Abstract
We analyze observations of the X9.3 solar flare (SOL2017-09-06T11:53) observed by SDO/HMI and Hinode/Solar Optical Telescope. Our aim is to learn about the nature of the HMI pseudocontinuum I-c used as a proxy for the white-light continuum.
From model atmospheres retrieved by an inversion code applied to the Stokes profiles observed by the Hinode satellite, we synthesize profiles of the FeI 617.3 nm line and compare them to HMI observations. Based on a pixel-by-pixel comparison, we show that the value of I-c represents the continuum level well in quiet-Sun regions only.
In magnetized regions, it suffers from a simplistic algorithm that is applied to a complex line shape. During this flare, both instruments also registered emission profiles in the flare ribbons.
Such emission profiles are poorly represented by the six spectral points of HMI and the MDI-like algorithm does not account for emission profiles in general; thus, the derived pseudocontinuum intensity does not approximate the continuum value properly.