ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-26-3707-2008 Nonlinear low-frequency wave aspect of foreshock density holes Lin N. 1 Lee E. 1 Mozer F. 1 Parks G. K. 1 Wilber M. 1 Rème H. 2 Space Sciences Laboratory, UC Berkeley, Berkeley, CA 94720, USA Centre d'Etude Spatiale des Rayonnement, Paul Sabatier University, Toulouse, France 25 11 2008 26 12 3707 3718 Copyright: © 2008 N. Lin et al. 2008 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/26/3707/2008/angeo-26-3707-2008.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/26/3707/2008/angeo-26-3707-2008.pdf

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 &delta;<I>E</I>/&delta;<I>B</I>. 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.

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