Hf-pump-induced Parametric Instabilities in the Auroral E-region

In November 1999 the EISCAT high-power, high-frequency (HF) facility located near Tromsø, Norway, was used to create artificial plasma turbulence in the ionosphere. During the experiment the EISCAT 224 MHz radar and sometimes the 931 MHz radar were used to obtain measurements of incoherent scatter ion and plasma lines, and artificially enhanced spectra of E-region plasma waves were measured for the first time at auroral latitudes with both radars. During periods with suitable peak E-region electron density, Z-mode propagation of the HF pump wave to the topside E-region occurred, and topside instability-enhanced plasma waves were observed. In addition to HF-pump-induced effects, an unusual F-region echo was seen in both the ion and plasma line channels, which appears to be due to an auroral arc intersecting the radar beam. INTRODUCTION At the high latitude European Incoherent Scatter (EISCAT) site near Tromsø, Norway, high frequency (HF) experiments involving the excitation of plasma instabilities using O-mode transmissions have been done mainly during the daytime. The reasons are a frequent lack of high enough electron density at night, especially near solar minimum, and the desire to avoid the ionospheric disturbances caused by auroral activity. Recently, however, such experiments have been attempted at night in order to study the artificial airglow and large electron temperature enhancements which may be created (e.g. Leyser et al., 2000). During a campaign in November 1999, a new data-taking program was implemented on the EISCAT very high frequency (VHF, 224 MHz) and ultra high frequency (UHF, 931 MHz) incoherent scatter radars which extended the scope of the observations to include ion and plasma lines from the E-region and the topside ionosphere with reasonable altitude resolution. E-region HFenhanced plasma and ion lines were observed. These observations are the first from high latitudes. Plasma waves excited in a sporadic-E layer by powerful HF wave injection were first reported using the 430 MHz incoherent scatter radar at Arecibo by Gordon and Carlson (1976). Further results, in both normal daytime and sporadic E-layers, were obtained by Djuth (1984) and their temporal evolution was studied by Djuth and Gonzales (1988). All the E-region plasma line observations at Arecibo showed narrow spectral features at 430 MHz ± fHF where fHF is the HF pump frequency. At the EISCAT facilities, enhanced E-region ion lines were observed with the UHF incoherent scatter radar by Schlegel et al. (1987), but spectra were not reported and there were no plasma line measurements. Based on the Arecibo results, two candidate interaction mechanisms have been postulated. One is the modulational instability, also known as the oscillating two-stream instability or the purely-growing parametric decay instability, where the HF wave decays into two oppositely directed Langmuir waves having frequencies equal to fHF and two ion acoustic waves shifted to zero frequency (Muldrew, 1978; Nishikawa, 1968). The other mechanism is direct conversion of the HF pump into Langmuir waves by ionospheric irregularities (Djuth, 1984). TECHNIQUE HF modulation On 11 Nov. 1999 the EISCAT HF facility was used to transmit an effective radiated power of 240 MW at 4.04 MHz with a 10 s on, 10 s off square wave modulation sequence. Before 18:27 UT only O mode was transmitted, after 18:27 UT the polarization alternated between X mode during the odd minutes and O mode during even minutes. The HF beam was directed vertically. Radar data taking program The data presented here were taken with programs created using software developed by T. Grydeland. The core of the experiment is a new correlator program, called EPLA2 (E-region plasma line experiment number 2) built from general purpose GEN (for the long pulse (Turunen, 1985, 1986)) and G2 (for the alternating code (Wannberg, 1993)) system subroutines. With the exception of the radar frequencies, the same program was run on both the VHF and UHF radars. The radar program combines long pulse, power profile (short pulse) and alternating code modulations, with three receiver channels receiving the power profile and the alternating code and one channel receiving the long pulse. This means that both the ion line and two different plasma line offsets can be monitored simultaneously. The alternating code channels (Lehtinen, 1986; Lehtinen and Häggström, 1987) allowed us to record high-resolution spectra of artificially induced ion and plasma lines in the E and F-regions. The long pulse was used to record ion line spectral data for analysis of background parameters. For the observations presented in this paper, we used 20 bits of a 32-bit alternating code with a baud length of 25 μs, resulting in a range resolution of 3.75 km, covering ranges from 90 to 311 km. The lag resolution is 25 μs, and lags from 25 to 475 μs are covered, resulting in a frequency resolution of nearly 2 kHz. This is sufficient for observation of the ion spectrum at these altitudes. The power profile used the same range resolution as the alternating code, but with the range extended to 475 km. The long pulse transmission was 420 μs in duration and was used to record data in the ranges from 180 to 780 km. The power profile short pulse and alternating code were transmitted 422 and 6,394 μs after the start of the long pulse, respectively. The VHF transmission frequencies were 223.6 (222.4) MHz (long pulse) and 222.6 (223.4) MHz (power profile and alternating code pairs) for set 1 (set 2). The UHF transmission frequencies were 930.5 (927.5) MHz (long pulse) and 928.0 (930.0) MHz (power profile and alternating code pairs) for set 1 (set 2). The pulse repetition period for a single pair of transmission sets was 38,506 μs; 256 repetitions of the transmission pairs (four complete alternating code cycles) were made during each 10-s integration period. The VHF radar antenna was pointed vertically while the UHF antenna was pointed at 81o elevation towards the south; the UHF radar ran only part of the time and suffered from low transmitter power and a noisy receiver. The magnetic dip angle at Tromsø is 13o. Supporting observations A co-located digital HF sounder (ionosonde) made fixed and swept frequency soundings. Other diagnostics included an all-sky airglow imaging system, stimulated electromagnetic emission (SEE) spectral measurements, and HF and VHF coherent scatter radar observations. Some of these supporting data will be published elsewhere. RESULTS VHF and UHF spectra Here we present EISCAT VHF and UHF radar results for 11 Nov. 1999. Figure 1 shows an overview of the intensity of the VHF echoes seen in the three power profile channels during one hour on 11 Nov. 1999. The bottom panel shows the ion line channel. The gray regions between 100 and 200 km are from the auroral E-region. The black dots at the lower edge of the gray are echoes from enhanced backscatter due to HF pumping. The center panel shows the downshifted plasma line (DPL) at –4.04 MHz and the top panel shows the upshifted plasma line (UPL) at +4.04 MHz. The HF-pump-enhanced signals are stronger and therefore seen more clearly in these channels. Around 18:17 and after 18:36 UT a number of enhanced signals come from F-region heights, 200 to 350 km, during periods when the E-region becomes underdense. After 18:50 UT the E-region echoes appear on both the bottom and topside E-region in all three channels. HF-induced enhancements are seen only during Omode pumping. The intense F-region echoes between 18:19 and 18:24 UT are discussed later. In the UPL channel (top panel) an artefact of the experiment causes the echoes to be repeated about 15 km above the real echoes. When echoes appear from both the bottom and topside E-region this technical fault makes it appear as if there are three scattering regions. Figures 2 and 3 show spectra from two 10-s intervals where HF-enhanced ion and plasma lines are excited in the E-region. In both figures the VHF plasma lines show a strong line offset from the HF frequency towards the radar frequency by the ion-acoustic frequency (about 1.5 kHz), which is not quite resolved. This feature, known as the decay line, is produced by the parametric decay instability (e.g. Hanssen et al., 1992). The UHF plasma line spectra in Figure 2 show both a decay line and a line at the HF pump frequency, which appears to be the purely growing line produced by the modulational instability, with the former being stronger. In the UHF spectra in Figure 3 only the decay line is present. The UHF ion line always shows strongly enhanced ion acoustic shoulders and a zero-frequency component, features produced by the parametric decay and modulational instabilities, respectively (Sprague and Fejer, 1995). The VHF UPL spectra show a first cascade, a feature offset towards the radar frequency (inshifted) from the decay line by twice the ion acoustic frequency or about 3 kHz. There is also a line outshifted from the decay line by about 3 kHz, which may be the image decay line, also known as the antiStokes line (Djuth, 1984; Stubbe et al., 1992; DuBois and Goldman, 1967; Goldman et al., 1995). The height difference of 7 km between UHF and VHF excitation in Figure 3 must be attributed in large measure to a spatially and temporally varying E-region, since the radars are pointing to regions 18 km apart. For example, twenty seconds later the excitation observed by the VHF radar had dropped from about 125 km to approximately 119 km altitude, while the UHF excitation height dropped from 119 km to 115 km. Furthermore, although the height of the VHF return was usually above that of the UHF signal by more than a kilometer, the VHF height was occasionally lower, although by a kilometer or less. Keeping this probably largely natural variability in mind, the height difference may be argued to be consistent with the relative difference in the altitudes where parametric Fig. 1. Overview of the backscatter intensity in the three VHF power profile channels on 11 Nov. 1999. The bottom panel shows the ion line, while the upper panels show the downshifted plasma line (DPL) and upshifted plasma line (UPL), which are offset by – and +4.03 MHz from the radar frequency, respectively. All three channels have a linear ±25-kHz bandwidth. 18:12 18:24 18:36 18:48 19:00 Time [UT] 30