Artificial optical emissions at HAARP for pump frequencies near the third and second electron gyro-harmonic

M J Kosch,F T Djuth,J Hughes,M Mccarrick,T Pedersen,D Sentman,A Senior,R Marshall,E Gerken

doi:10.5194/angeo-23-1585-2005

Abstract

Abstract. High-power high-frequency radio waves beamed into the ionosphere cause plasma turbulence, which can accelerate electrons. These electrons collide with the F-layer neutral oxygen causing artificial optical emissions identical to natural aurora. Pumping at electron gyro-harmonic frequencies has special significance as many phenomena change their character. In particular, artificial optical emissions become strongly reduced for the third and higher gyro-harmonics. The High frequency Active Auroral Research Program (HAARP) facility is unique in that it can select a frequency near the second gyro-harmonic. On 25 February 2004, HAARP was operated near the third and passed through the second gyro-harmonic for the first time in a weakening ionosphere. Two novel observations are: firstly, a strong enhancement of the artificial optical emission intensity near the second gyro-harmonic, which is opposite to higher gyro-harmonics; secondly, the optical enhancement maximum occurs for frequencies just above the second gyro-harmonic. We provide the first experimental evidence for these effects, which have been predicted theoretically. In addition, irregular optical structures were created when the pump frequency was above the ionospheric critical frequency.Keywords. Active experiments – Auroral ionosphere – Wave-particle interactions

Highlights

A major goal for active experiments in the ionosphere is to understand the fundamental physics governing the interaction between plasma and electromagnetic waves
High power high frequency pumping of the ionosphere causes several well-known phenomena. These include magnetic field-aligned density irregularities (Robinson et al, 1989; Frolov et al, 1997) of small scale size (1–10 m) (Kelley et al, 1995) called striations, stimulated electromagnetic emissions (Leyser, 2001), anomalous absorption of low power diagnostic radio waves when they propagate through the pumped volume (Stubbe, 1996), electron temperature enhancements (Stocker et al, 1992; Rietveld et al 2003) and artificial optical emissions, which are produced by electron collisions with the neutrals
The increased accelerated electron flux, as witnessed by the brighter optical emissions, indicates that the acceleration mechanism is more efficient when pumping just above the second gyro-harmonic. This suggests that the striations are enhanced when pumping near the second gyro-harmonic, consistent with Fialer’s (1974) observations, which is consistent with the theories by Das and Fejer (1979) and Grach (1979)

Summary

Introduction

A major goal for active experiments in the ionosphere is to understand the fundamental physics governing the interaction between plasma and electromagnetic waves. Istomin and Leyser (2003) predicted theoretically that upper-hybrid waves and striations, and artificial optical emissions, would be stronger for pump frequencies just above a gyro-harmonic frequency compared to below it, i.e. an asymmetry would exist. In the 1970s, experiments using the midlatitude Platteville heater in Colorado, USA, were performed at the fixed pump frequency of 2.85 MHz. They produced coherent radar backscatter (Fialer, 1974; Minkoff et al, 1974), i.e. striations and upper-hybrid waves, and artificial optical emissions (

Results

Conclusion