HF induced airglow: Double patches!

HF induced airglow has been observed at high latitudes on 21 February 1999 during a geomagnetically quiet period (Kp = 1). This observation is unique because two patches were generated simultaneously on one occasion. The EISC AT Heating experiment (69.59 degrees N, 19.23 degrees E) was operated at 4. 04 MHz (O-mode) in the local zenith from 16:48 to 18:28 UT using a LE-min. on and 4-min. off duty cycle. Simultaneously, the dynasonde measured the re flection height which increased from similar to 200 to similar to 290 lan d uring the experiment. The Digital All-Sky Imager (DASI) was recording at 63 0 nm from Skibotn (69.35 degrees N, 20.36 degrees E), about 50 km east of H eating. Whilst viewing in the local zenith with similar to 116 degrees fiel d of view, the development and fading of an airglow patch(es) corresponding to the Heater on and off times, respectively, could be clearly observed wi th an intensity of up to similar to 100 Rayleighs. Sometimes a diffuse airg low would appear approximately within the Heater beam and sometimes a brigh ter and smaller airglow patch would develop near the magnetic field aligned position, which is similar to 13 degrees south of vertical. For the latter direction, it is estimated that the Heater power was 10 dB down from the m aximum in the vertical. After 18:12 UT, the ionospheric critical frequency dropped below 4.04 MHz and no further airglow was observed. (C) 2000 Elsevi er Science Ltd. All rights reserved.