We have determined the physical mechanism responsible for plasma instabilities, which were first found by Ramani & Laval in 1978, associated with anisotropic velocity distributions induced by a temperature gradient in which there are growing low-frequency transverse magnetic waves, even in the absence of background magnetic fields. We have shown that the physical mechanism responsible for the growth of one of the modes is identical to the Weibel instability. The nonlinear saturation level of the instability is also provided by considering the wave-particle interactions. The nonlinear evolution of the magnetic fields after the saturation is speculated on. The results are applied to the cold fronts newly discovered by the Chandra X-Ray Observatory in clusters of galaxies. We predict the existence of a magnetic field of ∼10 μG tangential to the surface over the entire region of the cold front and that the heat conduction is significantly suppressed by the trapping of electrons by the generated magnetic fields. The instability may provide a new possibility for the origin of the cosmic magnetic field.
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