Resonant geomagnetic field oscillations and Birkeland currents in the morning sector
Authors: Potemra T. A. , Zanetti L. J. , Bythrow P. F. , Erlandson R. E. , Lundin R. , Marklund G. T. , Block L. P. , Lindqvist P.‐A. 
Published: February 2008

Magnetic field, electric field, and particle measurements acquired by the Viking satellite and magnetic field measurements acquired by the Active Magnetosphere Particle Tracer Explorers (AMPTE) CCE satellite have been used to study the relationship between large‐scale Birkeland currents and resonant oscillations in the Earth's magnetic field. The Viking observations were obtained when this polar‐orbiting satellite was near its ∼ 3‐RE apogee over the morning auroral zone and cusp region between approximately 0600 MLT and 1300 MLT. Region 1, region 2, and northward BZ (NBZ) Birkeland currents were identified with the data acquired by the Viking magnetic field instrument. The AMPTE CCE satellite, in an equatorial orbit with an 8.8‐RE apogee, obtained magnetic field measurements on several occasions when it was close to the same flux tube as Viking. These satellite conjunctions confirm that Viking and CCE can identify the same resonant oscillation at widely separated locations on the same flux tube, as done earlier by Zanetti et al. (1987). Magnetic field oscillations, present in each of the 10 consecutive Viking passes studied here, have periods between 1 min and 6 min (frequencies of 3 mHz to 16 mHz) and amplitudes from 5 nT to 60 nT. The periods and amplitudes decrease with decreasing L in seven cases. These oscillations extend from lower L shells where they correlate with the CCE observations (below L = 8.8) up to at least the interface between the region 1 and region 2 Birkeland current system. The Viking particle observations confirm that the region 1/region 2 interface maps closely to the interface between the low‐latitude boundary layer (LLBL) and the central plasma sheet (CPS). Electric and magnetic field variations are closely correlated in the region l Birkeland current. The ratio of their magnitudes provides an estimate of the height‐integrated Pedersen conductivity consistent with previously determined values in the ionosphere. This supports earlier suggestions that a voltage source in the LLBL drives the region 1 Birkeland currents. In the region 2 system of Birkeland currents, the northward electric and eastward magnetic field components show the same resonant oscillations with the electric field variations leading the magnetic field by approximately 90°. This is the correct phase relation for the fundamental mode of the toroidal standing wave oscillations of the geomagnetic field. There is evidence that the amplitudes of the oscillations observed by Viking are correlated with interplanetary magnetic field (IMF) cone angle. In one case, the energy‐time dispersion signature of temporal magnetosheath plasma injection into the low‐latitude boundary layer was associated with the resonant oscillations. These relationships and the presence of the resonant oscillations on field lines up to the region 1/region 2 (LLBL/CPS) interface (where Kelvin‐Helmholtz waves are likely to occur) lead us to conclude that there are several features in the solar wind and the direction of the IMF that can initiate magnetospheric pulsations. We believe that this is the first evidence for resonant geomagnetic field line oscillations on the same field lines that guide the region 2 Birkeland currents in the morning sector.