Aspect angle sensitivity of pump-induced optical emissions at EISCAT

Michael J Kosch,Yasunobu Ogawa,Michael T Rietveld,Carl Bryers,Timothy K Yeoman

doi:10.1186/s40623-014-0159-x

Abstract

Abstract We investigate the aspect angle sensitivity of the pump-induced artificial optical emissions in the ionosphere over the European Incoherent Scatter Scientific Association (EISCAT) high-frequency transmitter facility at Ramfjord, Norway, as a function of the pump beam launch angle relative to the magnetic field line direction. The highest intensity optical emissions occur when the pump beam pointing direction is in the magnetic zenith (approximately 12° S of local zenith). For pump beam directions further north from field aligned, the optical emission intensity decreases for the same pump power. In addition, the primary photon-emitting region becomes displaced towards the magnetic zenith relative to the pump beam and for larger aspect angles, the brightest emissions were found to be outside the −3-dB pump beam width. The Cooperative UK Twin-Located Auroral Sounding System (CUTLASS) coherent scatter high-frequency (HF) radar detected a quasi-constant level of backscatter power from the pumped ionosphere, indicating that saturated striations were formed for all pump beam directions. This indicates that the presence of upper-hybrid resonance is not sufficient to explain the angular sensitivity of the optical emissions.

Highlights

The production of artificial optical emissions by highpower, high-frequency (HF) radio waves has been observed at high-latitude locations such as the European Incoherent Scatter Scientific Association (EISCAT) facility (Rietveld et al 1993) since 1999 (Brändström et al 1999; Kosch et al 2000) and prior to this at lower-latitude facilities such as Platteville (Haslett and Megill 1974) and Arecibo (Bernhardt et al 1988)
By varying the EISCAT heater beam-pointing direction, it has been shown that the artificial pump-induced optical emission intensity is highly dependent upon the angular distance from the magnetic zenith
The optical emission intensity is usually maximum near the magnetic field-aligned direction even when there is more pump power in other directions. These observations are consistent with previous results from HAARP (Pedersen et al 2003, 2008) and Sura (Grach et al, 2007, 2012). These observations suggest that the mechanism which accelerates suprathermal electrons, believed to include upper-hybrid resonance, is aspect sensitive and more efficient parallel to the magnetic field line compared to the direction where most pump power is being transmitted

Summary

Introduction

The production of artificial optical emissions by highpower, high-frequency (HF) radio waves has been observed at high-latitude locations such as the European Incoherent Scatter Scientific Association (EISCAT) facility (Rietveld et al 1993) since 1999 (Brändström et al 1999; Kosch et al 2000) and prior to this at lower-latitude facilities such as Platteville (Haslett and Megill 1974) and Arecibo (Bernhardt et al 1988). The HF electromagnetic wave causes plasma wave instabilities (e.g. Langmuir turbulence and upper-hybrid (UH) waves) to grow close to the pump wave reflection height where the plasma frequency (fp) equals the pump frequency (fo) These electrostatic waves can be stimulated by the parametric decay instability (PDI), thermal parametric instability (TPI) or upper-hybrid resonance (UHR) (Kosch et al 2007a). The plasma waves heat the bulk plasma and efficiently accelerate ambient electrons to suprathermal energies great enough to excite atomic oxygen. Upon decaying to their ground state, a photon is emitted. The most commonly observed states are the O(1D) (red-line at 630 nm) and O(1S)

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