A complex structure of escaping helium spanning more than half the orbit of the ultra-hot Jupiter WASP-121b

This dataset contains the full-orbit helium phase-curve observations of the ultra-hot Jupiter WASP-121 b obtained with the James Webb Space Telescope (JWST) Near Infrared Imager and Slitless Spectrograph (NIRISS). The Python scripts used to produce the figures shown in the paper are provided with a README file, the extracted data from NAMELESS and exoTEDRF, the best-fit EVE toy-model. The data include flux measurements, wavelength calibrations, and associated uncertainties, capturing helium absorption over nearly 60% of the planetary orbit. These observations reveal a strong atmospheric outflow forming two distinct tails with different kinematics and provide critical constraints on hydrodynamic escape in highly irradiated exoplanets. The dataset supports analyses of mass-loss rates, atmospheric dynamics, and comparisons with theoretical models. It is intended for use in exoplanet atmosphere studies, model validation, and educational purposes.