Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm41c1888a&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SM41C-1888
Physics
[2407] Ionosphere / Auroral Ionosphere, [2721] Magnetospheric Physics / Field-Aligned Currents And Current Systems, [2790] Magnetospheric Physics / Substorms
Scientific paper
Magnetic field data from low Earth orbit returned by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) are used to assess time-dependent Birkeland current signatures at night times. AMPERE is based on magnetic field data from the Iridium Communications satellite constellation. The Iridium satellites are in 780-km-altitude, circular, near-polar orbits making the system ideal for monitoring Birkeland currents. Data processing yields signals with noise levels on the order of 50 to 70 nT such that strong current systems can be detected readily. A constellation-wide software modification designed and implemented by The Boeing Company created a new satellite network path for magnetometer data to increase the amount of data returned to the ground by factors of 10 to 100 compared to the standard engineering data telemetry. The operational constellation of Iridium satellites consists of 66 satellites distributed over six orbit planes, providing nine-minute separation along-track. On-orbit spare satellites provide additional sampling. The inter-spacecraft spacing of 9 minutes corresponds to the sampling revisit time, which is shorter than magnetosphere reconfiguration time scales as well as substorm growth and expansion phase time scales. Surprisingly, initial survey of AMPERE data indicated large, 100s of nT, transient signals, which often appeared in only one orbit plane within a few hours of local midnight and usually in both hemispheres simultaneously, within the 9-minute resampling time. These transient night-time current signatures display rotations in the horizontal vector consistent with filamentary current structures. In some cases the currents appear but vanish by the time the following satellite passes through the same region. In other cases the currents persist, change form, intensify and expand to adjacent orbit planes. These transient night-time currents are usually preceeded by the onset of currents on the dayside, which expand in local time from near noon, but the nightside onset of currents is often not contiguous in local time with the dayside currents. The onsets of persistent night-time current systems shows excellent correlation with the THEMIS substorm list for the northern hemisphere winter of 2009-2010 suggesting that these events are the Birkeland currents of the substorm current system. The inferred structure of the Birkeland currents of these events, the implications for the structure of convection, and opportunities for future observations of night-time current systems using AMPERE are discussed.
Anderson Benjamin J.
Barnes James R.
Dyrud Lars P.
Gjerloev Jesper W.
Korth Haje
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