  The isotropization process of a collisionless plasma with an electron
temperature anisotropy along an external magnetic field ($T_\| ^e\gg
T_\perp^e$, $\|$ and $\perp$ with respect to the background magnetic field) and
isotropic protons is investigated using a particle-in-cell(PIC) code.
Restricting wave growth mainly parallel to the external magnetic field, the
isotropization mechanism is identified to be the Electron Firehose Instability
(EFI). The free energy in the electrons is first transformed into left-hand
circularly polarized transverse low-frequency waves by a non-resonant
interaction. Fast electrons can then be scattered towards higher perpendicular
velocities by gyroresonance, leading finally to a complete isotropization of
the velocity distribution. During this phase of the instability, Langmuir waves
are generated which may lead to the emission of radio waves. A large fraction
of the protons is resonant with the left-hand polarized electromagnetic waves,
creating a proton temperature anisotropy $T_\|^p < T_\perp^p$. The parameters
of the simulated plasma are chosen compatible to solar flare conditions. The
results indicate the significance of this mechanism in the particle
acceleration context: The EFI limits the anisotropy of the electron velocity
distribution, and thus provides the necessary condition for further
acceleration. It enhances the pitch-angle of the electrons and heats the ions.
