Ulysses observations of electron and proton components in a magnetic cloud and related wave activity

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Solar Wind Plasma, Sources Of Solar Wind, Interplanetary Magnetic Fields

Scientific paper

Fifteen years ago, Burlaga defined magnetic clouds as interplanetary structures with enhanced magnetic field characterized by a smooth rotation of the magnetic field vector and a low proton temperature Tp. Their expansion in the solar wind leads to a depletion of plasma and a cooling of the ion component. Recently, Osherovich and colleagues showed that the electron component in magnetic clouds behaves differently: When the cloud expands, the electron temperature Te anticorrelates with the density and therefore Te increases in the cloud. Since Landau damping is not effective for Te/Tp>>1, they predicted an increase of ion-acoustic wave activity in magnetic clouds. Our paper presents evidence in support of this prediction. For the magnetic cloud observed by Ulysses on June 10-12, 1993 at 4.64 AU at S 32.5 deg, we present observations for both the electron and the proton components and the related plasma waves activity. Our results confirm the anticorrelation between Te and electron density: the data also exhibit high ratios of Te/Tp in the cloud (Te/Tp~10-20). Calculated frequencies of Doppler-shifted ion-acoustic waves compare well with the frequency range of the observed wave activity in the cloud. We find that the electron component in the cloud obeys a polytropic law with polytropic index γe<1(γe~0.35-0.37). The dynamics of this magnetic cloud are determined to a large degree by the dominating electron pressure with the contribution from halo electrons comparable to that from core electrons.

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