Science Objectives

Science objectives of the WIND 3-D Plasma and Energetic Particle Investigation include:

• Interplanetary Suprathermal Electrons: Sources and Transport

  Measurements on the earlier ISEE-3 spacecraft showed that there is a background population of 2-20 keV electrons in interplanetary space, at fluxes beyond what would be expected from the high energy tail of the thermal solar wind plasma. There are also impulsive events at energies below ~10 keV that do not appear to be associated with flares on the Sun. The flare electrons also exhibit the interesting behavior of apparently undisturbed propagation from the Sun below ~10-20 keV, but significant scattering (e.g. by irregularities in the interplanetary magnetic field) above these energies. The WIND 3-D experiment provides new more comprehensive information about these suprathermal electrons that will help to unravel the mystery of their origin and transport.

• Particle Acceleration, Storage, and Propagation at the Sun

  Small solar flares can accelerate electrons up to about 100 keV during the flash phase, but measurements of these solar flare particles are lacking at lower energy levels (below ~100keV for ions and below ~2 keV for electrons). Measurements of these lower energy particles will help answer questions about the acceleration mechanism of the particles.

• Langmuir Waves and Radio Bursts

  An important problem in plasma and solar physics is the generation of solar radio emission. Energetic particles are acclerated at the Sun and lead to electron plasma waves. These plasma waves are extremely bursty and the good angular resolution and high sensitivity and time resolution of the WIND 3-D Plasma experiment will allow study of how the elecron distribution evolves and what actually causes these bursts.

• Interplanetary Suprathermal Ions: Sources and Transport

  Like the electrons, the interplanetary population of protons includes an ambient suprathermal component at energies below ~40 keV but above the keV energy associated with the bulk solar wind. Some of these ions may be accelerated in the corotating interaction regions in the solar wind, but it is not apparent that this is the only source region. These In addition to telling us something about the nonthermal processes that are occurring in the Sun, the corona, and the solar wind, these particles provide the 'seed' population for acceleration to even higher energies by interplanetary shocks. They are thus a potential source for the SEP (Solar Energetic Particle) populations that are important in "space weather". The WIND 3-D experiment is sensitive enough to provide the first detailed measurements of the characteristics of the interplanetary suprathermal ions.

• Interplanetary Shocks and the Earth's Bow Shock

  At high energies, ions have often been observed to be accelerated by the passage of an interplanteary shock. similarly the Earth's bow shock accelerates ions and electrons to high energies. This instrument provides high time resolution and 3-D measurements which can be used to remotely sense the shock, trace particles leaving and entering the shock, and trace the ion distributions through the shock.

• Upstream Ions and Electrons

  Both interplanetary shocks and planetary bow shocks generate enhancements in the local ion and electron populations (which can be quite extensive for large shocks) over a broad range of energies. Interplanetary shocks can produce ions of 100s of keV to 10s of MeV energies, while the Earth's bow shock accelerates both ions and electrons to over 100 keV. The acceleration mechanisms at these shocks can be studied with the information obtained on the 3-D details of the particle distributions by this Wind spacecraft experiment. These measurements can also help to resolve the ongoing debate about the origin of the Earth's upstream ions, which are sometimes attributed to a magnetospheric, rather than a shock-related source.

• Particle Entry into the Magnetosphere

  Some interplanetary ions and electrons enter the Earth's magnetosphere, but where the entry takes place and how is not well understood. The detailed measurements of 3-D particle distribution outside and inside the magnetosphere that WIND can provide will be helpful in understanding this phenomenon.

• Magnetospheric Bursts in the Interplanetary Medium

  Occasionally bursts of ions and electrons are observed upstream from the Earth which extend to much higher energies than the bow shock related particles discussed above. The detailed particles distributions which can be measured by the WIND instrument will help determine the origins of these particles.

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