Title: The dehydration of water worlds via atmospheric losses,
Abstract: We present a three-species multi-fluid MHD model (H$^+$, H$_2$O$^+$ and
e$^-$), endowed with the requisite atmospheric chemistry, that is capable of
accurately quantifying the magnitude of water ion losses from exoplanets. We
apply this model to a water world with Earth-like parameters orbiting a
Sun-like star for three cases: (i) current normal solar wind conditions, (ii)
ancient normal solar wind conditions, and (iii) one extreme "Carrington-type"
space weather event. We demonstrate that the ion escape rate for (ii), with a
value of 6.0$\times$10$^{26}$ s$^{-1}$, is about an order of magnitude higher
than the corresponding value of 6.7$\times$10$^{25}$ s$^{-1}$ for (i). Studies
of ion losses induced by space weather events, where the ion escape rates can
reach $\sim$ 10$^{28}$ s$^{-1}$, are crucial for understanding how an active,
early solar-type star (e.g., with frequent coronal mass ejections) could have
accelerated the depletion of the exoplanet's atmosphere. We briefly explore the
ramifications arising from the loss of water ions, especially for planets
orbiting M-dwarfs where such effects are likely to be significant.