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We present the analysis of a full-orbit, spectroscopic phase curve of theultra hot Jupiter WASP-18b, obtained with the Wide Field Camera 3 aboard theHubble Space Telescope. We measure the planet's normalized day-night contrastas >0.96 in luminosity: the disk-integrated dayside emission from the planet isat 964+-25 ppm, corresponding to 2894+-30 K, and we place an upper limit on thenightside emission of <32ppm or 1430K at the 3-sigma level. We also find thatthe peak of the phase curve exhibits a small, but significant offset inbrightness of 4.5+-0.5 degrees eastward.  We compare the extracted phase curve and phase resolved spectra to 3D GlobalCirculation Models and find that broadly the data can be well reproduced bysome of these models. We find from this comparison several constraints on theatmospheric properties of the planet. Firstly we find that we need efficientdrag to explain the very inefficient day-night re-circulation observed. Wedemonstrate that this drag could be due to Lorentz-force drag by a magneticfield as weak as 10 Gauss. Secondly, we show that a high metallicity is notrequired to match the large day-night temperature contrast. In fact, the effectof metallicity on the phase curve is different from cooler gas-giantcounterparts, due to the high-temperature chemistry in WASP-18b's atmosphere.Additionally, we compare the current UHJ spectroscopic phase curves, WASP-18band WASP-103b, and show that these two planets provide a consistent picturewith remarkable similarities in their measured and inferred properties.However, key differences in these properties, such as their brightness offsetsand radius anomalies, suggest that UHJ could be used to separate betweencompeting theories for the inflation of gas-giant planets.

Climate of an Ultra hot Jupiter: Spectroscopic phase curve of WASP-18b with HST/WFC3