On the Kinetic Nature of Solar Wind Discontinuities

The most intense currents in the solar wind are carried by magnetic field discontinuities. The formation of such structures depends on whether they are rotational or tangential discontinuities. To distinguish between these two types, variations of plasma characteristics should be studied. We analyze data from a set of discontinuities observed by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun and Magnetospheric Multiscale missions in the near‐Earth solar wind. We show that these discontinuities have properties characteristic of both tangential and rotational discontinuities: (1) Jumps in the tangential velocity component are well correlated with jumps in the Alfven speed, and (2) Electron density and temperature vary significantly across these discontinuities. Suprathermal electron pitch angle distributions change across discontinuities, revealing the importance of electron kinetics to discontinuity structure. Investigation of the discontinuity formation needs to go beyond magnetohydrodynamics theory and highly likely involves both ion and electron kinetics. Plain Language Summary The most intense currents in the solar wind are carried by magnetic field discontinuities, coherent structures that contribute significantly to plasma heating and magnetic field fluctuations. Although investigation of discontinuities was started with the first spacecraft in situ measurements in the solar wind, we still do not know the primary mechanism of the discontinuity formation. Conclusions of the simplified fluid models of discontinuities often do not describe the observed discontinuity properties. In this study we investigate the nonfluid (kinetic) properties of these discontinuities. Such investigation becomes possible owing to unprecedented detailed plasma measurements provided by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun and Magnetospheric Multiscale missions in the near‐Earth solar wind. Our findings show that observed discontinuity classification needs to go beyond fluid theory and highly likely involves both ion and electron kinetics. Key Points Magnetic field discontinuities in solar wind are investigated using multispacecraft observations Observed discontinuities have properties characteristic of both rotational and tangential discontinuities Electron pitch angle distributions vary sharply across the observed discontinuities