Presentation #504.03 in the session Plenary 8.
Recent atmospheric models and observations of transiting exoplanets have shown that metastable helium is an invaluable tracer of their upper atmospheres. Ground-based, high-resolution spectrographs are able to resolve the strong absorption line of metastable helium at 10830 Angstroms, and probe the morphology and kinematics of escaping planetary winds. These data will help answer long-standing questions about stellar wind strengths; planetary magnetic fields; and the nature of sub-Neptunes. I will present three helium studies with Keck/NIRSPEC: a post-transit tail sculpted by a strong stellar wind; an emission spectrum of a hot Jupiter with implications for its magnetic field; and a transmission spectrum of a sub-Neptune which is inconsistent with a previously reported detection.