OBSERVATIONAL TEST OF STOCHASTIC HEATING IN LOW-β FAST-SOLAR-WIND STREAMS

Spacecraft measurements show that protons undergo substantial perpendicular heating during their transit from the Sun to the outer heliosphere. In this paper, we use Helios 2 measurements to investigate whether stochastic heating by low-frequency turbulence is capable of explaining this perpendicular heating. We analyze Helios 2 magnetic field measurements in low-β fast-solar-wind streams between heliocentric distances r = 0.29 AU and r = 0.64 AU to determine the rms amplitude of the fluctuating magnetic field, δ B p , near the proton gyroradius scale ρ p . We then evaluate the stochastic heating rate Q ⊥stoch using the measured value of δ B p and a previously published analytical formula for Q ⊥stoch . Using Helios measurements we estimate the "empirical" perpendicular heating rate that is needed to explain the T ⊥p profile. We find that Q ⊥stoch ∼ Q ⊥emp , but only if a key dimensionless constant appearing in the formula for Q ⊥stoch lies within a certain range of values. This range is approximately the same throughout the radial interval that we analyze and is consistent with the results of numerical simulations of the stochastic heating of test particles in reduced magnetohydrodynamic turbulence. These results support the hypothesis that stochastic heating accounts for much of the perpendicular proton heating occurring in low-β fast-wind streams.