Session CD4 - Recent advances in VLF observations of the ionosphere during space weather events

Carine Briand, onsite (LESIA, Observatoire de Paris-PSL), Mark Clilverd (British Antarctic Survey), Peter Gallagher (Dublin Institute for Advanced Studies)

Solar eruptions are the source of significant disturbances of the ionosphere through the rapid increase in energetic radiation, particularly the X-ray flux. Radio wave propagation effects and increases in D- and E-region electron density are the main perturbations generated by these events. From the space weather point of view, HF blackouts over extended regions are the major risks linked to solar eruptions. Being too high for balloons and too low for in situ measurements with spacecraft, the lower ionospheric D-layer, a key region of absorption of the HF emission, is mostly surveyed by VLF measurements. Networks of VLF receivers have been deployed by various groups, including the polar regions (AARDDVARK), Latin and South America (LATNET & SAVNET) or spread over the world (SUPERSID & AWESOME). VLF measurements have also proved to be a powerful tool to detect transients linked to thunderstorm regions (Elves, Sprites, and Terrestrial Gamma-ray Flashes). The interest in such events was recently raised for civil aviation since they can be the source of intense ionizing radiations. This session aims to present several aspects of recent achievements for solar flares or thunderstorm luminous events, from instrumentation to observations, and also modelling. We will also discuss the buildup of an international organization to better share and analyze the VLF data during specific space weather events.

Poster Viewing
Thursday October 27, 08:30 - 13:30, Poster Area

Talks
Thursday October 27, 16:30 - 17:45, Earth Hall

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Talks : Time schedule

Thursday October 27, 16:30 - 17:45, Earth Hall

16:30	The North American VLF array and polarization observations during space weather events	Moore, R et al.	Oral
	Robert Moore [1], Joshu Covey [1]
	[1]University of Florida
	Very low frequency (VLF) transmitter signals can propagate large distances within the Earth-ionosphere waveguide and provide a means to remote-sense the properties of the lower ionosphere. By comparing the VLF amplitude, phase, and polarization recorded at multiple VLF radio receiver locations, we are able to "image" the effective VLF reflection heights that characterize the ionosphere as a function of space and time. Ionospheric imaging provides great insight into the physical nature of the events being investigated. Here we present observations made using the newly-deployed North American VLF array and present the results of a physics-based ionospheric imaging technique. As time permits, we will present imaging results for lightning induced electron precipitation (LEP) events, solar X-ray flares, and day-night transition periods. Considerations for using lightning-radiated radio atmospherics as well as VLF transmitter signals will be discussed.
16:45	Improvements on the GIFDS‘ VLF network and its use for detecting solar flares	Hansen, M et al.	Oral
	M. Hansen[1], D. Banyś[1], D. Wenzel[1], L. Heinrich[1], M. Hoque[1]
	[1]Institute for Solar-Terrestrial Physics, German Aerospace Center (DLR), Germany.
	The burst of X-ray radiation during a solar flare leads to an enhanced ionization in the D-region of the ionosphere. The Global Ionospheric Flare Detection System (GIFDS) of the German Aerospace Center (DLR) provides continuous amplitude and phase measurements of VLF signals reflecting these ionospheric changes. To ensure daytime observations around the clock in the mid-latitudes, a network of receivers is deployed around the globe. The mid-latitude range is especially suitable for flare detection, as there are less other regional disturbances. To master the transition from a measured survey to a permanent warning system, the GIFDS instruments recently underwent a major technical upgrade resulting in the compact GIFDS receiver. The quiet day curve for VLF amplitudes changes over the course of a year, so same class flares can yield different pictures depending on the time of occurrence. One drastic change in the amplitude level is the so-called “October effect” – a persistent sharp decrease during fall at mid and high latitudes. The ongoing project “Analysis of the MEsosphere and Lower Ionosphere fall Effect” (AMELIE) investigates the influence of the seasonal ionospheric variations on the signal propagation for increasing the observability of solar events under varying conditions. This will improve the flare detection and size estimation.
17:00	Probing geo-storm driven ionospheric irregularities in the upper and lower ionosphere	Nwankwo, V et al.	Oral
	Victor U. J. Nwankwo[1], William Denig[2], Sandip K. Chakrabarti[3], Olanike Akinola[4], Olugbenga Ogunmodimu[5]
	[1]Space, Atmospheric Physics and Radio Wave Propagation Laboratory, Anchor University, Lagos, Nigeria, [2]St. Joseph College of Maine, Standish, ME 04084, U.S.A, [3]Indian Centre for Space Physics, Kolkata-700084, India, [4]Centre for Atmospheric Research, NARSDA, Anyigba, Nigeria, [5]Department of Electrical Engineering, University of Bolton, Bolton, UK
	We combined simultaneously observed diagnostics of the lower ionosphere (D-region) with high frequency (HF) radio pulses (ionosonde) in the E and F regions, and GNSS observations to investigate geomagnetic storm-induced ionospheric irregularities propagating from the upper to the lower ionosphere in the magnetosphere-ionosphere (I-T) system. We probed the D region using characterized metrics of very low frequency (VLF) signals (obtained from GQD-A118 and DHO-A118 propagation paths) such as the mean amplitude before sunrise (MBSR), daytime mean amplitude (DTMA) and mean amplitude after sunset (MASS). We analyzed trends in the variation of signal metrics during storms, to attribute the deviations in the signal amplitudes that were attributable to the storms. Relative to pre-storm day levels, the storm-day signal strength showed significant decrease in the DTMA, MBSR and MASS with characteristic strong responses of DTMA especially in DHO-A118 (Germany-France) propagation path. Analysis of the virtual heights (h’E, h’F1 and h’F2) and critical frequencies (foE, foF1, and foF2) from ionosondes located near the transmitter sites showed variations and/or fluctuations in the F-region parameters above the transmitters in association with the geomagnetic storms, with the largest increases in the virtual heights observed near the DHO transmitter. Measurements of the total election content (TEC) obtained from multiple stations near the transmitter and receiver sites also showed larger enhancements of electron density near the DHO transmitter. These results are consistent with the findings in previous investigation, and demonstrates the spaciotemporal variability of the I-T system, which produces local asymmetries in ionospheric regions.
17:15	Lower-ionosphere Electron Density from Multi-instrument Satellite Observations and Ground VLF Measurements during Solar Flares	Zigman , V et al.	Oral
	Vida Žigman [1], Marie Dominique [2], Davorka Grubor [3], Craig J. Rodger [4], Mark A. Clilverd [5]
	[1] University of Nova Gorica, Nova Gorica, Slovenia, [2] Royal Observatory of Belgium, Brussels, Belgium, [3] University of Belgrade, Faculty of Mining and Geology, Phys. Cathedra, Belgrade, Serbia, [4] Department of Physics, University of Otago, Dunedin, New Zealand, [5] British Antarctic Survey (UKRI-NERC) Cambridge, UK *
	We present a new model for predicting the electron density height-time profile of the lower ionosphere (55-100 km) during solar flares, throughout the flare development from beginning to end. The basic input data to the model are space-borne solar irradiance measurements in coincidence with ground recorded man-made Very Low Frequency (VLF), (< 30 kHz) signals. The observed time delay between the VLF amplitude/phase maximal perturbation and the solar flare irradiance maximum, gaining renewed interest recently, has proven to be a key quantity describing the ionospheric response to the flare forcing. The proposed model is based on the continuity equation coupled to the Appleton relation by the time delay parameter. In order to determine the effectiveness of the particular wavelength domain (X-ray and EUV range) in producing ionization changes in the lower ionosphere (D-region included), we use the irradiance measured by broad-band radiometers on board the satellites GOES, SDO, and PROBA2 over succeeding and partly overlapping wavelength intervals of the instrument bandpasses, altogether covering the wavelengths 0.1-20 nm. Simultaneous ground-based measurements are taken of the amplitude and phase of VLF signals propagating along great circle paths, utilizing four transmitters and two receiving sites. The solar spectral irradiance as modelled by the XUV Photometer System Level 4 Algorithm (Woods et al. 2008), is used to obtain a realistic estimate of the ionization efficiency. The effective recombination coefficient is deduced from the model itself. Both quantities are evaluated separately for each flare and for each instrument bandpass. The model output predicts the electron density time-height profile in the range 55 -100 km. The analysis of M to X class flares shows that the flare-enhanced electron densities due to a particular ionizing wavelength domain are in reasonable agreement in both cases where: (1) the irradiances are taken over the bandpass of either GOES (0.1-0.8 nm) or SDO/ESP (0.1-7 nm) when the range of heights up to 90 km is considered; (2) either SDO/ESP or PROBA2/LYRA (1-2 +6 -20 nm) irradiances are taken for heights above 90 km. The results agree within 22% in the height range up to 90 km, while differ by at most a factor of 2 above 90 km. Remarkable agreement is shown between measured and evaluated time delays, discrepancies generally amounting to less than 8%.
17:30	Numerical modeling of the solar flare impact on lower ionosphere as monitored by VLF propagation effects	Basak, T et al.	Oral
	Tamal Basak
	Amity University Kolkata, India
	Propagation of Very Low Frequency (VLF) radio signal through the earth-ionosphere wave-guide strongly depends on the plasma properties of the ionospheric D-region. The D-region characteristics gets affected due to energetic perturbations coming primarily from the sun. The soft solar x-ray perturbs the D-region especially during solar flares. Such solar energetic events can be investigated remotely by systematic monitoring and analysis of VLF signal. We numerically model the VLF perturbation due to the effects of solar flares on D-region. We present the outcomes of the numerical model as the D-region electron density perturbations during C, M and X classes of solar flares.

Posters

1	Filtering the useful flaring information from VLF signal	Guerrero, A et al.	Poster
	Sergio Núñez, Antonio Guerrero, Consuelo Cid
	Universidad de Alcalá, Space Weather group, Spain
	The VLF antenna of the University of Alcala has been recording signals since 2017. The information from this type of instrument has been extensively demonstrated to be of great interest for space weather purposes as they monitor the flaring activity of the Sun from very cheap ground instrumentation. Nevertheless, the raw signal that comes from the instrument is not useful as it is but needs the interpretation of the scientist and/or forecaster to add the useful value. In this work we present some preliminary filtering techniques develop for the use on automatic software to extract the useful information from the instrument.
2	AWESOME@Nancay: performances and first results	Briand, C et al.	Poster
	Carine Briand[1], Morris Cohen[2], Kevin Whitmore[2], Sangitiana F. RAKOTOZAFY HARISON [3]
	[1]LESIA/Obs. De Paris-PSL, CNRS, Sorbonne Univ.,Univ. Paris-Cité (France); [2]T Georgia Institute of Technology (USA); [3]Observatoire de Radioastronomie de Nançay (France)
	A SUPERSID receiver has been running for five years at the Meudon Observatory (France). We upgraded our observing capabilities with an AWESOME receiver implemented in Nançay (France) in June 2022. AWESOME can indeed observe at a much higher frequency rate, provides the phase variations and keeps the waveforms together with the spectra. These new capabilities enable us to follow the disturbances of the ionospheric D-layer arising from solar flares and transient events during Earth's stormy periods (lightning but also sprites, elves and TGF). During this presentation, we will present the performance of the instrument and the first observations of solar flares and lightning-induced disturbances.
3	VLF4IONS: a projet of VLF receivers around the equatorial region	Briand, C et al.	Poster
	Carine Briand[1], Germain Pham[2], Baptiste Cecconi[1], Sébastien Celestin[3], Mark Clilverd,[4] Morris Cohen[5], Kevin Whitmore[5]
	[1] LESIA/Obs. De Paris-PSL, CNRS, Sorbonne Univ.,Univ. Paris-Cité (France), [2] TelecomParisTech (France), [3] LPC2E, CNRS, Univ. d’Orléans (France), [4] BAS, UK, [5] Georgia Institute of Technology, USA
	Solar flares and lightning are two main sources of disturbances of the ionospheric D-layer. Solar flares produce the most significant increases in electron density of all transient events. Lightning can induce electron precipitation in the D-layer, either by direct heating of the lower ionosphere or through the emission of waves that couple with the electrons of the radiation belts. VLF4IONS is a project that aims to study the disturbances in the D-layer electron density during transient phenomena of solar and terrestrial origin. It has a fundamental societal objective: providing real-time alerts on communication degradations for civil aviation. To reach these goals, we will deploy a network of dedicated VLF/LF instrumentation completed by innovative automatic detection of faint amplitude signals, based on recent advances in Artificial Intelligence. Several professional VLF networks have been built: the AARDDVARK network (Clilverd et al. 2009) covering the polar zones, the SAVNET (Raulin et al. 2009) and LATNET (Borgazzi et al. 2014) networks, which cover South and Latin America respectively, the AVON (Asia VLF Observation Network), and AWESOME network (Cohen et al. 2010) with no privileged location. VLF4IONS network aims to cover the equatorial regions, taking advantage of some French overseas territories to monitor the Sun 24/7 and study the effects of stormy areas in the tropical regions. We will detail the project and give the timeline of deployment.
4	Detection of Solar Flares from the Analysis of Signal-To-Noise Ratio Records from the Ebro Observatory	Segarra, A et al.	Poster
	Antoni Segarra, Victor de Paula, David Altadill, Juan José Curto, Estefania Blanch
	Observatori de l’Ebre (OE), CSIC - Universitat Ramon Llull, Roquetes, Spain
	In this work, we propose a new indirect method of detection of solar flares via Ionospheric sounder data analysis. It is based on the analysis of the recorded signal-to-noise ratio (SNR) obtained from the Ebro Observatory ionospheric station (DPS4D, EB040). This method allows to detect and characterize the ionospheric absorption of High Frequency radio waves as a product of these energetic events, and provides observational data to the international Service of Rapid Magnetic Variations (SRMV) to confirm Sfe (Solar Flare Effects). The detection methodology is based on the estimation of the quiet conditions SNR pattern of the month, and then the comparison of this pattern with the current SNR for the analyzed day. To set up the method, we considered a data set containing the solar flares that occurred during 2011—2014 at daylight hours at EB040 (262 events: 17 X-class, 124 M-class, and 121 C-class). This led to impose a minimum threshold of -20dB in the absorption for at least four consecutive frequencies to confirm that a solar flare happened. The method is particularly sensitive to X-class solar flares detection, performs quite well with M-class flares and even detects some C-class flares with high solar altitude angles. Furthermore, we studied in depth some observational and energetic constraints from the analysis of GOES 15 hard X-Ray flux data about the considered flares. For each event, we computed the solar altitude angle at the time of the ionospheric sounding to get an estimation of the geoeffective irradiance which has an effect on the local ionosphere. According to these constraints, the methodology is more effective with flares that present a solar elevation angle higher than 18.94º, geoeffective hard X-Ray irradiance above of 2.47·10-6 W/m2, and a geoeffective hard X-Ray exposure of 1.43·10-3 J/m2, computed during the 5 minutes preceding the ionospheric sounding.
5	Study of the response of the lower ionosphere to solar-induced X-Ray using VLF data from A118 (France) and Anchor University Space Lab (Nigeria) receivers	Ovie, O et al.	Poster
	Oghenenyovwe Ovie[1],[2], Victor U. J. Nwankwo[2], Michael Olatunji[1],[2], Omodara E. Obisesan[2], Oluwaseun V. Fatoye[2]
	[1]Department of Physics, University of Medical Sciences, Ondo, Nigeria, [2]Space, Atmospheric Physics and Radio Waves Propagation Laboratory, Anchor University, Lagos, Nigeria
	The effect of solar flares on the propagation of sub-ionospheric VLF signals was investigated using very low frequency (VLF) radio waves from DHO (Germany) and HWU (France) transmitters, received at A118 (Southern France) and Anchor University Lagos (AUL) stations, respectively, between January 2019 and October 2021. Our goal was to probe changes in the ionospheric D regions that are associated with solar flare-induced X-ray flux during this interval. Prompt enhancement of signal amplitude was observed in both propagation paths (DHO-A118 and HWU-AUL), as a result of increased electron density of the D-region resulting from extra ionization. A total of 2-X, 18-M and 290-C class flares were recorded during the interval. The result of our analysis revealed that C class flares occurred most while X class was the least (most of which occurred in September 2021). The study showed that in addition to the size of the solar flare, the noise level of the transmission path, the state of the ionosphere before and during the perturbation, and the time of occurrence also determine the detectability of the perturbation on the received VLF signal.
6	Analysis of VLF disturbances using spectral methods and information entropy and perspectives of multiparameter ULF, VLF and HF Space Weather monitoring	Rapoport, Y et al.	Poster
	Yuriy Rapoport[1], Volodymyr Reshetnyk[2], Asen Grytsai[3], Volodymyr Grimalsky[4], Oleksandr Liashchuk[5], Alla Fedorenko[6], Masashi Hayakawa[7], Andrzej Krankowski[8], Leszek Błaszkiewicz[9], Sergiy Petrishchevskii[10], Paweł Flisek[11], Oleh Ivantyshyn[12]
	[1]Space Radio-Diagnostic Research Centre, University of Warmia and Mazury (UWM) in Olsztyn, Poland; Physics Faculty, Taras Shevchenko National University of Kyiv, Ukraine; Main Center of Special Monitoring (MCSM), National Space Facility Control and Tests Center, State Space Agency of Ukraine; [2]Physics Faculty, Taras Shevchenko National University of Kyiv, Ukraine; [3]Physics Faculty, Taras Shevchenko National University of Kyiv, Ukraine; [4]Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Mexico; [5]Main Center of Special Monitoring (MCSM), National Space Facility Control and Tests Center, State Space Agency of Ukraine; [6]Space Research Institute National Academy of Sciences of Ukraine and State Space Agency of Ukraine; [7]Hayakawa Institute of Seismo Electromagnetics Co. Ltd.(Hi-SEM), Japan; [8]Space Radio-Diagnostic Research Centre, University of Warmia and Mazury (UWM) in Olsztyn, Poland; [9]Space Radio-Diagnostic Research
	Data of 2014–2017 time range from the network of eight monitoring Japan VLF stations receiving signals of the JJI transmitter on Kyushu Island are analyzed. The applied spectral analysis includes Fourier and wavelet transforms which indicate periods of the observed disturbances. Nighttime data were processed by wavelet transform with a preliminary detrending to exclude influence of daily variations. We propose a theoretical interpretation of the observed variations, describing ultra-low frequency (ULF) modulation of VLF EMW spectra in waveguide Earth-Ionosphere (WGEI). Modulating oscillations with periods of 4 minutes belong to acoustic branch of acoustic-gravity waves (AGW) in opened Earth–Thermosphere waveguide; modulation of VLF with periods of 6–7 minutes correspond to global evanescent/reactive Brunt–Väisälä AGW oscillations; the oscillations with periods 20–60 min and ~3 hours may characterize evanescent/reactive Lamb gravity wave mode of AGW. Solar terminator passage can be responsible for exciting of the part of these disturbances; different lithospheric and atmospheric processes (including tropical cyclones and earthquakes) and magnetosphere influence can be other possible reasons. We also calculate the information entropy to identify main details in daily VLF variations and influence of solar flares. It is shown that the information entropy increases near sunrise and sunset with seasonal variation, and that solar flares also lead to the growth in information entropy. We proposed the models of: VLF EMW propagation in the opened WGEI accounting for boundary conditions of radiation into the upper ionosphere/magnetosphere, compatible with the causality principle; penetration of ULF through the Lithosphere (Earth)-Atmosphere-Ionosphere-Magnetosphere (LEAIM) system with given sources and limiting pass from dynamic to quasistatic modelling including quasimagnetostatic field; penetration to the ionospheric altitude of the multifrequency nonlinear packet of ULF AGW with random phases, excited by the strong ground acoustic source with ULF modulation. Our experimental and theoretical approaches and facilities, accounting for present VLF equipment, UWM radio telescopes as a part of the European LOFAR (Low-Frequency Array) network, and URAN KPMI radio telescope in Ukraine, and the perspectives of the developing additional VLF and ULF measurement equipment in MCSM pave a way for the multiparameter ULF/infrasound, VLF and HF Space Weather monitoring.