Articles | Volume 26, issue 12
Ann. Geophys., 26, 3707–3718, 2008
https://doi.org/10.5194/angeo-26-3707-2008

Special issue: STAMMS: Spatio-Temporal Analysis and Multipoint Measurements...

Ann. Geophys., 26, 3707–3718, 2008
https://doi.org/10.5194/angeo-26-3707-2008

  25 Nov 2008

25 Nov 2008

Nonlinear low-frequency wave aspect of foreshock density holes

N. Lin1, E. Lee1, F. Mozer1, G. K. Parks1, M. Wilber1, and H. Rème2 N. Lin et al.
  • 1Space Sciences Laboratory, UC Berkeley, Berkeley, CA 94720, USA
  • 2Centre d'Etude Spatiale des Rayonnement, Paul Sabatier University, Toulouse, France

Abstract. Recent observations have uncovered short-duration density holes in the Earth's foreshock region. There is evidence that the formation of density holes involves non-linear growth of fluctuations in the magnetic field and plasma density, which results in shock-like boundaries followed by a decrease in both density and magnetic field. In this study we examine in detail a few such events focusing on their low frequency wave characteristics. The propagation properties of the waves are studied using Cluster's four point observations. We found that while these density hole-structures were convected with the solar wind, in the plasma rest frame they propagated obliquely and mostly sunward. The wave amplitude grows non-linearly in the process, and the waves are circularly or elliptically polarized in the left hand sense. The phase velocities calculated from four spacecraft timing analysis are compared with the velocity estimated from δEB. Their agreement justifies the plane electromagnetic wave nature of the structures. Plasma conditions are found to favor firehose instabilities. Oblique Alfvén firehose instability is suggested as a possible energy source for the wave growth. Resonant interaction between ions at certain energy and the waves could reduce the ion temperature anisotropy and thus the free energy, thereby playing a stabilizing role.