Stochastic Heating of Protons by Alfven Waves Turbulence Near R = 0.3 AU in Fast Solar Wind

Several solar-wind and coronal observations show that ions undergo substantial perpendicular heating during their transit time from the Sun to the outer Heliosphere. In this work, we test the hypothesis that this perpendicular heating results from the dissipation of Alfven-wave (AW) turbulence via a process called stochastic heating. To do so, we examine in-situ measurements of velocity distribution functions and magnetic field vectors from the Helios satellite. We use these measurements to evaluate the magnitude of the various terms in a previously derived analytical formula for the stochastic heating rate --- terms such as the amplitude of the turbulence at the scale of the proton gyroradius. We then compare the resulting stochastic heating rate with the amount of heating that would be required to explain the radial profile of the perpendicular temperature of the protons as measured by Helios for heliocentric distances between 0.29 and 0.4 AU.