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A solar-flare-induced, high-latitude (peak at 70–75∘ geographic latitude – GGlat) ionospheric current system was studied. Right after the X9.3 flare on 6 September 2017, magnetic stations at 68–77∘ GGlat near local noon detected northward geomagnetic deviations (ΔB) for more than 3 h, with peak amplitudes of >200 nT without any accompanying substorm activities. From its location, this solar flare effect, or crochet, is different from previously studied ones, namely, the subsolar crochet (seen at lower latitudes), auroral crochet (pre-requires auroral electrojet in sunlight), or cusp crochet (seen only in the cusp). The new crochet is much more intense and longer in duration than the subsolar crochet. The long duration matches with the period of high solar X-ray flux (more than M3-class flare level). Unlike the cusp crochet, the interplanetary magnetic field (IMF) BY is not the driver, with the BY values of only 0–1 nT out of a 3 nT total field. The equivalent ionospheric current flows eastward in a limited latitude range but extended at least 8 h in local time (LT), forming a zonal current region equatorward of the polar cap on the geomagnetic closed region. EISCAT radar measurements, which were conducted over the same region as the most intense ΔB, show enhancements of electron density (and hence of ion-neutral density ratio) at these altitudes (∼100 km) at which strong background ion convection (>100 m s−1) pre-existed in the direction of tidal-driven diurnal solar quiet (Sq0) flow. Therefore, this new zonal current can be related to this Sq0-like convection and the electron density enhancement, for example, by descending the E-region height. However, we have not found why the new crochet is found in a limited latitudinal range, and therefore, the mechanism is still unclear compared to the subsolar crochet that is maintained by a transient redistribution of the electron density. The signature is sometimes seen in the auroral electrojet (AE = AU − AL) index. A quick survey for X-class flares during solar cycle 23 and 24 shows clear increases in AU for about half the > X2 flares during non-substorm time, despite the unfavourable latitudinal coverage of the AE stations for detecting this new crochet. Although some of these AU increases could be the auroral crochet signature, the high-latitude crochet can be a rather common feature for X flares. We found a new type of the solar flare effect on the dayside ionospheric current at high latitudes but equatorward of the cusp during quiet periods. The effect is also seen in the AU index for nearly half of the > X2-class solar flares. A case study suggests that the new crochet is related to the Sq0 (tidal-driven part) current.

It is important to monitor the extra-terrestrial dust flux in the Earth’s environment and into the atmosphere. Meteoroids threaten the infrastructure in space as hypervelocity hits by micron-sized granules continuously degrade the solar panels and other satellite surfaces. Through their orbital elements meteoroids can be associated to the interplanetary dust cloud, comets, asteroids or the interstellar space. The ablation products of meteoroids participate in many physical and chemical processes at dierent layers inthe atmosphere, many of them occurring in the polar regions. High-power large-aperture (HPLA) radars, such as the tristatic EISCAT UHF together with the EISCAT VHF, have been versatile instruments for studying many properties of the meteoroid population, even though they were not initially designed for this purpose. The future EISCAT_3D will comprise a phased-array transmitter and several phased-array receivers distributed in Northern Scandinavia. These will work at 233 MHz centre frequency with power up to 10 MW and run advanced signal processing systems. The facility will in many aspects be superior to its predecessors as the first radar to combine volumetric-, aperture synthesis- and multistatic imaging as well as adaptive experiments. The technical design goals of the radar respond to the scientific requests from the user community. The VHF frequency and the volumetric imaging capacity will increase the collecting volume compared to the earlier UHF, the high transmitter power will increase the sensitivity of the radar, and the interferometry will improve the spatial resolution of the orbit estimates. The facility will be able to observe and define orbits to about 10% of the meteors from the established mass flux distribution that are large or fast enough to produce an ionization mantle around the impacting meteoroid within the collecting volume. The estimated annual mean of about 190 000 orbits per day with EISCAT_3D gives many orders of magnitude higher detected orbit rates than the earlier tristatic UHF radar. Special issue: ACM Interrelated

Source at https://doi.org/10.1029/2018SW001937. We present ionospheric plasma conditions observed by the EISCAT radars in Tromsø and on Svalbard, covering 68°–81° geomagnetic latitude, during 6–8 September 2017. This is a period when X2.2 and X9.3 X‐ray flares occurred, two interplanetary coronal mass ejections (ICMEs) arrived at the Earth accompanied by enhancements of MeV‐range energetic particle flux in both the solar wind (SEP event) and inner magnetosphere, and an AL < −2,000 substorm took place. (1) Both X flares caused enhancement of ionospheric electron density for about 10 min. The X9.3 flare also increased temperatures of both electrons and ions over 69°–75° geomagnetic latitude until the X‐ray flux decreased below the level of X‐class flares. However, the temperature was not enhanced after the previous X2.2 flare in the prenoon sector. (2) At around 75° geomagnetic latitude, the prenoon ion upflow flux slightly increased the day after the X9.3 flare, which is also after the first ICME and a SEP event, while no outstanding enhancement was found at the time of these X flares. (3) The upflow velocity sometimes decreased when the interplanetary magnetic field (IMF) turned southward. (4) Before the first ICME arrival after the SEP event under weak IMF with Bz ~0 nT, a substorm‐like expansion of the auroral arc signature took place without local geomagnetic signature near local midnight, while no notable change was observed after the ICME arrival. (5) AL reached

Polar mesosphere winter echoes (PMWE) were observed at 70 km over Tromsø, Norway, on 8 January 2014 using the tristatic configuration of the European incoherent scatter VHF radar. For the interval 11:00–13:00 UT where the strongest patch of PMWE of about 6-min duration was detected, the spectra of the received signal were analysed for the Tromsø site and altitude profiles of spectral parameters were derived. For the remote sites Kiruna and Sodankylä, the Doppler velocities and their vertical shear were determined by using the measured autocorrelation functions. Ducted gravity waves with periods of 5–10 min were found in the vertical wind velocity between 66 and 81 km altitudes. The duct might be formed around 70 and 77 km altitude where horizontal wind maxima were observed with the Kiruna receiver. However, we did not find any close relation between wind shear at 70 km altitude and PMWE at the same height: the wind shear was present for 2 h, but PMWE for only 6 min. Enhanced spectral width in the vertical Tromsø beam was observed for the PMWE patch. We discussed these experimental findings in relation to the winter echo generation mechanism. Our conclusion is that the presence of patchy negatively charged small-sized dust might explain the observations although a gravity wave breaking mechanism cannot be completely rejected.

(c) American Geophysical Union, reprinted with permission. Article also available at source: https://doi.org/10.1002/2016JA023080

International audience; We determine the phase velocity and k vector for parallel and oblique broadband extremely low frequency, ELF, waves on nightside auroral magnetic field lines at altitudes around 4.6 R E. We use internal burst mode data from the EFW electric field and wave instrument onboard the Cluster spacecraft to retrieve phase differences between the four probes of the instrument. The retrieved characteristic phase velocity is of the order of the ion acoustic speed and larger than the thermal velocity of the protons. The typical wavelength obtained from interferometry is around the proton gyro radius and always larger than the Debye length. We find that in regions with essentially no suprathermal electrons above a few tens of eV the observed broadband waves above the proton gyro frequency are consistent with upgoing ion acoustic and oblique ion acoustic waves.

International audience; The k-filtering technique is a method to characterize stationary fluctuations in space plasmas in terms of the wave energy distribution in the frequency and wave vector space. It has the ability to distinguish between wave modes of the same frequency in the spacecraft frame of reference, but with different wave vectors. This method is based on simultaneous multi-point measurements of the wave field components, where a filter bank is used to enhance the spatial resolution. We have for the first time combined electric field data from the Electric Field and Wave (EFW) instrument and magnetic field data from the Spatio-Temporal Analysis of Field Fluctuation (STAFF) instrument on the four Cluster spacecraft in order to determine the wave energy distribution. The k-filtering technique has previously been performed with only the magnetic field measurements. The reason to include the electric field measurements is that it is important to include as much data as possible in order to get the best possible estimation of the wave energy distribution. Another reason is that it also enables comparisons between the electric and the magnetic part of the wave energy distribution to make it possible to differentiate the observed waves according to their polarization. The k-filtering method has been extended in order to allow for two measured components of the electric field, and also for the possibility that the electric field measurements from one or more satellites cannot be used for k-filtering. The technique has been applied on satellite data from the magnetosheath and the foreshock, and from these examples it is clear that k-filtering using both electric and magnetic field measurements is a good tool for characterizing the waves that are observed in space.

Observations by the Viking and Cluster satellites at altitudes up to 35000km show that Lower Hybrid Cavities (LHCs) are common in the inner magnetosphere. LHCs are density depletions filled with waves in the lower hybrid frequency range. The LHCs have, until recently, only been found at altitudes up to 2000km. Statistics of the locations and general shape of the LHCs is performed to obtain an overview of some of their properties. In total, we have observed 166 LHCs on Viking during 27h of data, and 535 LHCs on Cluster during 87h of data. These LHCs are found at invariant latitudes from the auroral region to the plasmapause. A comparison with lower altitude observations shows that the LHC occurrence frequency does not scale with the flux tube radius, so that the LHCs are moderately rarer at high altitudes. This indicates that the individual LHCs do not reach from the ionosphere to 35000km altitude, which gives an upper bound for their length. The width of the LHCs perpendicular to the geomagnetic field at high altitudes is a few times the ion gyroradius, consistent with observations at low altitudes. The estimated depth of the density depletions vary with altitude, being larger at altitudes of 20000-35000km (Cluster, 10-20%), smaller around 1500-13000km (Viking and previous Freja results, a few percent) and again larger around 1000km (previous sounding rocket observations, 10-20%). The LHCs in the inner magnetosphere are situated in regions with background electrostatic hiss in the lower hybrid frequency range, consistent with investigations at low altitudes. Individual LHCs observed at high altitudes are stable at least on time scales of 0.2s (about the ion gyro period), which is consistent with previous results at lower altitudes, and observations by the four Cluster satellites show that the occurrence of LHCs in a region in space is a stable phenomenon, at least on time scales of an hour.

International audience; We present the results of a study based on several events of broadband ULF/ELF emissions observed in the auroral region by the CLUSTER multi-spacecraft at distances around 4-5 RE. These emissions, observed below the ion plasma frequency, are similar to the broadband emissions observed at lower altitudes (800-4000 km) by rockets (e.g. AMICIST) and satellites (e.g. FREJA and FAST). As successive passages of the four CLUSTER satellites through nearly the same regions show, the intensity of the emissions depend on the thermal properties of the plasma and gradients thereof. The total Poynting flux is downward and is comparable to energy fluxes observed at lower altitudes. We believe the broadband emissions are the result of dispersed Alfvén waves (DAW), which propagates down the magnetic field lines, and emits higher frequency ion plasma wave modes.

Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have amplitudes at least up to 5 mV/m. These waves are similar on all four Cluster satellites, i.e. they are likely to be distributed over large areas of the boundary. The strongest electric fields occur during a few seconds, i.e. over distances of a few hundred km in the frame of the moving magnetopause, a scale length comparable to the ion gyroradius. The strongest magnetic oscillations in the same frequency range are typically found in the boundary layer, and across the magnetopause. During an event studied in detail, the magnetopause velocity is consistent with a large-scale depression wave, i.e. an inward bulge of magnetosheath plasma, moving tailward along the nominal magnetopause boundary. Preliminary investigations indicate that a rather flat front side of the large-scale wave is associated with a rather static small-scale electric field, while a more turbulent backside of the large-scale wave is associated with small-scale time varying electric field wave packets.Key words. Magnetospheric physics (magnetopause, cusp, and boundary layers) – Space plasma physics (waves and in-stabilities)