Stellar Drivers for Atmospheric Chemistry and Evolution: UV Exoplanet Characterization from Hubble to LUVOIR

High-energy stellar photons and particles regulate the atmospheric temperature structure and photochemistry on orbiting planets, influencing the long-term stability of planetary atmospheres and the production of potential "biomarker" gases. In the first part of the talk, I will describe results from an ongoing panchromatic survey (Chandra/XMM/Hubble/ground) of M and K dwarf exoplanet hosts, focusing on two main results: 1) the evolution of the high-energy spectral energy distribution as a star's habitable zone moves inward from 1 to 0.1 AU, including implications for the possible abiotic production of the suggested biomarkers O2 and O3, and 2) new estimates of the high-energy particle fluxes (from CMEs/SEPs) from these stars based on solar UV flare/particle flux measurements. I will conclude by discussing UV spectroscopic characterization of exoplanetary systems in the era beyond Hubble. Using the UV capability and high sensitivity of future large space observatories, e.g, the Large/Ultraviolet/Optical/InfraRed Surveyor (LUVOIR) LUMOS spectrograph, we will not only be able to carry out high-fidelity host star characterization of every potentially inhabited planetary system studied by LUVOIR, we will also open the door to statistical-scale studies of atmospheric escape physics for the first time.