Intermittent turbulence as the driver of stochastic heating in the near-Sun solar wind

Abstract

Understanding the mechanism by which the energy is dissipated at small scales to heat the outer corona and expanding solar wind, is a primary goal of the Parker Solar Probe (PSP) mission, and an outstanding question in heliophysics. Recent observational case studies have suggested that the dominant dissipation mechanism(s), stochastic heating (SH) or resonant heating (RH), may be modulated by the Alfvén `surface' ($M_A = 1$), while theoretical and simulation works predict that the mechanism should be modulated by the level of turbulence imbalance. We measure the local energy transfer (LET) rate from $9.8$ to $55$ solar radii from PSP observations, showing that the LET rate peaks at $M_A \sim 0.4 - 0.9$. We connect enhanced LET rates to signatures of intermittency, highlighting the key role that intermittent fluctuations play in heating the near-Sun solar wind. We compare the LET rate with estimates of the SH rate, finding that below $\sim 15$ solar radii the SH heating rate matches the LET rate provided intermittency is accounted for, regardless of $M_A$, while farther from the Sun, SH is unable to account for the observed LET rate. In contrast to predictions by recent theoretical works, this implies that `imbalanced' intermittent stochastic heating may be a viable mechanism to heat highly imbalanced, low-$β$ collisionless plasmas, and is likely an important mechanism of dissipation in the outer solar corona and near-Sun solar wind.