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Exoplanets on highly eccentric orbits offer unique insights into how planetary atmospheres respond to extreme variations in stellar forcing. By tracking the reradiation of heat during the planet.s closest approach to its host star (periapse passage) comma we can test how a planet.s atmosphere responds to extreme radiative forcing comma and directly measure the radiative timescales that remain ambiguous for planets on circular orbits. We propose to observe with HST-WFC3 a partial-orbit spectroscopic phase curve of the highly-eccentric transiting exoplanet comma HAT-P-2b. The planet.s dayside temperature changes from being a typical Hot-Jupiter (1300virgulK) at apoapse comma with no thermal inversion expected comma to that of an Ultra-Hot-Jupiter (2400virgulK) at periapse comma at which temperature thermal inversions are expected. Therefore comma this target offers an opportunity to explore the transition between Hot- and Ultra-Hot-Jupiters. This WFC3 phase curve observation comma will start before transit comma continue through periapse passage comma and end after secondary eclipse. The wavelength range offered by WFC3 will probe deep atmospheric pressure levels comma where dynamical processes become relevant in shaping the observed spatial and temporal flux variations. The spectroscopic phase curve observation through periapse passage will be the key to constraining the timescales of atmospheric response comma and to measure changes in the atmospheric vertical structure and opacities in this atmosphere. Our investigation will disentangle the contributions of radiative comma dynamical comma and chemical processes at work in strongly irradiated atmospheres comma enabling it to become a model for the broader class of Hot and Ultra-Hot Jupiters.