Local increase of field-aligned current intensity before a substorm onset

By using data treatment results of mid-latitude geomagnetic variations, the significant increase of the local current system intensity with the eastern direction current system, arising near the meridian of expected substorm center ~10-30 min before the break-up onset, has been found. A comparison of longitudinal sizes of two current system, forming the substorm disturbance before and after the onset of the substorm expansion phase, has been carried out. The longitudinal size of the current loop of the substorm growth phase is nearly twice as small as azimuth scale of the substorm current wedge, and the current strength is as week as 2.7-12 times.