Interchange reconnection within coronal holes powers the solar wind

The fast solar wind that fills the heliosphere originates from regions of open magnetic field on the Sun called 'coronal holes', driven by the pressure drop between the solar surface and the interstellar medium. The magnetic field near the solar surface within coronal holes is structured on angular scales associated with the boundaries of meso-scale supergranulation convection cells, where descending flows create intense bundles of magnetic field. The energy density in these 'network' magnetic field bundles is a likely candidate as an energy source of the wind. However, the mechanism responsible for energizing the low altitude plasma to become the solar wind is unknown. An often invoked candidate mechanism is the heating associated with Alfven waves (McKenzie et al, 1993; Axford et al, 1997). Here we report measurements from the Parker Solar Probe (PSP) spacecraft near the perihelion from encounter 10 (E10) to show that solar wind streams are phase correlated with the magnetic field at the solar footpoint of the spacecraft. The in situ measurements reveal a highly bursty radial wind with corresponding bursts of energetic ions that form powerlaw distributions at high energy. Supporting particle-in-cell simulations of interchange reconnection reveal key features of the observations, including the energetic ion spectra in the observations. Important characteristics of interchange reconnection in the low corona are inferred from the PSP data: reconnection is collisionless and the rate of energy release is sufficient to heat the ambient plasma and drive an energized wind. In this reconnection scenario of solar wind energization open magnetic flux undergoes continuous reconnection and the wind is driven both by the resulting plasma pressure and the radial Alfvenic flow bursts. Such bursty reconnection is also the likely source of the large amplitude Alfvenic 'switchback' structures observed in the inner heliosphere.