Narrowband stimulated electromagnetic emissions (NSEE), a component of radio emissions created during high‐frequency (HF) radiowave ionospheric modification experiments, occur within 1 kHz of the HF ...pump frequency. NSEE was observed and studied for the first time at the HAARP (High Frequency Active Auroral Research Program) facility in Alaska (Norin et al., 2009, https://doi.org/10.1103/PhysRevLett.102.065003; Bernhardt et al., 2010, https://doi.org/10.1103/PhysRevLett.104.165004). Magnetized stimulated Brillouin scatter (MSBS) is a component of NSEE, which was also first observed at HAARP (Norin et al., 2009, https://doi.org/10.1103/PhysRevLett.102.065003; Bernhardt et al., 2010, https://doi.org/10.1103/PhysRevLett.104.165004) and later at European Incoherent Scatter Scientific Association (Fu et al., 2015, https://doi.org/10.5194/angeo-33-983-2015). Ion‐acoustic and electrostatic ion cyclotron modes are the daughter products of MSBS and can be used for the determination of the electron temperature and of the presence of minor ion species, respectively, in the HF‐modified ionosphere. Here we present the first observations of the MSBS process at magnetic midlatitudes, excited during radio wave ionospheric modification experiments at the Arecibo Observatory. The NSEE observations, in combination with a theoretical model and the wave matching conditions, are used to estimate background ionospheric parameters. A qualitative comparison of the MSBS component of the NSEE spectrum with the thermal ion line measured by incoherent scatter radar is presented.
Key Points
First NSEE observations at the Arecibo Observatory is reported
Electron temperature estimation using NSEE spectral components is presented
Reproduction of ISR observations using MSBS IA spectral line is discussed
On 12 November 2001, the European Incoherent Scatter (EISCAT) high‐frequency (HF) radio wave transmitter facility, operating in O‐mode at 5.423 MHz with 550 MW effective radiated power, produced ...artificial optical rings which appeared immediately at transmitter turn‐on and collapsed into blobs after ∼60 s while descending in altitude. A similar descent in altitude was observed in the EISCAT ultra high frequency (UHF) ion line enhancements. Likewise, the stimulated electromagnetic emission (SEE) spectra changed as the pump frequency approached the fourth electron gyroharmonic due to pump‐induced variations in electron concentration. Optical recordings were made from Skibotn at 630.0 and 557.7 nm and from Ramfjord in white light. The altitude of the initial optical ring and steady state blob has been estimated by triangulation. The evolution in altitude of the optical emissions, ion line enhancements, and SEE spectra all show a similar morphology but are generally not at exactly the same height. Typically, the optical height is close to and a few kilometers below that of the radar backscatter but sometimes above it, both of which are above the SEE generation altitude. There is evidence that upper hybrid (UH) waves, which propagate perpendicular to the magnetic field line, and Langmuir (L) waves, which propagate parallel to the magnetic field line, act simultaneously to accelerate electrons even in the steady state.
Stimulated Brillouin scattering (SBS) and resonant phenomena are well known in the context of laser fusion, fiber optics, and piezoelectric semiconductor plasmas, as well as in various biological ...applications. Due to recent advances, active space experiments using high‐power high‐frequency (HF) radio waves may now produce stimulated Brillouin scattering (SBS) in the ionospheric plasma. The sensitivity of the narrowband SBS emission lines to pump frequency stepping across electron gyroharmonics is reported here for the first time. Experimental observations show that SBS emission sidebands are suppressed as the HF pump frequency is stepped across the second and third electron gyroharmonics. A correlation of artificially enhanced airglow and SBS emission lines excited at the upper hybrid altitude is observed and studied for second gyroharmonic heating. The SBS behavior near electron gyroharmonics is shown to have important diagnostic applications for multilayered, multi‐ion component plasmas such as the ionosphere.
Key PointsSensitivity of SBS emission lines to pump frequency stepping across nfceThe first observation of SBS EIC line produced by a minor ion speciesCorrelation of artificially enhanced airglow and SBS line excited at UH altitude
Stimulated electromagnetic emissions (SEEs) are secondary radiation produced during active space experiments in which the ionosphere is actively heated with high power high frequency (HF) ...ground‐based radio transmitters. Recently, there has been significant interest in ion gyro‐harmonic structuring the SEE spectrum due to the potential for new diagnostic information available such as electron acceleration and creation of artificial ionization layers. These relatively recently discovered gyro‐harmonic spectral features have almost exclusively been studied when the transmitting frequency is near the second electron gyro‐harmonic frequency. The first extensive systematic experimental investigations of the possibility of these spectral features for third electron gyro‐harmonic heating are provided here. Discrete spectral features shifted from the transmit frequency ordered by harmonics of the ion gyro‐frequency were observed for third electron gyro‐harmonic heating for the first time at a recent campaign at the High Frequency Active Auroral Research Program (HAARP) facility. These features were also closely correlated with a broader band feature at a larger frequency shift from the transmit frequency known as the downshifted peak (DP). The power threshold of these spectral features was measured, as well as their behavior with heater beam angle, and proximity of the transmit frequency to the third electron gyro‐harmonic frequency. Comparisons were also made with similar spectral features observed during second electron gyro‐harmonic heating during the same campaign. A theoretical model is provided that interprets these spectral features as resulting from parametric decay instabilities in which the pump field ultimately decays into high frequency upper hybrid/electron Bernstein and low frequency neutralized ion Bernstein IB and/or obliquely propagating ion acoustic waves at the upper hybrid interaction altitude. Coordinated optical and SEE observations were carried out in order to provide a better understanding of electron acceleration and precipitation processes. Optical emissions were observed associated with SEE gyro‐harmonic features for pump heating near the second electron gyro‐harmonic during the campaign. The observations affirm strong correlation between the gyro‐structures and the pump‐induced optical emissions.
Key PointsFirst observations of ion gyro‐structuring in the SEE spectrum for fp ≈ 3fceES waves & electron heating signatures from SuperDARN & optical measurementsCorrelation with other SEE features indicate extended altitude source region
Polar mesosphere winter echoes (PMWE), observed with the EISCAT VHF radar located near Tromsø, Norway on October 24, 2006, were modulated using the EISCAT Heating facility. The heater was turned on ...for 20 s and then off for 160 s. The experiment confirmed previous results that when the heater was switched on, the PMWE power dropped significantly compared to the undisturbed level measured before heating. The experiment showed for the first time that, after the heater was switched off, the PMWE strength overshot the value that it had before heating. By using data from 6 heating cycles we find that the overshoot effect is about 50% of the pre‐heating value, which is rather weak compared to 3–5 times for PMSE. No, or little recovery of PMWE strength during heating periods was observed. This may indicate the presence of very small dust particles, such as meteoric smoke particles.
First 100 ms of HF modification at Tromsø, Norway Djuth, F. T.; Isham, B.; Rietveld, M. T. ...
Journal of Geophysical Research - Space Physics,
November 2004, Volume:
109, Issue:
A11
Journal Article
Peer reviewed
Open access
Experiments were performed with the high‐power, high‐frequency (HF) facility at Tromsø, Norway to test theoretical predictions for the excitation of ion and Langmuir oscillations in the ionosphere. ...The principal diagnostic of wave‐plasma interactions was the VHF radar at the European Incoherent Scatter (EISCAT) facility. This radar is collocated with the HF facility. High‐resolution radar techniques were used to monitor the temporal development of the ion and Langmuir oscillations. HF pulses 100 ms in duration were periodically transmitted into a smooth background F region plasma. Measurements of the radar backscatter spectra show that all key spectral features predicted by strong Langmuir turbulence theory are simultaneously present in the plasma and that their evolution is in agreement with theoretical expectations. However, several new features have been observed that are not anticipated by current theory. The experimental results reinforce the notion that new theoretical developments are needed to accommodate the large HF electric fields produced at Tromsø and treat the electron acceleration process in a self‐consistent fashion.
Orbital Angular Momentum in Radio-A System Study Mohammadi, S.M.; Daldorff, L.K.S.; Bergman, J.E.S. ...
IEEE transactions on antennas and propagation,
02/2010, Volume:
58, Issue:
2
Journal Article
Peer reviewed
Recent discoveries concerning rotating (helical) phase fronts and orbital angular momentum (OAM) of laser beams are applied to radio frequencies and comprehensive simulations of a radio OAM system ...are performed. We find that with the use of vector field-sensing electric and magnetic triaxial antennas, it is possible to unambiguously estimate the OAM in radio beams by local measurements at a single point, assuming ideal (noiseless) conditions and that the beam axis is known. Furthermore, we show that conventional antenna pattern optimization methods can be applied to OAM-generating circular arrays to enhance their directivity.
In this paper we offer the first study of seldom‐recorded naturally occurring wave‐wave interactions in the polar ionosphere, using the first such data set acquired with the EISCAT Svalbard Radar ...(ESR). The simultaneous and co‐located observations of enhancements in the ion and plasma lines can only be explained in terms of wave‐wave interactions taking place within the scattering volume. These measurements strengthen previous indications that Langmuir turbulence triggered by a low energy electron beam associated with the aurora is probably the source of many of the Naturally Enhanced Ion Acoustic Lines (NEIAL) observed with incoherent scatter radars over the last decade.
The shape of the electron energy distribution has long been a central question in the field of high-frequency radio-induced optical emission experiments. This report presents estimates of the ...electron energy distribution function, fe(E), from 0 to 60 eV, based on optical multi-wavelength (6300, 5577, 8446, 4278Å) data and 930-MHz incoherent scatter radar measurements of ion temperature, electron temperature and electron concentration. According to our estimate, the electron energy distribution has a depression at around 2 eV, probably caused by electron excitation of vibrational states in N2, and a high energy tail that is clearly supra-thermal. The temporal evolution of the emissions indicates that the electron temperature still plays an important role in providing electrons with energies close to 2 eV. At the higher energies the electron energy distribution has a non-thermal tail. Keywords. Active experiments; Ionosphere atmosphere interaction; Ionospheric physics
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