Radial Evolution of Stochastic Heating in Low-β Solar Wind

We consider the radial evolution of perpendicular ion heating due to the violation of magnetic moment conservation caused by Alfvén and kinetic Alfvén wave turbulence. This process, referred to as stochastic heating (SH), can be quantified by the ratio between the average velocity fluctuations at the ion gyroradius and the perpendicular ion thermal speed . Using 17 yr of Helios observations, we constrain how much energy could be dissipated by this mechanism between 0.29 and 0.98 au. We find that SH likely operates throughout the entire inner heliosphere, but that its radial dependence is steeper than that of empirically derived dissipation rates, with r ^-2.5 being compared with r ^-2.08. This difference is significantly increased in fast solar wind streams to r ^-3.1 compared with r ^-1.8.