European Incoherent Scatter Research Articles

On the Production of Ionospheric Irregularities Via Kelvin‐Helmholtz Instability Associated with Cusp Flow Channels

AbstractWe present a multi‐instrument multiscale study of a channel of enhanced, inhomogeneous flow in the cusp ionosphere occurring on November 30, 2014. We provide evidence that strong Global Navigation Satellite System (GNSS) phase scintillations indices (σϕ>0.5 rad) can arise from such events, indicating that they are important in the context of space weather impacts on technology. We compare in detail two‐dimensional maps of ionospheric density, velocity, and temperatures obtained by the European Incoherent Scatter Scientific Association Svalbard Radar with scintillation indices detected from a network of four GNSS receivers around Svalbard and examine the different sources of free energy for irregularity creation. We observe that the strongest phase scintillations occur on the poleward side of the flow channel in a region of sheared plasma motion and structured low‐energy particle precipitation. As inhomogeneous plasma flows are evident in our observations, we perform a quantitative, nonlinear analysis of the Kelvin–Helmholtz instability (KHI) and its impact on phase scintillations using numerical simulations from the first principles‐based Geospace Environment Model of Ion‐Neutral Interactions and Satellite‐beacon Ionospheric‐scintillation Global Model of the upper Atmosphere. Using representative values consistent with the radar data, we show that KHI can efficiently create density structures along with considerable scintillations and is thus likely to contribute significantly under similar conditions, which are frequent in the cusp.

Plasma density gradients at the edge of polar ionospheric holes: the absence of phase scintillation

Abstract. Polar holes were observed in the high-latitude ionosphere during a series of multi-instrument case studies close to the Northern Hemisphere winter solstice in 2014 and 2015. These holes were observed during geomagnetically quiet conditions and under a range of solar activities using the European Incoherent Scatter (EISCAT) Svalbard Radar (ESR) and measurements from Global Navigation Satellite System (GNSS) receivers. Steep electron density gradients have been associated with phase scintillation in previous studies; however, no enhanced scintillation was detected within the electron density gradients at these boundaries. It is suggested that the lack of phase scintillation may be due to low plasma density levels and a lack of intense particle precipitation. It is concluded that both significant electron density gradients and plasma density levels above a certain threshold are required for scintillation to occur.

Multi-frequency SuperDARN radar observations of the modulated ionosphere by high-power radio-waves at EISCAT

This paper presents the first joint observations of multi-frequency SuperDARN (Super Dual Auroral Radar Network) radar of the heated ionosphere by high-power high-frequency (HF) ground-based radio-waves along with the stimulated electromagnetic emissions (SEE) measurements. The unique heating experiment design at EISCAT (The European Incoherent Scatter Scientific Association) including fine frequency stepping through the fourth electron gyro-frequency (4fce) provided the opportunity to directly determine the plasma waves responsible for SuperDARN radar echoes. Past experiments using a unique Kodiak SuperDARN receiver in Alaska with capability of data recording over a large bandwidth of frequencies different from the radar transmission frequency was able to detect some radar echoes due to pump-excited plasma waves. However, a precise characterization of these waves could not be reached in the past. Comparison of the behavior of the SEE data measured on the ground besides the multi-frequency SuperDARN observations above the heated ionosphere at EISCAT has shown a good correlation with the characteristics of upper-hybrid (UH)/electron Bernstein (EB) waves excited through parametric decay instability. The ray tracing model based on the EISCAT dynasonde data of the background ionospheric parameters has been used in order to determine the natural ionospheric effects on the propagation path of 9.9 MHz, 13.2 MHz, and 16.6 MHz signals associated with SuperDARN radar. By providing a more direct connection between SuperDARN echoes and the associated SEE measurements, this new technique potentially provides more quantitative characterization of plasma waves generated during ionospheric heating.

Electron heating by HF pumping of high-latitude ionospheric F-region plasma near magnetic zenith

Abstract. High-frequency electromagnetic pumping of ionospheric F-region plasma at high and mid latitudes gives the strongest plasma response in magnetic zenith, antiparallel to the geomagnetic field in the Northern Hemisphere. This has been observed in optical emissions from the pumped plasma turbulence, electron temperature enhancements, filamentary magnetic field-aligned plasma density irregularities, and in self-focusing of the pump beam in magnetic zenith. We present results of EISCAT (European Incoherent SCATter association) Heating-induced magnetic-zenith effects observed with the EISCAT UHF incoherent scatter radar. With heating transmitting a left-handed circularly polarized pump beam towards magnetic zenith, the UHF radar was scanned in elevation in steps of 1.0 and 1.5∘ around magnetic zenith. The electron energy equation was integrated to model the electron temperature and associated electron heating rate and optimized to fit the plasma parameter values measured with the radar. The experimental and modelling results are consistent with pump wave propagation in the L mode in magnetic zenith, rather than in the O mode.

On the radar frequency dependence of polar mesosphere summer echoes

Polar mesosphere summer echoes (PMSEs) are very strong radar echoes in the polar mesopause in local summer. Here we present the frequency dependence of the volume reflectivity and the effect of energetic particle precipitation on modulated PMSEs by using PMSEs observations carried out by European Incoherent SCATter (EISCAT) heating equipment simultaneously with very high frequency (VHF) radar and ultra high frequency (UHF) radar on 12 July 2007. According to the experimental observations, the PMSEs occurrence rate at VHF was much higher than that at UHF, and the altitude of the PMSEs maximum observed at VHF was higher than that at UHF. Overlapping regions were observed by VHF radar between high energetic particle precipitation and the PMSEs. In addition, high-frequency heating had a very limited impact on PMSEs when the UHF electron density was enhanced because of energetic particle precipitation. In addition, an updated qualitative method was used to study the relationship between volume reflectivity and frequency. The volume reflectivity was found to be inversely proportional to the fourth power of radar frequency. The theoretical and experimental results provide a definitive data foundation for further analysis and investigation of the physical mechanism of PMSEs.

Effects of Solar Activity on Ionospheric Ion Upflow During Geomagnetic Quiet Periods: DMSP Observations

Abstract Based on the Defense Meteorological Satellite Program (DMSP) observations during Solar Cycle 23, this paper examines solar activity dependence of ionospheric bulk ion upflow events (IUEs) in the Southern Hemisphere (SH). Much previous similar work was conducted over the Northern Hemisphere (NH) with measurements from European Incoherent Scatter (EISCAT). To eliminate the influence of geomagnetic disturbance on IUEs, we pick out observations during geomagnetic quiet periods (with Kp ≤ 2+). Results show that, ion upward densities and fluxes are dramatically elevated at times of high solar activity (HSA) but ion upward drifts and occurrences are increased at times of low solar activity (LSA) in the SH, which is consistent with the situation in the NH. The ratios between HSA and LSA for these four parameters (IUEs’ density, flux, upward drift and occurrence) are ~2.71, ~1.98, ~0.76 and ~0.57, respectively. Furthermore, lower flux event takes place frequently at LSA as the background ion density is low but the upward drift is large, while higher flux event happens commonly at times of HSA accompanied by high ion density but low upward velocity. Quantitatively, an increase in unit of solar activity (characterized by P index) causes a 4.2×108 m−3 increase in ion density and a 1.2×1011 m−2·s−1 enhancement in upward flux, together with a 0.6 m·s−1 and 0.02 % decrease in ion upward velocity and uprate, respectively. The acceleration from the ambipolar electric field is thought to be a possible mechanism affecting the dependence of IUEs on solar variations. For HSA, the acceleration from the ambipolar electric field weakens, but a large number of background ions provide abundant seeds for acceleration and upflow, which maintains a high IUE flux. It is inferred that upflow events and upward drifts are inhibited by the enhanced ionospheric background density.

Evidence of X-mode heating suppressing O-mode heating

In this study, we present three experiments carried out at the EISCAT (European Incoherent Scatter Scientific Association) heating facility on October 29 and 30, 2015. The results from the first experiment showed overshoot during the O-mode heating period. The second experiment, which used cold-start X-mode heating, showed the generation of parametric decay instability, whereas overshoot was not observed. The third experiment used power-stepped X-mode heating with noticeable O-mode wave leakage. Parametric decay instability and oscillating two-stream instability were generated at the O-mode reflection height without the overshoot effect, which implies suppression of the thermal parametric instability with X-mode heating. We propose that the electron temperature increased because X-mode heating below the upper hybrid height decreased the growth rate of the thermal parametric instability.

Experimental comparisons between AM and BW modulation heating excitation of ELF/VLF waves at EISCAT

In this paper, the experimental results of the extremely/very low frequency (ELF/VLF) radiation from the ionosphere by amplitude modulation (AM) mode and dual-beam beat-wave (BW) mode modulation heating using the European Incoherent Scatter Scientific Association (EISCAT) heating facility are presented. By comparing the dependence of the ELF/VLF radiation sources formed by AM and BW models on the geomagnetic field disturbance and the differences in different periods, this paper aims to examine some issues such as source regions and mechanisms addressed in the theoretical study of ionospheric high frequency modulated heating. The results show that (1) in the AM mode, the intensity of the ELF/VLF radiation source depends on the intensity of geomagnetic disturbance and thus on the magnitude of natural currents in the ionosphere, while in the BW mode, the intensity of the ELF/VLF radiation source is less dependent on the intensity of geomagnetic disturbance; (2) during the relatively quiet period of geomagnetic disturbance, the intensity of the ELF/VLF radiation source formed by AM mode modulation heating decreases and by BW mode modulation, heating increases when the ionization of the lower ionosphere decreases. The results of our EISCAT modulation experiments presented in the current paper and the earlier one by Yang et al. [“The polarization characteristics of ELF/VLF waves generated via HF heating experiments of the ionosphere by EISCAT,” Phys. Plasmas 25, 092902 (2018)] support that the BW mode heating mechanism mainly results from the ponderomotive nonlinearity, and the modulation region is located in the F layer.

Downshifted peak features of stimulated electromagnetic emissions during a two-pump wave heating experiment

Experimental results of the downshifted peak (DP) in stimulated electromagnetic emissions under two-pump wave ionospheric heating near the third electron gyroharmonic frequency are presented. The European Incoherent Scatter Scientific Association heating antenna array was divided into two parts, one of which worked at constant pump wave frequency f1 and the other part worked at varied pump wave frequency f2 which was not larger than f1. It was found that when the second pump wave was turned on at different frequency with f1, the f1 DP power declined by more than 10 dB with respect to the background noise level, while the downshifted maximum belonging to f1 was further enhanced. The time needed to reach a steady state for DP was shortened from approximately 10 s under cold background conditions belonging to f1, which was nearly consistent with growth time of small-scale artificially field-aligned irregularity (AFAI), to less than 1 s under the preconditioned heating belonging to f2 with pre-existing AFAI. According to the difference in DP temporal evolution under two experimental conditions, it could be deduced that AFAI plays an important role in the DP generation process. Similar to single-pump wave heating, the frequency offset of DP decreases as f2 increases toward the third electron gyroharmonic frequency. These experimental findings provide new insights into the theoretical study of ionospheric plasma nonlinearity.

Single‐Site IPS Power Spectra Analysis for Space Weather Products Using Cross‐Correlation Function Results From EISCAT and MERLIN IPS Data

AbstractInterplanetary scintillation (IPS) manifests itself as a variation in the radio signal received from a distant, compact radio source on the sky. The intensity fluctuations of the radio waves are caused by density inhomogeneities in the outflowing solar plasma across the heliosphere. IPS allows us to infer solar wind speed and density variations along the line of sight. There are two types of techniques in the literature to infer solar wind speed using IPS data sets: single‐site analysis (SSA), where the power spectra from single time series are analyzed to obtain solar wind parameters, and multisite analysis, where the cross‐correlation function of data from two or more widely separated sites is used. The selection of the analysis technique depends on the number of sites available to each IPS system. In order to combine and complement solar wind speed determinations from different instruments, it is important to validate results and methodologies of the two techniques. In this paper, we analyzed previously well‐studied European Incoherent SCATter (EISCAT) and Multi‐Element Radio‐Linked Interferometer Network (MERLIN) observations of IPS with well‐known results from the cross‐correlation function methodology. We applied the SSA technique to each of the individual EISCAT and MERLIN IPS power spectra. This work shows the capabilities of the SSA to describe complex events and seeks to obtain improved parameter fits using the SSA methodology.

What Do the New 2018 HIWIND Thermospheric Wind Observations Tell Us About High‐Latitude Ion‐Neutral Coupling During Daytime?

AbstractDaytime thermospheric winds observed by the balloon‐borne instrument HIWIND (High‐altitude Interferometer WIND experiment) during two flights in June 2011 and 2018 from Kiruna (68°N, 20°E), along with simultaneous European Incoherent SCATter radar ion drift data, are analyzed. National Center for Atmospheric Research TIEGCM (Thermosphere Ionosphere Electrodynamics General Circulation Model) simulations for both flights are compared with observations. The observed thermospheric winds from the two flights have many similarities. HIWIND‐observed thermospheric winds tend to be equatorward during the morning hours before noon and close to zero in the afternoon. In contrast, TIEGCM predicts poleward winds before noon and near zero in the afternoon. Southward interplanetary magnetic field Bz, occurring as the balloon passed through morning, was associated with greater equatorward meridional winds. The TIEGCM‐simulated zonal winds have large differences with observations under more active conditions. The second HIWIND flight confirms some important results from the first flight and further shows thermospheric wind variations under different interplanetary magnetic field conditions. HIWIND observations in general provide invaluable data for model validation and highlight deficiencies in current high‐latitude simulations.

Characteristics of the layered polar mesosphere summer echoes occurrence ratio observed by EISCAT VHF 224 MHz radar

Abstract. Polar mesosphere summer echoes (PMSEs) are strong radar echoes observed in the polar mesopause during the local summer. Observations of layered PMSEs carried out by the European Incoherent Scatter Scientific Association very-high-frequency (EISCAT VHF) radar during 2004–2015 in the latest solar cycle are used to study the variations of the PMSE occurrence ratio (OR). Different seasonal behavior of PMSEs is found by analyzing the seasonal variation of PMSE mono-, double-, and tri-layer OR. A method was used to calculate the PMSE mono, double-, and tri-layer OR under a different electron density threshold. In addition, a method to analyze the correlation of the layered PMSE OR with the solar 10.7 cm flux index (F10.7) and geomagnetic K index is proposed. Based on it, the correlation of the layered PMSE OR with solar and geomagnetic activities is not expected to be affected by discontinuous PMSEs. It is found that PMSE mono-, double-, and tri-layer ORs are positively correlated with the K index. The correlation of the PMSE mono- and double-layer OR with F10.7 is weak, whereas the PMSE tri-layer OR shows a negative correlation with F10.7.

Polarization Electric Field Inside Auroral Patches: Simultaneous Experiment of EISCAT Radars and KAIRA

AbstractThe primary focus of this study was the motion of auroral patches and the polarization electric field generated therein observed on 9 November 2015 in an experiment using the European incoherent scatter (EISCAT) radars, Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA), and an all‐sky imager simultaneously. Based on the all‐sky imager data, the drift speed of the auroral patches corresponded to a southward electric field of 14.1( ± 3.7)–17.2( ± 4.5) mV/m. The convective electric field derived from the EISCAT radars and KAIRA observation was approximately 14.6 mV/m in the southward direction. This suggest that the spatial distribution of the auroral patches reflects the distribution of the cold plasma in the magnetosphere. The electron density and the height‐integrated Hall conductance between 80 and 120 km were enhanced by a factor of 2–4 inside the auroral patches. In this situation, a polarization electric field was generated therein. Enhanced ion velocities due to the polarization electric field was observed at up to 200‐km altitude; however, the absolute values of the ion velocities were approximately 40% of what was expected from the polarization electric field. A field‐aligned current (FAC) from 5 to 10 μA/m−2 in the edges of the auroral patches could explain the weakening of the polarization electric field. Since a FAC of that order of magnitude corresponded with that observed by the Swarm satellite, it was suggested that the polarization electric field was weakened by the FAC. Furthermore, the polarization electric field propagated upward from the dynamo region to at least 200 km.

Characteristics of CME‐ and CIR‐Driven Ion Upflows in the Polar Ionosphere

AbstractWe investigated how velocity and flux of ionospheric ion upflows vary during magnetic storms driven by corotating interaction regions (CIRs) and coronal mass ejections (CMEs), using data from the European Incoherent Scatter (EISCAT) Tromsø UHF and Svalbard radars between 1996 and 2015. The characteristics of ion upflows were compared with ion and electron temperature variations measured with the EISCAT radars and also joule heating rate, electric field, and field‐aligned current distribution derived from the Weimer model. Upward ion velocity increases in the nighttime at Tromsø (66.2 °N geomagnetic latitude) just after the CIR‐ and CME‐driven storms, corresponding to electron temperature enhancements due to soft particle precipitation and also ion temperature enhancements in the strong westward electric field region. The CME‐driven storms have larger upward ion flux (~1.7 × 1013 m−2s−1) than those under the CIR‐driven storms (~0.3 × 1013 m−2s−1). In the daytime, ion upflows are seen at Longyearbyen, Svalbard (75.2 °N geomagnetic latitude), with an upward flux of typically 1013 m−2s−1 for small CIR and CME storm cases. Substantial ion upflows last for a few days after the storm onsets under small CIR storms, whereas they last for only a day under small CME storms. Under both the cases, the substantial ion upflows are associated with an enhancement of the Region 1 field‐aligned current, eastward electric field and Joule heating rate. For large CME storms, substantial ion upflows are absent in the daytime probably due to equatorward expansion of the auroral oval.

NSEE Yielding Electron Temperature Measurements at the Arecibo Observatory

AbstractNarrowband 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.

The 3‐D Distribution of Artificial Aurora Induced by HF Radio Waves in the Ionosphere

AbstractWe present 3‐D excitation rate estimates of artificial aurora in the ionosphericFlayer, induced by high‐frequency radio waves from the European Incoherent Scatter heating facility. Simultaneous imaging of the artificial aurora was done with four separate Auroral Large Imaging System stations, permitting tomography‐like 3‐D auroral reconstruction of the enhanced atomic oxygen emissions at 6,300, 5,577, and 8,446 Å. Inspection of the 3‐D reconstructions suggests that the distribution of energized electrons is less extended in altitude than predicted by transport calculations of electrons accelerated to 2–100 eV. A possible reason for this discrepancy is that high‐frequency pumping might induce an anisotropic distribution of energized electrons.

Statistical Modeling of the Coupled F‐Region Ionosphere‐Thermosphere at High Latitude During Polar Darkness

AbstractStatistical models have been developed for predicting the behavior of the coupled high‐latitude ionosphere‐thermosphere system. The modeled parameters were the F‐layer peak electron density, plasma structuring, ion temperature, neutral temperature, and the difference between these temperatures, which is a key term in the Joule heating equation. Ionospheric measurements from the European Incoherent Scatter Svalbard Radar and neutral atmosphere measurements from the colocated University College London Fabry‐Perot Interferometers have been made across a solar cycle. These data were all acquired during nighttime conditions as the observations with the Fabry‐Perot Interferometers are restricted to such times. Various geophysical proxies were tested to represent the processes that influence the modeled parameters. The dominant geophysical proxy for each modeled parameter was then determined. Multivariate models were also developed showing the combinations of parameters that best explained the observed variability. A comparison with climatology showed that the models give an improvement in every case with skill scores based on the mean square error of up to 0.88.

Effects of variations of geomagnetic field on VLF waves induced by beating of two HF waves

Beat wave (BW) high frequency (HF) ionospheric heating experiments were conducted to generate very low frequency (VLF) waves. The VLF waves were registered with a VLF receiver located ∼15 km east of the European Incoherent Scatter (EISCAT) heating facility in Tromsø, Norway. A fluxgate magnetometer was used to monitor auroral electrojet current, and ionospheric conditions were measured using a Dynasonde. Correlation coefficients between VLF amplitudes and the deviation of geomagnetic north–south components were calculated. Experimental results show that strong and positive correlation exists the majority of the time, but sometimes no correlation or even a negative correlation occurred. This is consistent with similar past experiments that took place with exclusively AM generation. These results therefore support the conclusion that BW generation of VLF waves is no different than with AM, likely occurring in the D or lower E ionospheric region.

Stimulated electromagnetic emissions spectrum observed during an X-mode heating experiment at the European Incoherent Scatter Scientific Association

An extraordinary (X-mode) electromagnetic wave, injected into the ionosphere by the ground-based heating facility at Tromso, Norway, was utilized to modify the ionosphere on November 6, 2017. The high-power high-frequency transmitter facility located at Tromso belongs to the European Incoherent Scatter Scientific Association. In the experiment, stimulated electromagnetic emission (SEE) spectra were observed. A narrow continuum occurred under cold-start conditions and showed an overshoot effect lasting several seconds. Cascading peaks occurred on both sides of the heating frequency only in the preconditioned ionosphere and also showed an overshoot effect. These SEE features are probably related to the ponderomotive process in the X-mode heating experiment and are helpful for understanding the physical mechanism that generated them during the X-mode heating experiment. The features observed in the X-mode heating experiments are novel and require further investigation.

The Intensities of High Frequency‐Enhanced Plasma and Ion Lines During Ionospheric Heating

AbstractAn ultrahigh frequency (UHF) observation during an ionospheric heating campaign at European Incoherent SCATter scientific association (EISCAT) demonstrates that the electron temperature and the intensities of the enhanced plasma and ion lines systematically vary as a function of the pump frequency near the fifth electron gyroharmonic. Specifically, with the change in the pump frequency, the weaker the enhancement in the electron temperature is, the more intense the intensities of the enhanced ion line is. The analysis shows that the enhanced electron temperature plays a significant role in determining the intensities of the enhanced Langmuir and ion acoustic waves by the parametric decay instability and the oscillation two‐stream instability, and the intensities of the enhanced plasma and ion lines depend on whether the enhanced Langmuir and ion acoustic waves satisfy the Bragg condition exactly or approximately. Moreover, an alternative explanation for the overshoots in the enhanced plasma and ion lines is presented, namely, the overshoots may be attributed not only to the anomalous absorption of the pump but also to the modifications of the plasma on the traveling path of the enhanced Langmuir and ion acoustic waves.