Introduction

The auroral zone is characterized by a number of different particle populations. In the upward current region down-going accelerated hot electrons of plasmasheet origin[1] and upflowing ionospheric ions are often observed. It has been found recently [2] that in the downward current regions ionospheric electrons are accelerated upwards, carrying the return current of the auroral current circuit. Beside those major populations, downward accelerated essentially field aligned electrons are often observed throughout the auroral region, both within auroral arcs [3,4] and outside them [5].

The pitch-angle distribution of this population is clearly different from the one of plasmasheet electrons. The electrons have energies in the range of 50 eV to several hundreds of eV, with transverse energy of just several eV. This is a strong indication of their ionospheric origin and suggests a local acceleration of background electrons. Sounding rocket observations of similar field-aligned electron bursts typically show a time-of-flight type dispersion [6], which is less pronounced at Freja altitudes (1500 km) and rarely observed at FAST altitudes [7].

On the other hand quasiperiodic electromagnetic structures in the ULF-frequency range are often observed together with these field aligned electrons. It has been suggested that dispersive Alfvén waves (DAW) may be responsible for the electron acceleration. DAW is described by dispersion relation [8]

$$\omega=k_{\parallel }v_{A}\sqrt{\frac{1+k_{\perp }^{2}\rho_i^2}{1+k_{\perp }^{2}\lambda_e^{2}}},$$ (1)

where $\lambda_e=c/\omega_p$ is the electron inertial length, $\rho_i$ the ion Larmor radius, and v_A the Alfvén velocity. Dispersive Alfvén waves propagate almost normal to the ambient magnetic field with $k_\parallel \ll k_\perp$ and have a very small transverse velocity $v_\perp=\omega/k_\perp$.

Several different configurations of the wave field in space and time were proposed to explain this turbulence. This includes solitary waves propagating from the magnetosphere [9], quasi-stationary structures [10], and waves in the ionospheric Alfvén resonator [11]. Single satellite observations cannot resolve the space-time ambiguity of the observations and it is difficult to distinguish these different mechanisms.