This work reports a holistic experimental investigation of stimulated Brillouin scattering (SBS) features and electron temperature inversion near the third electron gyroharmonic 3fce using the ...European Incoherent Scatter (EISCAT) heating facility. The evolution of SBS features including spectral offset, width, and power varies asymmetrically near 3fce. The asymmetries among SBS, electron temperature, and high frequency‐enhanced ion lines are clearly exhibited for pumping above f0 ≥ 3fce. Electron temperature Te at the resonance regime has been retrieved from the measured SBS spectra based on the wave matching theory. The inversion results by SBS are consistent with measurement by the EISCAT UHF incoherent scatter radar (ISR) at the resonance altitude. The comparison of electron temperature Te and ion temperature Ti between SBS and ISR enlightens great potentials for developing realistic ionospheric diagnostic technique.
Key Points
Clearly, SBS dynamics and spectral features are characterized near third electron gyroharmonics at EISCAT
Qualitative comparison of electron temperature inversion based on SBS agrees with ISR measurement
The asymmetries of SBS and DM are closely correlated with enhanced ion lines and electron temperature near 3fce
We report experimental results regarding the distinctive features and behaviors of high‐frequency (HF)‐induced Langmuir and ion‐acoustic turbulences induced by ordinary (O‐mode) and extraordinary ...(X‐mode) polarized powerful HF radio waves injected into the high‐latitude ionosphere F region toward the magnetic zenith at EISCAT (European Incoherent SCATter). Alternating O‐/X‐mode pumping was produced at high heater frequencies in the range fH = 6–8 MHz during magnetically quiet background geophysical conditions. An effective radiated power of 450–600 MW was utilized. The radical distinction between the temporal evolution of the O‐ and X‐mode Langmuir and ion‐acoustic plasma waves after the HF heater is switched on in conjunction with the development of artificial field‐aligned irregularities (FAIs) was analyzed. It was found that the excitation thresholds of the Langmuir and ion‐acoustic turbulences significantly differed for the O‐ and X‐mode HF pumping. We have revealed that the X‐mode excitation thresholds for HF‐induced plasma and ion line backscatter are 0.47 and 0.61 V/m, respectively. The persistent O‐mode plasma and ion line backscatters that coexisted with FAIs showed excitation thresholds of 0.62 and 0.73 V/m, respectively, which exceeded the thresholds for the X‐mode plasma and ion lines, while their intensity was 2 orders of magnitude less. For the same background conditions, the “classic” resonance parametric decay instability and modulation instability, excited as the momentary response to the O‐mode HF pump wave switching on (the so‐called overshoot), have thresholds of 0.17 and 0.25 V/m, respectively, which are much below the excitation thresholds of persistent O‐mode plasma and ion lines.
Key Points
Relation between the PDI and FAI development induced by X‐mode HF pumping is found
Different features and behaviors of the artificial O‐ and X‐mode turbulences are revealed
Different excitation thresholds of the artificial O‐ and X‐mode turbulences are found
The experimental phenomena involving the changes in electron temperature and electron density as a function of pump frequency during an ionospheric heating campaign at European Incoherent Scatter ...near Tromsø, Norway, are reported. When the pump frequency is slightly above the fifth electron gyrofrequency, the UHF radar observation shows some apparent enhancements over a wide altitude range in radar echo, ion line, and electron density respectively, which are apparently altitude independent and consistent temporally with the upshifting and spread of plasma line around the reflection altitude. However, they do not, in fact, correspond to true increase in electron density. Based on some existing theories, some discussions are presented to try to explain the above enhancements and the upshifting and spread of plasma line. Even so, the mechanism remains to be determined. In addition, the observation also shows some enhancements in electron temperature as a function of pump frequency around the reflection altitude of the pump, which are dependent on the behavior of dispersion of the upper hybrid wave near the fifth electron gyrofrequency.
Key Points
UHF radar observed enhancements in electron density with an altitude extent do not, in fact, correspond to a true increase in electron density
The upshifting and spread of plasma lines above pump frequency shows the correlation in heating cycle with the above enhancements
The enhancement in electron temperature occurring as a function of pump frequency around the reflection altitude of the pump
The ultrahigh‐frequency observation during an ionospheric heating experiment on 11 March 2014 at the European Incoherent Scatter Scientific Association Tromsø site illustrated a remarkable extension ...of observing altitudes of the enhanced plasma line and the ion line, implying that the enhanced ion acoustic wave and Langmuir wave should satisfy the Bragg condition within the extending altitude range. An analysis shows that the dependence of the wave number of the traveling ion acoustic wave on the profiles of enhanced electron temperature and ion mass, as are expected from the dispersion relation of the ion acoustic wave, leads to the extension of observing altitudes of the enhanced ion line. In addition, the altitude extension of the enhanced plasma line is dependent mainly on the profile of the electron density, although it is not independent of the profile of the electron temperature. Considering a small gradient profile of electron density, however, the enhanced electron temperature, as well as the thermal conduction along the magnetic field, may lead to the altitude extension of the enhanced plasma line.
Key Points
Enhanced plasma line and ion lines were observed by UHF ISR within the extending altitude range, where Bragg condition should be satisfied
Within the altitude range, electron temperature and ion mass compensate each other so that ion acoustic wave satisfies Bragg condition
For a small gradient profile of density, temperature and density compensate each other so that Langmuir wave satisfies Bragg condition
A limitation to the use of Global Navigation Satellite System (GNSS) for precise and real‐time services is introduced by irregularities in the ionospheric plasma density. An EISCAT UHF/ESR experiment ...was conducted to characterize the effect of electron density irregularities on temporal fluctuations in TEC along directions transverse to GPS ray paths in the high latitudes ionosphere. Two representative case studies are described: Enhancements in temporal TEC fluctuations originating (a) in the auroral ionosphere following auroral particle precipitation and (b) in the polar ionosphere following the drift of a polar patch as well as particle precipitation. The results indicate that the origin of enhancements in TEC fluctuations is due to the propagation through large‐to‐medium scale irregularities (i.e., ranging from few kilometres in the E region to few tens of kilometres in the F region) and occurring over spatial distances of up to approximately 400km in the E region and up to approximately 800km in the F region with a patchy distribution. Furthermore, the results indicate that enhancements in TEC fluctuations produced by polar plasma patches and particle precipitation occur over similar temporal scales, thus explaining the overall observation of higher phase scintillation indices in the high‐latitude ionosphere. The similarity in the temporal scales over which enhancements in TEC fluctuations occur in the presence of both particle precipitation and plasma patches suggests an intrinsic limitation in the monitoring and tracking of plasma patches through ground GNSS observations.
Key Points
Measurements indicate that the observed auroral and polar irregularities were distributed over large distances
Polar plasma patches and auroral particle precipitation enhance temporal fluctuations in Global Positioning System (GPS) Total Electron Content
Enhanced fluctuations in Total Electron Content occur over similar temporal scales for plasma patches and particle precipitation
We discuss the influence of charged dust on radar observations in the Earth ionosphere. This region in the upper Earth atmosphere can be described as a partially ionized, low‐temperature plasma. ...Plasma parameters vary by orders of magnitude spatially and in time. Dust particles influence the charge balance, in some cases dusty plasma condition is met. The polar mesospheric echoes are an example of dust plasma interactions observed with radar. The mesosphere is a region where atmospheric temperature decreases with altitude and can reach frost point temperature. The formation of the polar mesospheric radar echoes involves neutral atmosphere dynamics, which is latitude dependent and it involves charged dust particles, especially icy dust that forms in the polar summer mesosphere. Charged dust can also influence incoherent scatter that results from electromagnetic waves scattering off electrons, where the electrons are coupled to other charged components. Observers rarely report charged dust signatures in the incoherent scatter spectra; we show that there is a good chance for doing so with improved observations. The incoherent scatter can possibly also be used to estimate the amount of charged dust in the direct vicinity of a meteor, as we show based on the order of magnitude considerations. This prospect of new observational results makes theoretical investigations of radio‐wave scattering in the presence of charged dust with size distributions worthwhile.
Diurnal variation features of wintertime F2 peak electron density (NmF2) representative for solar minimum at both Zhongshan station, Antarctica, and Svalbard station are compared and analyzed. Both ...stations are located around cusp latitude and are almost on the same geomagnetic meridian plane in both hemispheres. For quiet time period, typical NmF2 diurnal variation features at Svalbard station show double peaks with a decrease of NmF2 around magnetic local noon (~UT + 3 h); NmF2 diurnal variation at Zhongshan station shows one major peak around magnetic local noon (~UT + 1.75 h), followed by a sharp decrease of NmF2, and a subpeak around 1500 UT. Simulation results of the high‐latitude ionospheres in both hemispheres agree well with observations at both stations. It is found that the major difference of NmF2 variation between both stations can be explained by the unique location of each station relative to the sunlit demarcation line during the day. For quiet time period, photoionization from lower latitude contributes to the major peak of NmF2 in the diurnal variation at Zhongshan station, while the interaction between horizontal convection and auroral precipitation is the main cause for NmF2 variation at Svalbard station. For active time period, both stations show the increase of NmF2 due to transportation of higher plasma density from lower latitudes on the dayside with the expansion of the polar cap and the additional ionization from soft‐precipitating electrons.
Key Points
Wintertime NmF2 in solar minimum in both hemispheres are compared and analyzed
The NmF2 peak at Zhongshan station is due to the transportation of photoionization
Auroral precipitation is the main cause for the major features of NmF2 at Svalbard station
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