Subionospheric Propagation Research Articles

Ionospheric perturbations in possible association with the 2010 Haiti earthquake, as based on medium-distance subionospheric VLF propagation data

Abstract. Ionospheric perturbations in possible association with the 2010 Haiti earthquake occurred on 12 January 2010 (with a magnitude of 7.0 and depth of 10 km) are investigated on the basis of subionospheric propagation data from the NAA transmitter on the east coast of the USA to a VLF receiving station in Peru. The local nighttime VLF amplitude data are extensively investigated during the period from the beginning of October 2009 to the end of March 2010, in which the trend (nighttime average amplitude), dispersion and nighttime fluctuation are analysed. It is found that a clear precursory ionosphere perturbation is detected just around New Years day of 2010, about 12 days before the main shock, which is characterised by the simultaneous decrease in the trend and the increases in dispersion and nighttime fluctuation. An additional finding might be the presence of the effect of the Earth's tide one and two months before the main shock, which can only be seen for a huge EQ.

Experimental simulation of satellite observations of 100 kHz radio waves from relativistic electron beams above thunderclouds

Abstract. Relativistic electron beams above thunderclouds emit 100 kHz radio waves which illuminate the Earth's atmosphere and near-Earth space. This contribution aims to clarify the physical processes which are relevant for the spatial spreading of the radio wave energy below and above the ionosphere and thereby enables an experimental simulation of satellite observations of 100 kHz radio waves from relativistic electron beams above thunderclouds. The simulation uses the DEMETER satellite which observes 100 kHz radio waves from fifty terrestrial Long Range Aid to Navigation (LORAN) transmitters. Their mean luminosity patch in the plasmasphere is a circular area with a radius of 300 km and a power density of 22 μW/Hz as observed at 660 km height above the ground. The luminosity patches exhibit a southward displacement of 450 km with respect to the locations of the LORAN transmitters. The displacement is reduced to 150 km when an upward propagation of the radio waves along the geomagnetic field line is assumed. This residual displacement indicates that the radio waves undergo 150 km sub-ionospheric propagation prior to entering a magnetospheric duct and escaping into near-Earth space. The residual displacement at low (L < 2.14) and high (L > 2.14) geomagnetic latitudes ranges from 100 km to 200 km which suggests that the smaller inclination of the geomagnetic field lines at low latitudes helps to trap the radio waves and to keep them in the magnetospheric duct. Diffuse luminosity areas are observed northward of the magnetic conjugate locations of LORAN transmitters at extremely low geomagnetic latitudes (L < 1.36) in Southeast Asia. This result suggests that the propagation along the geomagnetic field lines results in a spatial spreading of the radio wave energy over distances of 1 Mm. The summative assessment of the electric field intensities measured in space show that nadir observations of terrestrial 100 kHz radio waves, e.g., from relativistic electron beams above thunderclouds, are attenuated by at least 50 dB when taking into account a transionospheric attenuation of 40 dB.

Solar flare detection sensitivity using the South America VLF Network (SAVNET)

We present recent observations of Sudden Phase Anomalies due to subionospheric propagation anomalies produced by solar X‐ray flares. We use the new South America VLF Network (SAVNET) to study 471 ionospheric events produced by solar flares during the period May 2006 to July 2009 which corresponds to the present minimum of solar activity. For this activity level, we find that 100% of the solar flares with a X‐ray peak flux above 5 × 10−7 W/m2 in the 0.1–0.8 nm wavelength range produce a significant ionospheric disturbance, while the minimum X‐ray flux needed to do so is about 2.7 × 10−7 W/m2. We find that this latter minimum threshold is dependent on the solar cycle, increasing when the Sun is more active, thus confirming that the low ionosphere is more sensitive during periods of low solar activity. Also, our findings are in agreement with the idea that the ionospheric D‐region is formed and maintained by the solar Lyman‐α radiation outside solar flare periods.

On the correlation between ionospheric perturbations as detected by subionospheric VLF/LF signals and earthquakes as characterized by seismic intensity

The long-term data during seven years from January 2001 to December 2007, as observed by the Pacific VLF/LF network consisting of several Japanese stations and one station in Kamchatka, are extensively utilized to perform a statistical correlation study between the lower ionospheric perturbations as detected by subionospheric propagation and earthquakes (EQs). In this paper, we adopt a physical parameter, the maximum seismicity intensity observed ( I) to define the strength of an EQ unlike the previously and conventionally used EQ magnitude and depth, which is a combined effect of EQ magnitude and depth, together with the Earth’s surface information and geological condition around the EQ epicenter. After considering EQs only take place on the land because of the use of seismicity and by using the superimposed epoch analysis, it is found for the larger EQs with I from 5 to 7 (we feel serious trembling and we expect serious damage) that the most important VLF/LF parameter, trend (nighttime average amplitude), shows a definite decrease about 10 days before the EQ by exceeding 2 σ ( σ: standard deviation) criterion; the dispersion shows a maximum about 10 days before the EQ but not exceeding 2 σ line and finally the nighttime fluctuation shows an enhancement about 10 days before the EQ (with exceeding the 2 σ level). A definite statistical correlation is confirmed between the ionospheric perturbations and I when I is strong enough in a range from 5 to 7. Whereas, there is no significant correlation between the two when I is in a range from 3 to 4. Finally, together with the corresponding results for EQs in the sea, but close to the land, these results are discussed in the light of lithosphere–ionosphere coupling mechanism.

Fractal analysis of subionospheric LF propagation data and consideration of the lithosphere-atmosphere-ionosphere coupling

Abstract. Fractal analysis has been applied to the local nighttime data of subionospheric LF propagation, and the fractal dimension is estimated every day in the two distinct frequency ranges (AW: acoustic wave and AGW: atmospheric gravity wave). The data during several years are analyzed for the propagation paths from the Japanese transmitter of JJY to Moshiri (Hokkaido) and to Kochi. As the result of analysis, we come to the conclusion that when we pay attention to the period just around the earthquake, we sometimes detect some significant increases in the fractal dimension either in AW or AGW range. This indicates that the self – organization effect prior to an earthquake in the lithosphere, might be seen even in the lower ionosphere, probably in terms of atmospheric oscillation effect.

The ionospheric perturbations associated with Asian earthquakes as seen from the subionospheric propagation from NWC to Japanese stations

Abstract. Huge five earthquakes with magnitude greater than 6.0 took place in Asia (include Philippines, Indonesia, etc.) during the period from the beginning of August 2008 to the end of Junuary 2009, and the corresponding data of subionospheric VLF propagation between the NWC transmitter (Australia, 19.8 kHz) and a few Japanese stations (distance 6~8 Mm) are examined. As the result of our analysis by means of (1) trend (average nighttime amplitude), (2) dispersion, (3) nighttime fluctuation, and (4) atmospheric gravity wave enhancement, three earthquakes from the five taking place within the fifth Frenel zone are found to have accompanied a precursory signature in VLF propagation. On the other hand, there were observed no such precursory signatures for the remaing two earthquakes. One is too deep (>400 km) and another is too distant from the great-circle path. These characteristics of seismo-ionospheric perturbations would be of essential importance in studying the spatial/temporal properties of seismo-ionospheric perturbations for medium-distance propagation.

Ionospheric perturbations related to the Miyagi-oki earthquake on 16 August 2005, as seen from Japanese VLF/LF subionospheric propagation network

A case study has been performed on the lower ionospheric perturbations as seen from the Japanese subionospheric VLF (very low frequency, 3–30 kHz) /LF (low frequency, 30–300 kHz) propagation network for a recent powerful earthquake, i.e., the Miyagi-oki earthquake on 16 August 2005 (magnitude 7.2, and depth 36 km). Different propagation paths were examined and the two paths from a transmitter with call sign of JJY ( f = 40 kHz, in Fukushima prefecture) to both receiving stations of Kamachatka, Russia and Moshiri, Hokkaido, exhibit clear signatures of ionospheric perturbations a few days before and a few days after the earthquake. We have detected a clear nighttime amplitude decrease and an enhancement in nighttime fluctuation, both exceeding the corresponding three standard deviations from the mean. We discuss other seismogenic phenomena in order to study the lithosphere–ionosphere coupling.

On the generation mechanism of terminator times in subionospheric VLF/LF propagation and its possible application to seismogenic effects

Abstract. The signals from VLF/LF transmitters are known to propagate in the Earth-ionosphere waveguide, so that the subionospheric propagation characteristics are very sensitive to the condition of the lower ionosphere. We know that there appear the terminator times just around the sunrise and sunset in the diurnal variation of subionospheric VLF/LF signal (amplitude and phase). These terminator times are found to shift significantly just around an earthquake, which enables us to infer the change in the ionosphere during the earthquake. In this paper we try to understand the physical mechanism on the generation of those terminator times for relatively short propagation path (less than 2000 km) by means of wave-hop method. It is found that the lowering of the ionosphere boundary during an earthquake decreases the path length of the sky wave and this alters the interference condition of this wave with the ground wave, which lead to an appearance of terminator times as the destructive interference between the ground and sky waves. Finally, we suggest a possible use of terminator time shifts to investigate the lower ionospheric plasma changes during the earthquakes.

Initial results on solar flare effect on 24.8 kHz subionospheric propagation over long path to Suva

Solar flares are explosions on the surface of the sun that release a large amount of electromagnetic energy in the form of radio waves at the long wavelength end, through optical emission to X-rays at the short wavelength end. It has long been known that the solar flares, particularly associated with X-rays having wavelengths typically of tenths of nm, penetrate the lower region of the ionosphere (D-region) and increase the electron density via extra ionization (Mitra 1974). The normal unperturbed daytime D-region from which Very Low Frequency (VLF) signals are reflected is maintained mainly by Lyman-α radiation (121.6 nm) from the sun that partially ionizes the minor neutral constituent nitric oxide (at height around 70 km). Under normal conditions, the solar X-ray flux is too small to be a significant source for ionizing the D-region. However, when solar flare occurs, the X-ray flux increases significantly which with wavelengths below 1 nm penetrates down to the D-region and markedly increases the ionization of the neutral constituents particularly nitrogen and oxygen hence increases the electron density. The lower ionosphere can be characterized as the “Wait ionosphere” defined by a reference height H′in km and the exponential sharpness factor in km-1 (Wait and Spies 1964). Researchers have reported changes in the ionospheric parameters, H' and β as a function of solar X-ray flux (Thomson et al. 2004, 2005; Grubor et al. 2005; Zigman et al. 2007). The increase in the D-region electron density can produce significant perturbations in the phase and the amplitude of VLF signals propagating in the Earthionosphere waveguide.

Radiation belt electron precipitation into the atmosphere: Recovery from a geomagnetic storm

Large geomagnetic storms are associated with electron population changes in the outer radiation belt and the slot region, often leading to significant increases in the relativistic electron population. The increased population decays in part through the loss, that is, precipitation from the bounce loss cone, of highly energized electrons into the middle and upper atmosphere (30–90 km). However, direct satellite observations of energetic electrons in the bounce loss cone are very rare due to its small angular width. In this study we have analyzed ground‐based subionospheric radio wave observations of electrons from the bounce loss cone at L = 3.2 during and after a geomagnetic disturbance which occurred in September 2005. Relativistic electron precipitation into the atmosphere leads to large changes in observed subionospheric amplitudes. Satellite‐observed energy spectra from the CRRES and DEMETER spacecraft were used as an input to an ionospheric chemistry and subionospheric propagation model, describing the ionospheric ionization modifications caused by precipitating electrons. We find that the peak precipitated fluxes of >150 keV electrons into the atmosphere were 3500 ± 300 el cm−2 s−1 at midday and 185 ± 15 el cm−2 s−1 at midnight. For 6 d following the storm onset the midday precipitated fluxes are approximately 20 times larger than observed at midnight, consistent with observed day/night patterns of plasmaspheric hiss intensities. The variation in DEMETER observed wave power at L = 3.2 in the plasmaspheric hiss frequency band shows similar time variation to that seen in the precipitating particles. Consequently, plasmaspheric hiss with frequencies below ∼500 Hz appears to be the principal loss mechanism for energetic electrons in the inner zone of the outer radiation belts during the nonstorm time periods of this study, although off‐equatorial chorus waves could contribute when the plasmapause is L < 3.0.

Storm time, short‐lived bursts of relativistic electron precipitation detected by subionospheric radio wave propagation

In this study we report on ground‐based observations of short bursts of relativistic electron precipitation (REP), detected by a subionospheric propagation sensor in Sodankylä, Finland during 2005. In two ∼4 hour case study periods from L = 5.2, around local midnight, several hundred short‐lived radio wave perturbations were observed, covering a wide range of arrival azimuths. The vast majority (∼99%) of these perturbations were not simultaneous with perturbations on other paths, consistent with a precipitation “rainstorm” producing ionospheric changes of small spatial sizes around the Sodankylä receiver. The recovery time of these radio wave perturbations are ∼1.2 s, which is consistent with the modeled effects of a burst of >2 MeV precipitating electrons. This agrees with satellite observations of the microburst energy spectrum. The energetic nature of the precipitation which produces the FAST perturbations suggests that they should be observable in both day and night conditions. While it is widely assumed that satellite‐detected REP microbursts are due to wave‐particle interactions with very low‐frequency chorus waves, the energy spectra predicted by our current models of chorus propagation and wave‐particle interaction are not consistent with the experimentally observed radio wave perturbations presented here or previously reported satellite observations of REP microbursts. The results inferred from both the satellite and subionospheric observations, namely the absence of a large, dominant component of <100 keV precipitating electrons, fundamentally disagrees with a mechanism of chorus‐driven precipitation. Nonetheless, further work on the modeling of chorus‐driven precipitation is required.

The effect of subionospheric propagation on whistlers recorded by the DEMETER satellite – observation and modelling

Abstract. During a routine analysis of whistlers on the wide-band VLF recording of the DEMETER satellite, a specific signal structure of numerous fractional-hop whistlers, termed the "Spiky Whistler" (SpW) was identified. These signals appear to be composed of a conventional whistler combined by the compound mode-patterns of guided wave propagation, suggesting a whistler excited by a lightning "tweek" spheric. Rigorous, full-wave modelling of tweeks, formed by the long subionospheric guided spheric propagation and of the impulse propagation across an arbitrarily inhomogeneous ionosphere, gave an accurate description of the SpW signals. The electromagnetic impulses excited by vertical, preferably CG lightning discharge, exhibited the effects of guided behaviour and of the dispersive ionospheric plasma along their paths. This modelling and interpretation provides a consistent way to determine the generation and propagation characteristics of the recorded SpW signals, as well as to describe the traversed medium.

A Review of Progress in FDTD Maxwell's Equations Modeling of Impulsive Subionospheric Propagation Below 300 kHz

Wave propagation at the bottom of the electromagnetic spectrum (below 300 kHz) in the Earth-ionosphere waveguide system has been an interesting and important area of investigation for the last four decades. Such wave propagation is characterized by complex phenomena involving nonhomogeneous and anisotropic media, and can result in resonances of the entire Earth-ionosphere cavity. In the spirit of this Special Issue, the goal of this paper is to call attention to emerging finite-difference time-domain computational solutions of Maxwell's equations for wave propagation below 300 kHz which promise to complement and extend previous analyses by pioneers such as Profs. Wait and Felsen. The following topical areas are discussed: long-range two-dimensional propagation, lightning sources and radiation, global propagation, Schumann resonances, hypothesized pre-seismic lithosphere sources and radiation, detection of deep underground resource formations, and remote sensing of localized ionospheric anomalies. We conclude with a prospectus for future research, especially in incorporating the physics of the anisotropic, nonhomogeneous magnetized plasma in a global planetary ionosphere.

Subionospheric VLF/LF monitoring of ionospheric perturbations for the 2004 Mid-Niigata earthquake and their structure and dynamics

This paper reports on the detailed spatial structure and temporal dynamics of the ionospheric perturbations associated with the huge 2004 Mid-Niigata earthquake in Japan. The terminator time method in VLF/LF subionospheric propagation has been used to deduce the presence and dynamics of the seismo-ionospheric perturbations by making full use of our Japanese VLF/LF network observations. It is found that significant shifts were observed in terminator time for some selected paths, a few to several days before the earthquake, and we obtained the anisotropic shape of the inferred ionospheric perturbation likely elongated along the fault lines there. The temporal dynamics of inhomogeneity of the perturbations is suggested on the basis of a comparison of the observed terminator time shift and theoretical full-wave computations (2D FDTD and 3D full-wave scattering).

Solar flare induced D region perturbation in the ionosphere, as revealed from a short‐distance VLF propagation path

Short‐distance (propagation distance less than 1,000 km) VLF subionospheric propagation is, for the first time, used to study the solar flare induced D‐region perturbations of the lower ionosphere. We have used two years data for a propagation path from the Japanese VLF transmitter (call sign JJI, frequency = 22.2 kHz, and located at Ebino, Kyushu) to a receiving station of Chofu, Tokyo. For the sake of comparison, we have also used the long distance VLF path (NWC‐Chofu). First of all, the value of perturbation in amplitude (ΔA) for the short‐distance path seems to be much more enhanced (ΔA = 8∼9 dB) for this short‐path than the long‐distance paths previously studied. The experimental value of ΔA is found to monotonically increase with the logarithm of X‐ray flux. This dependence is numerically simulated by means of the FDTD (finite difference time domain) method to deduce the D‐region perturbation, {1 + C exp {−(z − z0)2/2β2} (C: enhancement factor, z0: reference height and β: vertical extent) (this modification seems to be much more realistic than the previous two parameter modeling in an exponential profile). The numerical computations with z0 ∼60 km and β ∼ 10 km have shown that the value of ΔA increase up to ∼6 dB in a range of X‐ray flux from C = 10 to C = 103. As the conclusion, the ΔA is closely related with the logarithm of solar X‐ray flux, so that we will be able to deduce the C value from the observed ΔA value.

Modeling a large solar proton event in the southern polar atmosphere

We have modeled the effects of the Sodankylä Ion Chemistry model (SIC) electron density profiles on VLF propagation across the southern polar region during the first few days of the 4–10 November 2001 solar proton event (SPE). The results show that the SIC model is accurately reproducing the changes in ionization during the SPE. These results were obtained by approximating the SIC electron density profiles to the Wait β and h′ profiles where the densities were below 1000 el cm−3, a limitation that means during SPEs the technique is typically sensitive in the altitude range 50–60 km. The calculated values of β and h′ were applied to the part of the propagation path poleward of the L = 4 boundary for the Hawaii (NPM)–Halley great circle path. Comparing the change in amplitude of NPM at Halley during the SPE with the GOES satellite proton flux measurements, we observe a good correlation and thus conclude that the variability observed in the VLF data is primarily caused by >50 MeV proton fluxes. This suggests that the SPE produced ionization dominates all other precipitation sources at these altitudes during 4 and 5 November 2001. Consequently, these results suggest that the assumption made during the SIC modeling runs of 4–5 November 2001 of only having proton precipitation and no significant energetic electron precipitation was reasonable. Our work strongly suggests that VLF subionospheric propagation is a reliable tool for the study of SPEs and that it is particularly effective when used in conjunction with an atmospheric model such as SIC.

The impact of PMSE and NLC particles on VLF propagation

Abstract. PMSE or Polar Mesosphere Summer Echoes are a well-known phenomenon in the summer northern polar regions, in which anomalous VHF/UHF radar echoes are returned from heights ~85km. Noctilucent clouds and electron density biteouts are two phenomena that sometimes occur together with PMSE. Electron density biteouts are electron density depletion layers of up to 90%, which may be several kms thick. Using the NOSC Modefndr code based on Wait's modal theory for subionospheric propagation, we calculate the shifts in received VLF amplitude and phase that occur as a result of electron density biteouts. The code assumes a homogeneous background ionosphere and a homogeneous biteout layer along the Great Circle Path (GCP) corridor, for transmitter receiver path lengths in the range of 500–6000km. For profiles during the 10h about midnight and under quiet geomagnetic conditions, where the electron density at 85km would normally be less than 500el/cc, it was found that received signal perturbations were significant, of the order of 1–4dB and 5–40° of phase. Perturbation amplitudes increase roughly as the square root of frequency. At short range perturbations are rather erratic, but more consistent at large ranges, readily interpretable in terms of the shifts in excitation factor, attenuation factor and v/c ratios for Wait's modes. Under these conditions such shifts should be detectable by a well constituted experiment involving multiple paths and multiple frequencies in the north polar region in summer. It is anticipated that VLF propagation could be a valuable diagnostic for biteout/PMSE when electron density at 85km is under 500el/cc, under which circumstances PMSE are not directly detectable by VHF/UHF radars. Key words. Electromagnetism (wave propagation) – Ionosphere (polar ionosphere) – Radioscience (ionospheric propagation)

Ground observations of chorus following geomagnetic storms

It has been suggested that whistler mode chorus waves play a role in acceleration and loss of radiation belt electrons during geomagnetic storms. In this paper we present data from a complete solar cycle (1992–2002) of nearly continuous (>95%) VLF/ELF observations from the VLF/ELF Logger Experiment (VELOX) instrument at Halley station, Antarctica (76°S, 27°W, L = 4.3), to determine whether there is statistical evidence for enhanced whistler mode chorus waves during geomagnetic storms. The data comprise 1 s resolution measurements of ELF/VLF wave power in eight frequency bands from 500 Hz to 10 kHz. The variations in chorus activity during several storms, including the well‐studied Bastille Day event (14 July 2000), show enhanced wave power but are variable from event to event. The average behavior has been found from a superposed epoch analysis using 372 storms with minimum Dst less than −50 nT, including 82 large storms with minimum Dst less than −100 nT. Compared with average prestorm levels, the chorus intensity decreases in the storm main phase but is enhanced in the recovery phase, typically maximizing a day after the storm onset. At 1 kHz the enhancement is independent of storm severity, suggesting a saturation effect, whereas larger storms produce larger wave intensities at higher frequencies in the chorus band (e.g., 3 kHz), which is interpreted as the effect of a chorus source region located on lower L shells than for weaker storms. The storm chorus enhancement maximizes at postdawn local times, leading to a 24 hour recurrence effect. A long‐enduring depression in wave intensities, of 10 days or more, is found near the top of the normal chorus band (∼5 kHz). We suggest that this is due to precipitation from enhanced relativistic particle fluxes affecting the subionospheric propagation of spherics from nearby thunderstorm regions across the L = 2–4 zone.

Ionospheric E region remote sensing with ELF radio atmospherics

Radio remote sensing of the ionospheric E region can be difficult, particularly in the nighttime E region valley which lies between two regions of higher electron density. We show that extremely low frequency (ELF) electromagnetic waves launched from below penetrate this region because of their low attenuation and can be reflected from both the D region and the top of the E region valley. This double reflection, also caused by sporadic E layers, produces a measurable effect on the subionospheric propagation of ELF waves, which we demonstrate with numerical Earth‐ionosphere waveguide simulations. This sensitivity opens the possibility of remotely sensing the E region and detecting sporadic E layers with broadband ELF propagation measurements using lightning discharges as the source. From nighttime observations of ELF lightning radiation over the same propagation path on different days, we extract E region electron density profiles that show significant variability not only from day to day but over the course of a single night. This technique is inherently a path‐integrated measurement, enabling tomographic large‐scale ionospheric remote sensing with multiple sources and receivers.

Effect of earthquakes on lower ionosphere as found by subionospheric VLF propagation

Abstract Reliable VLF subionospheric signal effect related to the seismic activity was first reported by Hayakawa et al. (1996 a, b) in association with the great Kobe earthquake. We have analyzed the similar data around 10 other great earthquakes (magnitude M >6), in order to understand the main features of such an effect, and the following characteristics are emerged from our analysis: 1. (a) This effect looks as transient oscillations with 5–10 days period, which are initiated a few days before a large earthquake and relaxed for a few days or weeks after it. 2. (b) It is mainly related to crust earthquakes. 3. (c) It cannot be observed outside the sensitivity zone of VLF transmitter (first Fresnel zone) even for very large earthquakes (M >7). 4. (d) It appears when the resonant atmospheric oscillations with a period in a range of 5–11 days exist before an earthquake. Probably, the seismic influence on the VLF signal is explained by the generation of long-term gravity waves during an earthquake process and their intensification at the heights of 70–90 km.