Global Energy Transfer from Pick-up Ions to Solar Wind Protons

Abstract

It has been known for years now that pick-up ions (PUIs) are produced by ionization of interstellar neutral atoms in the heliosphere and are then convected outwards with the solar wind flow as a separate suprathermal ion fluid. Only poorly known is the thermal behaviour of these pick-ups while being convected outwards. On the one hand they drive waves since their distribution function is unstable with respect to wave growth, on the other hand they also experience Fermi-2 energizations by nonlinear wave-particle interactions with convected wave turbulences. Here we will show that this complicated network of interwoven processes can quantitatively be balanced when adequate use is made of transport-kinetic results according to which pick-up ions essentially behave isothermally at their convection to large solar distances. We derive the adequate heat source necessary to maintain this pick-up ion isothermy and use the negative of that source to formulate the enthalpy flow conservation for solar wind protons (SWPs). This takes care of a consistent PUI-induced heat source guaranteeing that the net energy balance in the SWP–PUI two-fluid plasma is satisfied. With this PUI-induced heat input to SWPs we not only obtain the well-observed SWP polytropy, but we can also derive an expression for the percentage of intitial pick-up energy fed into the thermal proton energy. By a first-order evaluation of this expression we then can estimate that, dependent on the actual PUI temperature, about 40 to 50% of the initial pick-up energy is globally passed to solar protons within the inner heliosphere.