Data Reduction

Data Acquisition:

Lines selected: CaI 6122 Å line to derive the vector magnetograms and the photospheric Doppler maps. Ha or Ha-1Å for chromospheric imaging.

Measurements: Sets of 6 Stokes images I+Q, I-Q, I+U, I-U, I+V, I-V, recorded in both wings of the CaI line. 12 Ha or Ha-1Å filtergrams.

Preliminary reduction:

Dark current and flat field correction:

Computation of Stokes parameters V/I, Q/I, U/I. For example:

Compute LOS vector fields: Use V/I, Q/I, and U/I images from the red wing of the CaI line.

Calculate Longitudinal () and Transversal () field components:

L, T: calibration parameters. Determined by comparing FGE data with Hawaii IVM vector magnetograms of the same region acquired at the same time.

Calculate vector field direction:

Transformation from LOS to local coordinate frame:
  • Gamma to local Zenith Angle: 0° ==> vertical UP, 90° ==> horizontal, 180° ==> vertical DOWN.
  • Chi to local Azimuth: counted positive counterclockwise, starting from the local West.

    • Solution of the 180° ambiguity:

    1. Comparison of vectorfiled with a simple potential field extrapolation of the observed region. Derived by only considering the main magnetic polarities present in the field of view.
    2. Application of a smoothing filter to ensure continuity in the azimuthal vector field maps.
    3. Visual inspection to manually correct for local, small geometrical inconsistencies.

    Compute Doppler velocity maps: Average filtergrams recorded on both wings (Iblue, Ired) of the CaI line:

    V = c × (Iblue - Ired ), c : calibration parameter. Determined by comparing FGE and Hawaii IVM dopplergrams of the same region and recorded simultaneously. Positive values of V denote UP-flows.

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