Is the Near-Earth Current Sheet Prior to Reconnection Unstable to Tearing Mode?

The tearingmode instability plays a key role in the triggering process of reconnection. The triggering collisionless tearingmode instability has been theoretically and numerically analyzed by many researchers. However, due to the difficulty in obtaining the observational wave number, it is still unknown whether the tearingmode instability can be excited in an actual plasma sheetprior to reconnection onset. Using the data from four Cluster satellites prior to a magnetospheric reconnection event on 13 September 2002, we utilized the wave telescope technique to obtain the wave number which corresponds to the peak of power spectral density. The wavelength is about 18RE and is consistent with previous theoretic and numerical results. After substituting the wave vectorand other necessary parameters of the observed current sheetinto the triggering condition of tearingmode instability, we find that the near-Earth current sheetprior to reconnection is unstable to tearingmode.