Solar wind electrons: Parametric constraints

Solar wind electrons are often observed to consist of two distinguishable c omponents, a thermal, more dense core and a suprathermal, less dense halo. In this core/halo model linear Vlasov theory for the whistler heat flux ins tability predicts dimensionless heat flux thresholds which decrease as the electron core beta, <(beta)over tilde>parallel to(c), increases. It has bee n proposed that this theoretical threshold corresponds to an observable upp er bound on the electron heat flux. Linear theory also predicts that there is a critical value of <(beta)over tilde>parallel to c below which the whis tler heat flux instability does not have appreciable growth in the solar wi nd; there is another suggestion that this corresponds to an observable lowe r bound on <(beta)over tilde>parallel to(c). These two proposals are examin ed by comparison of linear theory and data from the initial in-ecliptic pha se of the Ulysses mission. The instability threshold does provide a statist ical constraint on observed solar wind heat fluxes, and the critical <(beta )over tilde>parallel to(c) Of theory is not inconsistent with a statistical lower bound on the observations of that parameter.