Radar observations of artificial E-region field-aligned irregularities

Abstract

Abstract. Artificial E region field aligned plasma density irregularities (FAIs) were generated using HAARP in four different experimental modes and observed with a coherent scatter radar imager located 450 km to the southwest where it could detect field-aligned backscatter. The experiments were conducted in July of 2008, during the Polar Aeronomy and Radio Science Summer School (PARS), during quiet conditions in the daytime when the E layer was dense and absorption was modest. The echoes observed during zenith and magnetic zenith heating experiments were deflected from their nominally anticipated horizontal positions toward the midpoint position. The occurrence of hysteresis when heating with amplitude modulated pulses implied the development of the resonance instability, although the threshold for the onset of instability appeared to be higher than what has been predicted theoretically. Heating experiments involving pump frequencies slightly above and below the second electron gyroharmonic frequency produced no significant differences in the observed echoes. Finally, heating with a pump frequency slightly above the E region critical frequency appears to have produced FAIs at two distinct altitudes where the upper-hybrid resonance condition could be satisfied.

Artificial E-region field-aligned plasma irregularities generated at pump frequencies near the second electron gyroharmonic

“…Other diagnostics unavailable for these experiments may do so, but the coherent scatter radar does not clearly distinguish between irregularities generated with pump frequencies above and below 2Ω e . As hysteresis effects indicative of wave trapping are evident in experiments involving pump frequencies below 2Ω e (Nossa et al, 2009), theory governing wave trapping in this frequency regime warrants additional investigation.…”

“…Most of the experiments performed so far were undertaken at heating frequencies below the second electron gyroharmonic. Preliminary results argue that the thermal OTSI, wave trapping, and the resonance instability are at work at these experiments (Nossa et al, 2009). …”

“…2 (the complete dataset, typified by this short segment, appears in Fig. 1 of Nossa et al, 2009). The echoes received this time had very small Doppler shifts, and the data in the figure were coherently integrated by a factor of 16 prior to detection to improve the overall signal-to-noise ratio.…”

“…Fialer, 1974;Hysell, 2008). For example, the FAIs observed by Nossa et al (2009) were mainly generated using a pump frequency of just 2.75 MHz. Theory in this area has mainly concentrated on F region applications, but renewed interest in E region heating experiments prompts reinvestigation of the experimental record and the underlying physics.…”

VHF Radar Images of Artificial Field‐Aligned Ionospheric Irregularities in the Subauroral E Region

“…AFAIs in the E region, meanwhile, can be observed with much less refraction and accompanying uncertainty in echo location. Observations of E region AFAIs at very high frequency (VHF) tend to be relatively consistent and repeatable so long as natural instabilities do not intervene (Djuth et al, ; Hibberd et al, ; Hysell & Nossa, ; Nossa et al, ). Conducting such experiments requires the ability to generate pump HF waves at frequencies below the E region critical frequency.…”