242 research outputs found
Low-frequency magnetic field fluctuations in Venus' solar wind interaction region: Venus Express observations
We investigate wave properties of low-frequency magnetic field
fluctuations in Venus' solar wind
interaction region based on the measurements made on board the Venus
Express spacecraft. The orbit geometry is very suitable to
investigate the fluctuations in Venus' low-altitude magnetosheath
and mid-magnetotail and provides an opportunity for a comparative
study of low-frequency waves at Venus and Mars. The
spatial distributions of the wave properties, in particular in the
dayside and nightside magnetosheath as well as in the tail and
mantle region, are similar to observations at Mars. As both planets
do not have a global magnetic field, the interaction process of the
solar wind with both planets is similar and leads to similar
instabilities and wave structures. We focus on the spatial
distribution of the wave intensity of the fluctuating magnetic field and
detect an enhancement of the intensity in the dayside magnetosheath
and a strong decrease towards the terminator. For a detailed
investigation of the intensity distribution we adopt an
analytical streamline model to describe the plasma flow around
Venus. This allows displaying the evolution of the intensity along
different streamlines. It is assumed that the waves are generated
in the vicinity of the bow shock and are convected
downstream with the turbulent magnetosheath flow.
However, neither the different Mach numbers upstream and downstream
of the bow shock, nor the variation of the cross sectional area and
the flow velocity along the streamlines play probably an important role
in order to explain the observed concentration of wave intensity in the
dayside magnetosheath and the decay towards the nightside
magnetosheath. But, the concept of freely evolving or decaying turbulence is in good
qualitative agreement with the observations, as we observe a power law decay
of the intensity along the streamlines. The observations support the assumption of
wave convection through the magnetosheath, but reveal at the same time
that wave sources may not only exist at the bow shock, but also in the magnetosheath
An Impacting Descent Probe for Europa and the other Galilean Moons of Jupiter
We present a study of an impacting descent probe that increases the science
return of spacecraft orbiting or passing an atmosphere-less planetary body of
the solar system, such as the Galilean moons of Jupiter. The descent probe is a
carry-on small spacecraft (< 100 kg), to be deployed by the mother spacecraft,
that brings itself onto a collisional trajectory with the targeted planetary
body in a simple manner. A possible science payload includes instruments for
surface imaging, characterisation of the neutral exosphere, and magnetic field
and plasma measurement near the target body down to very low-altitudes (~1 km),
during the probe's fast (~km/s) descent to the surface until impact. The
science goals and the concept of operation are discussed with particular
reference to Europa, including options for flying through water plumes and
after-impact retrieval of very-low altitude science data. All in all, it is
demonstrated how the descent probe has the potential to provide a high science
return to a mission at a low extra level of complexity, engineering effort, and
risk. This study builds upon earlier studies for a Callisto Descent Probe (CDP)
for the former Europa-Jupiter System Mission (EJSM) of ESA and NASA, and
extends them with a detailed assessment of a descent probe designed to be an
additional science payload for the NASA Europa Mission.Comment: 34 pages, 11 figure
Magnetospheric lion roars
International audienceThe Equator-S magnetometer is very sensitive and has a sampling rate normally of 128 Hz. The high sampling rate for the first time allows detection of ELF waves between the ion cyclotron and the lower hybrid frequencies in the equatorial dawnside magnetosphere. The characteristics of these waves are virtually identical to the lion roars typically seen at the bottom of the magnetic troughs of magnetosheath mirror waves. The magnetospheric lion roars are near-monochromatic packets of electron whistler waves lasting for a few wave cycles only, typically 0.2 s. They are right-hand circularly polarized waves with typical amplitudes of 0.5 nT at around one tenth of the electron gyrofrequency. The cone angle between wave vector and ambient field is nearly always smaller than 1°
A case study of a radially polarized Pc4 event observed by the Equator-S satellite
International audienceA 16 mHz Pc4 pulsation was recorded on March 17, 1998, in the prenoon sector of the Earth's magnetosphere by the Equator-S satellite. The event is strongly localized in radial direction at approximately L = 5 and exhibits properties of a field line resonance such as an ellipticity change as seen by applying the method of the analytical signal to the magnetic field data. The azimuthal wave number was estimated as m \approx 150. We discuss whether this event can be explained by the FLR mechanism and find out that the change in ellipticity is more a general feature of a localized Alfvén wave than indicative of a resonant process
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