Programme by Session

The solar wind is a highly-dynamic plasma supporting waves, instabilities, shocks and turbulent fluctuations over a broad range of scales. In-situ measurements of the solar wind provide unparalleled insights into these fundamental plasma processes, making it an excellent laboratory to better understand the flow of energy in other collisionless astrophysical plasmas.

We analyse over a decade of solar wind magnetic field and ion moment data from the Wind spacecraft to investigate the nature of solar wind fluctuations at ion-kinetic scales. We use a novel technique to recover wavevector information of these fluctuations using magnetic helicity. For the first time, we separate parallel- and perpendicular-propagating fluctuations with respect to the local mean magnetic field. We find that parallel-propagating fluctuations arise from Alfvén-ion cyclotron and parallel firehose instabilities driven by unstable proton temperature anisotropy and ?-particle drifts. In addition, we show that kinetic Alfvén wave-like fluctuations dominate the helicity signal of perpendicular-propagating fluctuations, consistent with an anisotropic turbulent cascade. These results imply that there is significant processing of energy by kinetic instabilities in the solar wind, which must be included to correctly model the global thermodynamics of expanding astrophysical plasmas.

The method we employ here will be especially useful for Solar Orbiter and Parker Solar Probe science - by probing kinetic-scale fluctuations in the outer corona and how they evolve with increasing heliocentric distance, we can diagnose the processes thought to be crucial for the formation and acceleration of the solar wind.