A method for monitoring the auroral electrojet from measurements at polar cap stations was developed. Since in the polar cap the auroral electrojet produces the magnetic field in both horizontal and vertical components, the method is accounting for all three components of the magnetic field. We studied statistically the correlation between hourly mean values of auroral electrojet AL index and magnetic disturbances at polar cap boundary for each magnetic field component and found that the best correlation takes place for total magnetic field disturbance T. Then we calculated mean values of the total magnetic field disturbances for each UT hour at four stations distributed along the polar cap boundary. The obtained values we called the T index of geomagnetic activity. We found high correlation between hourly mean values of the T index and the auroral electrojet AL/AE indices (correlation coefficients are ~0.9) for any time when at least one of the stations was located at the nightside. This index also shows good correlation with solar wind parameters. We suggest that the T index might be an important independent measure for deriving the auroral electrojet, which might be useful in the cases when ground-based measurements in the auroral zone are impossible as it takes place in the Southern Hemisphere. Since the T index is derived from an independent magnetometer network, it might also give important additional information on geomagnetic activity in both hemispheres.