Satellite observations (Freja)

The Freja satellite was launched to an orbit with perigee 600 km, apogee 1750 km, and inclination 63⁰ in 1992. Here we present data from Freja orbit 7279, April 10, 1994. The satellite was at 1380 km altitude, 20.9 hours MLT, 66⁰ magnetic latitude. A comprehensive description of the Freja scientific payload can be found in a special issue of Space Science Reviews (70, 405-602, 1994).

  

Figure 1: Electron energization and Alfvénic turbulence event observed on Freja orbit 7279, April 10, 1994. Upper panel - F7 TESP electron time-energy spectrogram for pitch-angles <20⁰, middle panel - parallel energy flux calculated for energies <5 keV, lower panel - magnetic field component ac f>1Hz measured along the satellite spin axis.

One of the typical electron energization events observed by Freja is presented on the upper panel of Figure 1. We can identify two main electron populations. One is an inverted-V population with energies above 1 keV. It is seen in a broad pitch-angle range ( $\alpha<90^0$) and clearly has a magnetospheric origin. The second population forms the Dispersive Electron Burst (DEB) with an energy falling from few keV down to a few tens of eV. This dispersion in energy is mainly due to a ''time of flight'' effect: if acceleration occurs somewhere above the satellite, the more energetic electrons will first hit the spacecraft and the less energetic will follow later. An estimation of the distance between the satellite and the acceleration source gives values in the range of 1,000 to 10,000 km. The observed distribution functions (Figure 2) are almost field aligned ( $\alpha<20^0$) in pitch-angle and are close to a plateau or a weak ``bump in tail'' distribution. Through a beam-plasma instability they may excite Langmuir waves which would quasilinearly thermalize the beam [12]. On the other hand the perpendicular temperature of this population is about several eV and remains constant. This suggests the ionospheric origin of the accelerated electrons which are accelerated either above the spacecraft or locally when there is no dispersion in energy.

  

Figure 2: Electron distribution function observed by Freja orbit 7279 on April 10, 1994 01:38:13 - 01:38:14.5.

DEBs carry relatively strong currents of several tens of $\mu$A/m². The associated paralell energy flux (Figure 1, middle panel) is usually about several mW/m² and sometimes reaches up to 20 mW/m². This is above the threshold of  1 mW/m² at which visible auroral arcs could be produced.

Simultaneously with DEBs an electromagnetic turbulence is observed with $\delta E$ up to 500 mV/m , $\delta B$ up to 50 nT and density variations about several tens of percent. By study the ratio $\delta E/\delta B$, which for DAW have following form

$$\left \vert\frac{\delta E_\perp}{\delta B_\perp}\right \vert= v_A \sqrt{(1 + k_\perp^2 \lambda_e^2)(1+k_\perp^2\rho_i^2)},$$ (2)

it has been possible to identify low frequency electromagnetic turbulence as a spatial turbulence of DAWs[13] which is Doppler-shifted by the fast motion of the spacecraft (7 km/sec) across the magnetic field lines.