Are the tearing and the Weibel instabilities the same?

The tearing instability, which takes free energy from oppositely directed magnetic fields, and the Weibel instability, which takes free energy from temperature anisotropies, at first glance, appear to be entirely different instabilities. However, the opposing magnetic fields enforce a current between them, and the associated drift of the plasma leads to an effective thermal spread that is larger along the direction of the flow. This modified thermal spread acts as a temperature anisotropy that helps drive the instability. We investigate the connection between the two instabilities using 2D semi-implicit particle-in-cell simulations (with the code ECSIM), starting from a Harris equilibrium and no guide field. We find that for thin current sheets (thinner than the ion Larmor radius), where the assumptions of the kinetic tearing instability from Zelenyi & Krasnosel'skikh (1979) break down, the Weibel theory gives a better estimate for the growth of the instability.