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A severe meteorological storm system on the frontal border of cyclone Fabienne passing above central Europe was observed on 23–24 September 2018. Large meteorological systems are considered to be important sources of the wave-like variability visible/detectable through the atmosphere and even up to ionospheric heights. Significant departures from regular courses of atmospheric and ionospheric parameters were detected in all analyzed datasets through atmospheric heights. Above Europe, stratospheric temperature and wind significantly changed in coincidence with fast frontal transition (100–110 km h−1). Zonal wind at 1 and 0.1 hPa changes from the usual westward before the storm to eastward after the storm. With this change are connected changes in temperature where at 1 hPa the analyzed area is colder and at 0.1 hPa warmer. Within ionospheric parameters, we have detected significant wave-like activity occurring shortly after the cold front crossed the observational point. During the storm event, both by Digisonde DPS-4D and continuous Doppler sounding equipment, we have observed strong horizontal plasma flow shears and time-limited increase plasma flow in both the northern and western components of ionospheric drift. The vertical component of plasma flow during the storm event is smaller with respect to the corresponding values on preceding days. The analyzed event of an exceptionally fast cold front of cyclone Fabienne fell into the recovery phase of a minor–moderate geomagnetic storm observed as a negative ionospheric storm at European mid-latitudes. Hence, ionospheric observations consist both of disturbances induced by moderate geomagnetic storms and effects originating in convective activity in the troposphere. Nevertheless, taking into account a significant change in the global circulation pattern in the stratosphere, we conclude that most of the observed wave-like oscillations in the ionosphere during the night of 23–24 September can be directly attributed to the propagation of atmospheric waves launched on the frontal border (cold front) of cyclone Fabienne. The frontal system acted as an effective source of atmospheric waves propagating upward up to the ionosphere.

The ionospheric effects induced by the September 2017 storm have been exceptional compared to other events in the solar cycle 24. This paper gathers results of the ionospheric observations at the European middle latitude station Pruhonice. It consists of evaluation of ionospheric vertical and oblique sounding, Digisonde drift measurement, and data obtained from the Continuous Doppler Sounding System. We observed strong ionospheric response with an unusual stratification of ionospheric layers, Large Scale Traveling ionospheric disturbances, changes in electron density, and increase and oscillations in plasma drift velocity.

This paper presents first results and test of passive detection of Digisonde signal obtained at the Průhonice station, Czech Republic. Described technique can be used to analyze signals transmitted by the Digisonde and further received by simple loop antenna. The receiving system is based on the software defined radio (SDR USRP N210). The device works as the two channels receiver. This method will be employed to record oblique ionograms at several locations in the Czech Republic forming a simple array for detection of fine structures within ionospheric plasma using common DPS 4D transmitter. Using relatively inexpensive passive detection helps to increase a number of reflection points which leads to a better spatial coverage of the ionosphere sounding. The device represents a complementary measurement for detection of signal transmitted by Digisondes in European sector.

The ionospheric weather is affected not only from above by the Sun but also from below by processes in the lower-lying atmospheric layers. One of the most pronounced atmospheric phenomena is the sudden stratospheric warming (SSW). Three major SSW events from the periods of very low solar activity during January 2009, February 2018, and December 2018/January 2019 were studied to evaluate this effect of the neutral atmosphere on the thermosphere and the ionosphere. The main question is to what extent the ionosphere responds to the SSW events with focus on middle latitudes over Europe. The source of the ionospheric data was ground-based measurements by Digisondes, and the total electron content (TEC). In all three events, the ionospheric response was demonstrated as an increase in electron density around the peak height of the F2 region, in TEC, and presence of wave activity. We presume that neutral atmosphere forcing and geomagnetic activity contributed differently in individual events. The ionospheric response during SSW 2009 was predominantly influenced by the neutral lower atmosphere. The ionospheric changes observed during 2018 and 2018/2019 SSWs are a combination of both geomagnetic and SSW forcing. The ionospheric response to geomagnetic forcing was noticeably lower during time intervals outside of SSWs.

The ionosphere represents part of the upper atmosphere. Its variability is observed on a wide-scale temporal range from minutes, or even shorter, up to scales of the solar cycle and secular variations of solar energy input. Ionosphere behavior is predominantly determined by solar and geomagnetic forcing. However, the lower-lying atmospheric regions can contribute significantly to the resulting energy budget. The energy transfer between distant atmospheric parts happens due to atmospheric waves that propagate from their source region up to ionospheric heights. Experimental observations show the importance of the involvement of the lower atmosphere in ionospheric variability studies in order to accurately capture small-scale features of the upper atmosphere. In the Part I Coupling, we provide a brief overview of the influence of the lower atmosphere on the ionosphere and summarize the current knowledge. In the Part II Coupling Evidences Within Ionospheric Plasma—Experiments in Midlatitudes, we demonstrate experimental evidence from mid-latitudes, particularly those based on observations by instruments operated by the Institute of Atmospheric Physics, Czech Academy of Sciences. The focus will mainly be on coupling by atmospheric waves.

Large Scale Travelling Ionospheric Disturbances (LSTIDs) are a frequent phenomenon during ionospheric storms, indicating strong electrodynamic processes in high latitudes. LSTIDs are signatures of Atmospheric Gravity Waves (AGW) observed in the changes of the electron density in the ionosphere. During ionospheric storms, large scale AGWs are often generated in the vicinity of the auroral region, where sudden strong heating processes take place.Many LSTIDs are observed during the ionosphere storm during the September 2017 Space Weather event. In this presentation, the LSTID occurrence on 8th September 2017 is analysed in more detail, based on a TID detection method using ground based Global Navigation Satellite System (GNSS) measurements. Fast LSTIDs are observed in midlatitudes between 0-3 UT and 13-16 UT. Slow LSTIDs are observed between 3-12 UT. A significant strong wave-like TEC perturbation occurred in high latitudes at noon, which vanished at around 50°N. A strong single LSTID in mid-latitudes generated in high latitudes around 18 UT. Consulting IMAGE magnetometer data, ionosonde measurements and Swarm field aligned current measurements, strong heating processes, the extension of the Auroral oval and unusual electrodynamic processes are discussed as source mechanisms for these LSTIDs.

We have analyzed time series of solar data together with the atmospheric and ionospheric measurements for solar cycles 19 till 23 according to particular data availability. For the analyses we have used long term data with 1-day sampling. By mean of Continuous Wavelet Transform (CWT) we have found common spectral domains within solar and atmospheric and ionospheric time series. Further we have identified terms when particular pairs of signals show high coherence applying Wavelet Transform Coherence (WTC). Despite wide oscillation ranges detected in particular time series CWT spectra we found only limited domains with high coherence by mean of WTC. Wavelet Transform Coherence reveals significant high power domains with stable phase difference for periods 1 month, 2 months, 6 months, 1 year, 2 years and 3–4 years between pairs of solar data and atmospheric and ionospheric data. The occurence of the detected domains vary significantly during particular solar cycle (SC) and from cycle to the following one. It indicates the changing solar forcing and/or atmospheric sensitivity with time.

A network of ionosondes in Europe has been established to monitor travelling ionospheric disturbances (TIDs) by simultaneously making vertical and oblique incidence HF sounding measurements. This network is the outcome of the Net-TIDE project, a collaboration between European Digisonde operators that have synchronised the sounding schedules of the Digisondes in order to record vertical and oblique ionogram traces simultaneously, and have added Digisonde-to-Digisonde (D2D) fixed frequency oblique-incidence measurements to the measurement schedule. The distances between the observatories involved in the project range from 500 km to over 2000 km. The technical feasibility of this network approach is explored. The challenge for the fixed-frequency D2D skymap measurements is the automatic selection of the sounding frequencies depending on the geometry of the sounding paths, the diurnal and seasonal ionospheric changes, and space weather induced events.

Global climate change affects the whole atmosphere, including the thermosphere and ionosphere. Calculations of long-term trends in the ionosphere are critically dependent on solar activity (solar cycle) correction of ionospheric input data. The standard technique is to establish an experimental model via calculating the dependence of ionospheric parameter on solar activity from the whole analysed data set, subtract these model data from observed data and analyse the trend of residuals. However, if the solar activity dependence changes with time, the solar correction calculated from the whole data set may result in miscalculating the ionospheric trends. To test this, data from two European ionospheric stations – Juliusruh and Slough/Chilton – which provide long-term reliable data, have been used for the period 1975–2014. The main result of this study is the finding that the solar activity correction used in calculating ionospheric long-term trends need not be stable, as was assumed in all previous investigations of ionospheric trends. During the previous solar cycle 23 and the current solar cycle 24, the solar activity correction appears to be different from that for the previous period and the Sun seems to behave in a different way than throughout the whole previous era of ionospheric measurements. In future ionospheric trend investigations the non-stability of solar activity correction has to be very seriously taken into account, because it can substantially affect calculated long-term trends of ionospheric parameters.

Different methods are used to research and monitor the ionospheric dynamics using ground measurements: Digisonde Drift Measurements (DDM) and Continuous Doppler Sounding (CDS). For the first time, we present comparison between both methods on specific examples. Both methods provide information about the vertical drift velocity component. The DDM provides more information about the drift velocity vector and detected reflection points. However, the method is limited by the relatively low time resolution. In contrast, the strength of CDS is its high time resolution. The discussed methods can be used for real-time monitoring of medium scale travelling ionospheric disturbances. We conclude that it is advantageous to use both methods simultaneously if possible. The CDS is then applied for the disturbance detection and analysis, and the DDM is applied for the reflection height control.

The ionospheric response to the solar eclipses of 20 March 2015 above the Czech Republic, 9 March 2016 above Taiwan, and 26 February 2017 above South Africa was studied. A distinct bipolar pulse was observed in ionospheric Doppler shift measurements above the Czech Republic (Central Europe) and above Taiwan (Eastern Asia). It is a local phenomenon clearly related with changes of electron density in the ionosphere induced by the passage of the Moon shadow above the measurement sites. The solar eclipse in Taiwan was rather small, with a maximum obscuration of 0.22. Yet, it obviously influenced the ionosphere on time scales above 100 min. The solar eclipse in South Africa occurred shortly before sunset and it is likely that ionospheric effects were masked by gravity waves generated by the evening solar terminator.

Ionosphere represents weakly ionized plasma that reflects both solar, geomagnetic activity and processes in the neutral atmosphere. Correlation coefficients of long time series of the critical frequencies from European stations measured by means of vertical sounding are analyzed with respect to latitudinal and longitudinal difference and surface distance of stations. Time series of critical frequencies are highly correlated reflecting the dominant solar influence. Correlation coefficients are high not only for raw data and subtracted mean courses but for fluctuations around mean as well. At the surface distance exceeding 1000 km and/or about 10° of latitudinal difference between stations, the correlation coefficient of fluctuations decrease rapidly. Such effect is less visible on the latitudinal dependence, where the correlation coefficients decrease with increasing distance with less pronounced threshold. We explain the existence of the ‘break pointʼ at 10° in longitude and/or 1000 km by the ‘localʼ influence of the neutral atmosphere and the wave activity. As a possible source of the common influence on scale 1000 km/10° we propose tropospheric systems that are known to be an important source of atmospheric waves in a broad period range. Large tropospheric mesoscale systems have typically up to 2000 km in diameter.

The ionospheric observatory Pruhonice serves to monitor the state of ionosphere using ground-based vertical sounding instrument - the Digisonde DPS-4D. Measured ionograms are automatically evaluated (scaled) and basic characteristics are derived. The ionograms and the scaled parameters are sent to the international databases. Especially during disturbed conditions the automatic scaling could give unreliable results. This paper presents simple method how to detect accidental errors in automatic scaling based on the parameters derived from ionograms and on an application of the finite impulse response filters. K e y w o r d s: ionogram, F2 layer, critical frequency, virtual height, total electron content, finite impulse response filter The Digisonde DPS4-D installed at the observatory Pruhonice (50 ◦ N, 14.5 ◦ E) is used to monitor the ionosphere with the regular interval of 15 minutes [1]. Ionograms obtained by the Digisonde are automatically scaled using the special software ARTIST which is implemented in the Digisonde and this information is used to compute profile of electron concentration [2]. Automatic scaling is based on identification of characteristic shapes of the wave reflection and the results are sent to the international databases. The data are later used for local/global mapping and forecast of the main ionospheric parameters. At times, especially during geomagnetically disturbed conditions, various problems occur despite relatively high quality of the software (eg, secondary reflection or only parts of primary reflections are identified, or presence of the sporadic E layer shields upper laying layers etc.). Manual correction of the automatically scaled ionograms is time consuming and, in general, only small amount of ionospheric data can be checked by human. The aim of our work is to facilitate the control of the quality of the automatically scaled Digisonde data by means of a preliminary identification of possibly incorrectly scaled (miscaled) ionograms. This will lead to reduction of the total number of ionograms designed for the manual correction to only ionograms with increased probability of miscalling. The ionograms identified as incorrectly scaled are manually checked and, in case of need, corrected. An example of application of the proposed method of the preliminary ionogram scaling control based on the Total Electron Content (TEC) obtained from the GNSS and the Digisonde is described in the paper [3]. The method presented in this paper is based on the using of data from the Digisonde without fusion of sensors. In this case, digital filters can be used for automatically control process of miscalled ionograms. Digital filters are described in [4] and [5]. For our purposes, we use the MA (Moving Average) filters. Filters are applied on ionospheric parameters, namely total electron content (TEC), maximum plasma frequency (critical frequency f0F2) of the F2 layer, and virtual height of the F2 layer (hF2).

Ionospheric disturbances, often associated with geomagnetic storms, may cause threats to radio systems used for communication and navigation. One example is the super storm on 20th November 2003, when plenty of strong and unusual perturbations were reported. This paper reveals additional information on the dynamics in the high latitude ionosphere over Europe during this storm. Here, analyses of wavelike Travelling Ionospheric Distrubances (TIDs) over Europe are presented, based on estimates of the Total Electron Content (TEC) derived from ground-based Global Navigation Satellite System (GNSS) measurements. These TIDs are ionospheric signatures of thermospheric surges initiated by space weather events. The source region of these TIDs is characterized by enhanced spatial gradients, TEC depression, strong uplift of the F2-layer, the vicinity of the eastward auroral electrojet and strong aurora E-layers. Joule heating is identified as the most probable driver for the TIDs observed over Europe during 20th November 2003. The sudden heating of the thermosphere leads to strong changes in the pressure and thermospheric wind circulation system, which in turn generates thermospheric wind surges observed as TID signatures in the TEC. Either the dissipation of the eastward auroral electrojet or particle precipitation are considered as the source mechanism for the Joule heating.In the course of the storm, the TEC observations show a southward shift of the source region of the TIDs. These meridional dislocation effects are obviously related to a strong compression of the plasmasphere. The presented results demonstrate the complex interaction processes in the thermosphere-ionosphere-magnetosphere system during this extreme storm.

The paper deals with comparison of two different methods for checking of automatically scaled ionograms. The first method is based on application of moving average filters on ionospheric characteristics. The second method uses complementary system for measurement of the ionospheric Total Electron Content (TEC). We show that both methods are comparable. However, the moving average filters based method gives slightly better results.

Ionosondes underwent significant improvements since their invention in 1924 (Breit and Tuve, 1926). At the beginning, ionospheric sounders measured only vertical profile of electron concentration. Contemporary modern digital ionosondes usually provide ionospheric drift measurement as a part of the routine monitoring in addition to the classical vertical ionospheric sounding (ionogram measurement). Ionospheric plasma motion monitoring has a large potential to improve our understanding of an ionospheric dynamics. However the measurement quality is highly variable. In this paper we deal with conditions and assumptions necessary for the correct drift velocity estimation, particularly differences in measurements for E and F regions. Correct and accurate drift velocity estimation requires recording of a sufficient number of reflection points during the measurement. We discuss how to obtain good quality drift data and how to estimate data quality. Large number of recorded drift measurements are eliminated from further use for drift velocity estimation due to low number of detected reflection points. Our analysis done for a large dataset strongly supports the idea that even low number of recorded reflection points may indicate an expressive horizontal stratification of the ionosphere. For our study we used drift data collected in 2006 from a mid-latitude station Pruhonice. Data were collected during a period of low geomagnetic and solar activity. We show statistical properties of drift velocity components for both E and F regions. We illustrate the influence of geomagnetic activity on drift velocities on maximal daily value of F region horizontal component (geomagnetic activity is represented by Kp). We also show statistics for poor-quality data associated with horizontal stratification of ionosphere.

Very high values of foEs had sometimes been reported in the past. These values, as well as all other ionogram-scaled values, had been derived from ionograms under the assumption of the vertical reflection of ordinary mode of sounding radio waves. In the past it was impossible to check the validity of this assumption. However, modern digisondes determine clearly the oblique or extraordinary mode reflections. To test the assumption of the vertical reflection of ordinary mode for high values of foEs, seven summers (June 2004–August 2010) from a midlatitude station Pruhonice are chosen. All hourly values of foEs≥6.0 MHz are selected from this data set, altogether 282 values. In 90% of cases the “classical” way of evaluation of foEs provides values higher than “true” values from modern digisonde (average difference about 1 MHz). 38% of “classical” foEs are oblique reflections, which however do not occur in direction perpendicular to geomagnetic field. The occurrence of high values of foEs varies very much from year to year between 0 and 130 events per year. As for diurnal variation, a pronounced maximum occurs at ∼10:00LT and 16:00LT (secondary), and a minimum after midnight.

Spread F is an ionospheric phenomenon which has been reported and analyzed extensively over equatorial regions on the basis of the Rayleigh-Taylor (R-T) instability. It has also been investigated over midlatitude regions, mostly over the Southern Hemisphere with its generation attributed to the Perkins instability mechanism. Over midlatitudes it has also been correlated with geomagnetic storms through the excitation of travelling ionospheric disturbances (TIDs) and subsequent F region uplifts. The present study deals with the occurrence rate of nighttime spread F events and their diurnal, seasonal and solar cycle variation observed over three stations in the European longitude sector namely Nicosia (geographic Lat: 35.29 °N, Long: 33.38 °E geographic: geomagnetic Lat: 29.38 °N), Athens (geographic Lat: 37.98 °N, Long: 23.73 °E geographic: geomagnetic Lat: 34.61 °N) and Pruhonice (geographic Lat: 50.05 °N, Long: 14.41 °E geographic: geomagnetic Lat: 47.7 °N) during 2009, 2015 and 2016 encompassing periods of low, medium and high solar activity, respectively. The latitudinal and longitudinal variation of spread F occurrence was examined by considering different instability triggering mechanisms and precursors which past literature identified as critical to the generation of spread F events. The main findings of this investigation is an inverse solar cycle and annual temporal dependence of the spread F occurrence rate and a different dominant spread F type between low and high European midlatitudes.

The changing state of the ionosphere is generally monitored by networks of vertical ionosondes that provide us with regular ionospheric sounding. Many ionospheric applications require determination of the true-height electron density profiles. Therefore, ionograms must be further inverted into real-height electron density profiles. The paper presents the comparison study of the true-height electron density profiles inverted from ionograms using two different methods POLAN (Titheridge, 1985) and NHPC (Huang and Reinish, 1996; Reinish et al., 2005), widely used by the ionospheric research community. Our results show significant systematic differences between electron density profiles calculated by these two inversion methods.

This paper deals with estimation of critical frequencies foF2 using the GPS measurement. The ionosphere affects the time of flight of the signals between the satellite and the Earth. It is therefore possible to use the signals from the GPS for monitoring of ionospheric characteristics. We use measurements from the digisonde DPS-4D and the dual frequency GPS receiver TOPCON at the station Pruhonice, Czech Republic (50°N, 14.5°E).

This paper summarizes the activities carried out by WP 3.1 of WG 3 of COST 296 action. The Work Package deals mostly with medium and large ionospheric structures that impacts on GNSS signals. In the research done by this European team, particular attention was given to the ionosphere/space weather monitoring, to the analysis of the variability of the ionospheric plasma during quiet and disturbed conditions and to the characterization of the behavior of low latitudes ionospheric depletions or bubbles and the spatial and temporal gradients of total electron contet.

[1] In this paper we concentrate on the plasma drift measurement performed by Digisonde Portable Sounder DPS 4 developed by the UMLCAR, Lowell. Special attention is paid to the quality control of data acquired prior to drift velocity calculations. Risks of carefree use of raw Doppler spectra for the plasma motion analysis have been recognized in the Digisonde community. In this paper we propose improvements of Digisonde drift data processing for velocity evaluation and provide a detailed step-by step description: (1) robust height range selection, (2) setting limits on the Doppler frequency shift, and (3) setting limits on the echo arrival angle. Raw data that passed our quality control were then used to derive three velocity components of the bulk plasma drift in the ionosphere. In the paper we discuss, in detail, application of the method on the DPS measurements. However, this technique can be efficiently applied on any other digital ionosondes drift data processing.

Traveling Ionospheric Disturbances (TIDs) reflect changes in the ionospheric electron density which are caused by atmospheric gravity waves. These changes in the electron density impact the functionality of different applications such as precise navigation and high-frequency geolocation. The Horizon 2020 project TechTIDE establishes a warning system for the occurrence of TIDs with the motivation to mitigate their impact on communication and navigation applications. This requires the identification of appropriate indicators for the generation of TIDs and for this purpose we investigate potential precursors for the TID occurrence. This paper presents a case study of the double main phase geomagnetic storm, starting from the night of 7th September and lasting until the end of 8th September 2017. Detrended Total Electron Content (TEC) derived from Global Navigation Satellite System (GNSS) measurements from more than 880 ground stations in Europe was used to identify the occurrence of different types of large scale traveling ionospheric disturbances (LSTIDs) propagating over the European sector. In this case study, LSTIDs were observed more frequently and with higher amplitude during periods of enhanced auroral activity, as indicated by increased electrojet index (IE) from the International Monitor for Auroral Geomagnetic Effects (IMAGE). Our investigation suggests that Joule heating due to the dissipation of Pedersen currents is the main contributor to the excitation of the observed LSTIDs. We observe that the LSTIDs are excited predominantly after strong ionospheric perturbations at high-latitudes. Ionospheric parameters including TEC gradients, the Along Arc TEC Rate (AATR) index and the Rate Of change of TEC index (ROTI) have been analysed for their suitability to serve as a precursor for LSTID occurrence in mid-latitude Europe, aiming for near real-time indication and warning of LSTID activity. The results of the presented case study suggest that the AATR index and TEC gradients are promising candidates for near real-time indication and warning of the LSTIDs occurrence in mid-latitude Europe since they have a close relation to the source mechanisms of LSTIDs during periods of increased auroral activity.