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We investigate the role of kinetic instabilities driven by a protonanisotropy on the onset of magnetic reconnection by means of 2-D hybridsimulations. The collisionless tearing of a current sheet is studied in thepresence of a proton temperature anisotropy in the surrounding plasma. Ourresults confirm that anisotropic protons within the current sheet region cansignificantly enhance/stabilize the tearing instability of the current.Moreover, fluctuations associated to linear instabilities excited by largeproton temperature anisotropies can significantly influence the stability ofthe plasma and perturb the current sheets, triggering the tearing instability.We find that such a complex coupling leads to a faster tearing evolution in aregime with larger perpendicular temperature when an ion-cyclotron instabilityis generated by the anisotropic proton distribution functions. On the contrary,in the presence of the opposite anisotropy, fire hose fluctuations excited bythe unstable background protons with larger parallel temperature are not ableto efficiently destabilize the current sheets, which remain stable for a longtime after fire hose saturation. We discuss possible influences of this novelcoupling on the solar wind and heliospheric plasma dynamics.

Proton temperature anisotropy and magnetic reconnection in the solar wind: effects of kinetic instabilities on current sheet stability