The Turbulent Alfvénic Aurora

It is demonstrated from observations that the Alfvénic aurora may be powered by a turbulent cascade transverse to the geomagnetic field from large MHD scales to small Alfvén wave scales of several electron skin depths and less. We show that the energy transport through the cascade is sufficient to drive the observed acceleration of electrons from near-Earth space to form the aurora. We find that regions of Alfvén wave dissipation, and particle acceleration, are localized or intermittent and embedded within a near-homogeneous background of large-scale MHD structures.

Figure 1

(a) UV auroral image from Polar UVI instrument and FAST spacecraft trajectory. (b) FAST $E_x$ (red) and $B_y$ (black) fields as defined in the text. (c),(d) FAST electron and ion spectrograms.

Figure 2

(a) Average $B_y^2(f_{\mathrm{sp}},k_x)$ spectra. (b) Average $E_x^2(f_{\mathrm{sp}})$ spectra for survey and burst data collection modes. The latter is down-shifted by 4 orders of magnitude. The black bars are composed of points representing individual measurements in each $f_{\mathrm{sp}}$ or $k_x$ bin.

Figure 3

Observed average $E_x/B_y(f_{\mathrm{sp}},k_x)$ ratio (red line). The blue line shows the predicted ratio given by local inertial Alfvén wave dispersion.

Figure 4

(a) Normalized PDFs. Solid lines show observations, dotted lines are fits of the Castaing model. (b) Structure functions based on PDFs in (a). The blue and red dotted lines correspond to fits of $S_p\propto \delta x^{\eta (p)}$. (c) The scaling exponent $\eta$ derived from the fits shown in (b).