Electron Distribution in the Solar Wind from the Sun to the Earth

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The solar wind is a stream of hot ionized gas, or plasma, expanding from the solar corona into the interplanetary medium. Since its discovery in 1959, space scientists have been studying the behavior of protons and electrons in the solar wind, their interaction with the magnetic fields pervading interstellar space, and with magnetospheres of planets. At approximately the same time, plasma physicists were thinking about containing plasma that is a hundred times hotter than that in the solar corona, to obtain controlled fusion reaction – a virtually inexhaustible source of relatively clean and safe energy. They came up with the so-called mirror machine, a magnetic configuration resembling a bottle with two openings at the opposite ends. However, such a device was not ideal – hot plasma could leak through the bottleneck openings and stream away from the confinement chamber. This project establishes the analogy between the plasma escaping the mirror machine and the plasma expanding from the hot solar corona. It will use the theoretical framework developed in plasma fusion studies to explain the behavior of electrons in the solar wind, one of the fundamental problems of space plasma physics. The project studies the solar wind plasma expanding from the hot solar corona. It addresses the following question: given the electron distribution function near the solar corona, what is the resulting distribution function at larger heliospheric distances? The project establishes the analogy with the plasma mirror machines. Similarly to the mirror-machine expander, the solar wind electron distribution function contains a beam of electrons streaming along the magnetic field lines and a quasi-isotropic core that, on one hand, is related to the electrons scattered from the beam and on the other, provides scattering for the streaming electrons. The project develops a new self-consistent theoretical framework for solving this problem. The outcome of the study will be the electron distribution as a function of radial distance. This will allow one to understand, in particular, the radial energy transfer and energy deposition in the background plasma, the plasma temperature profile in the expansion region, the electron instabilities and the resulting plasma turbulence. The results will be compared with available observations, and in particular, may be valuable for the interpretation of the Parker Solar Probe data.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.