An algorithm for estimating field-aligned currents from single spacecraft magnetic field measurements: a diagnostic tool applied to Freja satellite data

Introduces an algorithm which allows under certain assumptions to estimate the current density distribution along the track from single spacecraft magnetic field measurements. The assumptions are chosen that they meet at least partly the conditions encountered by the Freja satellite on its northernmost orbital segment, namely that all currents are field-aligned, ionospheric currents do not contribute significantly to the magnetic field measurements, and the velocity component perpendicular to the field direction is large (>5 km/s). Problems arise in the case of moving or temporally varying current systems. In those cases additional data like ground-based observations are needed to resolve the spatio-temporal ambiguity. With the help of simulated data the authors can show that for most of the encountered stationary current geometries the estimates fall into an uncertainty band of /spl plusmn/20%. Current density estimates are a local quantity with a spatial resolution of order 1 km. They are thus very suitable for use in studying the fine structure of auroral plasma processes.

[1]  Mario H. Acuna,et al.  Magnetic Field Experiment on the Freja Satellite , 1994 .

[2]  R. Lysak.  Electrodynamic coupling of the magnetosphere and ionosphere , 1990 .

[3]  G. Haerendel,et al.  Inverted‐V events simultaneously observed with the Freja satellite and from the ground , 1994 .

[4]  Wolfgang Baumjohann,et al.  Electric fields and currents at the Harang discontinuity - A case study , 1986 .

[5]  J. Samson.  6 – Geomagnetic Pulsations and Plasma Waves in the Earth's Magnetosphere , 1991 .

[6]  H. Höfner,et al.  The TESP electron spectrometer and correlator (F7) on Freja , 1994 .

[7]  C. Meng,et al.  The dynamics of the aurora—II Westward traveling surges , 1965 .

[8]  R. Hoffman,et al.  Finite geometry effects of field-aligned currents , 1992 .

[9]  Per-Arne Lindqvist,et al.  Fine structure of field-aligned current sheets deduced from spacecraft and ground-based observations: Initial FREJA results , 1994 .

[10]  M. Kelley,et al.  Alfvén waves in the auroral ionosphere: A numerical model compared with measurements , 1992 .

[11]  T. Potemra,et al.  Field‐aligned currents in the dayside cusp observed by Triad , 1976 .

[12]  Sven Grahn,et al.  The Freja Project , 1994 .

[13]  Michael C. Kelley,et al.  Distinguishing Alfvén waves from quasi‐static field structures associated with the discrete aurora: Sounding rocket and HILAT satellite measurements , 1990 .

[14]  D. Gurnett,et al.  Correlated low‐frequency electric and magnetic noise along the auroral field lines , 1984 .

[15]  L. J. Cahill,et al.  Initial results on the correlation between the magnetic and electric fields observed from the DE‐2 satellite in the field‐aligned current regions , 1982 .

[16]  R. Lundin,et al.  Observations of large‐scale Birkeland currents with Viking , 1987 .

[17]  J. Slavin,et al.  Correlation between magnetic and electric field perturbations in the field-aligned current regions deduced from DE 2 observations , 1992 .

[18]  Syun-Ichi Akasofu,et al.  The development of the auroral substorm. , 1964 .

[19]  F. Mozer,et al.  High-resolution sounding rocket observations of large-amplitude Alfven waves , 1990 .

[20]  M. Sugiura.  A fundamental magnetosphere‐Ionosphere coupling mode involving field‐aligned currents as deduced from DE‐2 observations , 1984 .

[21]  K. Glassmeier,et al.  Ground-based and satellite observations of traveling magnetospheric convection twin vortices , 1989 .