  The indefinite increase of temperature predicted by the solar wind expansion
in the direction parallel to the interplanetary magnetic field is already
notorious for not being confirmed by the observations. In hot and dilute
plasmas from space particle-particle collisions are not efficient in
constraining large deviations from isotropy, but the resulting firehose
instability provides in this case plausible limitations for the temperature
anisotropy of the thermal (core) populations of both the electron and proton
species. The present paper takes into discussion the suprathermal (halo)
electrons, which are ubiquitous in the solar wind. Less dense but hotter than
the core, suprathermals may be highly anisotropic and susceptible to the
firehose instability. The main features of the instability are here derived
from a first-order theory for conditions specific to the suprathermal electrons
in the solar wind and terrestrial magnetospheres. Unveiled here, new regimes of
the electron firehose instability may be exclusively controlled by the
suprathermals. The instability is found to be systematically stimulated by the
suprathermal electrons, with thresholds that approach the limits of the
temperature anisotropy reported by the observations. These results represent
new and valuable evidences for the implication of the firehose instability in
the relaxation of the temperature anisotropy in space plasmas.
