Influence of the Eccentric Geomagnetic Dipole on Longitude Variations of Compressional Upstream Wave Amplitudes in the Topside Ionosphere: Results From the MSS‐1A Satellite

Based on 2‐year magnetic data from the Macau science satellite, we investigate the compressional Pc3‐4 pulsations (between 16 and 100 mHz) driven by upstream waves (UWs). Our results confirm larger UWs amplitudes in the South Atlantic Anomaly (SAA) longitudes, agreeing with previous missions. We explain such longitudinal variations as consequence of a varying magnetopause stand‐off distance, oscillating by approximately 1,090 km over one day, caused by the Earth's eccentric geomagnetic dipole. After normalizing the UW amplitudes to a constant solar wind velocity ( V sw ), largest amplitudes occur right at the longitude where the magnetopause is closest to the Earth (near the SAA). Additionally, increased V sw that pushes the magnetopause inward by 1,090 km will produce a comparable UW amplitude growth. Both mechanisms result in wave amplitudes increases of 4–5 pT. This excellent agreement provides strong support for our hypothesis that the amplitude of UWs depends strongly on the distance to the magnetopause.