Atmospheric escape can play a significant role in the evolution of close-in small exoplanets. These planets experience a large amount of UV irradiation which heats their upper atmosphere and drives mass loss in the form of a hydrodynamic wind. This process can strip these planets of their primordial hydrogen/helium envelopes and has been used to explain the lack of short period Neptune sized planets (e.g hot Neptune desert) and the bimodal radius distribution of small planets (e.g radius valley). Planetary magnetic fields can substantially change the predicted mass loss rate for these planets and therefore is an uncertainty when using atmospheric escape models to match the location of these regions.
We present a method to detect the magnetic fields of close-in exoplanets undergoing atmospheric escape using transit spectroscopy in the 10830 Å line of helium. We suggest that planets with magnetic fields too weak to control the topology of the outflow lead to blue shifted transits on the order of the sound speed of the gas due to day-to-night flows. In contrast, strong magnetic fields prevent the day-to-night flow leading to red shifted transits. High resolution observations, available for helium 10830 Å, can distinguish between these profiles. Therefore our model provides a potential way to constrain the magnetic fields of these planets.