ULF/ELF waves and turbulence in the high‐latitude dayside magnetosphere

Abstract

The magnetospheric polar cusp has been recognized as a region of strong plasma wave activity. The paper briefly summarizes previous observations of waves in the high‐altitude cusp and the dayside magnetosheath. The main part of the work is concerned with low‐frequency waves found in and near the cusp at low altitudes (500–2000 km) and on the ground. Examining the ac electric field measurements made by polar‐orbiting satellites (mainly the DE‐2), the following groups of waves appear to be characteristic for the dayside high‐latitude magnetosphere: (1) ULF/ELF broadband noise extends from the low hertz range to approximately 1 kHz. The region of peak intensity is generally found to be consistent with the latitudinal extension of the cusp. (2) VLF hiss is found to be present in the cusp region, but generally extends outside the established projection of the cusp. (3) Narrow‐band ELF emissions, covering frequencies 200 Hz ≲f ≲ 1 kHz, are observed on the equatorward side of the cusp, in the closed field line regime. Ground‐based low‐frequency wave measurements at Λ = 75.4° show in the daytime the presence of strong micropulsation activity in the frequency range ∼ 1–100 mHz and polar ELF emissions, a narrow band of relatively unstructured noise at frequencies 300 Hz ≲ f ≲ 1 kHz. The latter is believed to correspond to the satellite‐observed narrow‐band ELF noise. The frequency range 1–7 kHz is, in general, characterized in both the satellite and ground recordings by very low emission activity in the time period ∼3 hours on either side of magnetic noon. This classification of the waves is independent of the level of magnetic disturbance. It is discussed how the characteristic appearance of the ULF/ELF noise and the narrow‐band ELF emissions can be used to localize the position of the polar cusp.