Understanding HMI pseudocontinuum in white-light flares
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| Number: | 324 |
| 1st Author: | Michal Švanda |
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| Published: | 28 May 2018 |
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Introduction
The most violent phenomena of solar activity are termed flares. Aside from the physical processes underlying them, these flares contain sources of electromagnetic radiation at all wavelengths from radio waves to gamma-rays. White-light flares (flares with emission in the visible continuum) are probably not a rare phenomenon. The enhancement relative to the photospheric brightness can be low, typically a few tens of percent, and it can be challenging to detect even in an X-class event. The energy contained in the white-light flare continuum represents a large fraction of the total flare energy.
These infrequent white-light flares have appeared to observers via many different instruments in the past. In order to properly study this phenomenon, continuous high-resolution data are definitely desireable. A synoptic experiment such as the HMI instrument on the SDO satellite therefore represents the holy grail of white-light flare observation. The HMI data have now been used in several studies of white-light flares. These data consist of sequences of exposures at different polarizations across six narrow wavelength bands capturing an Fe I line at 617.3 nm, an absorption line formed in the [photosphere]. Students of white-light flares in these data usually employ the standard data product called Ic, an estimate of the continuum level based upon this coarsely observed line profile. This is available every 45 s, a coarse sampling of the actual flare variations.
Comparison of HMI and Hinode observations
We have studied an X9.3-class flare that occurred on September 6, 2017 (SOL2017-09-06T11:53). We take advantage of the observations taken by the Solar Optical Telescope (SOT) aboard the Hinode satellite. For the first time, Hinode recorded detailed emission profiles during such an event, while HMI observed in its regular mode. We could use of the full set of HMI data across its polarization and wavelength settings for an exact comparison. Hinode/SOT observed the Stokes profiles of the two Fe I lines at 630.15 nm and 630.25 nm in a raster scan covering the flare ribbons. The two sets of observations were aligned both in space and time to ensure that we compared the profiles observed at the same pixel (see Fig. 1). Both HMI and SOT observe the solar photosphere, yet in different spectral lines. Given the similar formation height of the observed spectral lines, we use a complete model atmosphere derived from the Hinode observations to synthesise the Stokes profiles of the HMI line. This essentially calibrates the coarse HMI data product against a much more precise Hinode/SOT observation.
