Pronounced proton core temperature anisotropy, ion differential speed, and simultaneous Alfvén wave activity in slow solar wind at 0.3 AU

In the Helios 2 perihelion (0.3 AU) in May 1978, pronounced ion differential speeds Δv α p >0 and large proton temperature anisotropies T ∥ p / T ⊥ p <1 have been observed in slow solar wind, in distinct contrast to earlier observations during the phase of solar activity minimum. These features of ion distributions occurred simultaneously with high Alfvénic wave activity. Coulomb friction between the two ion species can be shown to play a minor role, thus obviously favouring the establishment of a large differential ion speed by preferentially accelerating wave forces. The observations may also indicate that strong waves sometimes only heat the protons without creating a fast solar wind.