Electron heat flux constraints in the solar wind

Enhanced fluctuations from electromagnetic heat flux instabilities may, through wave-particle scattering, constrain the electron heat flux which flows parallel to the background magnetic field in the solar wind. A corollary of this hypothesis is that instability thresholds should correspond to observable bounds on the heat flux. Here plasma and magnetic field data from February and March 1995 of the Ulysses mission is analyzed in terms of the core/halo electron model to yield scaling relations of dimensionless electron parameters and empirical upper bounds on the dimensionless heat flux as functions of the core beta. Use of these scaling relations in linear Vlasov theory for the whistler and Alfven heat flux instabilities in homogeneous plasmas yields threshold conditions on the dimensionless heat flux which are also functions of the electron core beta. The empirical bounds and the theoretical thresholds are similar and are therefore consistent with the hypothesis. (C) 1999 American Institute of Physics. [S1070-664X(99)03706-4].