A survey of Pc 5 pulsations in the dayside high‐latitude regions observed by Viking

Abstract

We have examined magnetic and electric field observations acquired with the polar orbiting Viking satellite near its 3 RE apogee and at local times from dawn through noon to dusk. Our objective was to determine the statistical properties of Pc 5 (150–600 s period) pulsations at different dayside local times and their relationship to large‐scale Birkeland currents. The Viking data examined in detail here were acquired from 41 orbits during the period from March 26, 1986, to July 19, 1986. Pulsations could be readily identified on prenoon orbits as evidenced in a series of 10 consecutive orbits in one case and six consecutive orbits in another. The waveforms in the postnoon hours are more complicated than in the morning, and this made it more difficult to identify pulsations in this region. The simultaneous electric and magnetic field measurements from Viking were used to distinguish the magnetic variations associated with large‐scale Birkeland currents from those associated with waves. These observations also made it possible to derive values for Pedersen and wave conductivities and Poynting fluxes for the waves. The average wave period for the 40 cases studied here is 4.8 min (frequency of 3.5 mHz), the average magnetic field amplitude is 30 nT, the average electric field amplitude (for the 11 cases when the electric field observations were available) is 7 mV/m, and the average incident Poynting flux is 85 μW/m2. The principal characteristics of the waves include the following: (1) From morning to noon, the magnetic field pulsations are polarized primarily in the east–west direction, and the associated electric field oscillations lag by 60°–80° and are polarized close to the north–south direction. Consequently, these pulsations are interpreted as the fundamental modes of resonant toroidal magnetic field oscillations. (2) In the afternoon hours, the magnetic field pulsations have a strong north–south component and include higher‐frequency components (possibly the second harmonic). (3) In both the morning and afternoon hours, the pulsations occur 2°–5° equatorward of the interface between the counterflowing region 1 and region 2 Birkeland currents. (4) The average wave periods increase from ∼3 min near 0800 magnetic local time (MLT) to ∼5 min near noon. (5) The average wave amplitudes range from ∼40 nT near 0900–1000 MLT to ∼25 nT near 1300–1600 MLT.