Lloyd Woodham

We use the magnetic field measurements from the FIELDS fluxgate and search-coil magnetometers onboard Parker Solar Probe (PSP) during its first solar encounter to probe the nature of Alfvénic turbulence at small scales in the near-Sun solar wind. These high-resolution, low-noise measurements provide a rare opportunity to be able to investigate the spectral properties of magnetic field fluctuations to scales smaller than the proton gyro-radius. After accounting for sampling effects arising from the unique nature of PSP’s orbit, we analyse a 12-hour interval during perihelion where the spacecraft encountered a highly Alfvénic slow wind stream. By comparing our results with a model spectrum of linear Alfvén waves, we find that the spacecraft observations are consistent with a transition from an Alfvénic to kinetic Alfvén turbulent cascade approaching the proton gyro-radius, as previously reported in simulations and elsewhere in the heliosphere. In addition, we find evidence for a possible second transition in the turbulence at even smaller scales, as suggested by a subsequent decrease in the amplitude of the coherent magnetic helicity signature toward zero from a peak close to the proton gyro-radius. A decrease in the helicity at these scales is seen by other spacecraft in the solar wind, but these data are often contaminated by an increasing contribution of noise to the physical signal toward smaller scales. After a detailed characterisation of the different sources of spacecraft and instrument noise throughout our interval, we conclude that the observed decrease in helicity observed by PSP is physical and extends over a larger range of scales compared to previous studies. We discuss the possible physical interpretations of this transition to a state of zero magnetic helicity and the potential implications for sub-ion scale turbulence in the solar wind.