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

A notable fraction of helium observations probing the evaporating atmospheres of short-period gas giants at 1083~nm exhibit a blueshift during transit, which might be indicative of a day-to-night side flow. In this study, we explore the gas dynamic effects of day-to-night temperature contrasts on the escaping atmosphere of a tidally locked planet. Using a combination of 3D hydrodynamic simulations and radiative transfer post-processing, we modeled the transmission spectra 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 and an increase in the blueshift of the line centroid of a few km~s$^{-1}$. (2) The velocity shift of the line depends on the line-forming altitude, with higher planetary mass-loss rates causing the line to form at higher altitudes, resulting in a more pronounced velocity shift. (3) A critical point of day-night anisotropy comes about when the blueshift saturates, due to turbulent flows 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 time variations in the velocity shift. Assuming that the day-night temperature gradient is the main cause of the observed blueshifts in the He-1083~nm triplet, the correlation between the velocity shift and day-night anisotropy provides an opportunity to constrain the temperature gradient of the line-forming region.