Drifting field-aligned density structures in the night-side polar cap.
Résumé
Spatio-temporal properties of density irregularities in
the night-side polar cap are inferred from multi-point
observations of auroral hiss at a radial distance of 5 Earth
radii. In this case study, we use high resolution data of
the wave instruments onboard the four Cluster spacecraft
to estimate the local plasma density, obtaining values
close to 1 cm3 with density depletions decreasing down
to 0.15 cm3. Combined analysis in different points in
space conclusively shows for the first time that these
density structures are field-aligned and that they
predominantly drift in the anti-sunward direction at speeds
of a few km/s. This corresponds to a dawn-to-dusk convection
electric field of approximately 1 mV/m. The transverse
dimension of the observed density depletions, mapped down
to the ionospheric heights, is of the order of a few tens of km.
Their lifetime is probably longer than tens of seconds.
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