Response of the low ionosphere to X-ray and Lyman-a solar flare emissions

International audience[1] Using soft X-ray measurements from detectors onboard the Geostationary Operational Environmental Satellite (GOES) and simultaneous high-cadence Lyman-a observations from the Large Yield Radiometer (LYRA) onboard the Project for On-Board Autonomy 2 (PROBA2) ESA spacecraft, we study the response of the lower part of the ionosphere, the D region, to seven moderate to medium-size solar flares that occurred in February and March of 2010. The ionospheric disturbances are analyzed by monitoring the resulting sub-ionospheric wave propagation anomalies detected by the South America Very Low Frequency (VLF) Network (SAVNET). We find that the ionospheric disturbances, which are characterized by changes of the VLF wave phase, do not depend on the presence of Lyman-a radiation excesses during the flares. Indeed, Lyman-a excesses associated with flares do not produce measurable phase changes. Our results are in agreement with what is expected in terms of forcing of the lower ionosphere by quiescent Lyman-a emission along the solar activity cycle. Therefore, while phase changes using the VLF technique may be a good indicator of quiescent Lyman-a variations along the solar cycle, they cannot be used to scale explosive Lyman-a emission during flares

Measuring global signals in the potential gradient at high latitude sites

Previous research has shown that the study of the global electrical circuit can be relevant to climate change studies, and this can be done through measurements of the potential gradient near the surface in fair weather conditions. However, potential gradient measurements can be highly variable due to different local effects (e.g., pollution, convective processes). In order to try to minimize these effects, potential gradient measurements can be performed at remote locations where anthropogenic influences are small. In this work we present potential gradient measurements from five stations at high latitudes in the Southern and Northern Hemisphere. This is the first description of new datasets from Halley, Antarctica; and Sodankyla, Finland. The effect of the polar cap ionospheric potential can be significant at some polar stations and detailed analysis performed here demonstrates a negligible effect on the surface potential gradient at Halley and Sodankyla. New criteria for determination of fair weather conditions at snow covered sites is also reported, demonstrating that wind speeds as low as 3m/s can loft snow particles, and that the fetch of the measurement site is an important factor in determining this threshold wind speed. Daily and seasonal analysis of the potential gradient in fair weather conditions shows great agreement with the “universal” Carnegie curve of the global electric circuit, particularly at Halley. This demonstrates that high latitude sites, at which the magnetic and solar influences are often present, can also provide globally representative measurement sites for study of the global electric circuit

Contribuição ao estudo de distúrbios ionosféricos utilizando a técnica de VLF

O sistema Terra-baixa ionosfera se comporta como um guia de onda para a propagação de ondas de rádio de frequências muito baixa (VLF). Se neste sistema a condutividade elétrica das fronteiras é perturbada, a propagação da onda é também perturbada. Existe uma variedade de fenômenos físicos transientes que alteram significativamente a condutividade elétrica da baixa ionosfera. Essas alterações são observadas como variações da fase e/ou amplitude com respeito ao nível quiescente. O presente trabalho tem como finalidade estudar a resposta da baixa ionosfera a fenômenos que produzidos na Terra, no sistema solar e até aqueles produzidos muito além do sistema solar. Com esse fim foram utilizados dados de VLF de fase e de amplitude fornecidos pela rede SAVNET (South America VLF NETwork) para o ciclo solar 24. Foi encontrado que a correção pelo fator de distância iluminada e o coeficiente de altura de referência permitem normalizar o efeito do distúrbio ionosférico a partir do sinal de VLF propagado em trajetos com direção de propagação norte-sul ou oeste-leste. O limiar de detecção das perturbações causadas pelo excesso na incidência dos raios-X é 1,8×10−9 Jm-2 para a ionosfera noturna e 2,6×10−7 Jm-2 para a ionosfera diurna. Perturbações ionosféricas observadas como alterações nos períodos do sinal de VLF, na faixa de infrassom, foram observadas entre 6 e 14 dias antes dos eventos sísmicos de magnitude 7 acontecidos no Haiti no ano 2010 e no Peru no ano 2011. Alterações nas periodicidades, da ordem de dezenas de minutos, foram observadas quando a sombra do eclipse solar total de 2010 se deslocava sobre a Terra. Devido ao eclipse, a altura de referência da ionosfera aumentou em ~3 km e a densidade eletrônica diminuiu em 60% com respeito do nível quiescente. Finalmente, foi encontrado que o coeficiente de recombinação efetiva, para o tempo do eclipse e para uma altura de 80 km, foi de 1,1×10−5 cm-3s-1, que é um valor intermediário entre as condições diurnas e noturnas.The Earth-Low ionosphere system behaves as a waveguide for the propagation of radio waves of very low frequency (VLF). If in this system the electrical conductivity of its boundaries is perturbed, the propagation of the VLF waves will also be perturbed. There is a diversity of transient physical phenomena that are able to alter significantly the electrical conductivity of the lower ionosphere. The disturbance in this region is able to produce phase and amplitude variations with respect to a quiescent level of these waves. The aim of the present work is to study the response of the lower ionosphere to phenomena originated in the Earth, our solar system or even much farther away. For this purpose, VLF data obtained by SAVNET (South American VLF Network) during the solar cycle 24 was used. It was found that the correction by both the length of the path illuminated by the flare and the reference height coefficient allows normalizing the effect of ionospheric disturbances observed in the VLF phase signals that propagated along trajectories with a north-south or west-east direction, separately. The lower limit of detection for disturbances caused by the X-ray radiation excess is 1.8×10−9 Jm-2 and 2.6×10−7 Jm-2 for the nighttime and daytime lower ionosphere, respectively. Changes in the periodicities of the VLF signal, in the infrasonic band, were observed between 6 and 14 days prior to the seismic events, of magnitude 7, occurred in Haiti in 2010 and in Peru in 2011. Increases in the periodicities of the order of few minutes were observed when the shadow of the total solar eclipse of 2010 was moving on the Earth. Due to the solar eclipse the ionospheric reference height increased in ~3 km and the electron density decreased in 60 % of its quiescent level. Finally, it was found that the effective recombination coefficient, for 80 km height, was 1.1×10−5 cm-3s-1 during the time of the eclipse, which is an intermediate value between the diurnal and nocturnal conditions.Conselho Nacional de Desenvolvimento Científico e Tecnológic

New contributions on VLF radio wave perturbations measured at high-latitudes

Abstract The Earth-ionosphere system behaves as a waveguide for the propagation of very low frequency (VLF) radio waves. If in this system the electrical conductivity of its boundaries is disturbed, the propagation of VLF waves is also disrupted, which is observed as phase and amplitude variations of VLF waves with respect to their quiescent levels. There is a diversity of physical phenomena that are able to alter significantly the conductivity of the upper boundary. These phenomena can have their origin at the Earth (e.g., lightning), in the solar system (e.g., solar flares) or even much farther away (e.g., galactic gamma-ray bursts). The aim of this thesis is to study short- and long-term VLF variations measured in Northern Finland (at the Sodankylä Geophysical Observatory) and their associations to different phenomena. The main results are as follows: [i] The minimum energy a solar flare should have in order to produce ionospheric disturbances depends on the solar cycle. This energy is understood as the ionospheric sensitivity and for daytime conditions its value lies in the range (1–12) × 10⁻⁷ J/m². [ii] The semiannual oscillation that appears in VLF measurements was determined to be related to geomagnetic activity variations. At the same time, it was found that the 27-day solar rotation oscillation is dominant during the declining phase of the solar cycle. [iii] The main characteristics of the observed VLF sunrise phase perturbation are derived from the shadowing of short wavelength solar UV radiation due to stratospheric ozone absorption when the Sun rises. [iv] VLF emissions with banded structure were observed in the 16–39 kHz frequency range, which are frequencies not usually used for the study of whistler mode VLF emissions coming from the magnetosphere. All these results are important since the VLF signals are related to variations of electron density in the ionospheric D-region, and thus have been used to identify the processes that influence the behavior of the upper atmosphere. The mentioned results can provide useful constraints on the long-term and short-term variability in coupled ion-neutral atmospheric models, thereby adding to our understanding of the response of the chemistry, dynamics and electrodynamics of the Earth’s ionosphere to solar and atmospheric forcing.Tiivistelmä Maa-ionosfäärijärjestelmä toimii aaltoputkena erittäin matalataajuisille (VLF) radioaalloille. Jos tässä järjestelmässä sen rajojen sähkönjohtavuus häiriintyy, myös VLF-aaltojen eteneminen häiriintyy, mikä havaitaan VLF-aaltojen vaihe- ja amplitudivaihteluina suhteessa niiden tasoon hiljaisina aikoina. On olemassa useita erilaisia fysikaalisia ilmiöitä, jotka pystyvät muuttamaan merkittävästi ylärajan johtavuutta. Nämä ilmiöt voivat olla peräisin maapallolta (esim. planetaariset aallot), aurinkokunnastamme (esim. auringon roihupurkaukset) tai jopa paljon kauempaa (esim. galaktiset gammapurkaukset). Tässä väitöskirjassa tutkijaan Pohjois-Suomessa (Sodankylän geofysiikan observatoriossa) mitattuja lyhyen ja pitkän aikavälin VLF-vaihteluita ja niiden yhteyksiä eri ilmiöihin. Tärkeimmät tulokset ovat seuraavat: [i] määritettiin, miten ionosfäärin päiväajan herkkyys ulkoisille häiriötekijöille riippuu auringon aktiivisuuden vaiheesta. Tämä tutkimus ymmärretään vähimmäisenergian suhteen, joka ulkoisella tapahtumalla, kuten auringon roihupurkauksella, tulisi olla, jotta se aiheuttaisi signaalin leviämiseen vaikuttavia ionosfäärisiä häiriöitä. [ii] VLF-mittauksissa esiintyvän vuotuisen vaihtelun havaittiin liittyvän mesofäärin lämpötilaan ja auringon Lyman-α-vuohon päivä- ja yöolosuhteissa. Puolivuosittaisen vaihtelun havaittiin liittyvän geomagneettisen aktiivisuuden muutoksiin. Samalla todettiin, että auringon 27 päivän pyörimisjaksoa vastaava värähtely on hallitseva auringon aktiivisuuden laskuvaiheessa. [iii] Auringonnousuvaiheen VLF-häiriöiden pääominaisuuksien havaittiin johtuvan lyhyen aallonpituuden auringon UV-vuon varjostumisesta D-alueen ionosfäärissä johtuen stratosfäärin otsonin absorptiosta auringon noustessa. [iv] VLF-aaltoja, joissa oli raitamainen rakenne, havaittiin taajuusalueella 16–39 kHz. Näitä taajuuksia ei yleensä käytetä magnetosfääristä tulevien vihellysmoodin VLF-aaltojen tutkimiseen. Tuloksemme ovat tärkeitä, koska VLF-signaalit liittyvät elektronitiheyden muutoksiin ionosfäärin D-alueella, ja siten niitä on käytetty tunnistamaan prosessit, jotka vaikuttavat ylemmän ilmakehän käyttäytymiseen. Mainitut tulokset voivat tarjota hyödyllisiä rajoja pitkä- ja lyhytaikaiseen vaihteluun ilmakehän yhdistettyissä ionineutraalimalleissa, mikä lisää ymmärrystämme Maan ionosfäärin kemiallisesta, dynaamisesta ja sähködynaamisesta vasteesta auringon ja ilmakehän pakotukseen

Periodicities in fair weather potential gradient data from multiple stations at different latitudes

Analysis of the variation of the potential gradient (PG) at ground level is important to monitor the global electric circuit and the different solar and geophysical phenomena affecting it. However, this is challenging since several local factors (e.g., meteorological) produce perturbations in the potential gradient. Time series and spectral analysis of PG at several stations can help to minimise local effects so that global effects may be more clearly observed. In this work, for the first time we performed spectral analysis of the potential gradient recorded at several sites located at Vostok, Concordia, Halley and Casleo (Southern Hemisphere), and Sodankyla and Reading (Northern Hemisphere). In order to find the main periodicities and how the amplitude of those periods change as a function of time we use the Lomb-Scargle periodogram and the wavelet transform, respectively. For all PG sites we found periodicities of 0.5-, 1-, ~180- and 365-day. Our results show that the 0.5-day (1-day) periodicity is more prominent during the months of June-July-August (December-January-February). Evidence of ~27- and ~45-day periods was also observed at multiple sites. Further analysis using the cross-wavelet transform for PG versus cosmic rays, PG versus Madden-Julian Oscillation index, and PG versus meteorological parameters, show clues that the 27- and 45-day periods are likely related to the solar rotation and Madden-Julian Oscillation, respectively. Furthermore, our results show that during the passages of co-rotating interaction regions, the 27-day period for PG vs cosmic rays XWT is stronger than for the other XWT analysis

Atmospheric electric field variations and lower ionosphere disturbance during the total solar eclipse of 2010 July 11

In this paper, we study the variations of atmospheric electric field during the total solar eclipse (TSE) of July 11, 2010, at Complejo Astronómico El Leoncito (CASLEO). These variations observed with two identical sensors separated by 0.4 km, show a significant increase (∼55 V/m) when compared with averaged values measured during previous and subsequent fair weather days. Furthermore, identical changes are detected on the measured phases of Very Low Frequency waves received at CASLEO. The latter suggests a possible connection between the lower ionosphere and the lower atmosphere during the period of the eclipse.Fil: Tacza, José C.. Universidade Presbiteriana Mackenzie; BrasilFil: Raulin, Jean Pierre. Universidade Presbiteriana Mackenzie; BrasilFil: Macotela, Edith L.. Universidade Presbiteriana Mackenzie; BrasilFil: Norabuena, Edmundo O.. Instituto Geofísico del Perú; PerúFil: Fernandez, German Enzo Leonel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Complejo Astronómico "el Leoncito". Casleo Sede Observatorio | Universidad Nacional de San Juan. Complejo Astronómico "el Leoncito". Casleo Sede Observatorio | Universidad Nacional de la Plata. Complejo Astronómico "el Leoncito". Casleo Sede Observatorio | Universidad Nacional de Córdoba. Complejo Astronómico "el Leoncito". Casleo Sede Observatorio; Argentin

Analysis of long-term potential gradient variations measured in the Argentinian Andes

Continuous measurements of the potential gradient are being recorded at the CASLEO astronomical observatory near to the Andes Mountain of Argentina, since 2010 (Latitude: 31°47.88′S, Longitude: 69°17.7′W, Altitude: 2552 masl). This study aims to use the potential gradient diurnal variation in fair weather conditions, which we named ‘standard curve’, to examine its correlation with the ‘universal’ Carnegie curve and to investigate its dependence with aerosol amount and lightning occurrences. In addition, a spectral analysis was performed to our data. The aerosol optical depth is recorded by an AERONET station at CASLEO, while the lightning occurrences was obtained from the STARNET network. For the analysis, the average of monthly, seasonal and annual electric field curves was determined. We found that the shape of these curves is preserved from year to year indicating their high reliability. The correlation between the standard curve for CASLEO and the Carnegie curve was found to be high (r = 0.94). However, some significant local effects were also found. We determined that these local effects in the standard curve may be associated to the convective process, which is more predominant during summertime than wintertime. The inspection of the seasonal variation of the potential gradient and the lightning occurrences showed a high similarity only after removing the effects of aerosols. Apart from this, the spectral analysis exposed a daily, annual and 165-day oscillation in the potential gradient.Fil: Tacza, Jose. Universidade Presbiteriana Mackenzie; BrasilFil: Raulin, Jeann Pierre. Universidade Presbiteriana Mackenzie; BrasilFil: Morales, Carlos. Universidade de Sao Paulo; BrasilFil: Macotela, Edith Liliana. University of Oulu; FinlandiaFil: Marun, Adolfo Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; ArgentinaFil: Fernandez, German Enzo Leonel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Complejo Astronómico "El Leoncito". Universidad Nacional de Córdoba. Complejo Astronómico "El Leoncito". Universidad Nacional de la Plata. Complejo Astronómico "El Leoncito". Universidad Nacional de San Juan. Complejo Astronómico "El Leoncito"; Argentin

VLF emissions with banded structure in the 16‐ to 39‐kHz frequency range measured by a high‐latitude ground‐based receiver

Abstract Very low frequency (VLF) emissions of natural origin were identified for the first time by analyzing 1‐hr ground‐based magnetic field spectrograms in the 0.2‐ to 39‐kHz frequency range. Data were used from the Kannuslehto radio receiver (L‐shell ~5.5), recorded during different campaigns between 2006 and 2019. The spectrograms exhibit banded structures, which consist of several strip elements that vary in time and frequency over the event duration. Statistical analysis of 95 events shows that they are observed in the frequency range that extends from 2 to ~37 kHz, and mainly appearing above 16 kHz. The events span from 4 to 110 min and occur in the evening sector (~17–01 magnetic local time), mostly during quiet geomagnetic conditions. Furthermore, they are primarily left‐handed polarized and are associated with bursts of lightning‐related radio emissions such as sferics and tweeks

Lower ionosphere sensitivity to solar X‐ray flares over a complete solar cycle evaluated from VLF signal measurements

Abstract The daytime lower ionosphere behaves as a solar X‐ray flare detector, which can be monitored using very low frequency (VLF) radio waves that propagate inside the Earth‐ionosphere waveguide. In this paper, we infer the lower ionosphere sensitivity variation over a complete solar cycle by using the minimum X‐ray fluence (FXmin) necessary to produce a disturbance of the quiescent ionospheric conductivity. FXmin is the photon energy flux integrated over the time interval from the start of a solar X‐ray flare to the beginning of the ionospheric disturbance recorded as amplitude deviation of the VLF signal. FXmin is computed for ionospheric disturbances that occurred in the time interval of December–January from 2007 to 2016 (solar cycle 24). The computation of FXmin uses the X‐ray flux in the wavelength band below 0.2 nm and the amplitude of VLF signals transmitted from France (HWU), Turkey (TBB), and U.S. (NAA), which were recorded in Brazil, Finland, and Peru. The main result of this study is that the long‐term variation of FXmin is correlated with the level of solar activity, having FXmin values in the range (1 − 12) × 10−7 J/m2. Our result suggests that FXmin is anticorrelated with the lower ionosphere sensitivity, confirming that the long‐term variation of the ionospheric sensitivity is anticorrelated with the level of solar activity. This result is important to identify the minimum X‐ray fluence that an external source of ionization must overcome in order to produce a measurable ionospheric disturbance during daytime