Deciphering transmission spectra by exploring the solar paradigm

TL;DR

This paper investigates how stellar magnetic activity, especially spots and faculae, affects the measurement of exoplanet atmospheres via transmission spectroscopy, using the Sun as a benchmark to understand chromatic radius variations.

Contribution

It introduces a detailed method to model the chromatic effects of stellar activity on apparent stellar radius, emphasizing the importance of facular CLV in accurate transmission spectroscopy analysis.

Findings

Facular CLV significantly impacts apparent radius estimates.

Activity-induced variations can exceed JWST's noise floor for Jupiter-like transits.

Simplified models underestimate the radius, especially for faculae.

Abstract

Transmission spectroscopy probes exoplanet atmospheres via the wavelength dependence of transit depths, but stellar contamination from magnetic activity can significantly bias these measurements. Activity-induced changes in the chromatic apparent stellar radius represent a major challenge for atmospheric characterisation. As surface distributions of magnetic features are generally unknown for stars other than the Sun, we adopt the Sun as a benchmark to study how the chromatic effect depends on the distribution of spots and faculae. Using spot and facular masks derived from SDO/HMI magnetograms and intensitygrams, combined with the SATIRE model, we compute the chromatic dependence of the Sun's apparent radius. We test different methods of convolving surface coverage with spectra the identify physical drivers of the effect. We find that simplified approaches, which neglect the CLV, underestimate the apparent radius, particularly for faculae, whose surface coverage dominates at near-solar activity levels. Proper treatment of facular CLV is therefore essential. The activity-induced variation between solar minimum and maximum reaches around 40 ppm for a Jupiter-like transit, exceeding JWST's expected 10 ppm noise floor, while remaining at around 0.4 ppm for an Earth-like transit.