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Auroral Physics
Auroral arc electrodynamics
Possible relationship between visible auroral arcs and ion beams
The altitude of the auroral acceleration region
Experimental investigation of auroral generator regions by Cluster/FAST conjunctions
Catalog of geomagnetic events recorded on-board the MAGION-2 satellite during 1990
Investigation of field-aligned currents in double-oval configuration
Small-scale structure of field aligned currents and discrete auroral arcs
Altitude of the Auroral Acceleration Region

 Fig. 1 Ground images (top), FAST electron and ion energy spectrograms (a, b), and fit parameters (c-g). The images are taken at 8:22:00 (A) and 8:23:00 (B), the time limits of the interval under study. Except for a 200m/s southward drift, the arc remains stable during this interval. FAST ionospheric footprint is shown as a square. '11' and '22' indicate the limits of the first two ion beams, as seen by the satellite. The sequence of ion beams visible in panel (b) suggests repeated encounters of FAST with the AARBB. The fit parameters panels show the fit quality, $\chi^2_{r_{min}}$ (c), the density (d), parallel and perpendicular temperature (e, f), and anisotropy (g). The scaled profiles of the potential drop above and below the satellite are dashed plotted in the panels (c) and (g), respectively.
 Fig. 2 Cartoon showing the 'dilation' and 'contraction' of the AAR. When the AARBB is located at low altitudes, below FAST, the AARTB is 'pushed' to higher altitudes. Experimental (e. g. McFadden et al., 1999; Mozer and Hull, 2001) and simulation (e. g. Ergun et al., 2000) studies suggest that one can speak about the top and bottom boundaries of the Auroral Acceleration Region (AAR). This is consistent with the fact that different plasmas, as in the ionosphere, AAR, and plasma sheet, tend to be separated by relatively narrow layers that concentrate substantial potential drops. A FAST case study (Marghitu et al., 2006) showed that the anisotropy of precipitating auroral electrons (obtained by fitting a bi-maxwellian function to the measured particle distributions) tends to raise up to 1 (circular contours in the velocity space) during ion beam events and to decrease to < 1 (elliptical contours) in between (Fig. 1). A possible explanation of this behavior relies on the altitudinal dilation of the AAR during ion beam events and contraction in between (Fig. 2). When the AAR top boundary (AARTB) is at a higher altitude, just a small fraction of the particles from above the AAR in a narrow, quasi-circular sector of the velocity space, around the magnetic field line makes it to the satellite, and the anisotropy information is lost. When the AARTB is at lower altitude, more particles, from a larger fraction of the velocity space, reach the satellite, and some of the anisotropy information is preserved. A configuration as sketched in Fig. 2 is consistent with simulation results obtained by Ergun et al. (2000), who found that the AAR bottom boundary (AARBB) consists of an electron transition layer , while the AARTB consists of an ion transition layer . The AARBB is located at the altitude where the density of backscattered and secondary electrons is equal to the beam ions density, of ionospheric origin, while the AARTB forms at the altitude where the density of beam ions is equal to the density of magnetospheric ions. The altitude of the AARBB depends on the field-aligned current density and on the atmospheric scale height. Assuming this scenario is true, when the AARBB is located closer to the Earth (i.e. FAST observes ion beams), the source of ionospheric ions that fill the flux tube inside the AAR is more abundant, therefore these ions dilute to magnetospheric densities at a higher altitude. This would explain the simultaneous expansion of the AAR both in downward and upward direction. However, more data need to be evaluated in order to check the relationship between the anisotropy of precipitating auroral electrons and the altitude of the AAR.
References:

Ergun, R., C. Carlson, J. McFadden, F. Mozer, and R. Strangeway, Parallel electric fields in discrete arcs, Geophys. Res. Lett., 27, 4053 4056, 2000.
Marghitu, O., B. Klecker, and J. McFadden, The anisotropy of precipitating auroral electrons: A FAST case study, Adv. Space Res., in press, 2006.
McFadden, J., C. Carlson, and R.E.Ergun, Microstructure of the auroral acceleration region as observed by FAST, J. Geophys. Res., 104, 14,453 14,480, 1999.
Mozer, F., and A. Hull, The origin and geometry of upward parallel electric fields in the auroral acceleration region, J. Geophys. Res., 106, 5763 5778, 2001.

Contact: Dr. Octav Marghitu
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