Quantified Energy Dissipation Rates in the Terrestrial Bow Shock
Résumé
We present the first observationally quantified measure of the energy dissipation rate due to wave-particle interactions in the transition region of the Earth's collisionless bow shock using data from the THEMIS spacecraft. Each of more than 11 bow shock crossings examined with available wave burst data showed both low frequency (<10 Hz) magnetosonic-whistler waves and high frequency (≥10 Hz) electromagnetic and electrostatic waves throughout the entire transition region and into the magnetosheath. The high frequency waves were identified as combinations of ion-acoustic waves, electron cyclotron drift instability driven waves, electrostatic solitary waves, and electromagnetic whistler mode waves. These waves were found to have: (1) amplitudes capable of exceeding δB ~ 10 nT and δE ~ 300 mV/m, though more typical values were δB ~ 0.1-1.0 nT and δE ~ 10-50 mV/m; (2) energy fluxes in excess of 2000 μW m−2; (3) resistivities > 9000 Ω m; and (4) energy dissipation rates > 3 μW m−3. The high frequency (>10 Hz) electromagnetic waves produce such excessive energy dissipation that they need only be, at times, < 0.01% efficient to produce the observed increase in entropy across the shocks necessary to balance the nonlinear wave steepening that produces the shocks. These results show that wave-particle interactions have the capacity to regulate the global structure and dominate the energy dissipation of collisionless shocks.