Very Low Frequency Measurements Research Articles

Examining the Capability of the VLF Technique for Nowcasting Solar Flares Based on Ground Measurements in Antarctica

Measurements of Very-Low-Frequency (VLF) transmitter signals have been widely used to investigate the effects of various space weather events on the D-region ionosphere, including nowcasting solar flares. Previous studies have established a method to nowcast solar flares using VLF measurements, but only using measurements from dayside propagation paths, and there remains limited focus on day–night mixed paths, which are important for method applicability. Between March and May of 2022, the Sun erupted a total of 56 M-class and 6 X-class solar flares, all of which were well captured by our VLF receiver in Antarctica. Using these VLF measurements, we reexamine the capability of the VLF technique to nowcast solar flares by including day–night mixed propagation paths and expanding the path coverage in longitude compared to that in previous studies. The amplitude and phase maximum changes are generally positively correlated with X-ray fluxes, whereas the time delay is negatively correlated. The curve-fitting parameters that we obtain for the X-ray fluxes and VLF signal maximum changes are consistent with those in previous studies for dayside paths, even though different instruments are used, supporting the flare-nowcasting method. Moreover, the present results show that, for day–night mixed paths, the amplitude and phase maximum changes also scale linearly with the logarithm of the flare X-ray fluxes, but the level of change is notably different from that for dayside paths. The coefficients used in the flare-nowcasting method need to be updated for mixed propagation paths.

Periodic Oscillation of VLF Transmitter Signals Measured in Low and Middle Latitude Regions

AbstractMeasurements of Very Low Frequency (VLF) signals from navy transmitters carry direct information about the D‐region ionosphere and have been widely utilized for detecting the electron density at D‐region altitudes, but not frequently for the atmospheric waves therein. Atmospheric waves have been extensively studied using the total ionospheric electron content, but if and how they are correlated with the D‐region ionosphere and VLF measurements still remains poorly investigated. In this study, we have conducted a comprehensive analysis using 7‐year measurements (2016–2022) of VLF signals from the JJI, NWC, and VTX transmitters as being recorded in Suizhou, China. These three transmitter‐receiver paths are representative and the corresponding observations constitute a valuable data set to investigate the periodicities of VLF data. Different from previous studies, we have utilized the ensemble empirical mode decomposition and Lomb‐Scargle methods to determine the periodicities of these data. By contrasting these paths, prominent periodicities ranging from 2 to 730 days have been found, with clear diurnal variation and suggestive latitudinal/longitudinal dependence. Moreover, we have found that the mesospheric temperature is closely related with the annual oscillation of VLF measurements, while this oscillation has a low correlation with solar Lyman‐α fluxes or geomagnetic activity. The oscillations with relatively shorter periods are likely atmospheric waves such as gravity waves, planetary waves, or harmonics of these waves. Our results suggest that, in addition to the electron density, the subionospheric VLF technique can be potentially utilized to remotely sense atmospheric waves that propagate up to or through the D‐region ionosphere.

Automatic Detection of VLF Tweek Signals Based on the YOLO Model

Tweek signals are a special type of VLF (very low frequency) pulse, originally produced by lightning discharge, which becomes dispersive after repetitive bounces within the waveguide between the Earth’s surface and lower ionosphere. As such, tweek signals carry critical information about the region near the reflection height of the VLF waves, namely the D-region ionosphere. Although tweek measurements have been widely utilized in studies of the D-region ionosphere and lightning discharge, few statistical studies have been conducted, mainly due to the difficulty of manually identifying tweek signals from the enormous amount of VLF data with heavy noise. Considering the importance of tweek signals and the lack of a high-precision detection model, in this study, we propose a method to automatically and accurately pick out tweek signals from VLF measurements. This method is explicitly developed based on the you only look once (YOLO) model and a post-tracing process. Using a total of 2495 randomly selected VLF spectrogram images as the testing set, we evaluated the performance of this method. The precision and recall are found to be 92.0% and 89.2% for the first-order mode, and 97.5% and 86.7% for the first-two-order mode tweek, respectively. The time needed to process 10-s VLF measurements with a cadence of 4 μs is only 6.5 s, allowing for identifying the tweek signals from continuous VLF measurements in real time. Therefore, this method represents a reliable means to automatically detect tweek signals and enables the opportunity to statistically investigate the D-region ionosphere and lightning discharge via these signals.

A Comparative Study of VLF Transmitter Signal Measurements and Simulations during Two Solar Eclipse Events

To monitor the Very-Low-Frequency (VLF) environment, a VLF detection system has been installed in Suizhou, China, a location with the longitude almost identical to that of the NWC transmitter in Australia. In the years 2019 and 2020, two solar eclipses crossed the NWC–Suizhou path at different locations. Each solar eclipse event represents a naturally occurring controlled experiment, but these two events are unique in that similar levels of electron density variation occurred at different locations along the VLF propagation path. Therefore, we conducted a comparative study using the VLF measurements during these two eclipses. Previous studies mostly estimated a pair of the reflection height (h′) and sharpness parameter (β) using the Long Wavelength Propagation Capability code, whereas, in this study, we use the VLF amplitude and phase as constraints in order to find the electron density change that best explains the VLF measurements. The eclipse measurements could be best explained if the path-averaged β value was 0.56 and 0.62 km−1 for the 2019 and 2020 eclipse, respectively. The VLF reflection height increased from 71.5 to 73.3 km for the 2019 eclipse and from 71.1 to 72.8 km for the 2020 eclipse. The best-fit β values were consistent with the Faraday International Reference Ionosphere model and statistical studies, and the h′ change was also consistent with previous studies and theoretical calculations. Moreover, present results suggested that VLF signals collected by a single receiver were not sensitive to where the electron density change occurs along the propagation path but reflected the average path condition. Therefore, a network of VLF receivers is required in order to monitor in real time the spatial extent of the space weather events that disturb the lower ionosphere.

Lower-ionosphere electron density and effective recombination coefficients from multi-instrument space observations and ground VLF measurements during solar flares

A new model to predict the electron density and effective recombination coefficient of the lower ionosphere under solar flare conditions is presented. This model relies on space-borne solar irradiance measurements in coincidence with ground recorded active transmissions of Very Low Frequency (VLF), (<30 kHz) signals. Use is made of the irradiance measured by broad-band radiometers onboard the satellites: GOES, SDO, and PROBA2. Measurements are made over succeeding and partly overlapping wavelength intervals of the instrument bandpass ranges altogether covering the range 0.1–20 nm. The aim is to determine the effectiveness of the particular instrument bandpass in producing changes in the ionization of the lower ionosphere (D-region) during solar X-ray flares. Ionization efficiency is evaluated using modelled Solar Spectral Irradiance for each flare separately and for each instrument as a function of its bandpass.The new model is based on coupling of the continuity equation with the Appleton relation and uses the concept of time delay – the time lag of the extreme VLF amplitude and phase behind the flare irradiance maximum. The solution of the continuity equation predicts the electron density time - height profile for 55–100 km altitude.An analysis of M to X class flares shows the flare-enhanced electron densities due to a particular ionizing wavelength domain are in good agreement for the case where irradiance is taken over the bandpass of (1) either GOES (0.1–0.8 nm) or SDO/ESP (0.1–7 nm) for up to 90 km (2) either SDO/ESP or PROBA2/LYRA (1–2 +6–20 nm) at heights above 90 km. The results agree within 22% for heights up to 90 km, and differ by at most a factor of 2 for heights above 90 km. Remarkable agreement is shown between measured and evaluated time delay; discrepancies are generally less than 8%. The effective recombination coefficient is deduced from the model itself and is found to be consistent with other independent estimates.

Measurements and Modeling of the Responses of VLF Transmitter Signals to X‐Class Solar Flares at the Great Wall Station in Antarctica

AbstractSolar flares can severely disturb the Earth's ionosphere, leading to changes of atmospheric chemistry and degradation of Global Navigation Satellite System (GNSS) signals. The key to better assess these effects is to more accurately quantify the electron density enhancements induced by solar flares, which is still uncertain. Thanks to the good coverage and quiet electromagnetic environment provided by the Great Wall Station (GWS, 62.22°S, 58.96°W) in Antarctica, our Very‐Low‐Frequency (VLF) receiver has recorded clean VLF signatures produced by three X‐class solar flares from March to May in 2022. These measurements motivate us to revisit the uncertain question related to the ionospheric effects induced by solar flares. Different from previous studies, we model solar flare events from satellite measurements of X‐ray fluxes to ground‐based measurements of VLF transmitter signals by combining a flare irradiance model, an X‐ray propagation model, a D‐region chemistry model, and a VLF propagation model. For eight transmitting frequencies and propagation paths, modeling results can satisfactorily explain our VLF measurements at GWS during the three X‐class flares, especially so for the X1.3 event occurring on 30 March 2022. Moreover, the electron density results that reconcile space‐ and ground‐based measurements suggest that the VLF reflection height can be lowered to ∼66 km by an X1.1 flare, in line with previous studies. The present set of numerical models can be thus utilized in future studies to improve the nowcasting capability of VLF sensing technique for monitoring solar flares, as well as the accuracy of GNSS navigation/positioning during solar flares.

Ionospheric D Region: VLF‐Measured Electron Densities Compared With Rocket‐Based FIRI‐2018 Model

AbstractGround‐based very low frequency (VLF) radio propagation in the Earth‐ionosphere waveguide has enabled extensive electron number densities in the D region of the Earth's ionosphere to be determined, by day typically below heights of 70–80 km and by night in the height range ∼75–90 km. Many rocket‐based electron density measurements have also been reported in the literature from ∼60 km upwards using current probes, and radio propagation at a few MHz between the rocket and ground. Recently these rocket measurements have been summarized, and supplemented with D region production‐loss modeling, giving rise to a near global model named FIRI‐2018 (Faraday‐International Reference Ionosphere) which provides electron number densities as functions of height, latitude (&lt;60°), solar zenith angle and F10.7 cm solar flux. These rocket‐based electron density values are here compared with corresponding values from VLF measurements, by day at a low‐latitude (∼20°) and a high mid‐latitude (∼55°), and by night mainly at mid‐latitudes. At night the average agreement (over 75–90 km) is remarkably good. By day, at low latitude the agreement is also fairly good (in the common height range ∼60–75 km), with the changes with solar zenith angle being moderately comparable. For daytime high mid‐latitudes, the agreement is less satisfactory, particularly at the lowest common altitudes, with the VLF measurements showing the expected effects of cosmic rays much more than the rocket‐based values. Overall, we find that the D region description in the FIRI‐2018 model is a significant advance on the earlier International Reference Ionosphere (IRI‐2016) model.

An Electron Density Model of the D‐ and E‐Region Ionosphere for Transionospheric VLF Propagation

AbstractTerrestrial Very‐Low‐Frequency (VLF) energy from both lightning discharges and radio transmitters has a role in affecting the energetic electrons in the Van Allen radiation belts, but quantification of these effects is particularly difficult, largely due to the collisional damping experienced in the highly variable electron density in the D‐ and E‐region ionosphere. The Faraday International Reference Ionosphere (FIRI) model was specifically developed by combining lower‐ionosphere chemistry modeling with in situ rocket measurements, and represents to date the most reliable source of electron density profiles for the lower ionosphere. As a full‐resolution empirical model, FIRI is not well suited to D‐ and E‐region ionosphere inversion, and its applicability in transionospheric VLF simulation and in remote sensing of the lower ionosphere is limited. Motivated by how subionospheric VLF remote sensing has been aided by the Wait and Spies (WS) profile (Wait &amp; Spies, 1964), in this study, we parameterize the FIRI profiles and extend the WS profile to the E‐region ionosphere by introducing two new parameters: the knee altitude hk and the sharpness parameter for the E‐region ionosphere βE. Using this modified WS profile, we calculate the expected signals at different receiver locations from the NAA, NPM, and NWC transmitters under the full range of possible ionospheric conditions. We also describe and validate a method about how these results can be readily used to translate VLF measurements into estimates of the lower ionosphere electron density. Moreover, we use this method to evaluate the sensitivity of different ground receiver locations in lower‐ionosphere remote sensing.

Measuring the Electron Density Roughness of the D‐Region Ionosphere

AbstractWe present a method of characterizing the horizontal and vertical electron density roughness of the D‐region ionosphere using Nationwide Differential Global Position System (NDGPS) transmitters as low‐frequency (LF; 30–300 kHz) and medium‐frequency (MF; 300–3,000 kHz) signals of opportunity. The horizontal roughness is characterized using an amplitude cross‐correlation method, which yields the correlation length scale metric. The vertical roughness is characterized using a differential phase height, which is needed to mitigate the effects of transmitter phase instability. The ranges and typical values of roughness metrics are investigated using data from several field campaign measurements. Finally, the roughness metrics for an NDGPS transmitter and very low frequency (VLF) transmitter are compared. It is found that the roughness detected by the VLF transmitter is significantly smoother and demonstrates the utility of this method to complement traditional VLF measurements.

Outer Radiation Belt Electron Lifetime Model Based on Combined Van Allen Probes and Cluster VLF Measurements

AbstractThe flux of energetic electrons in the outer radiation belt shows a high variability. The interactions of electrons with very low frequency (VLF) chorus waves play a significant role in controlling the flux variation of these particles. Quantifying the effects of these interactions is crucially important for accurately modeling the global dynamics of the outer radiation belt and to provide a comprehensive description of electron flux variations over a wide energy range (from the source population of 30 keV electrons up to the relativistic core population of the outer radiation belt). Here, we use a synthetic chorus wave model based on a combined database compiled from the Van Allen Probes and Cluster spacecraft VLF measurements to develop a comprehensive parametric model of electron lifetimes as a function of L‐shell, electron energy, and geomagnetic activity. The wave model takes into account the wave amplitude dependence on geomagnetic latitude, wave normal angle distribution, and variations of wave frequency with latitude. We provide general analytical formulas to estimate electron lifetimes as a function of L‐shell (for L = 3.0 to L = 6.5), electron energy (from 30 keV to 2 MeV), and geomagnetic activity parameterized by the AE index. The present model lifetimes are compared to previous studies and analytical results and also show a good agreement with measured lifetimes of 30 to 300 keV electrons at geosynchronous orbit.

Ionospheric D Region Parameters Obtained Using VLF Measurements in the South Pacific Region

AbstractThe diurnal variations in the phase and amplitude of very low frequency (VLF) transmissions with the call signs NWC, NPM, and NLK, received at Suva, Fiji, have been modeled using the Long Wave Propagation Capability (V2.1) code to determine the ionospheric D region parameters, H΄ (reference height), and β (rate of increase of electron density with height), for different daytime and nighttime conditions along the transmitter‐receiver great circle paths (TRGCPs). Measured VLF signal amplitude and phase show explicit variation over the day and nighttime along a TRGCP, also revealing amplitude minima and phase steps during sunrise and sunset as the day/night terminator traverses a TRGCP. While the daytime signal strength is reasonably smooth, at nighttime, the signal exhibits a great deal of variability. For three signal paths, the mean daytime H΄ and β were found to be 70.7 km and 0.40 km−1, respectively, while nighttime mean values of these parameters were found to be 84.2 km and 0.68 km‐1, respectively. The temporal and day‐to‐day variability of the nighttime D region parameters shows that H΄ and β ranges in between 83.0 and 85.0 km and 0.58 and 0.80 km−1, respectively. One of the possible sources of nighttime signal variability is increase in the number of modes propagating and relative complex interference between them along the TRGCPs, whereby the weaker modes also become significant at night due to reduced attenuation. In addition, the variations in the nighttime D region may also be a cause of high signal variability.

VLF Measurements and Modeling of the D-Region Response to the 2017 Total Solar Eclipse

In this paper, we report measurements in Colorado and Utah of the disturbed very-low-frequency (VLF) signals from the NML Navy transmitter in North Dakota during the 2017 solar eclipse. Using an occultation mask of solar fluxes together with detailed chemistry and VLF propagation simulations, we quantify the D-region response to the eclipse, in terms of electron density variation, as well as the expected signatures of VLF transmitter signals. The VLF measurements, including an anomalous amplitude enhancement recorded in UT, can be quantitatively explained using the Wait and Spies ionospheric profile with a sharpness parameter of $\beta = 0.3$ km−1 above ~55 km and an increase in the D-region ionosphere height of $\Delta h' \simeq 8$ km. This sharpness parameter is consistent with previously reported rocket measurements and first-principles calculations. The best-fit results suggest a reduction of D-region electron density by ~90% during the eclipse in the D-region, implying an occultation of Lyman- $\alpha $ by nearly 99%. This finding agrees with detailed calculations of time-dependent obscuration factors utilizing the He 30.4-nm images from Solar Dynamics Observatory as a proxy for the distribution of Lyman- $\alpha $ across the solar disk and limb. Moreover, the present results show that subionospheric VLF propagation is sensitive to the sharpness parameter of the electron density profile in the D-region. Previously reported first-principles simulations have shown that the sharpness parameter is mostly controlled by the background concentration of minor neutral species. Thus, the VLF technique can be likely used to remotely sense these neutral species at and below the effective reflection altitudes of VLF waves.

Fracture zones detection for groundwater exploration integrating Resistivity Profiling and Very Low Frequency electromagnetic methods (Errachidia basin, Morocco)

Abstract Resistivity Profiling and Very Low Frequency (VLF) electromagnetic methods were introduced to study fracture zones detection in Zaouia Jdida locality, within the Errachidia basin. The Horizontal Profiling was conducted in Wenner-α array, with AB = 300 m and profile lines oriented NW–SE and NE–SW. The resistivity measurements were taken using MAE advanced geophysics instruments. The VLF profiles were implanted with the length reaches 1000 m and profile lines oriented in NE–SW direction. The VLF measurements were collected using T-VLF iris instrument and the data filtering was done using KHFFILT software. Two filters, Karous-Hjelt and Fraser, were applied to the real component of the secondary electromagnetic field. The qualitative interpretation of resistivity results, showed the presence of subsurface targets; fracture zones were detected at 70m, 240m and 450m positions along the profile P1, at 180m, 340m and 450m positions from the profile P2. The semi-quantitative interpretation of VLF results revealed the presence of two principal fracture zones at L3 and L5 locations, oriented NW–SE, at a depth range of 30 m to 60 m. The VLF anomaly observed at L3 location is confirmed by the resistivity measurements from the profile P1 (at 70m station). The identified fractures represent the potential zones for groundwater supply and then will have an implication on storage and movement of groundwater in the prospect area.

D‐Region High‐Latitude Forcing Factors

AbstractThe subionospheric very low frequency (VLF) radio wave technique provides the possibility of investigating the response of the ionospheric D‐region to a diversity of transient and long‐term physical phenomena originating from above (e.g., energetic particle precipitation) and from below (e.g., atmospheric waves). In this study, we identify the periodicities that appear in VLF measurements and investigate how they may be related to changes in space weather and atmospheric activity. The powerful VLF signal transmitted from NAA (24 kHz) on the east coast of the United States, and received at Sodankylä, Finland, was analyzed. Wavelet transform, wavelet power spectrum, wavelet coherence, and cross‐wavelet spectrum were computed for daily averages of selected ionospheric, space weather, and atmospheric parameters from November 2008 until June 2018. Our results show that the significant VLF periods that appear during solar cycle 24 are the annual oscillation, semiannual oscillation, 121‐day, 86‐day, 61‐day, and solar rotation oscillations. We found that the annual oscillation corresponds to variability in mesospheric temperature and solar Lyman‐α (Ly‐α) flux and the semiannual oscillation to variability in space weather‐related parameters. The solar rotation oscillation observed in the VLF variability is mainly related to the Ly‐α flux variation at solar maximum and to geomagnetic activity variation during the declining phase of the solar cycle. Our results are important since they strengthen our understanding of the Earth's D‐region response to solar and atmospheric forcing.

Interpretation of Very Low Frequency Measurements Carried Out with an Unmanned Aerial System by 2D Conductivity Models

Very Low Frequency (VLF) measurements were successfully acquired using an Unmanned Aircraft System (UAS) as the measurement platform at two sites. The first site was characterized by strong anthropogenic anomalies and was located in Switzerland. The second site was a saltwater-freshwater transition zone in Northern Germany. The induction coils were used to measure the three components of the magnetic field of VLF transmitters. The magnetic field sensors and the corresponding data logger were suspended below a 50 kg rotary wing UAS. Prior to the test survey, noise measurements were carried out to establish a proper distance for the logger and the sensor, which were found at 2 m and 4 m from the body of the UAS, respectively. A suspension system was developed to enable stable flight characteristics in combination with the VLF system. The test flights were carried out with low speed (1 m/s) and at low altitude (approximately 3 m) above ground. A processing algorithm was developed to analyze the time series of the measured magnetic fields. Transfer functions for the vertical magnetic field were derived using both a scalar and a bivariate analysis approach. After rotating the transfer functions into the strike direction of the 2D subsurface anomaly, a two dimensional inversion method was applied for the interpretation of the conductivity structure to a depth of 15 m. In addition, radiomagnetotelluric (RMT) measurements were carried out on the same UAS-VLF profiles. The RMT data were interpreted by 2D inversion for comparison and are used here to derive the background resistivity, which cannot be achieved by VLF measurements. As a result, good correlation was found between the UAS-VLF and RMT conductivity models.

Global ionospheric flare detection system (GIFDS)

The Global Ionospheric Flare Detection System (GIFDS) is currently under development at the German Aerospace Center as a ground based detector for continuous monitoring of the solar flare activity in order to provide real time warnings on solar X-ray events. GIFDS is using Very Low Frequency (VLF) radio transmissions in the northern hemisphere which respond to enhanced ionization in the bottomside ionosphere caused by X-ray flares. Since solar flares can only be detected during daytime, VLF receivers have to be installed around the globe to guarantee continuous records at the dayside sector. GIFDS consists of a network of Perseus SDR (Software Defined Radio) receivers equipped with a MiniWhip antenna each. Reliable detection of solar flares is ensured by recording multiple frequency channels ranging from 0 to 500kHz. The applicability of the system is demonstrated in a first analysis by comparing VLF measurements with GOES's (Geostationary Operational Environmental Satellite) X-ray flux data. The high potential of GIFDS for a permanent monitoring of solar flares in near real time is discussed.

Delineation of Extension of Uranium Mineralization Zone using Resistivity and Very Low Frequency Electromagnetic Surveys around South Purulia Shear Zone, India

Abstract: During a random radiometric survey, nuclear radiation was detected near exposed rocks in an open-pit phosphatemine at Beldih (Purulia, W.B.), India. Radiometric analysis of rock samples reveals a significant concentration of uranium. Geological study suggests that Beldih phosphate mine lies near the South Purulia Shear Zone (SPSZ). Shear zones in the region are E-W striking elongated zone and associated with uranium mineralization (famous Singhbhum shear zone for several uranium mines). Electrical resistivity (profiling and sounding) and Very Low Frequency (VLF) electromagnetic surveys were performed around Beldih mine to delineate the suitable conducting zones which may be associated with uranium mineralization. Significant lateral extension of possible source is delineated; it could be exploited later on a large scale for commercial utilization. It is observed that gradient resistivity profiling (GRP) survey is the most suitable technique to explore the nearly vertical conducting structure. The GRP measurements were carried out at different locations. A good correlation of low resistive zones on various GRP was observed from the measured data. The study concludes that subsurface structures are nearly vertical and have alternate resistive and conducting bands. An inclined bore-hole drilled confirms the presence of alternate resistive and conducting structures and uranium mineralization is detected in conducting bands. This suggests that GRP can be conducted for precise location of the target zone. This will help in planning bore holes for drilling to find the exact thickness, lateral and vertical extent of the deposit for commercial utilization. VLF measurements were also carried out along various profiles. Even though VLF observation was quite noisy due to presence of power lines in the area; this method can be used to map the area and model the data for actual sub-surface conducting zones in less noisy areas as a support for drilling works.

Solar flare induced D-region ionospheric perturbations evaluated from VLF measurements

The results of very low frequency (VLF) wave amplitude measurements carried out at the low latitude station Varanasi (geom. lat. 14∘55′N, long. 154∘E), India during solar flares are presented for the first time. The VLF waves (19.8 kHz) transmitted from the NWC-transmitter, Australia propagated in the Earth-ionosphere waveguide to long distances and were recorded at Varanasi. Data are analyzed and the reflection height H′ and the sharpness factor β are evaluated. It is found that the reflection height decreases whereas sharpness factor increases with the increase of solar flare power. The H′ is found to be higher and β smaller at low latitudes than the corresponding values at mid and high latitudes. The sunspot numbers were low during the considered period 2011–2012, being the rising phase of solar cycle 24 and as a result cosmic rays may impact the D-region ionosphere. The increased ionization from the flare lowers the effective reflecting height, H′, of the D-region roughly in proportion to the logarithm of the X-ray flare intensity from a typical mid-day unperturbed value of about 71–72 km down to about 65 km for an X class flare. The sharpness (β) of the lower edge of the D-region is also significantly increased by the flare but reaches a clear saturation value of about 0.48 km−1 for flares of magnitude greater than about X1 class.

Three-component VLF using an unmanned aerial system as sensor platform

In a pilot project, very low frequency (VLF) measurements were performed using a prototype unmanned airborne system (UAS) at a test area near Neuchâtel, Switzerland. The VLF system operates over an extended frequency range of 1–250 kHz and consists of a three-component magnetic field sensor and a data logger, both deployed on an unmanned helicopter. Various developments had to be done prior to the first flight. The devices went through a weight reduction process, the noise influence of the aircraft on the devices had to be investigated, and an appropriate suspension had to be developed. This first experiment was conducted on a test site with anomalies due to man-made objects: a buried pipeline and a power cable with known orientation and location. The performance of the system in comparison with ground-based VLF data was investigated. First results demonstrate the feasibility of airborne VLF measurements with a UAS to detect buried pipelines and power cables.

Natural atmospheric noise statistics from VLF measurements in the eastern Mediterranean

[1] Very low frequency (VLF) (3–30 kHz) and extremely low frequency (ELF) (3–3000 Hz) electromagnetic transient signals and noise are generated by various natural and anthropogenic processes. On a global basis by far the most significant source is ELF/VLF radiation from lightning propagating in the Earth-ionosphere waveguide. This atmospheric “noise,” originating essentially from lightning discharges, is the main source of interference for VLF/LF telecommunications. One of the statistical measures that is used to define the properties of low-frequency radio noise is the voltage deviation Vd, which is a measure of the impulsiveness of the noise that is widely used to characterize radio noise, particularly in the International Radio Consultative Committee reports. In this paper we present atmospheric noise statistics based on VLF measurements at different temporal resolution (from minutes to seasonal variations). For the first time we present analysis of the statistical parameters of Vd from continuous broadband VLF measurements for a period extending more than 1 year. Our analysis shows that the long-term observed Vd characteristics can be reasonably estimated as the sum of two Gaussians distribution functions, while the hourly and seasonal distributions of Vd values can be fitted using a single Gaussian distribution with different mean and variance values.