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A notable fraction of helium observations probing the evaporating atmospheresof short-period gas giants at 1083~nm exhibit a blueshift during transit, whichmight be indicative of a day-to-night side flow. In this study, we explore thegas dynamic effects of day-to-night temperature contrasts on the escapingatmosphere of a tidally locked planet. Using a combination of 3D hydrodynamicsimulations and radiative transfer post-processing, we modeled the transmissionspectra of the metastable helium triplet. Our key findings are as follows: (1)Increasing the day-night anisotropy leads to a narrowing of the helium line andan increase in the blueshift of the line centroid of a few km~s$^{-1}$. (2) Thevelocity shift of the line depends on the line-forming altitude, with higherplanetary mass-loss rates causing the line to form at higher altitudes,resulting in a more pronounced velocity shift. (3) A critical point ofday-night anisotropy comes about when the blueshift saturates, due to turbulentflows generated by outflow material falling back onto the planet's night side.(4) A strong stellar wind and the presence of turbulent flows may induce timevariations in the velocity shift. Assuming that the day-night temperaturegradient is the main cause of the observed blueshifts in the He-1083~nmtriplet, the correlation between the velocity shift and day-night anisotropyprovides an opportunity to constrain the temperature gradient of theline-forming region.

Effects of planetary day-night temperature gradients on He 1083 nm transit spectra