The ULF wave foreshock boundary: Cluster observations

ANDRES, N.; MEZIANE, K.; MAZELLE, C. X.; ; BERTUCCI, C.;; GOMEZ, D. O.

Congreso; American Geophysical Union, Fall Meeting 2013; 2013

In the upstream region of the bow shock, the interaction of backstreaming ions with the incoming solar wind gives rise to a number of plasma instabilities from which ultra-low frequency (ULF) waves can grow. Due to the finite growth rate, it is expected that 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. Among other things, 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 aimed at revisiting the properties of the ULF wave foreshock boundary. For this purpose, we use 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) low-geometry factor side on board RUMBA (SC 1). We use a specific and accurate criterion for the determination of boundary crossings, and a 3-D structure bow shock model to reconstruct the foreshock geometry. In particular, our criterion is used to qualitatively measure the differences between the magnetic field in the wave and no-wave zones, taking into account possible rotations of the IMF. A new identification of the ULF wave foreshock boundary is presented and it is compared with previous results reported in the literature as well as with theoretical predictions.