First results from SO/PHI’s on-board data reduction

Abstract:

The Polarimetric and Helioseismic Imager (PHI), on-board Solar Orbiter (SO), is a spectropolarimeter imaging the solar photosphere at the wavelengths of the Fe I 617.3 nm Zeeman sensitive absorption line. SO/PHI's aim is to provide data about the magnetic structures and the line-of-sight (LOS) velocity in the solar atmosphere. For this, it takes time series of data sets consisting of 2048 x 2048 pixel images of the Sun at 6 wavelengths, each in 4 different polarisation states. With the minimum necessary 17 bits pixel depth, one data set amounts to approx. 0.2 GB. The guaranteed data telemetry for PHI, in contrast, is only 50 GiB/orbit which would also need to contain any calibration data obtained on-board, i.e. our flat and dark fields. To cope with this discrepancy, SO/PHI is performing full data reduction on-board, including the inversion of the radiative transfer equation. The downloaded results are science ready data, containing 5 final images: a total intensity image from nearby the spectral line, the magnetic field strength, azimuth and inclination (describing the magnetic vector) and the LOS velocity. This process maximises the science return by reducing the number of necessary images in a data set, as well as rendering the download of calibration data unessential. In the commissioning phase of SO/PHI we used the on-board data reduction system successfully for the first time. We have calibrated the instrument to its optimal operational parameters (calculation of exposure time, focus, etc.), acquired and processed calibration data (dark and flat fields), removed the most significant instrumental artefacts from the data (dark field, flat field, polarimetric modulation and polarimetric cross-talk), and performed the inversion of the radiative transfer equation. The data have then been compressed to further maximise the use of our telemetry. This contribution presents and discusses the final results from this process.