THE TEARING INSTABILITIES DRIVEN BY LOWER-HYBRID WAVES IN THE CURRENTSHEET

We have investigated the excitation of tearing instabilities driven by lower hybrid waves in the current sheet. Considering a two-dimensiona l equilibrium, we have shown that lower hybrid waves observed througho ut the plasma sheet can cause large anomalous resistivity as a consequ ence of nonlinear interactions and significantly influence the excitat ion of tearing mode in the layer, where B-x(z) reverses sign. We have found that the perturbed ponderomotive force gives two new physical ef fects, one due to current gradients in the inner core of the current s heet (z much less than delta much less than lambda, where lambda is th e current sheet width and delta is the thickness of the field reversal layer) and the other due to anomalous resistivity in the outer core ( delta much less than z less than or equal to lambda). In the presence of these effects, we have demonstrated that global tearing modes with growth times ranging from fraction of a second to several tens of mill iseconds are excited for wave electric fields of order of a few mV/m a nd scale lengths in the range k(0y)L(z) < 1 or k(0y)L(z) > 1, where k( 0y) is the wavenumber of the lower hybrid waves and L(z) is the pertur bation scale length. Finally, we have discussed the relevance of this wave-driven mode to the magnetic reconnection and the substorm onset p rocesses.