Beam-driven ECH waves: A parametric study

TL;DR

This study investigates how electron beams can excite electron cyclotron harmonic (ECH) waves in Earth's magnetosphere, revealing conditions that enhance wave growth and their potential role in auroral phenomena.

Contribution

It provides a comprehensive parametric analysis of beam-driven ECH wave instability, highlighting the influence of plasma parameters on wave growth, which was previously not well understood.

Findings

Wave growth is driven by beam electron cyclotron resonances of multiple orders.

ECH waves are unstable across a wide range of plasma conditions.

Growth rate increases with beam density, velocity, and hot electron temperature.

Abstract

Electron cyclotron harmonic (ECH) waves play a significant role in driving the diffuse aurora, which constitutes more than 75% of the particle energy input into the ionosphere. ECH waves in magnetospheric plasmas have long been thought to be excited predominantly by the loss cone anisotropy (velocity-space gradients) that arises naturally in a planetary dipole field. Recent THEMIS observations, however, indicate that an electron beam can also excite such waves in Earth's magnetotail. The ambient and beam plasma conditions under which electron beam excitation can take place are unknown. Knowledge of such conditions would allow us to further explore the relative contribution of this excitation mechanism to ECH wave scattering of magnetospheric electrons at Earth and the outer planets. Using the hot plasma dispersion relation, we address the nature of beam-driven ECH waves and conduct a comprehensive parametric survey of this instability. We find that growth is provided by beam electron cyclotron resonances of both first and higher orders. We also find that these waves are unstable under a wide range of plasma conditions. The growth rate increases with beam density, beam velocity, and hot electron temperature; it decreases with increasing beam temperature and beam temperature anisotropy, hot electron density, and cold electron density and temperature. Such conditions abound in Earth's magnetotail, where magnetospheric electrons heated by earthward convection and magnetic reconnection coexist with colder ionospheric electrons.