Abstract
We present the first observations of heater‐induced simultaneous topside and bottomside sporadic E layer enhancements at very high latitudes (78.15°N) using the Space Plasma Exploration by Active Radar (SPEAR) heating facility and the European Incoherent Scatter (EISCAT) Svalbard Radar. During the experiment the SPEAR heating facility was transmitting with O‐mode polarization in a field‐aligned direction with a constant effective radiated power of ∼16 MW. Results show distinct heater‐induced enhancements in both the ion and plasma line spectra. The plasma line enhancements are observed at the SPEAR heater frequency of 4.45 MHz. The plasma line observations represent the highest spatial resolution data (100 m) obtained of such heater‐induced enhancements and indicate simultaneous enhancements at both the topside and bottomside of the layer, respectively (located at ∼107.5 and 109 km altitude, respectively). It is postulated that the results represent evidence of O‐ to Z‐mode conversion of the heater wave occurring at the bottom of the E layer, allowing propagation through the layer resulting in simultaneous topside enhancements. The Z‐mode enhancements are observed outside the Spitze angle, which is thought to be a result of field‐aligned irregularities causing an increase in angular extent of the observations. Additional data from the Super Dual Auroral Radar Network (SuperDARN) HF Finland radar are also shown, which indicate that upon a thinning of the sporadic E layer, the heater beam propagated into the F region, where it induced artificial field‐aligned irregularities.
Highlights
The first is that of artificially generated field-aligned irregularities (AFAIs)
Such irregularities are thought to be generated through the thermal parametric instability (TPI) whereby the heater wave couples to upper hybrid waves at the upper hybrid resonance (UHR) height, which lies a few kilometers below the O-mode wave reflection height [Robinson, 1989, and references therein]
There have been observations made of such irregularities by the Super Dual Auroral Network (SuperDARN radars) [Greenwald et al, 1995], which have been generated by several ionospheric heating facilities including European Incoherent Scatter (EISCAT) [Baddeley et al, 2002], HAARP [Hughes et al, 2004], and Space Plasma Exploration by Active Radar (SPEAR) [Robinson et al, 2006]
Summary
[2] Experiments to artificially modify the Earth’s ionosphere by means of high-power HF Radio waves have been undertaken since the 1970s [e.g., Carlson and Duncan, 1977]. The second is called the Parametric Decay Instability (PDI) [e.g., Fejer, 1979] in which the heater wave decays into two oppositely directed highfrequency Langmuir waves and ion acoustic waves Both interactions are observable as enhancements in both the ion and plasma line spectra measured by Incoherent Scatter. It has been shown [e.g., Robinson, 2002] that the absorption of the heater beam at the upper hybrid height, resulting in the generation of AFAIs, will prevent the heater beam reaching the altitude with sufficient amplitude required for generation of the PGM and PDI While this seems to be true for ionospheric heaters located at lower latitudes, it has been shown [e.g., Robinson et al, 2006] that this is not always the case at SPEAR, with observations made of enhancements in both coherent and incoherent scatter occurring concurrently. [11] This paper will present observations of O- to Z-mode conversion in a sporadic E layer using the EISCAT Svalbard Radar, the SPEAR ionospheric heater, and the Co-operative UK Twin Auroral Sounding System (CUTLASS) SuperDARN radars
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