Subionospheric Signals Research Articles

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.

Poleward‐displaced electron precipitation from lightning‐generated oblique whistlers

New simulation results show that obliquely‐propagating lightning‐generated whistlers which fill vast volumes of the magnetosphere readily induce significant energetic electron precipitation over large regions of the ionosphere, depositing appreciable energy flux to the upper atmosphere at locations substantially poleward of the lightning source. These results provide a new interpretation for the observed spatial relationships between lightning discharges and subsequently disturbed ionospheric regions inferred from subionospheric signal perturbations (i.e. Trimpi events) reported in several lightning‐induced electron precipitation (LEP) studies.

Subionospheric VLF signal perturbations possibly related to earthquakes

A likely VLF subionospheric signal effect related to seismic activity was first reported by Hayakawa et al. [1996a, b] in association with the great Kobe earthquake. We have analyzed similar data during periods around 10 other great earthquakes (magnitude M>6) in order to understand the main features of such an effect. The following characteristics emerged from our analysis: The effect appears as a transient oscillation with a 5‐ to 10‐day period, which is initiated a few days before a large earthquake and decays over a few days to weeks after it. It is mainly related to crustal earthquakes. It appears when resonant atmospheric oscillations with periods in a range of 5–11 days exist before the earthquake. The seismic influence on the VLF signal is probably explained by the generation of long‐period gravity waves during the earthquake process and their intensification at heights of 70–90 km.

VLF and LF signatures of mesospheric/lower ionospheric response to lightning discharges

New evidence is presented of disturbances of the electrical conductivity of the nighttime mesosphere and the lower ionosphere in association with lightning discharges. In addition to extensive documentation of the characteristics of a class of events heretofore referred to as early/fast VLF events [Inan et al., 1993], our data reveal a new feature of these events, consisting of a postonset peak that typically lasts for 1–2 s. We also report the observation of short‐duration VLF or LF perturbations, in which the amplitude of the subionospheric signal exhibits a sudden change within 20 ms of the causative lightning discharge, and recovers back to its original level in < 3 s. These short‐duration events have characteristics similar to the previously observed rapid onset, rapid decay VLF signatures [Dowden et al.., 1994]. Both the typical and rapidly recovering events are observed primarily when the causative lightning discharge is within ±50 km of the VLF or LF great circle propagation path, indicating that the scattering from the localized disturbance is highly collimated in the forward direction. The latter in turn implies that for the parameters in hand, the transverse extent of the disturbance must be at least ∼ 100–150 km. The measured VLF signatures are compared with the predictions of a three‐dimensional model of subionospheric VLF propagation and scattering in the presence of localized ionospheric disturbances produced by electromagnetic impulses and quasi‐electrostatic (QE) fields produced by lightning discharges. The rapidly recovering or short‐duration events are consistent with the heating of the ambient electrons by quasi‐static electric fields, in cases when heating is not intense enough to exceed the attachment or ionization thresholds. When no significant electron density changes occur, the conductivity changes due to heating alone last only as long as the QE fields, typically less than a few seconds. When heating is intense enough so that attachment or ionization thresholds are exceeded, reductions or enhancements in electron density can respectively occur, in which case the medium would relax back to the ambient conditions with the time scales of the local D region chemistry, typically 10–100 s.

Focusing of nonducted whistlers by the equatorial anomaly

Impulsive ELF/VLF electric field bursts observed by the vector electric field instrument (VEFI) on the Dynamics Explorer 2 (DE 2) satellite on almost every crossing of the geomagnetic equator in the evening hours are interpreted as originating in lightning discharges. These signals that peak in intensity near the magnetic equator are observed within 5‐20° latitude of the geomagnetic equator at altitudes of 300–500 km with amplitudes of the order of ∼ mV/m in the 512‐ to 1024‐Hz frequency band of the VEFI instrument. Whistler‐mode ELF/VLF wave propagation through a horizontally stratified ionosphere predicts strong attenuation of subionospheric signals reaching the equator at low altitudes. However, ray tracing analysis shows that the presence of the equatorial density anomaly, commonly observed in the upper ionosphere during evening hours, leads to the focusing of the wave energy from lightning near the geomagnetic equator at low altitudes, thus accounting for all observed aspects of the phenomenon. The observations presented here indicate that during certain hours in the evening, almost all the energy input from lightning discharges entering the ionosphere at <30° latitude remains confined to a small region (in altitude and latitude) near the geomagnetic equator. The net wideband electric field, extrapolated from the observed electric field values in the 512‐ to 1024‐Hz band, can be ∼10 mV/m or higher. These strong electric fields generated in the ionosphere by lightning at local evening times may be important for the equatorial electrodynamics of the ionosphere.

A multiple‐mode three‐dimensional model of VLF propagation in the Earth‐ionosphere waveguide in the presence of localized D region disturbances

Transient localized D region disturbances, such as those associated with lightning discharges, affect the characteristics of VLF waves propagating in the Earth‐ionosphere waveguide. In particular, both phase and amplitude changes in the subionospheric signal can be observed at receiving sites as a result of the wave scattering that takes place in the disturbed region. In the present paper we present a multiple‐mode three‐dimensional model of VLF propagation in the Earth‐ionosphere waveguide in the presence of localized D region disturbances. The model takes into account great circle (GC) propagation paths with realistic ground and ionospheric conductivity changes that result in mode conversion along the path. It is assumed that conductivity changes transverse to the GC paths are negligible except in the vicinity of the D region disturbance and that mode coupling is negligible within the disturbed region. This new model is applied to experimental observations and is found to be in general agreement. The diagnostics potential of the model for characterizing energetic particle precipitation events is discussed.

The propagation of mixed polarization VLF (f ≤5 kHz) radio waves in the Antarctic Earth‐ionosphere waveguide

During 1986, a series of special VLF transmissions at ∼3 kHz and ∼5 kHz were made from the crossed dipole antenna at Siple Station, Antarctica, which simulated transmissions from a single horizontal dipole at a number of different orientations. The subionospheric signals thus excited were recorded at four Antarctic stations: Faraday, Halley, South Pole and Arrival Heights (McMurdo Sound). The signals excited broadside to the dipole were seen to exhibit characteristics notably different from those of signals excited along the axis of the antenna, showing a minimum in received power (the depth of the minimum decreasing for the more highly attenuating paths) and increases in apparent arrival azimuth error and elevation angle. A simple computer model for mode propagation close to 5 kHz in the Antarctic Earth‐ionosphere waveguide showed that these variations were the consequence of two effects: the preferential excitation of quasi‐transverse magnetic (QTM) modes along the axis of the antenna and the lower attenuation of quasi‐transverse electric (QTE) modes (with respect to QTM modes) over the Antarctic ice sheet. These results have important implications for studies involving the subionospheric propagation of signals with frequencies at the lower end of the VLF band and over highly attenuating surfaces, showing that QTE modes play a significant and sometimes dominant role. The propagation characteristics derived from the model may be applied to studies of the effect of burst energetic electron precipitation on subionospheric VLF signals (the Trimpi effect). Furthermore, the mode structure of subionospheric VLF signals radiated from the Siple transmitter, or a similar facility in Antarctica, may be controlled through the appropriate choice of signal frequency and antenna arrangement.

Signature of burst particle precipitation on VLF signals propagating in the Antarctic Earth‐ionosphere waveguide

The burst precipitation of energetic electrons (≥40 keV), induced by interactions with lightning‐generated whistler mode waves, has been observed to cause phase and amplitude perturbations on subionospheric VLF signals (“Trimpi” events). With a knowledge of the propagation characteristics of the subionospheric signal, analysis of the perturbation details can lead to estimates of the energy, extent, and location of the precipitation.Trimpi events have been observed on VLF signals propagating at high latitudes (L ≥ 4) over Antarctica, on 3.79‐kHz signals transmitted from the horizontal dipole at Siple station. A mode theory computer model for propagation of VLF waves in the Antarctic Earth‐ionosphere waveguide is used to illustrate the characteristics of 3.79‐kHz signals as they propagate from Siple toward VLF receivers at Halley and South Pole stations. To simulate the effects of precipitation, localized depressions in the ionospheric reflection height are introduced over the great circle propagation paths in the model, and it is seen that, while the amplitude and phase perturbations of some specific events are accurately reproduced, large positive amplitude (up to 6 dB) Trimpi events at Halley cannot be reproduced. Calculations are presented which show that signals echoing from precipitation patches located away from the great circle path could be the cause of such signatures.

Phase and amplitude perturbations on subionospheric signals produced by a moving patch of artificially heated ionosphere

The Ionospheric Heating Facility (1‐MW HF transmitter and steerable array) at Ramfjordmoen, Norway, was used to generate a localized and steerable discontinuity in the Earth‐ionosphere waveguide for scattering Omega signals. The phase of the scattered signal (“echo”) relative to the direct signal varied with position of the heated patch of ionosphere (the discontinuity) in proportion to the change in echo path length. Previous results by Barr et al. (1985), using the same technique, Omega signals and path, show very similar behavior when looked at in this way. In this respect, both experiments simulated the phase and amplitude perturbations (“trimpis”) produced by lightning‐induced (via electron precipitation) changes in the waveguide and show that measurements of the varying echo phase of trimpis can be used to track lightning‐induced perturbations. The steerable discontinuity can also be used as a waveguide probe to measure the phase velocity of the strongest waveguide mode received on the ground by such scattering.

Modal effects on amplitude perturbations on subionospheric signals (Trimpis) deduced from two‐frequency measurements

Interference between the first two modes of Earth‐ionosphere waveguide propagation at the high end of the VLF band (>18kHz) increases with distance from the transmitter out to very large distances and can add amplitude perturbations to the phase perturbations (trimpis) produced by lightning‐induced electron precipitation (LEP) on the great circle path. Since the two modes have slightly different phase velocities, an interference pattern or standing wave is formed which is shifted slightly along the propagation path by the LEP‐induced change in differential phase velocity. The modal effect at the receiver depends on the local gradient (along the great circle path) of amplitude with respect to the differential phase. Since this differential or mode beat phase varies with frequency, measurement of the resultant amplitude at two close frequencies enables an estimation of the modal effects. In this study, measurements were made at Dunedin at the two MSK frequencies, 22250 Hz and 22350 Hz, of the transmitter NWC, during a night of frequent “one‐dimensional” trimpis (i.e., those produced by large‐area LEP occurring close to the great circle path) and of strong and varying modal interference. Modal generation or modification of trimpi amplitude was related to the local gradient of amplitude as expected. From these results it was deduced that modal modification of “echo trimpis” (those produced by small area LEP occurring well off the great circle path), even under extreme conditions, is insignificant.

Phase and amplitude perturbations on subionospheric signals explained in terms of echoes from lightning‐induced electron precipitation ionization patches

Phase and amplitude perturbations (trimpis) on VLF signals propagating in the earth‐ionosphere waveguide are usually explained in terms of precipitation‐induced modification along the great circle path as though it were a one‐dimensional (1‐D) waveguide. The alternative 2‐D view presented here is that precipitation‐induced ionization patches, laterally displaced from the great circle path, produce echoes at the receiver slightly delayed relative to the direct signal. Interference between the direct and echo signals can produce both phase and amplitude trimpis of both signs. Single‐mode propagation is assumed here, since it is likely that the second mode amplitude exceeded that of the first by ∼12 dB. We apply this theory to a sequence of trimpis observed on the North West Cape (NWC)‐Dunedin path on July 10, 1987. This indicates a north–south drift of the precipitation region consistent with drift speeds of whistler ducts. The theory is also applied to events published in the literature.

Lightning‐associated precipitation of MeV electrons from the inner radiation belt

Transient perturbations of subionospheric very low frequency (VLF) radiowave signals provide new evidence for lightning‐induced electron precipitation (LEP) events involving short (<1 s) bursts of >1 MeV electrons from the earth's inner radiation belt at L ≤1.8. The signal amplitude changes are attributed to increased absorption in the earth‐ionosphere waveguide and/or alterations of the waveguide mode structure due to localized secondary ionization enhancements produced in the nighttime lower ionosphere and the mesosphere by the precipitating electrons. The otherwise stably trapped electrons are believed to be scattered in pitch angle during cyclotron resonant interactions in the magnetosphere with the lightning‐generated whistler waves. That some precipitation bursts consist partly of MeV electrons is suggested by (i) confinement of the perturbed subionospheric signal path to low magnetic latitudes (L ≤1.8), for which corresponding electron energies for gyroresonance with typical whistler‐wave frequencies in the magnetosphere are >1 MeV, and (ii) the temporal signatures of the perturbation events, which often exhibit an unusually rapid initial recovery (time constant of τ <1 s) followed by further recovery at rates believed characteristic of less energetic events (τ ∼5–20 s). The latter is interpreted as a manifestation of the rapid variation with altitude of the effective loss rate for excess ionization over an exceptionally wide range of mesospheric altitudes (40–70 km) penetrated by the >1 MeV electrons.

The Siple VLF transmitter as a multi-frequency probe of the Earth-ionosphere waveguide

1983 receptions of subionospheric signals radiated from Siple, Antarctica ( L = 4.3) to neighboring stations Palmer ( L = 2.3), Halley ( L = 4.3), and South Pole ( Λ = 74°), each ~ 1500 km from the horizontal (magnetically east-west) VLF transmitting antenna at Siple, were found to be strongly dependent upon azimuth and upon signal frequency. At Palmer, located equatorward in the broadside direction with respect to the antenna, signals near 2.5 kHz were often well defined, while the third harmonic of the transmitted signal, near 7.5 kHz, was not detected. Meanwhile, at Halley, the third harmonic was regularly observed and directionally stable, while the fundamental was often weak or undetectable. The field strength of the third harmonic component at Halley exceeded by ~ 40dB the level of the fundamental, when both were normalized to the same antenna input power. The large size of these effects is attributed in part to antenna properties that favor the endfire direction (toward Halley) at the 3d harmonic of the antenna half wave resonance frequency, and in general provide greater efficiency at higher frequencies. Other factors are high waveguide attenuation in the 2–4 kHz range and azimuth dependent differences in the propagating modes. The observed effects represent a way of extending the effective frequency range of the narrowband Siple antenna system, and also, by using the new crossed dipole configuration at Siple, of selectively probing certain regions of the Earth-ionosphere waveguide.

Seasonal, latitudinal and diurnal distributions of whistler‐induced electron precipitation events

The seasonal, latitudinal, and diurnal distributions of whistler‐induced electron precipitation events, detected as subionospheric signal perturbations (Trimpi events), have been studied by means of data sets acquired in 1982‐1983 at Palmer and Siple stations, Antarctica. The data sets, substantially larger than any previously examined, confirm previous indications of a broad (∼ 4 hour) maximum in hourly event occurrence rates. The peak was centered ∼ 1‐2 hours after local midnight at Palmer for the roughly north‐south Argentina Omega 12.9‐kHz path, but was shifted several hours later for the 23.4‐kHz NPM path, which has a westerly arrival bearing at Palmer. The previously reported seasonal variation, with peaks at the equinoxes, was confirmed; activity persisted in the austral winter, particularly following a magnetic storm, but in general occurred on fewer days and during shorter daily periods. Comparisons of occurrence data at Siple (L ≃ 4.3) and Palmer (L ≃ 2.4) for particular signal sources showed the number of days of activity to be larger at Palmer by a factor of ∼ 2 or more. Comparisons of five months of simultaneous Palmer and Siple receptions of 21.4‐kHz NSS signals on north‐south paths suggest that most of the events observed at Siple occurred as the result of ionospheric perturbations relatively close to the L shell of Palmer, that is, in the range L = 2 – 3. This result is consistent with the expected variation with latitude of the energy of electrons precipitated due to equatorial gyroresonance at typical whistler frequencies.

On the correlation of whistlers and associated subionospheric VLF/LF perturbations

Several periods of whistler‐associated subionospheric signal perturbations (i.e., Trimpi events) observed at Palmer, Antarctica (L ∼2.4) have been studied. In a case study of the time signature of the signal perturbations during a 10‐min recording period on March 30, 1983, the time delay between the whistler‐producing spheric and the onset of the change was found to be in the range 0.52–0.62 s, independent of the amplitude of the change. Event amplitude, as expected from previous work, was found to be well correlated with the associated whistler wave intensity. Other temporal features such as the rise and decay times of the perturbations were also found to be independent of the event amplitude. These results are consistent with a recent theoretical model of gyroresonant particle scattering interaction in the magnetosphere. The amplitudes of simultaneous perturbations on signals of different frequency and arrival bearing were well correlated in several cases, but exhibited case to case differences that appear to depend upon the spatial distribution of precipitation regions. Whistler occurrence times during two recording periods were found to approximately obey a Poisson distribution, while the statistics of Trimpi event occurrence in those periods showed significant deviations from Poisson behavior. The probability of random alignment in time of the whistlers subionospheric and perturbations is estimated to be < 10−14, in agreement with previous inferences of a cause and effect relation between whistlers and VLF/LF perturbations. Although nearly all of the whistlers observed originated in northern hemisphere lightning, the first example was found of a Trimpi event associated with a southern hemisphere source.

Whistler mode signals: Spectrographic group delays

VLF Doppler frequency shifts and group delays for individual whistler ducts can now be separated and displayed in a multi‐duct environment. These measurements of the simultaneous motions of the individual ducts show that there is quite commonly a significant and readily measurable spatial variation in the azimuthal electric fields in the magnetosphere. The experimental technique, which involves an improvement of the previously reported computer cross‐correlation method, also allows the use of MSK coded VLF transmissions as well as allowing the whistler mode signals to be received from any direction without a null in the direction of the subionospheric signal as before.

Siple VLF transmissions and their magnetospheric effects observed at Halley, Antarctica

Halley (75.5°S, 26.7°W, L = 4.3) is well placed for reception of subionospheric signals from the Siple transmitter. Occasionally a two-hop magnetospheric response to such signals can be observed. The data presented here are relevant to the problems of the radiation and propagation of VLF waves in the polar earth/ionosphere waveguide, duct stimulation and ducted wave amplification and the growth of triggered emissions. An upper sideband emission 20–40 Hz higher in frequency than the transmitted pulse was often received at a similar strength to, and simultaneously with the two-hop whistler mode echo.