Estimation of global field aligned currents using the Iridium((R)) System magnetometer data

The Iridium (R) System satellite constellation consists of 66 satellites in circular, polar, 780 km altitude orbits in six equally spaced planes. Each satellite carries an engineering magnetometer which has sufficient resolut ion to sense the Birkeland currents. This paper presents a spherical harmon ic fitting (SHF) technique for estimating field aligned currents (FACs) usi ng the cross track component of the magnetic field measurements from the Ir idium satellites. The SHF magnetic Field perturbations along with Ampere's law are used to derive the global FACs from 40 degrees MLAT to the pole in either hemisphere. Data for 10-11 UT, 23 August, 1999 were obtained from Ir idium, Defense Meteorology Satellite Program (DMSP) F13 and the Ultraviolet Imager (UVI) onboard the POLAR satellite. The SHF data were evaluated alon g the DMSP F13 track. The range of east-west component, SHF magnetic pertur bations from Iridium was (530,465) nT compared with (-630,634) nT from DMSP F13. The derived upward FACs ranged from -0.8 to 0.9 mu Am-2 Upward FACs w ere co-located with bright dayside UVI emissions.