The characteristics of three different modes of ELF and VLF excitation using the high‐power auroral simulation (HIPAS) HF heater array are compared under different ionospheric conditions. For each of these three methods, amplitude modulation (AM), phase demodulation (DM), and the double‐frequency excitation (DF), we observed signals on the order of a few picoteslas at two widely separated receiver sites for both X and O mode heater polarizations. Strong electrojet activity is essential for ELF generation at frequencies close to the Schumann range, whereas at higher frequencies, such as in the VLF range (greater than 1 kHz), signals could be observed under a wider range of ionosopheric conditions. The AM method generally produced the largest signal at both frequency ranges, while the DM mode was approximately half of this signal amplitude. The DF generated signals are comparable in strength with the AM signals at VLF frequencies and are more stable than either method as a function of time. The X mode polarization also produces a stronger signal than the O mode for these three methods of excitation by a factor of two. The polarization of the received signals follow the same variation at VLF frequencies for all three modes, indicating a common height of origin.

New measurements of stimulated electromagnetic emissions (SEE) in low duty cycle heating experiments performed at the HIPAS Observatory are presented. Two distinct types of spectra, a weak diffuse type and a stronger type with deep frequency modulations, were observed. These results have been compared with numerical predictions from a 1D driven and damped Zakharov model and are found to be consistent with Langmuir collapse processes (strong spectra) and Langmuir turbulence of the coexistence type (weak spectra). Through this joint experimental and numerical study, evidence of strong Langmuir turbulence via SEE measurements has been demonstrated. [S0031-9007(97)03881-7] PACS numbers: 52.35.Ra, 52.35.Mw, 94.20. -y Two commonly used diagnostics or measurements in high frequency (hf) ionospheric heating experiments are incoherent scatter radar (ISR) and stimulated electromagnetic emissions (SEE). ISR observations yield wave number resolved measurements of hf excited plasma waves (so called plasma lines) while SEE measurements are inherently wave number integrated. Results from the two diagnostics, then, are complementary to one another and are equally important in the understanding of hf induced turbulence in the ionosphere. Of the two types of measurements, ISR observations have received extensive experimental and theoretical attention. Strong Langmuir turbulence effects, characterized by collapsing cavitons and nucleation, predicted from strong Langmuir turbulence models [1-3] have recently been convincingly demonstrated in ISR experiments and have been shown to dominate the early-time evolution of hf induced turbulence [4-7]. SEE observations, on the other hand, have revealed many interesting spectral features of a different nature [8-15]. These features are typically observed during continuous (CW), high duty cycle, long heater pulse experiments [16]; some of them disappear for heater frequencies near multiples of the local electron gyro frequency f ce while others only exist near these frequencies.For a series of reasons, these features are currently associated with the occurrence of heater-induced small scale magnetic field aligned density striations that develop on a longer Ohmic heating time scale (see [17], and references therein). Similar manifestation of ponderomotive effects as those obtained in ISR measurements have not been reported in SEE observations. In this regard, the two diagnostics give apparently inconsistent results.In the present paper, new SEE measurements are reported, which aim at observing manifestations of ponderomotive or strong Langmuir turbulence, comparing them with theoretical predictions, and investigating to what extent the same interpretations can be supported by both SEE and ISR diagnostics. To this end, the experimen-tal method must be so as to exclude long time thermal effects such as the formation of striations. A series of controlled experiments have been conducted at the HIPAS Observatory near Fairbanks, Alaska [18]. These experiments are characterized by operating the heate...

New results of stimulated electromagnetic emissions (SEE) from the HIPAS Observatory are reported. A novel hf heating sequence was used to first precondition the ionosphere, and SEE was then excited with low-amplitude test pulses. Through this approach, the nonlinear physics of SEE was studied. The correlation between small-scale field-aligned density striations and SEE generation was demonstrated, and SEE was excited at power density of 24 dB less than normally required. The results compare well with theoretical predictions of SEE generation via trapped upper hybrid oscillations decay and cavitation within striations. [S0031-9007(98)06204-8] PACS numbers: 94.20.Tt, 52.35.Mw, 52.35.RaThe ionosphere provides an "outdoor laboratory" [1] ideally suited for the study and excitation of thermal and nonlinear plasma instabilities through high frequency (hf) ionospheric modification experiments. A manifestation of thermal instabilities is the observation of magnetic field-aligned density striations detected via radar scattering [2][3][4] and in situ rocket measurements [5]. A manifestation of nonlinear plasma processes is the observation of stimulated hf radiation from the heated ionosphere, commonly called stimulated electromagnetic emissions (SEE), through frequency analysis of the returned hf signal after its transit through the ionosphere [6][7][8][9][10][11][12][13][14][15][16]. Two of the more robust SEE spectral features observed under high duty cycle hf heating are a broad, diffuse, and down-shifted (with respect to the heating frequency, f hf ) continuum, with frequency extension of tens of kilohertz or more, called the broad continuum; and a discrete, narrow, down-shifted peak, called the downshifted maximum, with an offset frequency near the lower hybrid frequency, f lh [11,12].It has been suggested that these SEE features are associated with the occurrence of heater-induced small-scale (meter-size) striations [17][18][19][20][21] which act as resonators for electrostatic oscillations at the upper hybrid frequency, f uh . In contrast to Langmuir oscillations near the reflection layer where f hf ϳ f p ͑z o ͒ [f p ͑z o ͒ is the plasma frequency at the reflection layer z o ], these oscillations have wave vectors predominantly perpendicular to the ambient magnetic field. Recently, Mjølhus [22,23] proposes a SEE theory including the presence of striations and has calculated the radiation source current spectra. Trapped upper hybrid oscillations or eigenstates, with frequency of f f hf f uh ͑z͒, are first generated via conversion of pump wave off meter-size striations. The parametric decay of these states, with f 0 f hf 2 f dm (f 0 is the decay wave frequency), results in the generation of the down-shifted maximum feature. Optimum growth of this trapped upper hybrid decay instability is predicted to occur when the frequency offset, f dm ͑ f hf 2 f 0 ͒, is near f lh . The resultant radiation source current spectrum contains discrete spectral peaks that can be identified with the down-shifted maximum feature. As the p...

We report the double resonance excitation of two pump waves with frequencies separated from 5 to several hundred Hz. The cyclotron frequencies of NO+ and O2+, abundant at the bottom of the F layer (∼ 150 km), are within this range. The nonlinear mode coupling takes place at ∼ 150 km where the pump frequency matches the plasma frequency. The experiments show that when the beat frequency is close to the NO+ cyclotron frequency at ∼ 30 Hz, the energy transfer from the higher frequency to the lower frequency wave is enhanced through parametric coupling. When the two pump waves have equal power, discrete sidebands are symmetrically distributed around the pump frequencies. However, a burst of lower‐frequency sidebands, lasting for several seconds, frequently breaks the symmetry. We attribute this asymmetry to the cascade parametric process. The asymmetric spectra only occur only during O mode experiments.

[1] High Power Auroral Stimulation (HIPAS) Observatory and High Frequency Active Auroral Research Program (HAARP) are two radiating facilities in the Arctic region separated by a distance of the order of VLF wavelengths. The current-carrying plasma in the E region of the ionosphere above each facility can be modulated to radiate VLF waves via HF heating. Experiments demonstrated that VLF waves can be coherently excited by these two facilities through constructive interference that is sensitive to the phase difference between these two sources.