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Ann. Geophys., 26, 699-707, 2008
www.ann-geophys.net/26/699/2008/
© European Geosciences Union 2008
Lower hybrid waves at the shock front: a reassessment
S. N. Walker1, M. A. Balikhin1, H. St. C. K. Alleyne1, Y. Hobara1, M. André2, and M. W. Dunlop3
1Department of Automatic Control and systems Engineering, University of Sheffield, Sheffield, UK
2Swedish Institute for Radio Physics-Uppsala Division, University of Uppsala, Sweden
3Space Physics Division, SSTD, CCLRC Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK
Abstract. The primary process occurring at a collisionless shock is the redistribution
of the bulk upstream energy into other degrees of freedom. One part of this
process results in the acceleration of electrons at the shock front.
Accelerated electrons are observed at the terrestrial and other planetary
shocks, comets, and their effects are observed in astrophysical phenomena
such as supernova remnants and jets in the form of X-ray bremsstrahlung
radiation. One of the physical models for electron acceleration at
supercritical shocks is based on low-hybrid turbulence due to the presence of
reflected ions in the foot region. Since lower hybrid waves propagate almost
perpendicular to the magnetic field they can be simultaneously in resonance
with both the unmagnetised ions (ω=Vik⊥) and magnetised
electrons (ω=Vek||). In this paper, Cluster observations
of the electric field are used to study the occurrence of lower hybrid waves
in the front of the terrestrial bow shock. It is shown that the lower hybrid
waves exist as isolated wave packets. However, the very low level of the
observed lower hybrid turbulence is too small to impart significant
energisation to the electron population.
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