens
LCVR Polarimeter
Input array of fiber optics bundle
Jeff Kuhn and Haosheng Lin IFA, University of
Hawaii
Primary mirror
SHINE 2004, Big Sky, Montana
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Line-of-Sight Magnetic Fields measured in the Fe
XIII 1074.7 nm line
Samples 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 h0.17 R?
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Radial Variations of B and Comparison with Model
Calculations (Lin, Kuhn Coulter, ApJ 613,177,
2004)
Average B as a function of height from the limb
from the center of the FOV. The solid line with
errors plots the IR data. The dotted line shows
the Abbett et al. (2003) near-limb "breakout"
magnetic model scaled to an active region with
1000G longitudinal field strength at the
photosphere. It doesn't extend high enough for a
good comparison. The with error bars are the
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. The observed unsigned
field strength is qualitatively similar to that
of the Ledvina B model.
Averaged and fitted Stokes I V spectra from the
first 10 north-south columns used to construct
the B radial variation plot.
AAS 2004 Meeting, Denver, Colorado
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Orientation of Coronal Magnetic Fields

• Properties of Linear Polarization of Coronal
  Emission Lines
• Direction of linear polarization maps the
  orientation of coronal magnetic fields projected
  on the plane of sky
• The measurement of the orientation of B is
  subject to a 90 ambiguity due to the Van Vleck
  Effect.
• In this linear polarization map, the lengths of
  the lines are proportional to the degree of
  linear polarization P, while their orientation
  maps the direction of the linear polarization.
  The background gray scale intensity image is the
  EIT FeXVI 284 image. The loop structures seen in
  EIT or TRACE intensity images are usually
  interpreted as the tracer of the magnetic field
  lines. If this is indeed the case, then we would
  expect to see the orientation of the coronal
  magnetic fields, as inferred by the orientation
  of P, closely follow the EIT intensity
  structures. While this appears to be the case on
  the large scale, the polarization vector does
  not seem to follow the loop structure located at
  approximately (1100, 100) in the EIT image.
• The observed degree of linear polarization P
  increases as a function of height in general, as
  expected from the theory of CEL polarization.
  Notice that this is not the case in the first few
  rows of the southern-most field, where P
  decreases in height first. This could be due to
  the magnetic field angle approaching the Van
  Vleck angle.

AAS 2004 Meeting, Denver, Colorado
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Southwest limb, Q
Qgt0 ?perpendicular polarization to limb
(Display range -10, 10)
Scattered Photospheric Si
He 1083
Faint He I 1083 nm coronal component Kuhn,
Arnaud, Jaeggli, Lin, SPW4, Sept 2005
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Sac Peak 20 cm One Shot Coronagraph
1024 ?1024 Rockwell Detector 1.5 Rsun
Field-of-View, 4 arcsec Pixels Augment with
Spectroscopy at Evans
HAO (NCAR, Boulder) project, PI Steve Tomczyk
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Birefringent filter bamdpasses Yellow 1074.7
nm FeXIII line Red near-by continuum Doted
lines line bandpass shifted to measure V. This
filter is well suited for continuum
substraction and precise lines width and radial
velocities measurements.
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Stokes polarimetry of an eruptive prominence in
He I 1083.0 nm
Steven Tomczyk
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CoMP 1074.7, 31 Aug 2004
Steven Tomczyk
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• Intensité (image de gauche) de la raie 1074.7 nm
  du Fe XIII et quantité
• de lumière polarisée dans cette raie. La raie
  1079.8 nm du même ion a été également observée.
  Le rapport d'intensité de ces deux raies,
  indicateur de la densité électronique, varie ici
  entre 1.5 et 3 environ.
• Observation du 21 avril 2005.

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Projets aux Etats-Unis

• ATST télescope solaire généraliste de 4 mètres
  de diamètre installé à Hawaii, horizon 2014.
  Magnétométrie coronale dans les raies IR proche
  ou thermique,
• à haute résolution spatiale sur programme.
• Magnétomètre coronal dédié d'environ un mètre de
  diamètre. Projet
• également à installer à Hawai, observations
  systématiques dans les raies du proche IR,
  horizon 2010 .

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Telluric absorptions and thermal atmopheric
emissions arevery weak in the Infrared Very
pure, stable and dark skies near to the Sun (no
aerosols) Outstanding image quality 0.2 arcsec
seeing possible for several hours during the day
Possibility of observing 24 hours a day and up
to 15 days in a rowAntarctica is likely to be
the only location where very high resolution
observations of the innermost visible and IR
corona may be performed. Such observations are
not possible from space.
Dôme C
Photo Lucia S. Simion
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Some questions coronal magnetometry will
adress - Coronal loops magnetic structure are
loops flux tubes ? How much twist in the field ?
Energy storage? How loops are rooted in the
underlaying atmosphere? - Prominence cavities
magnetic structure - Dynamics of coronal magnetic
field during Prominences eruptions, flaring
active regions, CMEs - How does the magnetic
field expend from the photosphere into the
corona? - Extrapolations need to be compared
with magnetic field observations - Intensity and
magnetic field oscillations MHD modes, energy
transfert - Large scale magnetic fields
magnetic coupling between the corona and the
solar dynamo Very low corona observations
will strongly benefit to underlined topics

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Pic du Midi

• La magnétométrie coronale est considérée pour ce
  site dans le cdre du projet 2010.
• Ce projet complémentera parfaitement en
  longitude les instruments installés à Hawai,
• dans un site reconnu pour la pureté du ciel.

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The Sky Brigthness Monitor (SBM)
The SBM was built for ATST site testing It is a
small, automated, externally occulted coronagraph
which measures the sky brightness from 4 Rsol to
8 Rsol from the blue to the near IR Improved SBM
are planned to test two sites in Hawaii, Pic du
Midi and Dome C.