VLF Measurements Research Articles

Contribution of VLF electromagnetic survey to the investigation of Hessdalen lights (Norway)

Hessdalen valley in Norway is known for luminous phenomena suddenly and evenly appearing temporarily. Since several decades, these phenomena are observed by many witnesses, and they are sometimes traced by geophysical devices. The first appearance in modern times was reported in 1981 but systematic observations started during winter of 1984 when the Hessdalen project was launched. Later, Østfold University College led the project and yearly organized one to two field campaigns, with the objective to systematically record and investigate the phenomenon. Till that epoch, detailed tectonics, fault systems and superficial conductive structures remained unknown. Therefore, during the last years, VLF surveys have been performed in Hessdalen valley as part of six geophysical field campaigns which sometimes also included Total Magnetic Field and Self Potential spatial ground measurements. VLF measurements have been carried out on a 20 m average spacing along many traces totaling to100 km length. The entire covered area was about 100 km2.In this paper, we focus on the results of the VLF measurements. Several conductive zones have been found. They are mainly related to mineral deposits (mainly sulfides). The trace at the ground surface of these conductive zones could suggest that they draw an ellipse of 6 by12 km, related to the shape of the gabbro intrusion present in the area and oriented in the SW-NE direction. The results combined with other geophysical data contribute to better understand how the near surface structures (depth less than a few hundred meters) could supply the generation of the so-called Hessdalen lights (‘HL’) and explain why these lights appear inside this valley. The particular geological structure detected in Hessdalen valley may encourage similar campaigns in other areas where similar phenomena are observed.

Multi-instrumental investigation of the solar flares impact on the ionosphere on 05–06 December 2006

The sudden increase of X-radiation and EUV emission following solar flares causes additional ionization and increased absorption of electromagnetic (EM) waves in the Earth’s atmosphere. The solar flare impact on the ionosphere above Europe on 05 and 06 December 2006 was investigated using ground-based (ionosonde and VLF) and satellite-based data (Vertical Total Electron Content (VTEC) derived from GNSS observations and VLF measurements from DEMETER satellite). Based on the Kp and Dst indices, 05 December 2006 was a quiet day, while there was a geomagnetic storm on 06 December 2006. The total fade-out of the EM waves emitted by the ionosondes was experienced at all investigated stations during an X9 class flare on 05 December 2006. The variation of the fmin parameter (first echo trace observed on ionograms, it is a rough measure of the “non-deviative” absorption) and its difference between the quiet period and during the flares have been analyzed. A latitude dependent enhancement of fmin (2–9 MHz) and Δfmin (relative change of about 150%–300%) was observed at every station at the time of the X9 (on 05 December) and M6 (on 06 December) flares. Furthermore, we analyzed VTEC changes during and after the flare events with respect to the mean VTEC values of reference quiet days. During the X9 solar flare, VTEC increased depending on the latitude (2–3 TECU and 5%–20%). On 06 December 2006, the geomagnetic storm increased ionization (5–10 TECU) representing a “positive” ionospheric storm. However, an additional peak in VTEC related to the M6 flare could not be detected. We have also observed a quantifiable change in transionospheric VLF absorption of signals from ground transmitters detected in low Earth orbit associated with the X9 and M6 flare events on 05 and 06 December in the DEMETER data. Moreover, amplitude and phase of ground-based, subionospherically propagating VLF signals were measured simultaneously during the investigated flares to analyze ionosphere reaction and to evaluate the electron density profile versus altitude. For the X9 and M6 flare events we have also calculated the ionospheric parameters (sharpness, reflection height) important for the description and modelling of this medium under forced additional ionization.

COMPARISON OF ON-SITE PARTIAL DISCHARGE MEASUREMENTS USING VLF-, DAC- AND SLOPE-VOLTAGE FORMS AND DISSIPATION FACTOR MEASUREMENTS ON SERVICE-AGED MEDIUM VOLTAGE CABLES

In order to make the results of different partial discharge measurement methods comparable and to increase the informative value of the results, 41 aged-but-still-in-service medium voltage cables were measured, using VLF, DAC and Slope technology for partial discharge measurements and VLF technology for dissipation factor measurements. The detected partial discharge defects were examined for their localization, their apparent charge and their inception voltage. Occurring deviations between the measuring methods were documented. Furthermore, the results of the partial discharge measurements were compared with the results of the dissipation factor measurements and checked for dependencies. It was determined, that the different partial discharge measurement methods detect mostly the same defects, while DAC and Slope measurements seems to have a high sensibility in PILC components. Furthermore, it was found out that the inception voltages are often identical, but at some defects the VLF method seems to have higher inception voltages. While the results of Slope and DAC measurements are similar in levels of apparent charges, those levels are generally higher than the results of VLF measurements. There were no coherent dependencies found between partial discharge behaviour and the results of the dissipation factor measurements.

Chemical Response of the Upper Atmosphere Due to Lightning‐Induced Electron Precipitation

AbstractTerrestrial lightning frequently serves as a loss mechanism for energetic electrons in the Van Allen radiation belts, leading to lightning‐induced electron precipitation (LEP). Regardless of the specific causes, energetic electron precipitation from the radiation belts in general has a significant influence on the ozone concentration in the stratosphere and mesosphere. The atmospheric chemical effects induced by LEP have been previously investigated using subionospheric VLF measurements at Faraday station, Antarctica (65.25°S, 64.27°W, L = 2.45). However, there exist large variations in the precipitation flux, ionization production, and occurrence rate of LEP events depending on the peak current of the parent lightning discharge, as well as the season, location, and intensity of the thunderstorm activity. These uncertainties motivate us to revisit the calculation of atmospheric chemical changes produced by LEP. In this study, we combine a well‐validated LEP model and first‐principles atmospheric chemical simulation, and investigate three intense storms in the year of 2013, 2015, and 2017 at the magnetic latitude of 50., 32., and 35., respectively. Modeling results show that the LEP events in these storms can cumulatively drive significant changes in the , , and concentration in the mesosphere. These changes are as high as , , and at 75–85 km altitude, respectively, and comparable to the effects typically induced by other types of radiation belt electron precipitation events. Considering the high occurrence rate of thunderstorms around the globe, the long‐term global chemical effects produced by LEP events need to be properly quantified.

Ionospheric perturbations observed due to Indonesian Earthquake (Mw = 7.4) using GPS and VLF measurements at multi-stations

Ionospheric perturbations observed due to Indonesian Earthquake (Mw = 7.4) using GPS and VLF measurements at multi-stations

Imaging of hydrothermal altered zones in Wadi Al-Bana, in southern Yemen, using remote sensing techniques and very low frequency–electromagnetic data

Economic mineralization and hydrothermally altered zones are areas of great economic interests. This study focusses on hydrothermal altered zones of high mineralization potentials in Wadi Al-Bana, in southern Yemen. An azimuthal very low frequency–electromagnetic (AVLF-EM) data acquisition was conducted in search for mineralization in the study area. The study integrated observations from geophysical field data with others extracted from object-oriented principal component analysis (PCA) to better map and understand mineralization in the investigated area. This technique was applied to two data sets, ASTER and Landsat 8 Operational Land Imager (OLI) imagery. The results of PCA revealed high accuracy in detecting alteration minerals and for mapping zones of high concentration of these minerals. The PCA-based distribution of selected alteration zones correlated spatially with high conductivity anomalies in the subsurface that were detected by VLF measurements. Finally, a GIS model was built and successfully utilized to categorize the resulted altered zones, into three levels.

Solar flare effects on D-region ionosphere using VLF measurements during low- and high-solar activity phases of solar cycle 24

The effects of solar flares on the propagation of subionospheric VLF signals from NWC and NLK transmitter stations monitored at a low-latitude station, Suva (18.2°S, 178.4°E), Fiji, between December 2006 and December 2010 (an unprecedented solar minimum of solar cycles 23 and 24) and between January 2012 and December 2013 (moderate solar activity at the peak of solar cycle 24) have been analyzed to find solar flare time D-region changes. The amplitude and phase enhancements associated with solar flares were observed in the signals from both stations which are due to an increase in the electron density of the D-region as a result of extra ionization caused by the solar flares. The solar flare-induced perturbations in both the amplitude and phase of VLF signals were used to determine D-region ionospheric parameters: H′ (the ionospheric reflection height) and β (rate of increase in electron density with height) using Long Wave Propagation Capability (LWPC) version 2.1 modeling. A comparative analysis of the ionospheric D-region parameter changes carried out for this location shows a greater increase in β and decrease in H′ during low-solar activity period than during moderate-solar activity period, for the same class of flares. Our results also show greater differences in the values of β and H′ for strong flares in comparison with weak flares under both low- and moderate-solar activity conditions.

Synthetic Empirical Chorus Wave Model From Combined Van Allen Probes and Cluster Statistics

AbstractChorus waves are among the most important natural electromagnetic emissions in the magnetosphere as regards to their potential effects on electron dynamics. They can efficiently accelerate or precipitate electrons trapped in the outer radiation belt, producing either fast increases of relativistic particle fluxes or auroras at high latitudes. Accurately modeling their effects, however, requires detailed models of their wave power and obliquity distribution as a function of geomagnetic activity in a particularly wide spatial domain, rarely available based solely on the statistics obtained from only one satellite mission. Here we seize the opportunity of synthesizing data from the Van Allen Probes and Cluster spacecraft to provide a new comprehensive chorus wave model in the outer radiation belt. The respective spatial coverages of these two missions are shown to be especially complementary and further allow a good cross calibration in the overlap domain. We used 4 years (2012–2016) of Van Allen Probes VLF data in the chorus frequency range up to 12 kHz at latitudes lower than 20°, combined with 10 years of Cluster VLF measurements up to 4 kHz in order to provide a full coverage of geomagnetic latitudes up to 45° in the chorus frequency range 0.1fce–0.8fce. The resulting synthetic statistical model of chorus wave amplitude, obliquity, and frequency is presented in the form of analytical functions of latitude and Kp in three different magnetic local time sectors and for two ranges of L shells outside the plasmasphere. Such a synthetic and reliable chorus model is crucially important for accurately modeling global acceleration and loss of electrons over the long run in the outer radiation belt, allowing a comprehensive description of electron flux variations over a very wide energy range.

The effectiveness of the very low frequency electromagnetic method (VLF-EM) in the exploration of sulphide mineralization in arid environments, case study from South Sinai Peninsula, Egypt

Very low frequency electromagnetic (VLF-EM) measurements were carried out in Wadi Isbayia area, south Sinai Peninsula, to test the efficiency of the VLF-EM method in the exploration of sulphide mineralization in arid environments. The VLF-EM field measurements, including tilt angle, real and imaginary components of the received VLF field, were carried out along fifteen profiles covering a quartz monzonite bedrock. Interpretation of the VLF measurements, in the light of geological information, has showed that sulphide minerals in the Wadi Isbayia area extend from the ground surface to a depth of about 200 m. However, the structural lineaments, especially faults, have controlled the distribution of the sulphide mineralization. A few of polished sections for rock samples, collected from the sites of the VLF-EM anomalies in the study area, have been prepared and examined by ore microscopy which confirmed the presence of pyrite and chalcopyrite as well as iron oxides, disseminated in the quartz monzonite bedrock. These results showed that the VLF-EM method is an effective tool in the exploration of sulphide minerals in the arid environments.

Empirical model of lower band chorus wave distribution in the outer radiation belt

AbstractAccurate modeling of wave‐particle interactions in the radiation belts requires detailed information on wave amplitudes and wave‐normal angular distributions over L shells, magnetic latitudes, magnetic local times, and for various geomagnetic activity conditions. In this work, we develop a new and comprehensive parametric model of VLF chorus waves amplitudes and obliqueness in the outer radiation belt using statistics of VLF measurements performed in the chorus frequency range during 10 years (2001–2010) aboard the Cluster spacecraft. We used data from the Spatio‐Temporal Analysis of Field Fluctuations‐Spectrum Analyzer experiment, which spans a total frequency range from 8 Hz to 4 kHz. The statistical model is presented in the form of an analytical function of latitude and Kp (or Dst) index for day and night sectors of the magnetosphere and for two ranges of L shells above the plasmapause, from L = 4 to 5 and from L = 5 to 7. This model can be directly applied for numerical calculations of charged particle pitch angle and energy diffusion coefficients in the outer radiation belt, allowing to study with unprecedented detail their statistical properties as well as their important spatiotemporal variations with geomagnetic activity.

Geothermal evolution of an intruded dike in the rift zone of Kilauea volcano, Hawaii from VLF and self-potential measurements

Self-potential (SP) and VLF measurements were made in 1973, 1975, 1995, 1997 and 2012 across a basaltic dike that intruded into the Koae fault zone of Kilauea volcano, Hawaii in May 1973. The SP anomaly remained strong throughout. In 2012 it was at about 60% of the strength it had in 1973. In contrast, the VLF anomaly, though diminished, was still observable in 1995/1997, but by 2012 it had disappeared. A hydrothermal dike model, with parameters calibrated by modeling the solidification of Kilauea Iki lava lake, is used to calculate temperatures and conductivity variation. Following Jaeger's (1957) method, we find that the time in years for a dike of width W (m) to solidify is 0.0075W2. Thus, a 1m dike solidifies within the first few days, and after 39years is only tens of degrees above ambient. Given the orders of magnitude difference between the conductivities of wet and dry basalt, we infer, that after solidification, the VLF anomalies were caused by induction in a localized veil of wet, hot basalt enveloping the dike, that was generated initially by condensation of steam, and subsequently by condensation of evaporated water as temperatures reduced. The conductivity anomaly persisted until the mid-nineties. By 2012, temperatures and condensation were too small for a VLF signal. The persistent SP anomaly is attributed to localized fluid disruption, with evaporation mainly at the water table and in the vadose zone. Streaming potentials are associated with evaporative circulation in the vadose zone. Next to the dike a positive potential is generated by upward flow of moisture-laden air, with a smaller negative potential on its flanks from downward infiltrating rainwater. The analysis indicates that the combination of SP and VLF measurements can characterize the evolving geothermal regime of intrusions above the water table.

Nighttime sensitivity of ionospheric VLF measurements to X‐ray bursts from a remote cosmic source

AbstractOn 22 January 2009, a series of X‐ray bursts were emitted by the soft gamma ray repeater SGR J1550‐5418. Some of these bursts produced enhanced ionization in the nighttime lower ionosphere. These ionospheric disturbances were studied using X‐ray measurements from the Anti‐Coincidence Shield of the Spectrometer for Integral onboard the International Gamma‐Ray Astrophysics Laboratory and simultaneous phase and amplitude records from two VLF propagation paths between the transmitter Naval Radio Station, Pearl Harbor (Hawaii) and the receivers Radio Observatorio do Itapetinga (Brazil) and Estação Antarctica Commandante Ferraz (Antarctic Peninsula). The VLF measurements have been obtained with an unprecedented high time resolution of 20 ms. We find that the illumination factor I (illuminated path length times the cosine of the zenith angle), which characterizes the propagation paths underlying the flaring object, is a key parameter which determines the sensitivity threshold of the VLF detection of X‐ray bursts from nonsolar transients. For the present VLF measurements of bursts from SGR J1550‐5418, it is found that for I ≥ 1.8 Mm, all X‐ray bursts with fluence in the 25 keV to 2 MeV range larger than F25_min ~ 1.0 × 10−6 erg/cm2 produce a measurable ionospheric disturbance. Such a lower limit of the X‐ray fluence value indicates that moderate X‐ray bursts, as opposed to giant X‐ray bursts, do produce ionospheric disturbances larger than the sensitivity limit of the VLF technique. Therefore, the frequency of detection of such events could be improved, for example by increasing the coverage of existing VLF receiving networks. The VLF detection of high‐energy astrophysical bursts then appears as an important observational diagnostic to complement their detection in space. This would be especially important when space observations suffer from adverse conditions, like saturation, occultation from the Earth, or the passage of the spacecraft through the South Atlantic anomaly.

3D geoelectrical model of geothermal spring mechanism derived from VLF measurements: A case study from Aggistro (Northern Greece)

The most important task of a survey concerning the exploitation and protection of a thermal spring is the understanding of the mechanism of the outflow of thermal fluids. In this concept, geophysical methods are conscripted to contribute to the understanding of the hydrogeological model, since they can provide information without interferences which could potentially react on the flow of the hot water (i.e. wells, pits).In the most of the cases, the occurrence of such springs is related to the local faulting system that allows the upwards flow of the thermal fluids.In this concept, the VLF method is suggested since it is a very fast, low cost and can detect faulting zones with high accuracy.In this work, VLF method is applied in order to study the faulting system that is related to the mechanism of the hot springs in Aggistro (Northern Greece). VLF measurements have been done on parallel lines in order to examine the possible existence of conductive zones, such as faults, that potentially could be used for the outflow of the thermal fluid. VLF data have been inverted allowing the calculation of 2D resistivity models where conductive zones can be accurately located. Finally, a pseudo-3D presentation of the resistivity model allows the understanding of the mechanism of the hot spring.Electrical resistivity tomography method has been used to cross check the inversion results of VLF method and the correlation was very good. This fact, supplemented by hydrogeological data deriving from adjacent wells, verified the resistivity model revealed by VLF data, suggesting that VLF method can be a fast and accurate application for the study of a hot spring.A productive well, drilled at a location suggested by the geophysical data, found groundwater of a temperature of 34.3°C and discharge of 200m3/h.

Simultaneous observation of chorus and hiss near the plasmapause

On 4 August 2010 a moderate geomagnetic storm occurred with minimum Dst of −65 nT and maximum Kp of 7−. Shortly after the onset of this storm, VLF chorus was observed at Marion Island (L= 2.6). Over time the spectral structure of the chorus transformed into a hiss band spanning the same frequency range. The observation of overlapping chorus and hiss suggests that Marion Island was close to the plasmapause at the time of this event, and provides ground‐based observational confirmation of the generation mechanism of plasmaspheric hiss from chorus waves outside of the plasmasphere. Chorus observations at Marion Island were not common during this period of the solar cycle and so this event was investigated in detail. The geomagnetic conditions are discussed and geosynchronous particle data and broadband data from two other stations are presented. Empirical models are employed to predict the location of the plasmapause, and its location is inferred from a knee whistler recorded at Dunedin, New Zealand. These show that Marion Island is in the vicinity of the plasmapause during the event. The event is also compared to chorus observed at similarL after the Halloween storms of 2003. The rarity of the chorus observation is quantified using DEMETER VLF data. The DEMETER data, along with the various ground based VLF measurements, allows us to infer temporal and spatial variations in the chorus source region.

Contrasting the responses of three different ground‐based instruments to energetic electron precipitation

In order to make best use of the opportunities provided by space missions such as the Radiation Belt Storm Probes, we determine the response of complementary subionospheric radiowave propagation measurements (VLF), riometer absorption measurements, cosmic noise absorption, and GPS‐produced total electron content (vTEC) to different energetic electron precipitation (EEP). We model the relative sensitivity and responses of these instruments to idealized monoenergetic beams of precipitating electrons, and more realistic EEP spectra chosen to represent radiation belts and substorm precipitation. In the monoenergetic beam case, we find riometers are more sensitive to the same EEP event occurring during the day than during the night, while subionospheric VLF shows the opposite relationship, and the change in vTEC is independent. In general, the subionospheric VLF measurements are much more sensitive than the other two techniques for EEP over 200 keV, responding to flux magnitudes two‐three orders of magnitude smaller than detectable by a riometer. Detectable TEC changes only occur for extreme monoenergetic fluxes. For the radiation belt EEP case, clearly detectable subionospheric VLF responses are produced by daytime fluxes that are ∼10 times lower than required for riometers, while nighttime fluxes can be 10,000 times lower. Riometers are likely to respond only to radiation belt fluxes during the largest EEP events and vTEC is unlikely to be significantly disturbed by radiation belt EEP. For the substorm EEP case both the riometer absorption and the subionospheric VLF technique respond significantly, as does the change in vTEC, which is likely to be detectable at ∼3–4 total electron content units.

Analysis of daytime ionosphere behavior between 2004 and 2008 in Antarctica

In this paper we present 5-year study of the lower ionosphere behavior obtained from VLF sounding done at Comandante Ferraz Brazilian Antarctic Station. The results suggested a strong influence of ‘meteorological processes’ especially during wintertime, when VLF measurements showed that the D-region was strongly affected by planetary waves in all years. This effect was superposed to the well known control by solar radiation. The 16-day wave was the most important PW modulating the lower ionosphere behavior. Since planetary waves are very predominantly of tropospheric origin, this result is an evidence of the vertical couplings in the atmosphere–ionosphere system.

Delineation of subsurface structures using resistivity, VLF and radiometric measurement around a U-tailings pond and its hydrogeological implication

The hydrogeological characteristics of the uranium mill tailings pond in the vicinity of Jaduguda (Jharkhand, India) were investigated to examine possible contamination and suggest suitable remedial measures, if required. As the hydrogeological characteristics of subsurface geology are closely related to the electrical properties of the subsurface, geophysical measurements using electrical resistivity coupled with Very Low Frequency electromagnetic method and radiation study were used to investigate the geophysical and geological condition of mill tailings in order to characterize the subsurface structures of the tailings pond. The resistivity interpretation depicted the thickness of the soil cover and thickness of tailings in the pond, as well as the depth to the basement. It also suggested the possible flow direction of leachate. It was observed that the resistivity of the top layer decreases in the direction opposite to the dam axis, which in turn, indicated that the groundwater movement occurs in the opposite direction of the dam axis (in the northwest direction). The VLF method depicted the fractures through which groundwater moves, and also showed the current density alignment in the northwest direction at 10 m depth. The radiation measurement showed relatively higher counts in the northwest direction. This correlated well with the resistivity measurement. The current density at a depth of 20 m showed a closed contour suggesting no groundwater movement in the area at this depth, and that high conductivity material was confined to the tailings area only. It was concluded that groundwater moves in opposite direction of the dam axis at shallower depth only. It was found that continuation of fractures do not extend to deeper depths, which suggested that the tailings storage facility at Jaduguda was reasonably safe from any downward contamination.

Integrated geophysical exploration for the Longtoushan Ag-Pb-Zn deposit in the southeast of the Da Xing’an Ling mountains, Inner Mongolia, northern China

Geophysical and geological studies attempting to search and explore new mineralisation resources were carried out in the Longtoushan area, Inner Mongolia, northern China. The Longtoushan area is located within a large alteration zone, which is covered by a variety of altered lava, bioclast limestone and Quaternary sediments. In this case study, ground geophysical surveys played an important role in defining the mineralised structures beneath Quaternary sediment cover. The aim of this study was to investigate the subsurface mineralisation in the area using a combination of very low frequency electromagnetic (VLF-EM), Stratagem EH4 and controlled source audiofrequency magnetotelluric (CSAMT) measurements. These successful surveys in the area revealed that the VLF, Stratagem EH4 and CSAMT methods were effective in detecting the unseen mineralised system and results from the different methods confirmed each other. VLF measurements at 2875 stations along 35 lines identified the principal mineralised shear system as two sub-parallel linear conductive belts, tracing the principal mineralised system for a further 650 m of strike length beneath cover loess, in a NW–SE-trending direction. The subsequent Stratagem EH4 soundings along four parallel traverses perpendicular to the mineralised trend indicated that the principal mineralised shear structure, extending for more than 600 m in the dip direction, was almost vertically dipping above the 200 m level but changed to the SSW direction at a steep angle at depth. A CSAMT survey on two lines confirmed that the ore-bearing mineralised zone was a conductive belt and contained a low resistivity anomaly of less than 500 Ωm.

Airborne VLF measurements and mapping of ground conductivity in Sweden

Airborne VLF data are routinely collected by The Geological Survey of Sweden (SGU) as part of its bedrock mapping programme. In this paper we demonstrate that the novel Tensor VLF technique developed at Uppsala University and SGU can provide useful qualitative and quantitative information about the electrical conductivity distribution in the upper few hundred meters. Single transmitter scalar VLF maps emphasize those conductive structures that have dominant strikes in the direction of the transmitter. The tensor tipper (essentially the vertical magnetic field from currents along the strike direction) calculated from multiple transmitters is dependent only upon the underlying conductivity structure. Transformation of the tipper into the peaker (the horizontal divergence) has proven to enhance the lateral resolution while the transformation to the apparent resistivity can be used to discriminate different rock types. Two case histories from the application of VLF data are presented in this study. Two dimensional structures can be quantitatively modelled by modern inversion methods developed originally for deep electromagnetic MT soundings. Direct inversion of the real and imaginary parts of the tipper provides more quantitative information about the subsurface resistivity distribution.

Nighttime ionospheric D region parameters from VLF phase and amplitude

Nighttime ionospheric D region heights and electron densities are determined from an extensive set of VLF radio phase and amplitude observations. The D region parameters are characterized by the traditional H′ (height in kilometers) and β (sharpness in km−1) as used by Wait and by the U. S. Navy in their Earth‐ionosphere waveguide programs. The VLF measurements were made with several frequencies in the range 10 kHz to 41 kHz on long, mainly all‐sea paths, including Omega La Reunion and Omega Argentina to Dunedin, New Zealand, NAU (Puerto Rico) and NAA (Maine, USA) to Cambridge, UK, and NPM (Hawaii) to San Francisco. Because daytime VLF propagation on such paths is readily measured and predicted, the differences between night and day amplitudes and phases were measured and compared with calculations for a range of nighttime ionospheric parameters. This avoided the problem of uncertainties in the transmitter powers. In this way the height, H′, and the sharpness, β, when averaged over periods of several days, at least for the midlatitude D region near solar minimum, were found to be 85.1 ± 0.4 km and 0.63 ± 0.04 km−1, respectively.