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Recently, citizen scientist photographs led to the discovery of a new auroral form called "the dune aurora" which exhibits parallel stripes of brighter emission in the green diffuse aurora at about 100 km altitude. This discovery raised several questions, such as (i) whether the dunes are associated with particle precipitation, (ii) whether their structure arises from spatial inhomogeneities in the precipitating fluxes or in the underlying neutral atmosphere, and (iii) whether they are the auroral manifestation of an atmospheric wave called a mesospheric bore. This study investigates a large-scale dune aurora event on 20 January 2016 above Northern Europe. The dunes were observed from Finland to Scotland, spanning over 1500 km for at least four hours. Spacecraft observations confirm that the dunes are associated with electron precipitation and reveal the presence of a temperature inversion layer below the mesopause during the event, creating suitable conditions for mesospheric bore formation. The analysis of a time lapse of pictures by a citizen scientist from Scotland leads to the estimate that, during this event, the dunes propagate toward the west-southwest direction at about 200 m/s, presumably indicating strong horizontal winds near the mesopause. These results show that citizen science and dune aurora studies can fill observational gaps and be powerful tools to investigate the least-known region of near-Earth space at altitudes near 100 km.

We report on the observations of 188 sporadic layers of either Ca atoms and/or Ca ions that we have observed during 112 nights of lidar soundings of Ca, and 58 nights of Ca+ soundings, at Kühlungsborn, Germany (54° N, 12° E). The Ca+ soundings have been performed simultaneously and in a common volume with the Ca soundings by two separate lidars. Correlations between sporadic neutral and ionized metal layers are demonstrated through four case studies. A systematic study of the variations of occurrence of sporadic Ca and Ca+ layers reveals that neutral and ionized Ca layers are not as closely correlated as expected earlier: (a) The altitude distribution shows the simultaneous occurrence of both sporadic Ca and Ca+ layers to be most likely only in the narrow altitude range between 90 and 95 km. Above that region, in the lower thermosphere, the sporadic ion layers are much more frequent than atom layers. Below 90 km only very few sporadic layers have been observed; (b) The seasonal variation of sporadic Ca layers exhibits a minimum of occurrence in summer, while sporadic Ca+ layers do not show a significant seasonal variation (only the dense Ca+ layers appear to have a maximum in summer). At mid-latitudes sporadic Ca layers are more frequent than sporadic layers of other atmospheric metals like Na or K. For the explanation of our observations new formation mechanisms are discussed.Key words. Ionosphere (ion chemistry and composition; ionosphere-atmosphere interactions; mid-latitude ionosphere)

Range spread-F (RSF) and occurrence of "satellite" traces prior to RSF onset were studied at the southern peak of the ionospheric equatorial anomaly (EA). Ionograms recorded in September 2007 at the new ionospheric station of Tucumán, Argentina (26.9° S, 294.6° E, dip latitude 15.5° S), by the Advanced Ionospheric Sounder (AIS) developed at the Istituto Nazionale di Geofisica e Vulcanologia (INGV), were considered. Satellite traces (STs) are confirmed to be a necessary precursor to the appearance of an RSF trace on the ionograms. Moreover, an analysis of isoheight contours of electron density seems to suggest a relationship between RSF occurrence and gravity wave (GW) propagation. Fil: Cabrera, Miguel Angel. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Ionósfera; Argentina. Universidad Tecnológica Nacional. Facultad Regional Tucuman; Argentina Fil: Pezzopane, Michael. Istituto Nazionale Di Geofisica E Vulcanologia; Italia Fil: Zuccheretti, Enrico. Istituto Nazionale Di Geofisica E Vulcanologia; Italia Fil: Ezquer, Rodolfo Gerardo. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Ionósfera; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; Argentina

An unusual nighttime impulsive electron density enhancement was observed on 6 March 2010 over a wide region of South America, below the southern crest of the equatorial anomaly, under low solar activity and quiet geomagnetic conditions. The phenomenon was observed almost simultaneously by the F2 layer critical frequency ( foF2) recorded at three ionospheric stations which are widely distributed in space, namely Cachoeira Paulista (22.4°S, 44.6°W, magnetic latitude 13.4°S), São José dos Campos (23.2°S, 45.9°W, magnetic latitude 14.1°S), Brazil, and Tucumán (26.9°S, 65.4°W, magnetic latitude 16.8°S), Argentina. Although in a more restricted region over Tucumán, the phenomenon was also observed by the total electron content (TEC) maps computed by usingmeasurements from 12 GPS receivers. The investigated phenomenon is very particular because besides being of brief duration, it is characterized by a pronounced compression of the ionosphere. This compression was clearly visible both by the virtual height of the base of the F region (h′F) recorded at the aforementioned ionospheric stations, and by both the vertical electron density profiles and the slab thickness computed over Tucumán. Consequently, neither an enhanced fountain effect nor plasma diffusion from the plasmasphere can be considered as the single cause of this unusual event. A thorough analysis of isoheight and isofrequency ionosonde plots suggest that traveling ionospheric disturbances (TIDs) caused by gravity wave (GW) propagation could have likely played a significant role in causing the phenomenon Fil: Pezzopane, M.. Istituto Nazionale di Geofisica e Vulcanologia; Italia Fil: Fagundes, P. R.. Física E Astronomia, Universidade Do Vale Do Paraiba; Brasil Fil: Ciraolo, L.. Ifac-crn; Italia Fil: Correia, E.. Física E Astronomia, Universidade Do Vale Do Paraiba; Brasil Fil: Cabrera, Miguel Angel. Laboratorio de Telecomunicaciones; Argentina Fil: Ezquer, Rodolfo Gerardo. Centro de Investigación de Atmósfera Superior y Radiopr; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; Argentina

This study is the first which gives the climatology of the ionosphere at the northern tropical crest of ionization in the Asian sector. We use the data from Phu Thuy station, in Vietnam, through three solar cycles (20, 21 and 22), showing the complete morphology of ionosphere parameters by analyzing long term variation, solar cycle variation and geomagnetic activity effects, seasonal evolution and diurnal development. Ionospheric critical frequencies, foF2, foF1 and foE, evolve according to the 11-year sunspot cycle. Seasonal variations show that foF2 exhibits a semiannual pattern with maxima at equinox, and winter and equinoctial anomalies depending on the phases of the sunspot solar cycle. ΔfoF2 exhibits a semiannual variation during the minimum phase of the sunspot solar cycle 20 and the increasing and decreasing phases of solar cycle 20, 21 and 22. ΔfoF1 exhibits an annual variation during the maximum phase of solar cycles 20, 21 and 22. Δh'F2 shows a regular seasonal variation for the different solar cycles while Δh'F1 exhibits a large magnitude dispersion from one sunspot cycle to another. The long term variations consist in an increase of 1.0 MHz for foF2 and of 0.36 MHz for foF1. foE increases 0.53 MHz from solar cycle 20 to solar cycle 21 and then decreases −0.23 MHz during the decreasing phase of cycle 21. The diurnal variation of the critical frequency foF2 shows minima at 05:00 LT and maxima around 14:00 LT. foF1 and foE have a maximum around noon. The diurnal variation of h'F2 exhibits a maximum around noon. The main features of h'F1 are a minimum near noon and the maximum near midnight. Other minima and maxima occur in the morning, at about 04:00 or 05:00 LT and in the afternoon, at about 18:00 or 19:00 LT but they are markedly smaller. Only during the maximum phase of all sunspot solar cycles the maximum near 19:00 LT is more pronounced.

International audience; On 24 November 2006, simultaneous observations of Pc5 pulsations, electron precipitation and whistler-mode chorus, as well as solar wind and IMF parameters have been analyzed based on data from IMAGE magne-tometers, riometer array and temporal VLF station. This paper focuses on the Pc5 pulsations detected at the same time, in the 1–25 millihertz range, by the [SQUID] 2 system (SQUID magnetometer within a Shielding QUalified for Ionosphere Detection) installed 518 m underground at 43.92 • N, 5.48 • E. As expected, the 3-D-frequency spectrum of these mid-latitude [SQUID] 2 signals exhibits frequency peaks quasi identical to those observed by polar stations of close geomagnetic longitude. The signal/noise ratio allows the observation of the wave polarization and the beatings of the frequencies. As a result, the possibility of studying, at mid-latitude, magnetic Pc5 pulsations linked with an event in the magnetosphere can improve the description of both behaviour and propagation of these waves.

On 24 November 2006, simultaneous observations of Pc5 pulsations, electron precipitation and whistler-mode chorus, as well as solar wind and IMF parameters have been analyzed based on data from IMAGE magnetometers, riometer array and temporal VLF station. This paper focuses on the Pc5 pulsations detected at the same time, in the 1–25 millihertz range, by the [SQUID]2 system (SQUID magnetometer within a Shielding QUalified for Ionosphere Detection) installed 518 m underground at 43.92° N, 5.48° E. As expected, the 3-D-frequency spectrum of these mid-latitude [SQUID]2 signals exhibits frequency peaks quasi identical to those observed by polar stations of close geomagnetic longitude. The signal/noise ratio allows the observation of the wave polarization and the beatings of the frequencies. As a result, the possibility of studying, at mid-latitude, magnetic Pc5 pulsations linked with an event in the magnetosphere can improve the description of both behaviour and propagation of these waves.

Several possible mechanisms for the production of sporadic sodium layers have been discussed in the literature, but none of them seem to explain all the accumulated observations. The hypotheses range from direct meteoric input, to energetic electron bombardment on meteoric smoke particles, to ion neutralization, to temperature dependent chemistry. The varied instrumentation located on Andøya and near Tromsø in Norway gives us an opportunity to test the different theories applied to high latitude sporadic sodium layers. We use the ALOMAR Weber sodium lidar to monitor the appearance and characteristics of a sporadic sodium layer that was observed on 5 November 2005. We also monitor the temperature to test the hypotheses regarding a temperature dependent mechanism. The EISCAT Tromsø Dynasonde, the ALOMAR/UiO All-sky camera and the SKiYMET meteor radar on Andøya are used to test the suggested relationships of sporadic sodium layers and sporadic E-layers, electron precipitation, and meteor deposition during this event. We find that more than one candidate is eligible to explain our observation of the sporadic sodium layer.

This is the publishers version/PDF (Published in Annales Geophysicae, An Open Access Journal of the European Geosciences Union) Lower-thermospheric winds at high latitudes during moderately-disturbed geomagnetic conditions were studied using data obtained with the European Incoherent Scatter (EISCAT) Kiruna-Sodankylä-Tromsø (KST) ultrahigh frequency (UHF) radar system on 9–10 September 2004. The antenna-beam configuration was newly designed to minimize the estimated measurement error of the vertical neutralwind speed in the lower thermosphere. This method was also available to estimate the meridional and zonal components. The vertical neutral-wind speed at 109 km, 114 km, and 120 km heights showed large upward motions in excess of 30ms−1 in association with an ionospheric heating event. Large downward speeds in excess of −30ms−1 were also observed before and after the heating event. The meridional neutral-wind speed suddenly changed its direction from equatorward to poleward when the heating event began, and then returned equatorward coinciding with a decrease in the heating event. The magnetometer data from northern Scandinavia suggested that the center of the heated region was located about 80 km equatorward of Tromsø. The pressure gradient caused the lower-thermospheric wind to accelerate obliquely upward over Tromsø in the poleward direction. Acceleration of the neutral wind flowing on a vertically tilted isobar produced vertical wind speeds larger by more than two orders of magnitude than previously predicted, but still an order of magnitude smaller than observed speeds.

We investigate the seasonal, local solar time, and geomagnetic activity variations of the average Doppler velocity measured by an HF digital ionosonde deployed at Bundoora, Australia. The Doppler velocities were heavily averaged to suppress the short-term effects (hours) of atmospheric gravity waves, and thereby obtain the diurnal variations attributed to the tidally-driven ionospheric dynamo and electric fields generated by magnetic disturbances. The observed seasonal variations in Doppler velocity were probably controlled by variations in the lower thermospheric winds and ionospheric conductivity above Bundoora and in the magnetically conjugate location. The diurnal variations of the meridional (fieldperpendicular) drifts and their perturbations exhibited a complex structure, and were generally smaller than the variations in the zonal drifts. The latter were basically strongly westward during the evening to early morning, and weakly eastward during the late morning to just past noon. The zonal perturbations were strongly enhanced by increasing geomagnetic activity, and closely resembled the perturbation drifts measured by the incoherent scatter radar (ISR) at Millstone Hill. There was also some resemblance between the diurnal variations in the meridional drifts. Overall, the comparisons suggest that with sufficient averaging, Doppler velocities measured with digital ionosondes at mid-latitudes correspond to true ion motions driven by ionospheric electric fields. This is a useful result because apart from the ISRs located in the American-European sector, there are no ground-based instruments capable of measuring electric fields in the mid-latitude ionosphere.

Strong wind shears may result in dynamic instability, often characterised by the Richardson number lying between zero and 0.25. The extent to which electric-field driven ion flow may induce such neutral wind shears is examined. Further, it is proposed that, in the ionosphere, it is possible for electric fields to drive ion winds such that the collisionally induced neutral air response may be comparable to viscous damping of neutral motion. We shall present an analogy to the Reynolds Number Re to quantify this effect. In the same way that Re may be used to evaluate the likelihood of a flow being turbulent, the analagous metric may also indicate where in the atmosphere plasma dynamics may be strong enough to destabilise the neutral dynamics.Key words: Ionosphere (auroral ionosphere; ionosphere-atmosphere interactions) - Meteorology and atmospheric dynamics (turbulence)