Abstract
Using observations from different Galileo experiments (plasma wave
system, magnetometer and energetic particle detector), we analyze
a strong magnetospheric disturbance that occurs on day 311 of 1996
as Galileo was close to Jupiter (less than 15 Jovian radii). This
perturbation is characterized by multiple injections of energetic
particles in the inner magnetosphere and has been described as a
possible analog of the terrestrial magnetic storm by Mauk et al.
1999. We show here that it also corresponds to a large-scale magnetospheric
perturbation similar to the "energetic events" described by Louarn
et al., 1998, 2000. It is associated with the development of a
particular magnetic activity in the outermost part of the to torus,
over periods of 2-4 hours and in sectors of longitude with a typical
30degrees-80degrees longitudinal extension. At distances ranging
from 10 to 13 R-j the activity itself is characterized by the generation
of low-frequency magnetic oscillations (18 min periodicity in the
present case) that correlate with dispersionless energetic electron
injections and modulations of the auroral radio flux. When they are
observed a few hours after their formation, these injections present
a weak energy-time dispersion and are still periodic. They then progressively
mix and finally define a region of limited longitudinal extension
where the density of energetic particles is particularly large. We
show that this region corresponds to the source of the narrowband
kilometric radiation (n-KOM). By combining remote sensing radio observations,
in situ particle, and magnetic field measurements, we show that the
active zone where the large scale disturbance initially develops
most probably does not corotate and would even be almost fixed in
local time. In the present case, the magnetospheric event is the
consequence of two activations separated by a few hours. They occur
in two separated longitude sectors and give rise to two different
n-KOM sources. During the event, some 10 W are transferred to the
electron population. It is proposed that this set of phenomena is
the manifestation of a sporadic dissipation of a part of the to torus
rotational energy and would be thus associated with the development
of a large-scale instability in the external part of the to torus.
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