Pc 1 waves in the ionosphere: A statistical study

A statistical study of Pc 1 waves has been performed using electric field data recorded in the ionosphere by the Dynamics Explorer 2 satellite. The study was performed by applying an automated wave detection algorithm 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 frequency 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 events at high latitudes (INV > 65 degrees) was limited by noise associated with auroral zone electric fields. Significant differences in the properties of ionospheric Pc 1 waves were observed in the dawn and noon sectors. First, noon sector waves were clearly ordered by the equatorial gyrofrequencies (f < f(eqO+) and f(eqO+) < f < f(eqHe+)), whereas in the dawn sector the ordering was not as clear. Second, a magnetic field 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 between (Delta E/Delta B) and the Alfven velocity was similar in both sectors. Fourth, more short-duration events (2-8 s) were observed in the noon than in the dawn sector. We conclude that the source of noon sector ionospheric Pc 1 waves is electromagnetic ion cyclotron waves generated in the equatorial magnetosphere. The source of the dawn sector waves is less certain, as many waves were observed only in the electric field.