| Abstract: | Since the launch of JWST, the community has found that the main source of noise in transmission spectroscopy of exoplanet atmospheres is magnetic activity. So far, all efforts to mitigate this influence have been limited to the study of the influence of spots, faculae and flares. However, the fact that each star has an optically thin outer layer, the chromosphere and corona, has thus far been neglected, mostly due to the assumption that these layers emit only at wavelengths in the UV and X-ray, and hence do not have any influence on transmission spectra acquired in the near-IR and IR. In this presentation, I show that this assumption is false, and that the neglection of chromosphere and corona is the dominant source of bias in JWST transmission spectra. I demonstrate this using a combination of the NIRISS/SOSS data set of HAT-P-18 b in conjunction with a CHIANTI plasma model. |