Observations of Preferential Heating and Acceleration of α-particles in the Young Solar Wind by Parker Solar Probe

Abstract Remote sensing observations of the nascent solar wind in the solar corona imply that heavy ions are often faster and hotter than the plasma protons. In situ observations of the near-Earth solar wind with low collisional age show that hotter and faster states of heavy ions usually do not occur at the same time: heavy ions are either faster or hotter than the protons. This discrepancy highlights the need to understand the competing effects of heating, acceleration, Coulomb collisions, and instabilities on heavy ions in the inner heliosphere, from the solar corona to Earth. The Parker Solar Probe (PSP) is currently the closest spacecraft to the Sun and is anticipated to cross the outer boundary of the preferential heating and acceleration zone. Using in situ measurements from PSP, we find that larger drift speeds between α -particles and protons ( V α p ) correspond to larger α -to-proton temperature ratios ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfrac> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>α</mml:mi> </mml:msub> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>p</mml:mi> </mml:msub> </mml:mfrac> </mml:math> ) in the young solar wind close to the corona. This observation differs from in situ observations farther from the Sun (e.g., with Helios and Wind at 0.3–1 au). We believe that this difference is related to the speed of the solar wind, as well as the distance from the Sun. The anti-correlation between V α p and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfrac> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>α</mml:mi> </mml:msub> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>p</mml:mi> </mml:msub> </mml:mfrac> </mml:math> in fast wind away from the Sun is potentially related to the energy transfer from the drift kinetic energy of α-particles to proton thermal energy due to drift instabilities. In the young slow wind, heavy ions exhibit signs of both preferential heating and preferential acceleration, but are not affected much by drift instabilities. We identify the outer boundary of the preferential heating and acceleration zone for heavy ions in the young slow wind to be at a distance of around 0.16 au, beyond which the effects of Coulomb collisions weaken the effects of preferential heating and acceleration.