We conducted a feasibility study to estimate the horizontal ion velocity field in the ionospheric F region from ion velocities observed by the EISCAT_3D radar. We assumed a 27-beam configuration with a minimum elevation angle of 30 degrees. The ion velocity observation data from 200 km to 500 km altitude were projected to 200 km altitude, assuming ions above 200 km altitude follow the E x B drift. Then, we reconstructed ion velocity vectors for ±250 km in the east-west direction and ±500 km in the north-south direction at 200 km altitude. The resolution in north-south and east-west directions was 25 km. The reconstruction was conducted by maximizing a posterior probability based on Bayes’ theorem. The constraints were set to minimize the L2 norms of the following four vectors in the horizontal plane: (1) the first and (2) the second derivative of three components of the ion velocity vector, (3) the divergence of the ion velocity perpendicular to the magnetic field lines, and (4) the ion velocity parallel to the magnetic field lines. We investigated which combination of the four constraints would reconstruct the shear flow field most correctly and found that the best combination was (2), (3), and (4).

How to cite: Fukizawa, M., Ogawa, Y., Nishimura, K., Ueno, G., Nishiyama, T., Hashimoto, T., and Tsuda, T.: Feasibility study to estimate the ion velocity field in the F region from EISCAT_3D radar observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7480, https://doi.org/10.5194/egusphere-egu24-7480, 2024.

The mesosphere/lower-thermosphere/ionosphere (MLTI) region is a critical boundary in the coupling of the atmosphere, climate and space weather, however it is one of the least understood regions, making it hard to include in whole atmosphere models. The EISCAT radars at Tromsø (UHF and VHF) have been measuring ionospheric parameters, such as electron density, for almost 4 decades making them an excellent resource to study changes in the ionosphere over a long time period.  We have generated two data archives using 20 years of observations of EISCAT Tromsø from 2001 to 2021; the data have been re-analysed at 10-minutes and 1-hour integrations. These archives are used to study the different sources of variability in the MLTI from 50-200 km. This is the first time the mainland EISCAT data has been converted into a format that allows for long term statistical study. We have created electron density climatologies split by solar, geomagnetic and atmospheric indices to investigate the different drivers of variability in the MLTI region. We show seasonal averages of the electron density altitude profiles and compare our results to the Empirical Canadian High Arctic Ionospheric Model (E-CHAIM) and the Whole Atmosphere Community Climate Model.

How to cite: Reidy, J. and Kavanagh, A.: Investigating seasonal to decadal variability in the electron density of the mesosphere using historical EISCAT data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17646, https://doi.org/10.5194/egusphere-egu24-17646, 2024.

The important question of how much of the variability in the high latitude ionosphere is driven by atmospheric processes as opposed to space weather impacts remains unanswered.  The EISCAT-3D radar provides a unique opportunity to probe this variability across multiple spatial and temporal scales. One of the key aims of the DRIIVE project (DRivers and Impacts of Ionospheric Variability with EISCAT-3D) is to determine the balance of energy input to the lower ionosphere and quantify the variability under different atmospheric and geomagnetic conditions.  Here we present a preliminary study of the variability using historic data from the EISCAT UHF radar taken over the course of several years in the winter months. We identify wave like signatures that occur simultaneously with Travelling Ionospheric Disturbances (TID) as seen in coherent radar data (SuperDARN), alongside enhancements due to energetic precipitation.  The magnitude of the variations are compared for different years and different driving conditions. This study will allow us to optimize the design of future experiments for EISCAT-3D to study the variability while developing effective analysis techniques to maximise utility of the new radar system.

How to cite: Kavanagh, A. J., Grocott, A., Walach, M.-T., Reidy, J., and Clilverd, M.: Identifying sources of Ionospheric Variability: planning for EISCAT-3D operations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17680, https://doi.org/10.5194/egusphere-egu24-17680, 2024.

How to cite: Gupta, M. and Guio, P.: Modelling ISR plasma line frequency using ALPS dispersion relation solver , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14792, https://doi.org/10.5194/egusphere-egu24-14792, 2024.

The auroral emissions are due to electron precipitations into the upper atmosphere in polar regions. These electrons populations, called suprathermal, due to their high energies (10 eV to 50/100 keV) generates some emissions mainly in the green (O I 557 nm), the red (O I 630-636 nm) and the purple (N2+ 427 nm). However the spectra contains much more lines especially molecular bands such as the 1^st , 2^nd postive bands and the Vegard Kaplan band of molecular nitrogen which produces structured vibrational emissons. It is then important to be able to get quasi exhaustive simulations of these emissions. Based on kinetic calculations from the Transsolo code, we implemented simulations of almost all the visible emissions lines of the auroras parametized by the particle precipitations. The code used MSIS 2.0 as atmospheric model, IRI  2020 for the ionosphere and a very large set of updated cross sections. We propose in this work to present the result of these simulatins in different conditions and to consider the potential uses of such calculations for different auroral physics applications.

How to cite: Barthelemy, M. and Robert, E.: Synthetic spectra of the auroras, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15254, https://doi.org/10.5194/egusphere-egu24-15254, 2024.

In October 2023, a spectrograph has been permanently installed at the Skibotn Observatory (Norway) in order to regularly monitor the auroral spectrum between ~ 400 and 700 nm with a time resolution of 30 seconds. Using a 300 lines/mm grating and a slit of 100 nm width, the wavelength resolution is approximately of 0.3 nm.  The instrument is pointing field-aligned.

The characteristics of the instrument will be provided as well as examples of spectra obtained during quiet, moderate and strong geomagnetic conditions.  A relative flux calibration is currently under way and will be discussed as well.  This will allow the computation of line ratios or a comparison with synthetic spectra obtained using kinetic transport codes such as e.g. Transsolo. Both approaches will provide an estimate of the precipitating fluxes (for electrons but also possibly for protons).

Later on we aim to provide a database of low resolution auroral spectra accessible to the community, which will nicely complement data obtained with the new EISCAT_3D radar located a few kilometers from the observatory and data from the other optical instruments located at the observatory itself.  We will also consider the possibility to obtain higher resolution spectra using a 1800 lines/mm grating during specific requested campaigns.

This project is a joint collaboration between BIRA-IASB (Belgium), IPAG (France) and UiT (Norway).

How to cite: Lamy, H., Cessateur, G., Bosse, L., Bolsée, D., Barthélemy, M., Sequies, T., and Johnsen, M. G.: Optical auroral spectra obtained at the Skibotn Observatory, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6117, https://doi.org/10.5194/egusphere-egu24-6117, 2024.

We have constructed a new high-resolution auroral imager, called Embla, to simultaneously measure energy and flux of auroral precipitation, neutral temperature, and electric fields in fine scale aurora. Embla is designed primarily for studies of auroral electrodynamics, substorm onset, and neutral heating by aurora. The instrument has recently been installed at Skibotn, Norway, very close to the EISCAT_3D radar transmitter site, and we expect the combination of radar and optical observations to enable better measurements for our science than either instrument can provide alone.

Embla builds on work done using the Auroral Structure and Kinetics (ASK) instrument, which has been stationed at the EISCAT Svalbard Radar since 2007. Embla has a spatial resolution in the E-region of ~30 m and a planned temporal resolution of at least 32 frames per second, allowing us to resolve the fine-scale structure and rapid dynamics of auroral features. It consists of 4 co-aligned imagers with identical 9 degree fields of view centred on magnetic zenith. Each imager is equipped with a different narrow passband interference filter, targetting emissions in N₂ 1P (2 imagers), OI 777.4 nm, and O^{+ 2}P. The combination of N₂ 1P and OI 777.4 nm observations allows us to image the characteristic energy and flux of auroral electron precipitation. The O^{+ 2}P emission has a long lifetime of 5 s, providing a means to observe ion drift perpendicular to the magnetic field and therefore a way to determine ionospheric electric fields at very high cadence in localised regions around the aurora. Finally, by combining observations from two of the imagers in separate regions of the N₂ 1P band we can image the neutral temperature at the altitude of the auroral emission. The simultaneous measurement of these properties of the aurora and ionosphere will allow us to investigate auroral electrodynamics in detail.

How to cite: Whiter, D., Rae, J., Samaddar, S., Krcelic, P., Lanchester, B., Carlyle, I., Brindley, N., Thillaimaharajan, K., and Fear, R.: Embla: A new optical instrument to measure auroral precipitation, neutral temperature and electric fields at high resolution, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19428, https://doi.org/10.5194/egusphere-egu24-19428, 2024.

Ground based optical observations of aurora reveal fine scale structures with brightness width less than 10 kms in the direction perpendicular to B. These fine scale structures exhibit a phenomenon called arc splitting, also known as bifurcating elemental arcs or packets. One can witness the arc splitting in the auroral image when elemental arcs peel away from a central bright arc which is then followed by the generation of new arcs. Dispersive Alfvén waves have been suggested as a possible generation mechanism for this phenomenon. Semeter et al., JGR 2008 interpreted the observations of splitting elemental arcs with respect to inertial Alfvén waves. They suggested that the energy of the precipitating electrons should decrease as the arc packets move away from the central bright arc. We tested this theory by using the data from the multi spectral imager called ASK (Auroral Structure and Kinetics) stationed on Svalbard. The energy and flux of the precipitating electrons are calculated from the ASK data and Southampton ionospheric model. We used empirical number density and IGRF (International Geomagnetic Reference Field) model to calculate the properties of the inertial Alfvén waves. A comparison between the splitting elemental arcs and Alfvén waves indicates that the wave particle interaction between Alfvén waves and the precipitating electrons is a possible generation mechanism for the production of these splitting elemental arcs. From our data and calculations, we infer an acceleration height of precipitating electrons just under 3000 km.

References:

Semeter, J., M. Zettergren, M. Diaz, and S. Mende (2008), Wave dispersion and the discrete aurora:

New constraints derived from high-speed imagery, J. Geophys. Res., 113, A12208.

How to cite: Thillaimaharajan, K., Whiter, D., Brindley, N., and Krcelic, P.: Splitting elemental arcs of aurora and their association with inertial Alfvén waves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18848, https://doi.org/10.5194/egusphere-egu24-18848, 2024.

Ionospheric sporadic E layers (Es) are intense plasma irregularities between 80 and 130 km in altitude and are generally unpredictable. Reconstructing the morphology of sporadic E layers is not only essential for understanding the nature of ionospheric irregularities and many other atmospheric coupling systems, but is also useful for solving a broad range of demands for reliable radio communication of many sectors reliant on ionosphere-dependent decision-making. Despite the efforts of many empirical and theoretical models, a predictive algorithm with both high accuracy and high efficiency is still lacking. Here we introduce a new approach for Sporadic E Layer Forecast using Artificial Neural Networks (SELF-ANN). The prediction engine is trained by fusing observational data from multiple sources, including a high-resolution ERA5 reanalysis dataset, Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation (RO) measurements, and integrated data from OMNIWeb. The results show that the model can effectively reconstruct the morphology of the ionospheric E layer with intraseasonal variability by learning complex patterns. The model obtains good performance and generalization capability by applying multiple evaluation criteria. The random forest algorithm used for preliminary processing shows that local time, altitude, longitude, and latitude are significantly essential for forecasting the E-layer region. Extensive evaluations based on ground-based observations demonstrate the superior utility of the model in dealing with unknown information. The presented framework will help us better understand the nature of the ionospheric irregularities, which is a fundamental challenge in upper-atmospheric and ionospheric physics. Moreover, the proposed SELF-ANN can make a significant contribution to the development of the prediction of ionospheric irregularities in the E layer, particularly when the formation mechanisms and evolution processes of the Es layer are not well understood.

How to cite: Tian, P.: Ionospheric irregularity reconstruction using multisource data fusion via deep learning, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3145, https://doi.org/10.5194/egusphere-egu24-3145, 2024.

The sudden increase of Solar X-ray and EUV emission following solar flares causes ionization and increased absorption of electromagnetic (EM) waves in the sunlit hemisphere of the Earth’s ionosphere. Solar flares are also accompanied by energetic particles which can lead to additional ionization and absorption especially at the higher latitudes (> 60 °). A novel method has been developed by Buzás et al. [1] based on the amplitude data of the EM waves measured by Digisondes to calculate and investigate the relative absorption changes (compared to quiet period) occurring during solar flares. The effect of 13 intense (>C4.8) solar flares that occurred between 06:00 and 16:30 (UT, daytime LT = UT+2 h) from 04 to 10 September 2017 have been studied using the so-called "amplitude method". Total and partial radio fade-outs, furthermore, +20%–1400% amplitude changes (measured at 2.5 and 4 MHz) were experienced at three Digisonde stations (Juliusruh (54.63° N, 13.37° E), Průhonice (49.98° N, 14.55° E) and San Vito (40.6° N, 17.8° E)) during and after the investigated flares.

In the present study we compare the results of the amplitude method with the absorption changes measured by the Finnish Riometer Chain and determined by the NOAA D-RAP model during the same solar flare events. The X-class flares caused 1.5–2.5 dB attenuation at 30–32.5 MHz based on riometer data, while the absorption changes were between 10 and 15 dB in the 2.5–4.5 MHz frequency range (thus 10 times higher) according to the amplitude data measured by the Digisondes. The impact caused by the energetic particles after the solar flares are clearly seen in the riometer data, while it can be observed only at Juliusruh (~55°) at some certain cases among the Digisonde stations. Therefore, the absorption changes as a result of the particle precipitation is significant at high latitudes, but decreases rapidly with decreasing latitude, and is no longer detectable below the sub-auroral region. The main conclusion from the comparison of the amplitude method with the D-RAP model is that the model underestimates the values obtained from the Digisonde's measurements at both 2.5 and 4 MHz in almost every case. The differences varied between 0.2 and 15 dB at 2.5 MHz and 2.9–10 dB at 4 MHz and they did not show any systematic trend with the intensity of the flare, or with the latitude of the station. 

[1] Buzás, A., Kouba, D., Mielich, J., Burešová, D., Mošna, Z., Koucká Knížová, P., & Barta, V. (2023). Investigating the effect of large solar flares on the ionosphere based on novel Digisonde data comparing three different methods. Frontiers in Astronomy and Space Sciences, 10.

How to cite: Barta, V., Bozóki, T., Süle, D. P., Kouba, D., Mielich, J., Raita, T., and Buzás, A.: Ionospheric absorption variation as measured by European Digisondes, riometers and determined by the NOAA D-RAP model during intense solar flares in September, 2017, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8057, https://doi.org/10.5194/egusphere-egu24-8057, 2024.

We have analyzed all the existing European incoherent scatter (EISCAT) radar VHF measurements carried out together with the neutral temperature measurements from LIDAR collocated at Tromsø, Norway. Incoherent scatter radar (ISR) spectral parameters are estimated from the backscattered signal autocorrelation function by fitting the D-region lag profiles (pulse-to-pulse fitting). This study focuses on the ISR spectral width which is a function of the ion-to-neutral collision frequency, neutral temperature, and ion mass. Using the neutral temperatures obtained from LIDAR, we have measured for the first time the ion-to-neutral collision frequency in the mesosphere lower thermosphere (MLT) altitude range (80-100 km) by fitting the ISR spectral width. The model ion-to-neutral collision frequencies estimated using MSIS neutral densities are found to be underestimated on average by 1.5 in comparison to the measurements. Also, the ISR collision frequencies showed large temporal variations caused by neutral density fluctuations, which are absent in the model values. These large-scale neutral density fluctuations which are thought to be caused by atmospheric waves are found to have amplitudes as large as 50% of the background density. This indicates that the ISR spectral width is largely influenced by the neutral density fluctuations. In light of these observations, the inherent limitations of inferring the neutral temperatures from ISR spectral width are studied. This study showcases the challenges of estimating neutral temperatures from ISR spectral width in the MLT altitudes, which is significant in the context of the upcoming EISCAT_3D.

How to cite: Thomas, N., Kero, A., Virtanen, I., and Nozawa, S.: Incoherent scatter radar measurements of ion-to-neutral collision frequencies and neutral temperatures in the D-region ionosphere, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10825, https://doi.org/10.5194/egusphere-egu24-10825, 2024.