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).
![]() |
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 (
)
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 (
)
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.
![]() |
DEBs carry relatively strong currents of several tens
of 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
up to 500 mV/m ,
up to
50 nT and
density variations about several tens of percent.
By study the ratio
,
which for DAW have following form
![]() |
(2) |