Nonlinear Processes in Geophysics www.nonlin-processes-geophys.net 1023-5809 1607-7946 16 2 2009 10.5194/npg-16-373-2009 http://www.nonlin-processes-geophys.net/16/373/2009/ http://www.nonlin-processes-geophys.net/16/373/2009/npg-16-373-2009.html http://www.nonlin-processes-geophys.net/16/373/2009/npg-16-373-2009.pdf 373 380 2009-04-30 Nonlinear electron acoustic structures generated on the high-potential side of a double layer R. Pottelette raymond.pottelette@lpp.polytechnique.fr M. Berthomier LPP-CNRS/INSU, 4 avenue de Neptune, 94107 Saint-Maur des Fossés, France High-time resolution measurements of the electron distribution function performed in the auroral upward current region reveals a large asymmetry between the low- and high-potential sides of a double-layer. The latter side is characterized by a large enhancement of a locally trapped electron population which corresponds to a significant part (~up to 30%) of the total electron density. As compared to the background hot electron population, this trapped component has a very cold temperature in the direction parallel to the static magnetic field. Accordingly, the differential drift between the trapped and background hot electron populations generates high frequency electron acoustic waves in a direction quasi-parallel to the magnetic field. The density of the trapped electron population can be deduced from the frequency where the electron acoustic spectrum maximizes. In the auroral midcavity region, the electron acoustic waves may be modulated by an additional turbulence generated in the ion acoustic range thanks to the presence of a pre-accelerated ion beam located on the high-potential side of the double layer. Electron holes characterized by bipolar pulses in the electric field are sometimes detected in correlation with these electron acoustic wave packets. Bale, S. D., Hull, A., Larson, D. E., Lin, R. P., Muschietti, L., Kellogg, P. J., Goetz, K., and Monson, S. J. : Electrostatic turbulence and Debye-scale structures associated with electron thermalization at collisionless shocks, Astrophys. J., 575, L25–L28, 2002. Berthomier, M., Pottelette, R., Malingre, M., and Khotyaintsev, Y.: Electron-acoustic solitons in an electron-beam plasma system, Phys. Plasmas, 9, 2987–2994, 2000. Berthomier, M., Pottelette, R., Muschietti, L., Roth, I., and Carlson, C.: Scaling of three~dimensional electron phase space density holes observed by FAST in the auroral downward current region, Geophys. Res. Lett., 30(22), 2148–152, doi:10.1029/2003GL018491, 2003. Carlson, C. W., Pfaff, R., and Watzin, J. G.: The Fast Auroral Snapshot (FAST) mission, Geophys. Res. Lett., 25, 2013–2016, 1998. Cattell, C., Crumley, J., Dombeck, J., Wygan, J., and Mozer, F. S.: Polar observations of solitary waves at the Earth's magnetopause, Geophys. Res. Lett., 29(5), 1065, doi:10.1029/2001GL014046, 2002. Dubouloz, N., Treumann, R. A., and Pottelette, R.: Turbulence generated by a gas of electron-acoustic solitons, J. Geophys. Res., 98, 17415–17422, 1993. Ergun, R. E., Carlson, C. W., McFadden, J. P., et al.: FAST satellite observations of large amplitude solitary structures, Geophys. Res. Lett., 25, 2061–2064, 1998. Ergun, R. E., Anderson, L., Main, L., Su, Y.-J., Newman, D. L., Goldman, M. V., Carlson, C. W., McFadden, J. P., and Mozer, F. S.: Parallel electric fields in the upward current region of the aurora: Numerical solutions, Phys. Plasmas, 9, 3695–3704, 2002. Ergun, R. E., Anderson, L., Main, L., Su, Y.-J., Newman, D. L., Goldman, M. V. , Carlson, C. W., Hull, A. J., McFadden, J. P., and Mozer, F. S.: Auroral particle acceleration by strong double layers: The upward current region, J. Geophys. Res., 109, A12220, doi:10.1029/2004JA010545, 2004. Kakad, A. P., Singh, S. V., Reddy, R. V., Lakhina, G. S., Tagare, S. G., and Verheest, F.: Generation mechanism for electron acoustic solitary waves, Phys. Plasmas, 14, 052305-9, doi:10.1063/1.2732176, 2007. Lakhina, G. S., Kakad, A. P., Singh, S. V., and Verhest, F.: Ion- and electron-acoustic solitons in two temperature space plasmas, Phys. Plasmas, 15, 062903, doi:10.1063/1.2930469, 2008. Matsumoto, H., Kojima, H., Miyatake, T., Omura, Y., Okada, M. et al.: Electrostatic solitary waves~(ESW) in the magnetotail: BEN wave forms observed by GEOTAIL, Geophys. Res. Lett., 21, 2915–2918, 1994. Mozer, F. S. and Hull, A.: Origin and geometry of upward parallel electric fields in the auroral acceleration region, J. Geophys. Res., 106, 5763–5775, 2001. Muschietti, L., Ergun, R. E., Roth, I., Carlson, C. W.: Phase-space electron holes along magnetic field lines, Geophys. Res. Lett., 26(8), 1093–1096, 1999. Muschietti, L., Roth, I., Carlson, C. W., and Berthomier, M.: Modeling stretched solitary waves along magnetic field lines, Nonlinear. Proc. Geoph., 9, 101–109, 2002. Omura, Y., Matsumoto, H., Miyake, T., and Kojima, H.: Electron beam instabilities as generation mechanism of electrostatic solitary waves in the magnetotail, J. Geophys. Res., 101(A2), 2685–2697, 1996. Pickett, J. S., Menietti, J. D., Gurnett, D. A., Tsurutani, B., Kintner, P. M., Klatt, E., and Balogh, A.: Solitary potential structures observed in the magnetosheath by the Cluster spacecraft, Nonlinear. Proc. Geoph., 10, 3–11, 2003. Pottelette, R., Ergun, R. E., Treumann, R. A., Berthomier, M., Carlson, C., McFadden, J. P., and Roth, I.: Modulated electron acoustic waves in auroral density cavities: FAST Observations, Geophys. Res. Lett., 26, 2629–2632, 1999. Pottelette, R. and Treumann, R.: Electron holes in the auroral upward current region, Geophys. Res. Lett., 32, L12104, doi:10.1029/2005GL022547, 2005. Shukla, P. K., Mamun, A., and Eliasson, B.: 3-D electron acoustic solitary waves introduced by phase space electron vortices in magnetized space plasmas, Geophys. Res. Lett., 31, L07803, doi:10.1029/2004GL019533, 2004. Singh, S. V. and Lakhina, G. S.: Generation of electron acoustic waves in the magnetosphere, Planet. Space Sci., 49, 107–114, 2001. Singh, S. V. and Lakhina, G. S.: Electron acoustic solitary waves with non-thermal distribution of electrons, Nonlinear. Proc. Geoph., 11, 275–279, 2004. Tokar, R. L. and Gary, S. P.: Electrostatic hiss and the beam driven electron acoustic instability in the dayside polar cusp, Geophys. Res. Lett., 11, 1180–1183, 1984. Treumann, R. A. and Baumjohann, W.: Advanced Space Plasma Physics, Imperial College Press, London, UK, 82–84, 1997.