Warwick researchers explain why solar wind heats up

New research led by a University of Warwick physicist gives insight into why solar wind heats up when travelling away from the sun. The solar wind rushes outwards from the raging inferno that is our Sun, logically the wind should only get cooler as it expands beyond our solar system since there are no particle collisions to dissipate energy. However, the solar wind is surprisingly hotter than it should be, which has puzzled scientists for decades. Two new research papers led by Dr Kareem Osman of the University of Warwick’s Centre for Fusion, Space and Astrophysics, Department of Physics and co-authors may have solved this mystery. The research papers explaining this phenomenon are titled ‘Intermittency and Local Heating in the Solar Wind’ by K.T. Osman, W.H. Matthaeus, M. Wan, and A.F. Rappazzo and ‘Kinetic Signatures and Intermittent Turbulence in the Solar Wind Plasma’ by K.T. Osman, W.H. Matthaeus, B. Hnat, and S.C. Chapman. Solar wind is much hotter than would be expected if it were just expanding outward from the sun, the researchers say. Turbulence is the likely source of this heating. For neutral fluids such as fast flowing water, energy dissipation occurs through many microscopic collisions. As is the case for many astrophysical plasmas, the near-Earth solar wind is thin and spread out, which means collisions between particles are rare to the point that the plasma is considered collisionless. A major outstanding problem is how, in the absence of those collisions, does plasma turbulence move energy to small scales to heat the solar wind. The new research reveals how turbulence heats the solar wind. Dr Osman said, “Turbulence stretches and bends magnetic field lines, and often two oppositely directed field lines can come together to form a current sheet. These current sheets, which are distributed randomly in space, could be sites where the magnetic field snaps and reconnects transferring energy to particle heating. There are also many more ways that current sheets can heat and accelerate the plasma.” The researchers set thresholds in the strength of these current sheets, to determine how proton temperature was related to current sheet strength. The results show convincingly that these current sheets are associated with temperature enhancements, and that the strongest are also the hottest. While each current sheet does not provide a lot of heating, collectively the current sheets account for 50% of the solar wind internal energy despite only representing 19% of all the solar wind data. Even more striking, the strongest current sheets which only make up 2% of the solar wind were found to be responsible for 11% of the internal energy of the system. The researchers also found that current sheets heat the solar wind unequally in all directions.