Prospective mesure du champ magnétique coronal Jean ARNAUD Laboratoire d’Astrophysique Observatoire Midi-Pyrénées Toulouse, France

The solar corona is a high temperature and low β 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. Coronal Emission Lines (CEL) give access to coronal magnetic fields

Coronal magnetic field measurements in the inner corona The Fe XIII nm line is sensitive to Zeeman and Hanle effects: very few V measurements yet (Lin et al. 2000, Lin et al 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 ?

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 90 0 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 nm Fe XIII line (W 2 =1 and g= 1.5) magnetometry, complemented by the nm Fe XIII line intensity to constrain the coronal model as the ratio of the two Fe XIII lines depends on N ion. The nearby nm He I line can nicely complement those data.

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. LCVR Polarimeter Input array of fiber optics bundle Re-imaging lens Prime focus inverse occulter/field stop Secondary mirror Primary mirror SHINE 2004, Big Sky, Montana Jeff Kuhn and Haosheng Lin IFA, University of Hawaii

Line-of-Sight Magnetic Fields measured in the Fe XIII nm line B B 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 h=0.17 R 

Radial Variations of B and Comparison with Model Calculations ( Radial Variations of B and Comparison with Model Calculations (Lin, Kuhn & Coulter, ApJ 613,177, 2004) AAS 2004 Meeting, Denver, Colorado 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.

Orientation of Coronal Magnetic Fields AAS 2004 Meeting, Denver, Colorado 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.

Southwest limb, Q (Display range -10, 10) Scattered Photospheric Si He 1083 Q>0  perpendicular polarization to limb Faint He I 1083 nm coronal component Kuhn, Arnaud, Jaeggli, Lin, SPW4, Sept 2005

1024  1024 Rockwell Detector ± 1.5 Rsun Field-of-View, 4 arcsec Pixels Augment with Spectroscopy at Evans Sac Peak 20 cm “One Shot” Coronagraph HAO (NCAR, Boulder) project, PI: Steve Tomczyk

Birefringent filter bamdpasses: Yellow: 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.

Steven Tomczyk Stokes polarimetry of an eruptive prominence in He I nm

Steven Tomczyk CoMP , 31 Aug 2004

Intensité (image de gauche) de la raie nm du Fe XIII et quantité de lumière polarisée dans cette raie. La raie 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.

Projets aux Etats-Unis ATST: télescope solaire généraliste de 4 mètres de diamètre installé à Hawaii, horizon 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

* Telluric absorptions and thermal atmopheric emissions are very 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 row Antarctica 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

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

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

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.