Observations of solar wind directly driven auroral electrojets and gravity waves

One of principal objectives in studies of atmospheric gravity waves is to identify their sources. It has been hypothesized for many years that variations of the interplanetary magnetic field (IMF) can result in occurrence of the auroral electrojets in the high-latitude ionosphere and the auroral electrojets can generate gravity waves, but identification of this relationship in experiments has been difficult since gravity waves are generally observed far from the source region. In this paper, we present simultaneous observations of the IM;, auroral electrojets, and gravity waves on December 7, 1994. It is found that when the IMF B-z component turned southward, auroral electrojet currents appeared after a time delay of about 9 min. The Saskatoon HF radar observed quasiperiodic gravity waves after each burst of the auroral electrojet with a time delay of about 60-70 min. The IMF southward turnings, the auroral electrojets, and the gravity waves had a recurrence period of about 40 min. These observations show clearly the cause and effect relationship between the IMF southward turnings, the auroral electrojets, and the gravity waves. Another important phenomenon observed in this event is that the quasiperiodic occurrence of the gravity wave pulses in far-field points was determined by the repetitive energy input in the source region rather than by the wave dispersion. It seems that each burst of the auroral electrojet generated a single gravity wave pulse.