[1] We test the existing theories regarding the thresholds for the parametric decay instability (PDI), the oscillating two-steam instability (OTSI), and the thermal parametric instability (TPI) using the European Incoherent Scatter (EISCAT) facility's ionospheric heater. In these processes, the pump wave can couple to various electrostatic waves in the F layer ionosphere, which can be observed using the EISCAT UHF radar (PDI and OTSI) or by HF radar (TPI). On 19 October 2012, the heater power was stepped from 0.5 MW to 100 MW effective radiated power in seven steps using a 1 min on, 1 min off cycle. We use an electric field model, taking into account D region absorption, to compare theory with our observations. In all three cases, we find good agreement. In addition, the growth of striations formed during the TPI causes anomalous absorption of the heater wave, which we observe as decreased UHF ion line and plasma line backscatter power. We show evidence that heating for a prolonged period of time reduces the UHF ion line intensity throughout the experiment.Citation: Bryers, C. J., M. J. Kosch, A. Senior, M. T. Rietveld, and T. K. Yeoman (2013), The thresholds of ionospheric plasma instabilities pumped by high-frequency radio waves at EISCAT,

[1] We compare the height-integrated electron heating rates from O-and X-mode HF pump waves to extract the components due to resonant and nonresonant heating mechanisms in the ionosphere. We present results from a November 2011 campaign in Norway, using the European Incoherent Scatter (EISCAT) heater facility and UHF radar. We show that the theoretical nonresonant, ohmic heating due to the electromagnetic pump wave electric field agrees with observations for X-mode pumping. For O-mode pumping, the observed height-integrated heating rate exceeds the theoretical ohmic electron heating rate by a factor of 2-5, the excess being attributed to resonant heating mechanisms. In addition, a persistent UHF ion-line enhancement is observed for O-mode and, more unusually, X-mode pumping. We attribute the latter to O-mode leakage in the X-mode pulse. For O-mode, we see a descent in altitude of the ion-line enhancement and show that this is most likely due to ionization from pump-induced fluxes of suprathermal electrons. We estimate the ionization rate and determine an enhanced electron flux showing that approximately 10-20% of the pump power is transferred to high energy suprathermal electrons.

[1] We analyze optical emissions and enhanced electron temperatures induced by high power HF radio waves as a function of power flux using the EISCAT heater with a range of effective radiated powers. The UHF radar was used to measure the electron temperatures and densities. The Digital All Sky Imager was used to record the 630.0 nm optical emission intensities. We quantify the HF flux loss due to self-absorption in the D-region (typically 3-11 dB) and refraction in the F-region to determine the flux which reaches the upper-hybrid resonance height. We find a quasi-linear relationship between the HF flux and both the temperature enhancement and the optical emission excitation rate with a threshold at $37.5 mWm À2 . On average $70% of the HF flux at the upper-hybrid resonance height goes in to heating the electrons for fluxes above the threshold compared to $40% for fluxes below the threshold.Citation: Bryers, C. J., M. J. Kosch, A. Senior, M. T. Rietveld, and T. K. Yeoman (2012), EISCAT observations of pumpenhanced plasma temperature and optical emission excitation rate as a function of power flux,

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.

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