THE EARTH'S ULF WAVE FORESHOCK: CLUSTER OBSERVATIONS

Abstract:

The interaction between backstreaming ions and the incoming solar wind in the upstream region of the bow shock, gives rise to a number of plasma instabilities from which ultra-low frequency (ULF) waves can grow. The region of ULF wave activity is spatially localized in the ion foreshock. Observational evidence of the ULF wave foreshock boundary has accumulated over the last three decades. In particular, it has been shown that the geometrical characteristics of the boundary are very sensitive to the interplanetary magnetic field (IMF) cone angle. In the present work, we aim at investigating the statistical properties of the ULF wave foreshock boundary. For this purpose, we make use of the first three years of magnetic field data from the flux gate magnetometer (FGM), and the plasma densities and velocities from the Hot Ion Analyzer (HIA) on board Cluster (SC-1). A new identification of the ULF wave foreshock boundary is presented, using specific and accurate criteria for a correct determination of boundary crossings. In particular, the criteria are based on the degree of IMF rotation as Cluster crosses the boundary. To reconstruct the foreshock geometry, we use two different 3-D gas dynamic bow shock models. The ULF wave foreshock boundary is compared with previous results reported in the literature as well as with theoretical predictions.