Kelvin-Helmholtz instability at the magnetotail boundary: MHD simulation and comparison with Geotail observations

On March 24, 1995, the Geotail spacecraft observed large fluctuations of th e magnetic field and plasma propel-ties in the low-latitude boundary layer about 15 R-E tailward of the dusk meridian. Although the magnetospheric and magnetosheath magnetic fields were strongly northward, the B-Z component s howed strong short-duration fluctuations in which B-z could even reach nega tive values. We have used two-dimensional magnetohydrodynamic simulations w ith magnetospheric and magnetosheath input parameters specifically chosen f or this Geotail event to identify the processes which cause the observed bo undary properties. It is shown that these fluctuations can be explained by the Kelvin-Helmholtz instability if the k vector of the instability has a c omponent along the magnetic field direction. The simulation results show ma ny of the characteristic properties of the Geotail observations. In particu lar, the quasi-periodic strong fluctuations are well explained by satellite crossings through the Kelvin-Helmholtz vortices. It is illustrated how the interior structure of the Kelvin-Helmholtz vortices leads to the rapid flu ctuations in the Geotail observations. Our results suggest an average Kelvi n-Helmholtz wavelength of about 5 R-E, with a vortex size of close to 2 R-E for an average repetition time of 2.5 min. The growth time for these waves implies a source region of about 10-16 R-E upstream from the location of t he Geotail spacecraft (i.e., near the dusk meridian). The results also indi cate a considerable mass transport of magnetosheath material into the magne tosphere by magnetic reconnection in the Kelvin-Helmholtz vortices.