Brightening of an auroral arc and changes in its structure during substorm intensification

Data are presented on the end motion of an auroral arc during the commencement of substorm intensification on auroral field lines. It is shown that the arc-brightness enhancement consists in the successive brightening of the individual filaments of the arc and their movement in the azimuthal direction. The most northerly filaments are the first to brighten, followed by the more southerly filaments. The displacement velocity of the filament ends varies in the 1-10-20 km/s range, and can change by an order of magnitude in the course of about 1 s.