Entropy Generation Across Earth's Bow Shock

Earth's bow shock is a transition layer that causes an irreversible change in the state of plasma that is stationary in time. Theories predict entropy increases across the bow shock but entropy has never been directly measured. Cluster and Double Star plasma experiments measure 3D plasma distributions upstream and downstream of the bow shock that allow calculation of Boltzmann's entropy function H and his famous H-theorem, dH/dt ~ O. We present the first direct measurements of entropy density changes across Earth's bow shock. We will show that this entropy generation may be part of the processes that produce the non-thermal plasma distributions is consistent with a kinetic entropy flux model derived from the collisionless Boltzmann equation, giving strong support that solar wind's total entropy across the bow shock remains unchanged. As far as we know, our results are not explained by any existing shock models and should be of interests to theorists

Plasma waves associated with density holes upstream of Earth's bow shock

The regions upstream of Earth's bow shock provide a natural laboratory for a variety of fundamental plasma processes, which include the interaction between particle populations that stream away from the shock with the solar wind and low frequency waves. Recent observations from Cluster and Double Star have exhibited novel short-duration (~4-20 s) density holes. These density holes are accompanied by a variety of plasma waves at ~ 0.01 Hz to several tens kHz. Preliminary analysis shows that the gross-scale structures appear fast magnetosonic-like, while in the interior ion cyclotron waves, whistler mode waves, and strong electrostatic waves are observed. A broad spectrum of electrostatic turbulence (a few kHz to ~50 kHz)is observed at the upstream edges of density holes.These waves are likely to play important roles in determining the holes' properties, including heating of the suprathermal particles seen. Some of the wave activity, especially at the upstream edges of density holes, are similar to those observed at the Earth's bow shock. We have studied the wave characteristics in a number of density holes, and will discuss their possible mechanisms, influences on the plasma properties, and role in the structures' formation

ULF waves in the Io torus: ULYSSES observations

International audienceThroughout the Io torus, Ulysses has observed intense ultralow frequency (ULF) wave activity in both electric and magnetic components. Such ULF waves have been previously suggested as the source of ion precipitation leading to Jovian aurorae. The peaks of the wave spectra are closely related to the ion cyclotron frequencies, which is evidence of the waves being ion cyclotron waves (ICWs). Analysis of the dispersion relation using a multicomponent density model shows that at high latitudes (approximately 30 deg), peak frequencies of the waves fall into L mode branches of guided or unguided ICWs. Near the equator, in addition to the ICWs below fcO(2+), there are strong signals at approximately 10 Hz which require an unexpectedly large energetic ion temperature anistropy to be explained by the excitation of either convective or nonconvective ion cyclotron instabilities. Their generation mechanism remains open for the future study. Evaluation of the Poynting vector and the dispersion relation analysis suggest that the waves near the equator had a small wave angle relative to the magnetic field, while those observed at high latitudes were more oblique. The polarization of the waves below fcH(+) is more random than that of the whistler mode waves, but left-hand-polarized components of the waves can still be seen. The intensity of the ICWs both near the equator and at high latitudes are strong enough to meet the requirement for producing strong pitch angle scattering of energetic ions