Structure of ULF Pc3 waves at low altitudes

Abstract

Recent low‐orbiting observations at satellites with high‐accuracy magnetometers onboard (Oersted, CHAMP, and ST5) have provided a detailed picture of the Pc3 wave structure in the topside ionosphere. Pc3 waves were detected very clearly in the compressional component of the satellite magnetic field data, whereas on the ground their signature was found in the H component. The occurrence of a significant compressional component in Pc3 pulsations in the topside ionosphere was also evidenced by radio‐sounding measurements of ionospheric plasma oscillations. The following possibilities of ULF compressional disturbance excitation are considered: (1) an incident Alfvén wave generates an evanescent fast mode as a result of its interaction with the anisotropically conducting ionosphere; (2) transport of ULF wave energy from a distant source toward the ionosphere predominantly occurs by a fast mode. We estimate quantitatively the expected relationships between the Pc3 wave magnetic components above the ionosphere and on the ground produced by these different mechanisms and have derived simple analytical relationships between the compressional and ground signals for both mechanisms. Numerical modeling with the use of exact formulas has shown that these approximations work well over a wide range of wave scales. This model has been applied to the interpretation of Pc 3 waves observed by CHAMP in the upper ionosphere and by ground stations at midlatitudes. In general, the observed ratio between the compressional component in space and the ground signal corresponds better to the scenario of direct fast mode transmission to the ground.