Ionospheric D-region Research Articles

Effects of the Great American Solar Eclipse on the lower ionosphere observed with VLF waves

The altitude of the ionospheric lower layer (D-region) is highly influenced by the solar UV flux affecting in turn, the propagation of Very Low Frequency (VLF) signals inside the waveguide formed between this layer and the Earth surface. The day/night modulation observed in these signals is generally used to model the influence of the solar irradiance onto the D-region. Although, these changes are relatively slow and the transitions are “contaminated” by mode coalescences. In this way, a rapid change of the solar irradiance, as during a solar eclipse, can help to understand the details of the energy transfer of the solar radiation onto the ionospheric D-layer. Using the ”Latin American VLF Network” (LAVNet-Mex) receiver station in Mexico City, Mexico, we detected the phase and amplitude changes of the VLF signals transmitted by the NDK station at 25.2 kHz in North Dakota, USA during the August 21, 2017 solar eclipse. As the Sun light was eclipsed, the rate of ionization in the ionosphere (D-region) was reduced and the effective reflection height increased, causing a considerable drop of the phase and amplitude of the observed VLF waves. The corresponding waveguide path is 3007.15 km long and crossed almost perpendicularly the total eclipse path. Circumstantially, at the time of the total eclipse a C3 flare took place allowing us to isolate the flare flux from the background flux of a large portion of the disk. In this work we report the observations and present a new model of the ionospheric effects of the eclipse and flare. The model is based on a detailed setup of the degree of Moon shadow that affects the entire Great Circle Path (GCP). This relatively simple model, represents a new approach to obtain a good measure of the reflection height variation during the entire eclipse time interval. During the eclipse, the maximum phase variation was −63.36° at 18:05 UT which, according to our model, accounts for a maximum increase of the reflection height of 9.3 km.

GNSS and SAR Signal Delay in Perturbed Ionospheric D-Region During Solar X-Ray Flares

We investigate the influence of the perturbed (by a solar X-ray flare) ionospheric D-region on the global navigation satellite systems (GNSS) and synthetic aperture radar (SAR) signals. We calculate a signal delay in the D-region based on the low ionospheric monitoring by very-low-frequency (VLF) radio waves. The results show that the ionospheric delay in the perturbed D-region can be important and, therefore, should be taken into account in modeling the ionospheric influence on the GNSS and SAR signal propagation and in calculations relevant for space geodesy. This conclusion is significant because numerous existing models ignore the impact of this ionospheric part on the GNSS and SAR signals due to its small electron density which is true only in quiet conditions and can result in significant errors in space geodesy during intensive ionospheric disturbances.

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.

Variations in ionospheric D-region recombination properties during increase of its X-ray heating induced by solar X-ray flare

In this paper we present an analysis of parameters describing the effective recombination processes in the upper ionospheric D-region in the period of its additional heating by the X-radiation emitted during a solar X-ray flare. We present a procedure for calculation of the effective recombination coefficient and electron loss rate in the period when the X-radiation flux detected by the GOES satellite in the wavelength domain between 0.1 and 0.8 nm increases. The developed procedure is based on observational data obtained in the low ionospheric monitoring by the very low/low frequency radio waves and it is related to the considered area and time period. The obtained expressions are applied to data for the very low frequency signal emitted in Germany and recorded in Serbia during the solar X-ray flare detected by the GOES-14 satellite on May 5, 2010.

Comparison of Numerical Simulations of the Electron Density in the D-Region Ionosphere with Measurements of the Radar of Partial Reflections

The paper reports results of a comparison of the measured electron density in the ionospheric D-region measured using the partial reflection facility at the observatory. Tumanny of the Polar Geophysical Institute (69.0°N, 35.7°E) with numerical simulations performed using the theoretical model of the Polar Geophysical Institute (PGI) (Murmansk, Russian Federation). The model was examined using experimental data obtained under quiet geomagnetic conditions in March, 2017. The comparative analysis carried out in this study shows a very good agreement of the PGI model with experimental data and indicates that the IRI-2016 model fails to adequately reproduce measurements in regions with high electron density gradients.

Анализ результатов расчетов концентрации электронов по детерминированно-вероятностной модели среднеширотной невозмущенной D-области ионосферы

The work is devoted to the development of a fundamentally new way of modeling the ionospheric D-region — deterministic-probabilistic. The results of electron density (Ne) calculations using this technique are analyzed. Research of this kind is of fundamental importance, related to the rejection of a purely deter-ministic description of a continuously changing envi-ronment such as the ionosphere. In this work, Ne is cal-culated using a five-component system of ionization-recombination cycle equations. Probability density functions (PDFs) of input parameters of the model are used to solve the system. The most important sources of the D-region ionization are taken into account to calculate PDFs of the ionization rate. The necessary number of iterations is determined by the convergence of PDFs of the electron density from 50 to 85 km at midlatitudes under different heliogeophysical conditions. Theoretical Ne PDFs have been shown to be in good agreement with two experimental databases on electron density, especially at large D-region heights. The next important stage of modeling is the thorough verification of PDFs from experimental radiophysical data on VLF—LF propagation.

Analysis of electron density calculations using deterministic-probabilistic model of the ionospheric D-region

The work is devoted to the development of a fundamentally new way of modeling the ionospheric D-region – deterministic-probabilistic. The results of Ne calculations using this technique are analyzed. Research of this kind is of fundamental importance, related to the rejection of a purely deterministic description of a continuously changing environment such as the ionosphere. In this work, the electron density is calculated using a five-component system of ionization-recombination cycle equations. Probability density functions (PDFs) of input parameters of the model are used to solve the system. The most important sources of the D-region ionization are taken into account to calculate PDFs of the ionization rate. The necessary number of iterations is determined by the convergence of PDFs of the electron density from 50 km to 85 km at midlatitudes under different heliogeophysical conditions. Theoretical Ne PDFs have been shown to be in good agreement with two experimental databases on electron density, especially at large D-region heights. The next important stage of modeling is the thorough verification of Ne PDFs from experimental radiophysical data on VLF–LF propagation.

Analysis of the Relationship Between the Solar X-Ray Radiation Intensity and the D-Region Electron Density Using Satellite and Ground-Based Radio Data

Increases in the X-ray radiation that is emitted during a solar X-ray flare induce significant changes in the ionospheric D region. Because of the numerous complex processes in the ionosphere and the characteristics of the radiation and plasma, the causal-consequential relationship between the X-ray radiation and ionospheric parameters is not easily determined. In addition, modeling the ionospheric D-region plasma parameters is very difficult because of the lack of data for numerous time- and space-dependent physical quantities. In this article we first give a qualitative analysis of the relationship between the electron density and the recorded solar X-ray intensity. After this, we analyze the differences in the relationships between the D-region response and various X-ray radiation properties. The quantitative study is performed for data observed on 5 May 2010 in the time period between 11:40 UT – 12:40 UT when the GOES 14 satellite detected a considerable X-ray intensity increase. Modeling the electron density is based on characteristics of the 23.4 kHz signal emitted in Germany and recorded by the receiver in Serbia.

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.

The Role of Excited Oxygen Molecules in the Formation of the Secondary Ozone Layer at 87 to 97 km

The secondary ozone layer is located at elevations of 87 to 97 km in the upper mesosphere – lower thermosphere. It overlaps with the ionospheric D-layer. Daytime intensive UV radiation is dissociating O2 molecules to O atoms and photoexcitating O2 molecules up to 11.07eV level. Ozone photolysis between the wavelengths of 118.7–121.6 nm produces three oxygen atoms from one ozone molecule. Collision reactions of and with produce additional oxygen atoms. The number of oxygen atoms is maintained at such a high level that a small but significant ozone concentration survives. UV radiation weakens radically during the night. The number of O atoms shows no diurnal variation in the MLT. This leads to a ten-fold increase of ozone concentration over the course of the night. Dissociative recombination of O2+ (entered via diffusion from above) and reactions of O (3P) atoms with excited O2 molecules generate O(1S) atoms. The quenching of O(1S)→O(1D) emits the green nightglow. The reactions of O(1D) with ozone and O2 absorption of UV nightglow produce . When these molecules relax, they emit the O2 UV nightglows. The relaxations of and emit infrared nightglows.

Long-term climate change in the D-region

Controversy exists over the potential effects of long-term increases in greenhouse gas concentrations on the ionospheric D-region at 60–90 km altitudes. Techniques involving in-situ rocket measurements, remote optical observations, and radio wave reflection experiments have produced conflicting results. This study reports a novel technique that analyses long-distance subionospheric very low frequency radiowave observations of the NAA 24.0 kHz transmitter, Cutler, Maine, made from Halley Station, Antarctica, over the period 1971–2016. The analysis is insensitive to any changes in the output power of the transmitter, compensates for the use of different data logging equipment, and can confirm the accuracy of the timing systems operated over the 45 year long record. A ~10% reduction in the scale size of the transmitter nighttime interference fringe pattern has been determined, taking into account the quasi-11 year solar cycle. Subionospheric radiowave propagation modeling suggests that the contraction of the interference fringe pattern about the mid-latitude NAA transmitter is due to a 3 km reduction in the effective height of the nighttime ionospheric D-region over the last 45 years. This is consistent with the effect of enhanced infra-red cooling by increasing greenhouse gases.

Changes of atmospheric properties over Belgrade, observed using remote sensing and in situ methods during the partial solar eclipse of 20 March 2015

Measurements of atmospheric parameters were carried out during the partial solar eclipse (51% coverage of solar disc) observed in Belgrade on 20 March 2015. The measured parameters included height of the planetary boundary layer (PBL), meteorological parameters, solar radiation, surface ozone and air ions, as well as Very Low Frequency (VLF, 3–30 kHz) and Low Frequency (LF, 30–300 kHz) signals to detect low-ionospheric plasma perturbations. The observed decrease of global solar and UV-B radiation was 48%, similar to the solar disc coverage. Meteorological parameters showed similar behavior at two measurement sites, with different elevations and different measurement heights. Air temperature change due to solar eclipse was more pronounced at the lower measurement height, showing a decrease of 2.6 °C, with 15-min time delay relative to the eclipse maximum. However, at the other site temperature did not decrease; its morning increase ceased with the start of the eclipse, and continued after the eclipse maximum. Relative humidity at both sites remained almost constant until the eclipse maximum and then decreased as the temperature increased. The wind speed decreased and reached minimum 35 min after the last contact. The eclipse-induced decrease of PBL height was about 200 m, with minimum reached 20 min after the eclipse maximum. Although dependent on UV radiation, surface ozone concentration did not show the expected decrease, possibly due to less significant influence of photochemical reactions at the measurement site and decline of PBL height. Air-ion concentration decreased during the solar eclipse, with minimum almost coinciding with the eclipse maximum. Additionally, the referential Line-of-Sight (LOS) radio link was set in the area of Belgrade, using the carrier frequency of 3 GHz. Perturbation of the receiving signal level (RSL) was observed on March 20, probably induced by the solar eclipse. Eclipse-related perturbations in ionospheric D-region were detected based on the VLF/LF signal variations, as a consequence of Lyα radiation decrease.

Diagnostics of plasma in the ionospheric D-region: detection and study of different ionospheric disturbance types

Here we discuss our recent investigations of the ionospheric plasma by using very low and low frequency (VLF/LF) radio waves. We give a review of how to detect different low ionospheric reactions (sudden ionospheric disturbances) to various terrestrial and extra-terrestrial events, show their classification according to intensity and time duration, and present some methods for their detections in time and frequency domains. Investigations of detection in time domain are carried out for intensive long-lasting perturbations induced by solar X-ray flares and for short-lasting perturbations caused by gamma ray bursts. We also analyze time variations of signals used in the low ionospheric monitoring after earthquake events. In addition, we describe a procedure for the detection of acoustic and gravity waves from the VLF/LF signal analysis in frequency domain. The research of the low ionospheric plasma is based on data collected by the VLF/LF receivers located in Belgrade, Serbia.

Экспериментальное определение эффективных коэффициентов рекомбинации в D-области ионосферы высоких широт во время солнечных затмений по данным метода частичных отражений

Экспериментальное определение эффективных коэффициентов рекомбинации в D-области ионосферы высоких широт во время солнечных затмений по данным метода частичных отражений

India's Contribution to Geomagnetism and Allied Studies in Antarctica-A Review

In this paper, we review the progress in Geomagnetism and allied areas based on the past thirty five Indian Antarctic expeditions and summarize the scientific results obtained in the last decade. Various dynamic processes in the near-Earth space environment, driven by the transient changes in geomagnetic field such as storms and substorms, severely affect the space weather over the Polar Regions. Magnetopsheric substorms lead to the intensification of ionospheric currents and auroral outbursts within the auroral oval, causing energetic charged particles in the auroral region to come down to D-region ionosphere, which is reflected as Cosmic Noise Absorption (CNA) as monitored by imaging RIOmeter at Indian Antarctic station, Maitri (Geog. 70.75o S, 11.73oE; Geom. 66.84oS, 56.29oE). A systematic rapid decline in The Earth’s complex main magnetic field at Maitri (~110 nT/yr) is important for monitoring the evolution of reverse magnetic flux patches due to physical processes occurring in the outer core of the Earth. The Global Electric Circuit (GEC) studies were started to understand the solar-terrestrial relationship and associated changes in surface weather and the near-earth electrical environment. Schumann resonances (SRs), the AC part of GEC reveal a strong UT variation of amplitude in seasonal as well as yearly time scales. The observed diurnal variation is explained in terms of the dominant thunderstorm activity centered over the three convectively active regions, viz. Asia/Maritime Continent (Indonesia), South America and Africa. The velocity and strain distribution of Schirmarchaer Glacier was investigated during two GPS campaigns in the year 2003 and 2004. The studies indicate that the horizontal velocity is in the range of 1.89-10.88 ma–1 with an average velocity of 6.21 ma–1.

Space weather effects on lower ionosphere: First investigation from Bharati station during 34th Indian scientific expedition to Antarctica

We investigate the solar flare effects on the D-region of the ionosphere with the help of VLF (Very Low Frequency) radio waves using a portable E-field system from Antarctica during the summer period of 34th Indian scientific expedition. Two GPS time synchronized VLF receivers, one located at Bharati, Antarctica (geographical latitude 69.40°S, longitude 76.18°E) and another located at Tripura, India (geographical latitude 23.84°N, longitude 91.28°E) were operated simultaneously to infer common mode changes in the lower ionosphere for a number of solar flares events. The two systems constantly monitored the carrier amplitude and phase of the MSK (Minimum Shift Keying) modulated navy transmitter located in Australia (Callsign: NWC, 19.8kHz, geographical latitude 21.88°S, longitude 114.13°E), around 5.6Mm great circle distance from the two receivers. The results are interpreted in terms of Earth-ionosphere wave-guide characteristics. A Long Wave Propagation Capability (LWPC) model study is also performed to infer the changes in the daytime electron density in polar D-region ionosphere during the solar flares. The exponential fit of the modeled electron density change with average X-ray flux change shows an excellent correlation (R2 value 0.95). The exponential fit is utilized to infer the daytime electron density change in the polar ionosphere during solar flare events. The analyses indicate that small solar flares of class ‘C’ can be very effectively detected with the portable antenna system even if the receiver is located in polar coastal region compared to equatorial region. The expedition results also demonstrate the feasibility of using portable VLF receivers from the coastal stations for monitoring the polar lower ionosphere from Antarctica and open up new opportunities for long term exploration.

A Study on the VLF/LF Long Term Amplitude Oscillations Associated with Frequencies 37.5 kHz and 45.9 kHz Received at Keil Longwave Monitor, Keil, Germany

The middle and upper atmosphere of Earth displays many large-scale oscillations in several parameters. Of these oscillations, Atmospheric Oscillation (AO) and Semi-annual Oscillation (SAO) are prominent ones. In this work, we have analyzed the Very Low Frequency/Low Frequency (VLF/LF) data from two of the transmitters of frequency 45.9 kHz at NSY, Sicily, Italy and 37.5 kHz at Grindavik, Iceland. The selected Trasmitter-Receiver Great Circle Path (TRGCP) is middle latitude which is marine in case of Grindavik TRGCP and terrestrial in case of NSY TRGCP. The VLF/LF signals are bounced back and forth from D-layer of ionosphere (altitude at ~65 km during day and ～95 km during night) in Earth-ionospheric waveguide. This infers the presence of atmospheric oscillations as a consequence of change in ionization and recombination rates. Many works related to AO and SAO are mostly done only for equatorial region of the ionosphere and authors have reported the elegant dominancy of AO and SAO in the VLF/LF amplitude pattern over years. To our surprise, in our work it is seen that not only AO and SAO but also other oscillations are necessary to model the oscillation pattern of middle latitude ionosphere.

Responses of the ionospheric D-region to periodic and transient variations of the ionizing solar Lyα radiation

Solar radiation has the most important role in periodical variation of terrestrial atmospheric properties. Under unperturbed ionospheric conditions, the solar Lyα line has a dominant influence on ionization processes in the lowest ionospheric layer, the so called D-region. In this paper, we present periodical and transient variations in influences of the Lyα radiation on this ionospheric layer. In the case of periodical lower ionospheric changes we consider diurnal, seasonal and solar cycle variations and show analysis of acoustic and gravity waves induced by solar terminator. Influences of solar flares and eclipses on this atmospheric layer are analyzed as examples of sudden ionospheric disturbances. For decades, Very Low Frequency radio signals (3 – 30 kHz) are successfully used as a tool for monitoring of changes in the lower ionosphere, based on radio wave propagation through Earth-ionosphere waveguide along given trajectories and registration of their physical parameters (amplitude and phase delay). For the analysis conducted in this paper, we used records of the VLF DHO signal, emitted on 23.4 kHz frequency from transmitter in Germany and received in Serbia.

AC and DC global electric circuit properties and the height profile of atmospheric conductivity

An apparent discrepancy is pointed out - at all heights, and by up to an order of magnitude - between the height profiles of atmospheric conductivity derived at AC using ELF propagation studies, especially from information on Schumann resonance of the Earth-ionosphere cavity, and using a model of the DC global atmospheric electric circuit. This serious issue is resolved by creating a hybrid profile of these two mid-latitude profiles, the first of which refers to conditions by day and the second by night. This hybrid profile is thus a first order attempt to represent globally averaged conditions. Close to the Earth’s surface, where the resistance of the atmosphere is largest, the properties of the DC global model exert the greatest influence, whereas in the middle atmosphere, at heights between 40 and 100 km, full wave computations show that the AC results are the more crucial. The globally averaged hybrid profile presented here has some limitations, and the physical reasons for these are addressed. They are due to the presence of aerosol particles of ice and/or of meteoric material which reduce the ionospheric D-region conductivity by an order of magnitude over only ~2 km of height, thereby causing ledges of ionisation. In the context of the globally averaged profile, published observations of the ionospheric effects of the giant gamma-ray flare from SGR 1806-20 (a neutron star having an enormously large magnetic field) occurring at 21:30 U.T. on December 27, 2004, are briefly discussed.

On the Use of VLF Narrowband Measurements to Study the Lower Ionosphere and the Mesosphere–Lower Thermosphere

The ionospheric D-region (~60 km up to ~95 km) and the corresponding neutral atmosphere, often referred to as the mesosphere–lower thermosphere (MLT), are challenging and costly to probe in situ. Therefore, remote sensing techniques have been developed over the years. One of these is based on very low frequency (VLF, 3–30 kHz) electromagnetic waves generated by various natural and man-made sources. VLF waves propagate within the Earth–ionosphere waveguide and are extremely sensitive to perturbations occurring in the D-region along their propagation path. Hence, measurements of these signals serve as an inexpensive remote sensing technique for probing the lower ionosphere and the MLT region. This paper reviews the use of VLF narrowband (NB) signals (generated by man-made transmitters) in the study of the D-region and the MLT for over 90 years. The fields of research span time scales from microseconds to decadal variability and incorporate lightning-induced short-term perturbations; extraterrestrial radiation bursts; energetic particle precipitation events; solar eclipses; lower atmospheric waves penetrating into the D-region; sudden stratospheric warming events; the annual oscillation; the solar cycle; and, finally, the potential use of VLF NB measurements as an anthropogenic climate change monitoring technique.