  Recent statistical studies of observational data unveil relevant correlations
between whistler fluctuations and the anisotropic electron populations present
in space plasmas, e.g., solar wind and planetary magnetospheres. Locally,
whistlers can be excited by two sources of free energy associated with
anisotropic electrons, i.e., temperature anisotropies and beaming populations
carrying the heat flux. However, these two sources of free energy and the
resulting instabilities are usually studied independently preventing a
realistic interpretation of their interplay. This paper presents the results of
a parametric quasilinear study of the whistler instability cumulatively driven
by two counter-drifting electron populations and their anisotropic
temperatures. By comparison to individual regimes dominated either by beaming
population or by temperature anisotropy, in a transitory regime the instability
becomes highly conditioned by the effects of both these two sources of free
energy. Cumulative effects stimulate the instability and enhance the resulting
fluctuations, which interact with electrons and stimulate their diffusion in
velocity space, leading to a faster and deeper relaxation of the beaming
velocity associated with a core heating in perpendicular direction and a
thermalization of the beaming electrons. In particular, the relaxation of
temperature anisotropy to quasi-stable states below the thresholds conditions
predicted by linear theory may explain the observations showing the
accumulation of these states near the isotropy and equipartition of energy.
