The range of Alfvénic turbulence scales in the topside auroral ionosphere

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The range of Alfvénic turbulence scales in the topside auroral ionosphere

Golovchanskaya, I. V. / Kozelov, B. V.

Abstract The minimal scale of the Alfvénic turbulence transverse to the external magnetic field in the topside auroral ionosphere is investigated using electric field observations of the FAST spacecraft (the resolution 512 s–1). The events in which the power law form of the electric fluctuation spectra with a 2.0–2.5 slope (typical of Alfvénic turbulence) remains unchanged down to acoustic gyroradius ρs or ion gyroradius ρi local values are illustrated for the first time. In this case, the character of spectrum variation does not change at the electron inertial length λe, which is much larger than ρs and ρi for FAST altitudes (apogee ~4000 km). We have tried to explain this experimental fact by consideration of the known scenarios of the appearance of a small transverse scale in an Alfvénic perturbation. It has been noted that the effects of front steepening in an inertial Alfvén wave with a finite amplitude, which propagates at an angle smaller than (me/mi)1/2 with respect to the transverse direction, can result in small transverse scales comparable with acoustic gyroradius appearing in a perturbation.