ULF Waves in the Topside Ionosphere: Satellite Observations and Modeling

Abstract

Low-orbiting observations at satellites with high-accuracy magnetometers onboard (Oersted, CHAMP, ST5) have provided the possibility to examine the ULF wave (Pc3, Pi2, Pc1) structure in the topside ionosphere. Pc3 waves were detected very clearly in the compressional component of the satellite magnetic data, whereas on the ground, their signature was found in the H component. The relationships between the Pc3 wave compressional magnetic component above the ionosphere and the ground response can be produced by two different mechanisms: (a) an incident Alfven wave generates an evanescent fast mode as a result of its interaction with the anisotropically conducting ionosphere; (b) transport of ULF wave energy from a distant source towards the ionosphere predominantly occurs via the fast mode. Numerical modeling and analytical estimates have been applied to the interpretation of Pc3 waves observed by CHAMP in the upper ionosphere and by ground stations at mid-latitudes. 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. To interpret simultaneous observations of low-latitude Pi2 pulsations at CHAMP and on the ground a simple analytical model which couples an incident compressional wave above the equatorial ionosphere with the ground response can be used. The amplitude and phase relationships between Pi2 signals in the upper ionosphere and on the ground at conjugate stations Tihany and Hermanus (L∼1.8) match the theoretical predictions.