Quantified Energy Dissipation Rates: Electromagnetic Wave Observations in the Terrestrial Bow Shock

TL;DR

This study quantifies the energy dissipation rates caused by high frequency electromagnetic waves in Earth's bow shock, demonstrating their potential to regulate shock structure through detailed wave observations from the THEMIS spacecraft.

Contribution

First measurement of energy dissipation rates by high frequency waves in Earth's bow shock using spacecraft data, highlighting their role in shock regulation.

Findings

High frequency waves have large amplitudes and energy fluxes exceeding 2000 μW/m².

Dissipation rates surpass the entropy increase needed across shocks by over four orders of magnitude.

Waves include ion-acoustic, whistler, and electrostatic solitary waves, capable of influencing shock dynamics.

Abstract

We present the first quantified measure of the rate of energy dissipated per unit volume by high frequency electromagnetic waves in the transition region of the Earth's collisionless bow shock using data from the THEMIS spacecraft. Every THEMIS shock crossing 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 waves in both frequency ranges had large amplitudes, but the higher frequency waves, which are the focus of this study, showed larger contributions to both the Poynting flux and the energy dissipation rates. The higher frequency waves were identified as combinations of ion-acoustic waves, electron cyclotron drift instability driven waves, electrostatic solitary waves, and whistler mode waves. These waves were found to have: (1) amplitudes capable of exceeding dB ~ 10 nT and dE ~ 300 mV/m, though more typical values were dB ~ 0.1-1.0 nT and dE ~ 10-50 mV/m; (2) energy fluxes in excess of 2000 x 10^(-6) W m^(-2); (3) resistivities > 9000 Ohm m; and (4) energy dissipation rates > 3 x 10^(-6) W m^(-3). The dissipation rates were found to be in excess of four orders of magnitude greater than was necessary to explain the increase in entropy across the shocks. Thus, the waves need only be, at times, < 0.01% efficient to balance the nonlinear wave steepening that produces the shocks. Therefore, these results show for the first time that high frequency electromagnetic and electrostatic waves have the capacity to regulate the global structure of collisionless shocks.