A theory has been constructed for standing quasi-Alfven waves excited in the magnetosphere by a broadband source localized in the ionosphere. Formulas are obtained which describe the oscillation field of individual standing waves excited by a source arbitrarily distributed in the ionosphere and with an arbitrary behavior in time. It is shown that oscillations from an impulsive source that is narrowly localized across geomagnetic field lines can occur in two different regimes. The initial regime is characterized by the fact that the excited standing wave oscillates throughout the space with the same frequency. In the asymptotic regime, the oscillation field of the standing Alfven wave occupies a distinctly delineated region across field lines. At every point of the region the waves are oscillating with their eigenfrequency. A numerical calculation is made of the total field of standing quasi-Alfven waves excited in the magnetosphere by an impulsive source localized in the ionosphere. It is shown that the fundamental harmonic of standing waves reaches an asymptotic regime already after the first oscillation period. All other harmonics oscillate in the initial regime during many oscillation periods of the fundamental harmonic. Such a picture of the oscillations adequately depicts the MHD disturbances recorded as part of the active MASSA1 experiment (at 023100 UT on November 28, 1981) aboard the AUREOLE-3 satellite. The point at which the leading edge of a strong electromagnetic impulse was recorded is quite well associated with the equatorward boundary of localization of the fundamental harmonic of standing waves. The form and duration of the impulse recorded aboard the satellite is also adequately described. ¿ 2000 American Geophysical Union