PC-1-WAVES IN THE IONOSPHERE - A STATISTICAL STUDY

A statistical study of Pc 1 waves has been performed using electric fi eld data recorded in the ionosphere by the Dynamics Explorer 2 satelli te. The study was performed by applying an automated wave detection al gorithm to over 900 hours of data recorded at invariant latitudes (INV ) greater than 40 degrees from December 6, 1981, to February 16, 1983. A total of 390 Pc 1 waves in the electric field were identified using a selection technique based on spectral peak detection in the frequen cy range from 0.2 to 6.0 Hz. Most events were observed at frequencies between 0.4 and 2.0 Hz and in the dawn (0400-0600 MLT) and noon (1000- 1500 MLT) sectors from 50 degrees to 62 degrees INV. Detection of even ts at high latitudes (INV > 65 degrees) was limited by noise associate d with auroral zone electric fields. Significant differences in the pr operties of ionospheric Pc 1 waves were observed in the dawn and noon sectors. First, noon sector waves were clearly ordered by the equatori al gyrofrequencies (f < f(eqO+) and f(eqO+) < f < f(eqHe+)), whereas i n the dawn sector the ordering was not as clear. Second, a magnetic fi eld spectral peak was observed in 90% of the large-amplitude events, i .e., E(x) > 10 (mV/m)(2)/Hz, in the noon sector compared with just 23% in the dawn sector. Third, for events with a magnetic field component , the median value of Delta E/Delta B was 410 km/s in the dawn and 330 km/s in the noon sector, although the median value of the ratio betwe en (Delta E/Delta B) and the Alfven velocity was similar in both secto rs. Fourth, more short-duration events (2-8 s) were observed in the no on than in the dawn sector. We conclude that the source of noon sector ionospheric Pc 1 waves is electromagnetic ion cyclotron waves generat ed in the equatorial magnetosphere. The source of the dawn sector wave s is less certain, as many waves were observed only in the electric fi eld.