Polarization of the eclipse corona (June 21, 2001) by Denis Fiel Société Astronomique de France

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1 Solar Eclipse Conference SEC 004 Polarization of the eclipse corona (June 1, 001) by Denis Fiel Société Astronomique de France. Observations - nstruments - Lens : MTO 1000 (f = 1000 mm, D = 100mm) - Camera : Olympus OM 1 - Film : Kodak Supra Linear polarizing filter (diam : 160mm, in front of the lens) - 6 orientations of the polarizer (α = 0, 30, 60, 90, 10, 150 ) and 3 exposure times for each orientation (1/15 s, 1/4 s, 1 s). Scan of the negatives The negatives were scanned with the Nikon Super CoolScan α = 0 α = 30 α = 60 α = 90 α = 10 α = 150 Scan of the negatives (exposure time :1/4 s ) for six orientations of the linear polarizer.. Conversion from photographic densities to coronal intensities Density of the negatives for 3 exposure times and α = 0 ntensity scale for 3 exposure times and α = 0 1/15 s 1/15 s Conversion (linear response only) γ = 1.57 Conversion to intensity + 1/4 s 1/4 s Due to its large dynamics, the intensity is stored in two 16 bits files (File 1 and File ) : = "File 1" + "File " 16 1 s 1 s page 1/6

2 Polarization of the eclipse corona (June 1, 001) Denis FEL Solar Eclipse Conference SEC 004 V. Polarization As the light emitted by the photosphere is not polarized, the polarisation of the light scattered by the corona is given by : T R p = + where T and R are the tangential and the radial components of the intensity. T We can also get p from 3 α, where α is the intensity measured through the linear polarizing filter oriented in the α direction. For example, with α = 0, 60 and 10 : p = R Here, the polarization was obtained by a χ method from the 6 orientations of the polarizer. The polarization rate error p is about This error comes mainly from the conversion from density to intensity Solar Corona The solar corona radiation has three components : - The K-corona (light scattered by the free electrons around the Sun), - The F-corona (light scattered by the interplanetary dust), - The E-corona (emission lines). The contribution of the E corona is negligible in white light. To get information about the free electrons distribution, as shown by Van de Hulst (Bull. Astron. nst. Neth. 11, 135 (1950)), we need the polarization of the K component. 1. Polarization of the K+F corona Coronal aureola and sky background. - The coronal aureola is the light coming from the inner corona and scattered by the Earth atmosphere. - The sky background is the light doubly scattered by the Earth atmosphere. ts intensity S is assumed to be constant. For more details, see O. Koutchmy and S. Koutchmy, 1974, Astron. Astrophys. Suppl. Ser. 13, S is measured far from the centre of the Sun on a wide field picture. The intensity A(r) of the coronal aureola has been determined by experiments and models. To calibrate A, we need only to know A(0), the value of A(r) on the Moon background. f b and d are respectively the intensity of the bright and the dark zones on the Moon, we have : b = A(0) + S + Bb d = A(0) + S + Bd where B b and B d are the intensity of the light coming from the bright and the dark zones of the Moon see Koutchmy and Laffineur 1970 (Nature 6, 551, 1141). As we can assume that B = B, A(0) = S. b d d b Note : A(0) was got from a wide field picture taken with a MTO 500 (see Fig. 1, page 4). Polarization of the Solar Corona (K + F components) We assumed that the coronal aureola and sky background light are not polarized. We got the polarization of the K + F Corona in subtracting the contribution of S and A(r) from the intensity α (r) : ( A(r) + S) α (r) = α (r) page /6

3 Polarization of the eclipse corona (June 1, 001) Denis FEL Solar Eclipse Conference SEC 004 K + F Corona Polarization (%) isopleths, superposed on a map of polarization ratio sopleths curves superposed on the image of coronal structure. Polarization of the K Corona alone Assuming that the F component is not polarized, the polarization of the K component is deduced from : F(r) α (r) = α (r) Where F(r) is the intensity of the F component (see for example the model of Koutchmy and Lamy, AU Coll. n 85, pp 63-74, 1985). K Corona Polarization (%) isopleths, superposed on a map of polarization ratio sopleths curves superposed on the image of coronal structure page 3/6

4 Polarization of the eclipse corona (June 1, 001) Denis FEL Solar Eclipse Conference SEC 004 ntensity of the K corona along different radial directions : intensity (arbitrary unit) ; to compare to the mean intensity and the Koutchmy model r : radial distance from the Sun center (solar radius) ; θ : azimuthal angle of the radial cuts (see Fig., page 5) Fig. 1 : structure of the Solar Corona in white light. page 4/6

5 Polarization of the eclipse corona (June 1, 001) Denis FEL Solar Eclipse Conference SEC 004 Polarization of the K corona along different radial directions P : polarization rate (%) ; to compare to Van de Hulst model (maximum phase) r : radial distance from the Sun center (solar radius) ; θ : azimuthal angle of the radial cuts (see Fig., page 5) Fig. : Radial directions page 5/6

6 Polarization of the eclipse corona (June 1, 001) Denis FEL Solar Eclipse Conference SEC 004 We can explain the difference between the experimental results and the Van de Hulst model as follows : - As indicated before, p is about But, in some areas (in green on the figure) we deduced a larger p. This can be seen too by computing the polarization angle : the polarization must be tangential ; on the green areas, the orientation of the observed polarization vector can differ from the expected one by The polarization rate given by Van de Hulst assumes a spherical distribution of the free electrons. f we assume that there is only one scattering structure on the line of sight, p can be smaller or larger. β = 90 β = 80 (polarization rate for a spherical distribution : the intensity is integrated over the line of sight). β = 70 β = 60 p β = 50 Sun Plane of the sky β = 40 β = 30 β = 0 β = 10 Earth r Scattering structure β r Polarization rate p due to different positions of structure with respect to the plane of the sky. Van de Hulst model - Bull. Astron. nst. Neth. 11, 135 (1950) - page 6/6