Convective Amplification of Electromagnetic Ion Cyclotron Waves From Ring-distribution Protons in the Inner Magnetosphere

TL;DR

This paper investigates how ring-distribution protons in the inner magnetosphere can significantly amplify electromagnetic ion cyclotron waves, especially at oblique angles, potentially explaining observed equatorial noise.

Contribution

It demonstrates that ring distributions can produce greater EMIC wave amplification than bi-Maxwellian distributions due to their non-monotonic velocity profile, especially near the Alfven speed.

Findings

Maximum growth rate occurs at oblique angles for ring speeds near the Alfven speed.

Amplification is stronger for non-monotonic ring distributions compared to bi-Maxwellian.

Strong wave gain is associated with moderate ring speeds close to the Alfven speed.

Abstract

The growth of electromagnetic ion cyclotron waves (EMIC) due to a ring distribution of Hydrogen ions is examined. Though these distributions are more commonly implicated in the generation of equatorial noise, their potential for exciting EMIC waves is considered here. It is shown that since the ring distribution is non-monotonic in perpendicular velocity, the amplification achieved by this instability is greater than bi-Maxwellian distributions for typical anisotropies, because the waves can maintain resonance over a much longer part of its trajectory. For ring speeds V_R close to the Alfven speed V_A, the growth rate is maximum at parallel propagation but is much larger at oblique angles compared with a bi-Maxwellian, and can have a second peak approximately at kperp c/omegaPH V_R/V_A ~2.3 for ring speeds about the parallel thermal speed. Strong wave gain is achieved for moderate ring speeds V_R~V_A . The analysis suggests that EMIC wave activity should be closely associated with equatorial noise.