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Meteoric ablation in the Earth's atmosphere produces particles of nanometer size and larger. These particles can become charged and influence the charge balance in the D region (60–90 km) and the incoherent scatter observed with radar from there. Radar studies have shown that, if enough dust particles are charged, they can influence the received radar spectrum below 100 km, provided the electron density is sufficiently high (>109 m 3). Here, we study an observation made with the EISCAT VHF radar on 9 January 2014 during strong particle precipitation so that incoherent scatter was observed down to almost 60 km altitude. We found that the measured spectra were too narrow in comparison to the calculated spectra. Adjusting the collision frequency provided a better fit in the frequency range of ± 10–30 Hz. However, this did not lead to the best fit in all cases, especially not for the central part of the spectra in the narrow frequency range of ±10 Hz. By including a negatively charged dust component, we obtained a better fit for spectra observed at altitudes of 75–85 km, indicating that dust influences the incoherent-scatter spectrum at D-region altitudes. The observations at lower altitudes were limited by the small number of free electrons, and observations at higher altitudes were limited by the height resolution of the observations. Inferred dust number densities range from a few particles up to 104 cm -3 , and average sizes range from approximately 0.6 to 1 nm. We find an acceptable agreement with the dust profiles calculated with the WACCM-CARMA (Whole Atmosphere Community Climate Model-Community Aerosol Radiation Model for Atmospheres) model. However, these do not include charging, which is also based on models. [ABSTRACT FROM AUTHOR]

EISCAT_3D is a project to build a multi-site phased-array incoherent scatter radar system in northern Fenno-Scandinavia. We demonstrate via numerical simulation how useful monochromatic images taken by a multi-point imager network are for auroral research in the EISCAT_3D project. We apply the generalized aurora computed tomography (G-ACT) method to modelled observational data from real instruments, such as the Auroral Large Imaging System (ALIS) and the EISCAT_3D radar. G-ACT is a method for reconstructing the three-dimensional (3D) distribution of auroral emissions and ionospheric electron density (corresponding to the horizontal two-dimensional (2D) distribution of energy spectra of precipitating electrons) from multi-instrument data. It is assumed that the EISCAT_3D radar scans an area of 0.8° in geographic latitude and 3° in longitude at an altitude of 130 km with 10 × 10 beams from the radar core site at Skibotn (69.35° N, 20.37° E). Two neighboring discrete arcs are assumed to appear in the observation region of the EISCAT_3D radar. The reconstruction results from G-ACT are compared with those from the normal ACT as well as the ionospheric electron density from the radar. It is found that G-ACT can interpolate the ionospheric electron density at a much higher spatial resolution than that observed by the EISCAT_3D radar. Furthermore, the multiple arcs reconstructed by G-ACT are more precise than those by ACT. In particular, underestimation of the ionospheric electron density and precipitating electrons' energy fluxes inside the arcs is significantly improved by G-ACT including the EISCAT_3D data. Even when the ACT reconstruction is difficult due to the unsuitable locations of the imager sites relative to the discrete arcs and/or a small number of available images, G-ACT allows us to obtain better reconstruction results. [ABSTRACT FROM AUTHOR]

The EISCAT (European Incoherent Scatter Scientific Association) Heating facility was used to transmit powerful high-frequency (HF) electromagnetic waves into the F-region ionosphere to enhance optical emissions at 557.7 and 630.0 nm from atomic oxygen. The emissions were imaged by several stations of ALIS (Auroral Large Imaging System) in northern Sweden, and the EISCAT UHF incoherent scatter radar was used to obtain plasma parameter values. The ratio of the 557.7 to 630.0 nm column emission rates changed from I5577/I6300≈0.2 for the HF pump frequency f0=6.200 MHz ≈4.6fe to I5577/I6300≈0.5 when f0=5.423 MHz ≲4fe , where fe is the ionospheric electron gyro frequency. The observations are interpreted in terms of decreased electron heating efficiency and thereby weaker enhancement at 630.0 nm for f0=5.423 MHz ≲ 4 fe. The emissions at 557.7 nm are attributed to electron acceleration by upper hybrid waves of metre-scale wavelengths that can be excited with f0=5.423 MHz ≲ 4 fe. [ABSTRACT FROM AUTHOR]

Simultaneous knowledge of the temperatures of electrons, ions and neutrals is key to the understanding and quantification of energy transfer processes in planetary atmospheres. However, whereas electron and ion temperature measurements are routinely obtained from ground-based incoherent scatter radars, simultaneous measurements of electron, ion and neutral temperature measurements can only be made in situ. For the Earth's lower thermosphere–ionosphere, the only available comprehensive in situ dataset of electron, ion and neutral temperatures to date is that of the Atmosphere Explorers C, D and E and the Dynamics Explorer 2 missions. In this study we first perform a cross-comparison of all co-temporal and co-spatial measurements between in situ electron and ion temperature measurements from the above in situ spacecraft missions with corresponding measurements from the Arecibo, Millstone Hill and Saint-Santin incoherent scatter radars, during times of overflights of these spacecraft over the radar fields of view. This expands upon a previous study that only considered data from the Atmosphere Explorer C. The results indicate good agreement between satellite and ground-based radar measurements. Subsequently, out of the above datasets, all instances where ion temperatures appear to be lower than neutral temperatures are identified and are studied statistically. Whereas current understanding indicates that ion temperatures are generally expected to be higher than neutral temperatures in the lower thermosphere–ionosphere, a non-negligible number of events is found where this does not hold true. The distribution of all such cases in altitude, latitude and longitude is presented and discussed. Potential causes leading to neutral temperatures being higher than ion temperatures are outlined, including both instrumental effects or measurement errors and physical causes. Whereas a conclusive case cannot be made based on the present analysis, it is speculated from the results presented herein that not all cases can be attributed to instrument effects or measurement errors. This can have significant implications for the current understanding that the energy of the ions is expected to be higher than that of the neutrals and points to the need for additional simultaneous in situ measurements in the lower thermosphere–ionosphere (LTI). [ABSTRACT FROM AUTHOR]

Volumetric measurements of the ionosphere are important for investigating spatial variations of ionospheric features, like auroral arcs and energy deposition in the ionosphere. In addition, such measurements make it possible to distinguish between variations in space and time. While spatial variations in scalar quantities such as electron density or temperature have been investigated with incoherent scatter radar (ISR) before, spatial variation in the ion velocity, which is a vector quantity, has been hard to measure. The upcoming EISCAT3D radar will be able to do volumetric measurements of ion velocity regularly for the first time. In this paper, we present a technique for relating volumetric measurements of ion velocity to neutral wind and electric field. To regularize the estimates, we use Maxwell's equations and fluid-dynamic constraints. The study shows that accurate volumetric estimates of electric field can be achieved. Electric fields can be resolved at altitudes above 120 km, which is the altitude range where auroral current closure occurs. Neutral wind can be resolved at altitudes below 120 km. [ABSTRACT FROM AUTHOR]

Energetic particle precipitation associated with pulsating aurora (PsA) can reach down to lower mesospheric altitudes and deplete ozone. It is well documented that pulsating aurora is a common phenomenon during substorm recovery phases. This indicates that using magnetic indices to model the chemistry induced by PsA electrons could underestimate the energy deposition in the atmosphere. Integrating satellite measurements of precipitating electrons in models is considered to be an alternative way to account for such an underestimation. One way to do this is to test and validate the existing ion chemistry models using integrated measurements from satellite and ground-based observations. By using satellite measurements, an average or typical spectrum of PsA electrons can be constructed and used as an input in models to study the effects of the energetic electrons in the atmosphere. In this study, we compare electron densities from the EISCAT (European Incoherent Scatter scientific radar system) radars with auroral ion chemistry and the energetics model by using pulsating aurora spectra derived from the Polar Operational Environmental Satellite (POES) as an energy input for the model. We found a good agreement between the model and EISCAT electron densities in the region dominated by patchy pulsating aurora. However, the magnitude of the observed electron densities suggests a significant difference in the flux of precipitating electrons for different pulsating aurora types (structures) observed. [ABSTRACT FROM AUTHOR]

Fragmented aurora-like emissions (FAEs) are small (few kilometres) optical structures which have been observed close to the poleward boundary of the aurora from the high-latitude location of Svalbard (magnetic latitude 75.3 ∘ N). The FAEs are only visible in certain emissions, and their shape has no magnetic-field-aligned component, suggesting that they are not caused by energetic particle precipitation and are, therefore, not aurora in the normal sense of the word. The FAEs sometimes form wave-like structures parallel to an auroral arc, with regular spacing between each FAE. They drift at a constant speed and exhibit internal dynamics moving at a faster speed than the envelope structure. The formation mechanism of FAEs is currently unknown. We present an analysis of high-resolution optical observations of FAEs made during two separate events. Based on their appearance and dynamics, we make the assumption that the FAEs are a signature of a dispersive wave in the lower E-region ionosphere, co-located with enhanced electron and ion temperatures detected by incoherent scatter radar. Their drift speed (group speed) is found to be 580–700 m s -1 , and the speed of their internal dynamics (phase speed) is found to be 2200–2500 m s -1 , both for an assumed altitude of 100 km. The speeds are similar for both events which are observed during different auroral conditions. We consider two possible waves which could produce the FAEs, i.e. electrostatic ion cyclotron waves (EIC) and Farley–Buneman waves, and find that the observations could be consistent with either wave under certain assumptions. In the case of EIC waves, the FAEs must be located at an altitude above about 140 km, and our measured speeds scaled accordingly. In the case of Farley–Buneman waves a very strong electric field of about 365 mV m -1 is required to produce the observed speeds of the FAEs; such a strong electric field may be a requirement for FAEs to occur. [ABSTRACT FROM AUTHOR]

Measurements of height-dependent electric field (E) and neutral wind (u) are important governing parameters of the Earth's upper atmosphere, which can be used to study, for example, how auroral currents close or how energy flows between the ionized and neutral constituents. The new EISCAT 3D (E3D) incoherent scatter radar will be able to measure a three-dimensional ion velocity vector (v) at each measurement point, which will allow less stringent prior assumptions about E and u to be made when estimating them from radar measurements. This study investigates the feasibility of estimating the three-dimensional electric field and neutral wind vectors along a magnetic field-aligned profile from E3D measurements, using the ion momentum equation and Maxwell's equations. The uncertainty of ion drift measurements is estimated for a time and height resolution of 5 s and 2 km. With the most favourable ionospheric conditions, the ion wind at E region peak can be measured with an accuracy of less than 1 m/s. In the worst case, during a geomagnetically quiet night, the uncertainty increases by a factor of around 10. The uncertainty of neutral wind and electric field estimates is found to be strongly dependent on the prior constraints imposed on them. In the lower E region, neutral wind estimates have a lower standard deviation than 10 m/s in the most favourable conditions. In such conditions, also the F region electric field can be estimated with uncertainty of about 1 mV/m. Simulated measurements of v are used to demonstrate the ability to resolve the field-aligned profile of E and u. However, they can only be determined well at the heights where they dominate the ion drift, that is above 125 km for E and below 115 km for u. At the other heights, the results are strongly dependent on the prior assumptions of smoothness. [ABSTRACT FROM AUTHOR]

Energetic particle precipitation (EPP) has the potential to change the neutral atmospheric temperature in the mesopause region. However, recent results are inconsistent, leaving the mechanism and the actual effect still unresolved. In this study we have searched for electron precipitation events and investigated a possible correlation between D-region electron density enhancements and simultaneous neutral temperature changes. The rotational temperature of the excited hydroxyl (OH) molecules is retrieved from the infrared spectrum of the OH airglow. The electron density is monitored by the European Incoherent Scatter Scientific Association (EISCAT) Svalbard Radar. We use all available experiments from the International Polar Year (IPY) in 2007–2008 until February 2019. Particle precipitation events are characterized by rapid increases in electron density by a factor of 4 at an altitude range of 80–95 km, which overlaps with the nominal altitude of the infrared OH airglow layer. The OH airglow measurements and the electron density measurements are co-located. Six of the 10 analysed electron precipitation events are associated with a temperature decrease of 10–20 K. Four events were related to a temperature change of less than 10 K. We interpret the results in terms of the change in the chemical composition in the mesosphere. Due to EPP ionization the population of excited OH at the top of the airglow layer may decrease. As a consequence, the airglow peak height changes and the temperatures are probed at lower altitudes. The observed change in temperature thus depends on the behaviour of the vertical temperature profile within the airglow layer. This is in agreement with conclusions of earlier studies but is, for the first time, constructed from electron precipitation measurements as opposed to proxies. The EPP-related temperature change recovers very fast, typically within less than 60 min. We therefore further conclude that this type of EPP event reaching the mesopause region would only have a significant impact on the longer-term heat balance in the mesosphere if the lifetime of the precipitation was much longer than that of an EPP event (30–60 min) found in this study. [ABSTRACT FROM AUTHOR]

Pulsating aurora (PsA) is a diffuse type of aurora with different structures switching on and off with a period of a few seconds. It is often associated with energetic electron precipitation (>10 keV) resulting in the interaction between magnetospheric electrons and electromagnetic waves in the magnetosphere. Recent studies categorize pulsating aurora into three different types – amorphous pulsating aurora (APA), patchy pulsating aurora (PPA), and patchy aurora (PA) – based on the spatial extent of pulsations and structural stability. Differences in precipitation energies of electrons associated with these types of pulsating aurora have been suggested. In this study, we further examine these three types of pulsating aurora using electron density measurements from the European Incoherent Scatter (EISCAT) VHF/UHF radar experiments and Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) cosmic noise absorption (CNA) measurements. Based on ground-based all-sky camera images over the Fennoscandian region, we identified a total of 92 PsA events in the years between 2010 and 2020 with simultaneous EISCAT experiments. Among these events, 39, 35, and 18 were APA, PPA, and PA types with a collective duration of 58, 43, and 21 h, respectively. We found that, below 100 km, electron density enhancements during PPAs and PAs are significantly higher than during APA. However, there are no appreciable electron density differences between PPA and APA above 100 km, while PA showed weaker ionization. The altitude of the maximum electron density also showed considerable differences among the three types, centered around 110, 105, and 105 km for APA, PPA, and PA, respectively. The KAIRA CNA values also showed higher values on average during PPA (0.33 dB) compared to PA (0.23 dB) and especially APA (0.17 dB). In general, this suggests that the precipitating electrons responsible for APA have a lower energy range compared to PPA and PA types. Among the three categories, the magnitude of the maximum electron density shows higher values at lower altitudes and in the late magnetic local time (MLT) sector (after 5 MLT) during PPA than during PA or APA. We also found significant ionization down to 70 km during PPA and PA, which corresponds to ∼200 keV of precipitating electrons. [ABSTRACT FROM AUTHOR]

We provide an explanation for a mysterious phenomenon that has been recognized in recent years in European Incoherent Scatter (EISCAT) UHF incoherent scatter radar (ISR) measurements during many high-power high-frequency (HF) ionospheric pumping experiments. The phenomenon is an apparent increase in electron density observed above the HF reflection altitude, extending up to the observable limits usually in the range 400–650 km, as shown in several publications in recent years. It was shown by Senior et al. (2013) that several examples of these enhanced backscatter could not be explained by increases in electron density. A summary of characteristics of the backscatter enhancements is presented as well as the results of a survey of events. We propose that medium- to large-scale HF-induced field-aligned irregularities (tens to hundreds of metres scale) act to refract the radar signals along the magnetic field, thereby acting as a guide so that the free-space r-2 spreading of the signals no longer applies. The nature of the irregularities and the physical mechanism of their production by powerful HF waves is an exciting topic for future research since, surprisingly, they appear to be preferentially excited by X-mode waves. The explanation proposed here involving HF-induced irregularities may well apply to other ISR observations of the ionosphere in the presence of specific natural irregularities. [ABSTRACT FROM AUTHOR]

On 8 April 2005, strong gravity wave (GW) activity (over a period of more than 3 h) was observed in São João do Cariri (7.4 ∘ S, 36.5 ∘ W). These waves propagated to the southeast and presented different spectral characteristics (wavelength, period and phase speed). Using hydroxyl (OH) airglow images, the characteristics of the observed GWs were calculated; the wavelengths ranged between 90 and 150 km, the periods ranged from ∼26 to 67 min and the phase speeds ranged from 32 to 71 m s -1. A reverse ray-tracing analysis was performed to search for the possible sources of the waves that were detected. The ray-tracing database was composed of temperature profiles from the Naval Research Laboratory Mass Spectrometer Incoherent Scatter (NRLMSISE-00) model and SABER measurements as well as wind profiles from the Horizontal Wind Model (HWM) and meteor radar data. According to the ray tracing result, the likely source of these observed gravity waves was the Intertropical Convergence Zone, which caused intense convective processes to take place in the northern part of the observatory. Also, the observed preferential propagation direction of the waves to the southeast could be explained using blocking diagrams, i.e. due to the wind filtering process. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We have quantitatively evaluated generation mechanisms of a sporadic sodium layer (SSL) based on observational data obtained by multiple instruments at a highlatitude station: Ramfjordmoen, Tromsø, Norway (69.6° N, 19.2° E). The sodium lidar observed an SSL at 21:18 UT on 22 January 2012. The SSL was observed for 18 min, with a maximum sodium density of about 1.9×1010 m-3 at 93 km with a 1.1 km thickness. The European Incoherent Scatter (EISCAT) UHF radar observed a sporadic E layer (Es layer) above 90 km from 20:00 to 23:00 UT. After 20:00 UT, the Es layer gradually descended and reached 94 km at 21:18 UT when the SSL appeared at the same altitude. In this event, considering the abundance of sodium ions (10% or less), the Es layer could provide only about 37% or less of the sodium atoms to the SSL. We have investigated a temporal development of the normal sodium ion layer with a consideration of chemical reactions and the effect of the (southwestward) electric field using observational values of the neutral temperature, electron density, horizontal neutral wind, and electric field. This calculation has shown that those processes, including contributions of the Es layer, would provide about 88% of sodium atoms of the SSL. The effects of meteor absorption and auroral particle sputtering appear to be less important. Therefore, we have concluded that the major source of the SSL was sodium ions in a normal sodium ion layer. Two processes - namely the downward transportation of sodium ions from a normal sodium ion layer due to the electric field and the additional supply of sodium ions from the Es layer under relatively high electron density conditions (i.e., in the Es layer) - played a major role in generating the SSL in this event. Furthermore, we have found that the SSL was located in a lower-temperature region and that the temperature inside the SSL did not show any remarkable temperature enhancements. [ABSTRACT FROM AUTHOR]