Ampere Mission: Using Commercial Spacecraft Data To Image the Birkeland Currents

Field-aligned or Birkeland currents provide the electromagnetic link transferring stress between the magnetosphere and the ionosphere. The Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) uses magnetic field data from the constellation of 66 Iridium Communications satellites to image the distributions of the Birkeland currents in the northern and the southern hemisphere simultaneously. The satellites are located in 6 equally-spaced, circular, near-polar orbits with an altitude of 780 km. As part of the avionics, each satellite is equipped with a magnetometer, which samples the magnetic field at a resolution of 29 nT, and data are returned to the ground at commandable intervals of 19.4 s or 2.16 s. The re-sampling cadence of a location within the orbit planes is 9 min, dictated by the inter-satellite spacing. The magnetic field observations are processed into magnetic field residuals by subtracting the International Geomagnetic Reference Field model and applying automated inter-calibrations, gain, offset, and alignment corrections. The resulting magnetic perturbations are fit by spherical harmonic basis functions to a scalar potential function, the Laplacian of which yields the global distribution of the Birkeland currents at the satellite altitude. Birkeland current distributions were derived using 10 min intervals with a resolution of 3° in latitude and 1h in local time for the period 1 January 2010 to 31 May 2013 and are available >99% of the time. Continuous availability into the future is anticipated through at least 2018. The AMPERE observations provide unprecedented opportunities to explore the coupling between the magnetosphere and the ionosphere and allow global, continuous observations of the development of storm-time currents for the first time. In addition, AMPERE data have been used for a variety research projects, including the determination of the relationship between currents and particle precipitation and dynamics of the polar cap magnetic flux. We discuss the data processing, derivation of the Birkeland currents, and selected research highlights to date.