Aspect sensitivity of the first 100 ms HF-modification measured with the MUIR at HAARP

Oyama, S.1 and Watkins, B. J.2

1 STEL, Nagoya University, Nagoya, Japan
2 GI UAF, Fairbanks, USA

In this paper we present results of new height-resolved observations of F-region Langmuir turbulence measured with the MUIR (Modular UHF Ionospheric Radar; 446 MHz) at HAARP (High frequency Active Auroral Research Program) in Alaska, USA. The scientific objective of this paper is to study aspect sensitivity of the first 100 ms of HF modification. An outstanding advantage of the MUIR is the phased array system, which enables us to change the beam direction every IPP (interpulse period). The HAARP HF transmitter system was operated on 25-26 March 2006 during geomagnetically quiet conditions. HF pulses of O-mode polarization at 4.95 MHz were periodically transmitted in duration of 100 ms at 8 degree off from the geographic vertical to the geomagnetic south. The duty cycle is 1 % (IPP = 10 s). Three MUIR beam directions were selected for the experiment; geographical vertical, up B (elevation angle = 75 degree), and between them (elevation angle = 82 degree). The coded long-pulse technique was employed with 10 ms IPP, 998 micro-second pulse length, and 1 micro-second baud length, which gives approximately 150 m range resolution. The radar backscatter spectrum was obtained every IPP of 10 ms; but the spectrum at each beam position was derived every 30 ms because the beam direction was sequentially changed every IPP among three directions mentioned above. Thus three HF-pulse sequences or 30 s provide a series of the spectra during HF-on period (100 ms) with time resolution of 10 ms at each MUIR beam position. The radar backscatter spectra along the geomagnetic field line were characterized by well-developed parametric decay instability and the Langmuir decay instability cascade features. While a few cascade lines were identified in the spectra at the middle beam position, number of the cascade line was smaller than that for the field-aligned spectra. The cascade feature was not seen in the vertical-beam spectra. These results are consistent with spectral features predicted by the modern Langmuir turbulence theory. The temporal growth and saturation of the spectra during heating and the decay of the spectra after the HF turn-off are also studied.