Birkeland field-aligned current as an attractor of Alfvénic coherent structures: mechanism for aurora brightening and structuring I.V. Golovchanskaya, О.V. Mingalev, М.N. Melnik, B.V. Kozelov Polar Geophysical Institute, Apatity

Broadband ELF turbulence: Alfvénic turbulence (f ~ 0), broadband electrostatic noise (f = kHz) Figure 1. Event of the BBELF turbulence observed by FAST in the near-midnight auroral zone; [Ergun et al., 1998]

Broadband ELF turbulence: Alfvénic turbulence, electrostatic noise Figure 2. Events of the BBELF turbulence observed by FAST in the auroral zone at different MLTs, [Golovchanskaya et al., 2011]

Alfvénic turbulence generator: observations (1). It should be current rather than voltage generator. Figure 3. Seasonal variation in the Alfvénic turbulence, [Golovchanskaya et al., 2012]

Alfvénic turbulence generator: observations (2). Magnetospheric source (Poynting flux is downward) Figure 4. Direction of the Poynting flux in two events of Alfvénic turbulence observed by DE-2 [Golovchanskaya and Maltsev, 20041].

Alfvénic turbulence generator: observations (3). It should operate at the observed scales of Alfvénic turbulence: s = ~ 100 m – ~ 100 km ρ i < s ~ 100 m < λe in the FAST environment [Lund, 2010] Figure 5. Output of the magnetic receiver from 1.7 to 5.6 kHz during a portion of the Hawkey 1orbit, [Kintner, 1976]

Alfvénic turbulence generator: observations (4). It should provide complexity (signatures of intermittent turbulence) in the magnetic (and electric) fields (a) Power law form of the logarithmic diagrams constructed by the DWT Figure 6. Scaling indices of the magnetic b E component observed by FAST in three events of Alfvénic turbulence, [Golovchanskaya et al., 2011]. The approximation error is ~ Scaling index α somewhat varies from event to event, keeping in average ~ 2. Generally, it differs from those predicted by the classical models of turbulence. [ Kozelov and Golovchanskaya, 2006; Golovchanskaya, et al., 2006 ].

(b) Non-Gaussian probability density functions (PDFs) of the field fluctuations dX and an approximate collapse of the re-scaled PDFs: P(dX/σ) Figure 8. Collapse of normalized PDFs of E-fluctuations on scales km observed by DE2 in the (a) auroral zone and (b) polar cap in the above event. Figure 7. Event of Alfvénic turbulence observed by Dynamics Explorer 2 in the polar ionosphere, [Golovchanskaya and Kozelov, 2010]

Alfvénic turbulence generator: observations (5). It should provide a polarization pattern of magnetic perturbations as the observed one (disordered) Figure 9. (a) Hodograms of the perpendicular magnetic field with components b N, and b E observed in the event of Alfvénic turbulence , UT from 07:03:10 to 07:03:14. The hodograms are constructed for the data filtered in the pass bands: (top) 4-8 Hz, (middle) 2-4 Hz, and (bottom) 1-2 Hz, [Golovchanskaya et al., 2011].

Alfvénic turbulence: theory 1.Dubinin, Volokitin et al., Planet.Space Sci., Pokhotelov et al., J. Geophys. Res., Chang et al., Phys.Plasma Instead of: Non-linear interactions of Alfvénic coherent structures provide signatures of intermittent turbulence

Inertial Alfvén mode was considered for altitudes < 3 R E, where Non-linear equations for the inertial Alfvén mode: (1) (2) Non-linear equations for the Alfvénic coherent structures: (1)'(2)'

The macroparticle method was applied to solve numerically the set of equations (1)‘, (2)‘ (6) (4) (3) (5) (7) Algorithm: A macroparticle is a field-aligned current of a given value.

Turbulence in the magnetic fields after ~230 s of Alfvénic coherent structures non-linear interaction [Golovchanskaya et al., 2011] Figure 10. Coarse-graining process development

Figure 11. (a) Observed and (b) modeled hodograms of the magnetic fields of the Alfvénic turbulence, [Golovchanskaya et al., 2011]

Coarse-graining process in the field-aligned currents in case j ||,0 = 0

Time evolution of the scaling index Figure 12 from [Kozelov et al., 2011]

Alfvénic coherent structure evolution in the presence of the background j ||,0

Conclusions 1.Alfvénic turbulence can be understood as a non-linear transient process, which exhibits signatures of intermittent turbulence 2.Major features of the Alfvénic turbulence can be explained in the model of non-linearly interacting Alfvénic coherent structures 3. In the presence of external field-aligned current, Alfvénic structures of corresponding polarity migrate to this current, which leads to its strengthening and structuring. This feature may be relevant to the brightening and structuring of the auroral arcs.

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