LOW FREQUENCY WAVES UPSTREAM FROM AND : CASSINI SPACECRAFT OBSERVATIONS

Bertucci Cesar,Nick Achilleos,Edward Smith,Michele Dougherty, Christian Mazelle,Christopher Russell,Bruce Tsurutani Space & Atmospheric Physics Group, , The , Prince , Consort Road, London, United Kingdom SW7 2BZ

ABSTRACT

Cassini is the first spacecraft to study Saturn\'s magnetosphere and its interaction with the solar wind in detail. It is well known that this wind is supersonic and superalfvenic and then a bow shock forms ahead of the planet. However, the planet\'s presence can be perceived before the solar wind flow reaches this boundary as charged particles from the planet are injected into the solar wind generating low frequency electromagnetic waves. In the upstream region of magnetized planets like the Earth, these waves are mainly concentrated in the foreshock, i.e., the sector in the upstream space where the interplanetary magnetic field lines intersect the planet\'s bow shock surface. In this work we analyse observations from the Cassini investigation (MAG) upstream from the bow shock of Saturn during the first orbits of its mission around that planet. In particular, we study the properties of phase-steepened, non-linear, low frequency plasma waves. These \"tooth-shaped\" waves have periods of the order of several minutes in the spacecraft frame and they show a linear polarisation. In addition, packets containing highly coherent oscillations were found attached to the sharper edge of the steepened wave. The oscillations within the packets have periods of the order of the tens of seconds in the spacecraft frame and show a circular polarisation. Interestingly, Cassini MAG measurements reveal that the amplitude and the period of these oscillations decrease with increasing distance from the sharper edge of the steepened wave. During these observations, the prolongation of the local background magnetic field intersects the nominal position of the Kronian bow shock. This suggests that these waves are generated in the foreshock of Saturn. Following an analysis of the experimental properties of these waves, we comment on the instabilities capable of generating them in their linear stage. Then we investigate the process of wave steepening which takes place in the nonlinear stage of the wave growth. As a result, the dispersive wave packet attached to the steepened wave would be a consequence of the steepening process rather than a result of an instability generated by foreshock particles. In addition, we comment on the type of ion distribution function to which these waves are associated and we analyse the role of these waves in the formation of the quasi parallel bow shock. Finally, we compare these recent results with similar observations obtained by Cassini MAG during the flyby of Jupiter between November 2000 and February 2001.