HST

Stellar activity produces two main diagnostics within low-resolution exoplanet transmission spectra. Its highly chromatic nature imparts trends in the underlying spectrum that become most noticeable at shorter wavelengths due to the increased contrast between the flux from the active regions (i.e. spots and/or faculae) and the quiet photosphere. The second characteristic is that activity is inherently time-variable, predominantly modulated by stellar rotation as active regions rotate into and out of view but also with contributions from longer timescales of evolution/decay and even long-term activity cycles e.g. maxima and minima (although certain configurations e.g. polar spots and active latitudes will be more resistant to this). This variability can reveal itself through subsequent observations of the system and is both useful and challenging; it can further help us to identify and constrain potential contamination but also means that observations at different epochs may require individual corrections before they can robustly be combined and analysed together which will become increasingly difficult for smaller planets as the SNR of each visit progressively decreases. In this seminar I will present how both diagnostics have been used with archival HST WFC3 and STIS datasets to create two new activity metrics that are highly complementary to existing indicators. I will also show preliminary results surrounding how this work is being extended to Tier 2 and Tier 3 Ariel simulations to explore how stellar variability will impact our ability to stack visits to obtain the required SNR for these tiers.