Nature of the Solar Wind Electron Distribution Functions at 1 AU: Wind Observations.

The characteristics of the solar wind electron distribution functions (EDFs) at 1 AU are of great importance in many aspects, for instance in understanding heat conduction, plasma microinstabilities and transport, and origin and acceleration of the solar wind plasma. It has been known for a long time that, in the free solar wind, EDFs display both thermal (``core") and suprathermal (``halo" and ``strahl") populations; more recently a ``super-halo'' population has also been identified. The usual simplified model used to characterize the observed solar wind EDF is a sum of two bi-Maxwellians -the core-halo model-, with a core-halo drift velocity oriented along the interplanetary magnetic field. Other recent works have emphasized the Lorentzian nature of EDFs, i.e. the importance of their suprathermal tails, which should play a crucial role in the exospheric expansion of the slow and fast solar wind. Based on either the core-halo or the Lorentzian (or Kappa) models, kinetic instabilities in space plasma have been discussed in the literature and wave growth rates have been calculated. However neither model correctly characterizes the different features of the observed EDFs, in particular their suprathermal tails, which are well underestimated by the core-halo model and on the contrary well overestimated by the Kappa model. It is therefore important to determine and characterize more precisely the nature of the EDF suprathermal tails. The 3DP experiment on the WIND spacecraft provides measurements of the full 3D electron distributions from energies of the order of few eV to above 100 keV, with a high-sensitivity, wide dynamic range, good energy and angular resolutions, and high time resolution (3s). Wind's in-ecliptic orbits cover prolonged periods in the ambient -slow and fast- solar wind near L1, during the last minimum of solar activity. New characteristics of EDFs are established and their consequences in different field of space plasma processes are discussed.