Existing indices of magnetic activity (AE, Kp, Dst, etc.) are bad pointers to the instantaneous magnetospheric configuration. In this paper we introduce a new activity index, MT-index, which is defined as the invariant latitude of the Isotropic Boundary (IB) of≈100 keV protons reduced to magnetic midnight. The IB observed by low-altitude polar spacecraft is a boundary between the regions of adiabatic and chaotic regimes of particle motion in the tail current sheet, and is controlled by the magnetic field in the equatorial near-Earth tail. We have investigated the local time and activity dependence of the IB using data from NOAA spacecraft. We compared the MT-index with magnetic field measured simultaneously by geosynchronous GOES-2 spacecraft and found that MT-index displays a good correlation with the magnetic field inclination at the nightside part of the geosynchronous orbit and is thus a good measure to characterize quantitatively the stretching of the tail magnetic field. We conclude that the MT-index is the best known indicator of the instantaneous magnetic configuration in the near-Earth magnetotail. It is available on regular basis and can be implemented for scientific studies as well as for monitoring purposes.