VLF Paths Research Articles

Preliminary Results of Solar Flare Induced D-Region Perturbations over UKM Using Stanford AWESOME Receiver

We present the preliminary results of VLF signal perturbations produced due to solar flare. The data were recorded by the Stanford VLF AWESOME receiver located at National University of Ma-laysia, Selangor. Two new long distance (>1000 km) VLF paths, JJI-UKM (2700 km) and NWC-UKM (3300 km) were analyzed simultaneously. Data from the GOES satellite were used to determine the onset time and type of each of these flares. Results indicated that all five solar flare events with an X-ray peak flux above 10-5 W/m2 (M-class) were recorded, 37.5% for X-ray flux greater than 10-6 W/m2 (C-class), while the weakest X-ray flux recorded was 2.6 × 10-7 W/m2 (B-class) with 0.24% probing potentiality. We found a strong positive correlation (0.84) between solar flare radiation intensity and the values of amplitude and phase perturbations for both paths. The values of amplitude and phase perturbations time-correlated with solar flare, varied from 0.2 to 5 dB and 0.15 to 20 degree respectively. These findings are in complete agreement with previous works and demonstrate that the data obtained by the UKM AWESOME observation station will provide addi-tional contribution to the study of ELF/VLF waves phenomena in the ionosphere/magnetosphere, especially at low latitudes region.

Lower ionosphere monitoring by the South America VLF Network (SAVNET): C region occurrence and atmospheric temperature variability

AbstractDaily profiles of phase measurements as observed on fixed VLF paths generally show a transient phase advance, followed by a phase delay, for about 90 min after sunrise hours. This is indicative of a reflecting ionospheric C region developing along the terminator line at an altitude below the normal D region. The suggested occurrence of a C region is consistent with rocket measurements made in the 1960s, showing a maximum of the electron density between 64 and 68 km, and by radio sounding in the 1980s. In order to correctly describe the properties of the phase effect associated with the presence of a C region, it is important to understand the subionospheric propagation characteristics of the VLF paths. In this paper, we analyze the variations presented by the temporal properties of the VLF narrowband phase effect and determined a parameter associated with the appearance of the C region at sunrise hours observed by receivers from the South America VLF Network. Periodic patterns emerge from the parameter curves. Two distinct temporal behavior regimes can be identified: one exhibiting slow variations between March and October, and another one exhibiting faster variations between October and March. Solar illumination conditions and the geometrical configuration of the VLF paths relative to the sunrise terminator partly explain the slow variation regime. During periods of faster variations, we have observed good association with atmospheric temperature variability found in the measurements of the Thermosphere Ionosphere Mesosphere Energetics and Dynamics and Sounding of the Atmosphere using Broadband Emission Radiometry satellite instrument, which we assume to be related to the winter anomaly atmospheric phenomenon. However, when comparing the parameter time series with temperature curves, no direct one‐to‐one correspondence was found for transient events.

A relationship between solar proton events, ionospheric uplift observed at VLF and negative ionospheric storms

Three consecutive magnetic storms during the month of September 1982 were found to be associated with solar proton events (SPE) observed over a number of high latitude VLF propagation paths. The penetration of solar protons into the auroral zone produced a marked reduction in reflection height at night for high latitude VLF paths resulting in a reduced diurnal phase shift. This effect has been known for some 50 years. However in this paper, a previously unidentified response is described consisting of an increase in the night 90km reflection height over middle latitude and transequatorial VLF paths. Solar protons do not penetrate to these latitudes and this slight increase in VLF reflection height was associated with typical negative ionospheric storm effects in the F2 region. Dynamics at the 90km base of the night ionosphere are little known and difficult to investigate except at VLF. These results are the first to suggest a response of the night ionospheric base to events leading to the well known negative ionospheric storm seen at greater heights. Such negative storms seen in the F2 region have been associated with an equatorward wind surge and change in neutral atmospheric chemistry driven by joule heating in the auroral zone produced by solar proton precipitation.

Large solar flares and their ionospheric D region enhancements

On 4 November 2003, the largest solar flare ever recorded saturated the GOES satellite X‐ray detectors, making an assessment of its size difficult. However, VLF radio phase advances effectively recorded the lowering of the VLF reflection height and hence the lowest edge of the Earth's ionosphere. Previously, these phase advances were used to extrapolate the GOES 0.1–0.8 nm (“XL”) fluxes from saturation at X17 to give a peak magnitude of X45 ± 5 for this great flare. Here it is shown that a similar extrapolation, but using the other GOES X‐ray band, 0.05–0.4 nm (“XS”), is also consistent with a magnitude of X45. Also reported here are VLF phase measurements from two paths near dawn: “Omega Australia” to Dunedin, New Zealand (only just all sunlit) and NPM, Hawaii, to Ny Alesund, Svalbard (only partly sunlit), which also give remarkably good extrapolations of the flare flux, suggesting that VLF paths monitoring flares do not necessarily need to be in full daylight. D region electron densities are modeled as functions of X‐ray flux up to the level of the great X45 flare by using flare‐induced VLF amplitudes together with the VLF phase changes. During this great flare, the “Wait” reflection height, H′, was found to have been lowered to ∼53 km or ∼17 km below the normal midday value of ∼70 km. Finally, XL/XS ratios are examined during some large flares, including the great flare. Plots of such ratios against XL can give quite good estimates of the great flare's size (X45) but without use of VLF measurements.

A model for the electron temperature variation along geomagnetic field lines and its effect on electron density profiles and VLF paths

A realistic model for the temperature variation along geomagnetic field lines is described. For high altitudes (>1500 km) the temperature is taken to increase as the nth power of radial distance ( n−2), giving temperatures consistent with those measured in situ by high altitude satellites. For realistic temperatures at low altitude an extra term is included. The temperature gradient along the field line is then 0.9–1.6° km −1 during the day and 0.5–0.7° km −1 during the night at 1000 km, reducing to about half these values at 2000 km, for the latitude range 35–50°. This is consistent with calculations made from nearly simultaneous satellite measurements at 1000 and 2500 km. It is shown that assuming diffusive equilibrium, including the new temperature model, more realistic equatorial electron density profiles result than for isothermal field lines. The temperature gradient model is also purposely formulated to be of a form that enables the temperature modified geopotential height to be obtained without numerical integration. This renders the model particularly suitable for ray-tracing calculations. A ray-tracing model is developed and it is shown that unducted ray paths are significantly altered from the corresponding paths in an equivalent isothermal model; there is greater refraction and magnetospheric reflection takes place at lower altitudes. For summer day conditions, an inter-hemispheric unducted ray path becomes possible from 26° latitude that can reach the ground at the conjugate.

Perturbations of subionospheric LF and MF signals due to whistler‐induced electron precipitation bursts

First evidence of whistler‐induced burst precipitation effects on subionospherically propagating signals in the LF and MF ranges have been observed at Palmer (L ∼ 2.4) and Siple stations (L ∼ 4.3), Antarctica. The occurrence rate on a 37.2‐kHz LF signal originating in California was usually comparable to that on the more disturbed VLF paths. At Palmer, examples at 37.2 kHz were seen on 70% of the nights during a March‐April 1983 observing period. Perturbations on a ∼1800 km‐long 780‐kHz MF path to Palmer occurred on nights of high activity on VLF paths, but fewer than 10 MF events were usually detected as compared to >50 on the active VLF paths. The MF perturbations were of order 50% in amplitude and were not in general followed by a ∼30‐s decay toward a pre‐event level, as is usually the case for the VLF signals. Test particle modeling of the whistler‐particle interaction supports the idea that the MF signals are affected by ionization extending above the nominal ∼85‐km reflection height for VLF signals and also are consistent with the observed time delay between the 780‐kHz signal perturbation and the whistler‐generating lightning stroke. Fast phase advances correlated with the whistlers were regularly observed on a 12.9‐kHz Omega signal. The data provide a means of inferring the existence of multiple precipitation regions and offer new evidence that whistlers originating in the northern hemisphere can cause significant precipitation in the southern hemisphere, at least in the vicinity of the South Atlantic magnetic anomaly.

Energetic electron precipitation and vlf phase disturbances at middle latitudes following the magnetic storm of December 16, 1971

Enhanced fluxes of electrons precipitating over middle latitudes (L ∼ 3–4) were detected by the polar-orbiting satellite 1971-089A following a period of magnetic activity starting on December 16, 1971. The electron fluxes measured in 256 differential channels between 130 and 2800 keV have been coordinated with phase observations of VLF radio waves propagating in the earth-ionosphere wave guide. The VLF paths in question, NLK (near Seattle, Washington) and GBR (at Rugby, England) to APL (near Washington, D. C.), cover ≈120° in longitude and range from L ∼ 2.5 to L ∼ 4.0 in invariant latitude. These paths showed marked daytime and nighttime phase advances from 1650 UT on December 17 (in excess of 10 µs during maximum disturbance). The phase values did not return to prestorm levels before December 22–23. The unusual presence of these daytime VLF disturbances is offered as evidence for the widespread precipitation at low L shell values of nearly relativistic electrons (Ee ≳ 200 keV) which would be required to penetrate below ∼70-km altitude to affect the daytime VLF transmissions. Wave guide mode calculations using D region electron density profiles deduced from the satellite particle data predict phase advances which agree reasonably well with the observed values. It is concluded that the observed long-lived VLF phase disturbances can be explained by excess D region ionization caused by energetic electrons precipitating from the earth's radiation belt following their injection deep into the magnetosphere during the magnetic storm.

Localized nighttime D-region disturbances and ELF propagation

ELF radio wave propagation measurements at northeastern U.S. sites within 1600 km of the U.S. Navy test transmitter in Wisconsin occasionally display an unexpected early-morning null behavior in which a decrease of more than 6 dB in signal strength occurs over a period of several hours. These severe ELF disturbances occur only occasionally during the several days following magnetic storms when similar, but less pronounced behavior is found to coincide with phase disturbances on VLF paths across the northern U.S. These disturbances appear to be caused by energetic electrons precipitated into middle latitudes from the radiation belt. The magnitude of the severe ELF disturbances requires an explanation involving a twofold discontinuity in the Earth-Ionosphere waveguide. It is suggested that the effective ionosphere reflecting height is lowered immediately to the east of the receiving locations by particulate bombardment but then is raised further east at the geomagnetic conjugate of the South Atlantic anomaly, beyond which quasi-trapped particles are no longer available at lower D-region heights.

VLF propagation disturbances and electron precipitation at mid-latitudes

Observations of nighttime phase disturbances to long-distance mid-latitude (2 ≲ L ≲ 4) VLF transmissions during a magnetospheric substorm on January 2, 1971, are presented. The onset of the phase disturbances to a number of VLF paths began at the time of the substorm expansion. Whistler measurements during this event indicate that all the paths were below the plasmapause. Bremsstrahlung X rays and VLF emissions were observed above the plasmapause, near L = 4, simultaneously with the VLF propagation disturbances. A previous analysis of the X ray and VLF emission data indicates that electron precipitation at L = 4 was caused by cyclotron resonance between energetic trapped electrons and whistler mode VLF waves near the equatorial plane. The data presented here suggest that the mid-latitude ionospheric disturbances are due to energetic electron precipitation, which is possibly caused by this same mechanism. For this event it is estimated that a flux of electrons J(>40 kev) ≲ 300/cm² sec ster with e-folding energy equal to 45 kev will account for the measured phase change along a path nearly parallel to L = 3. Calculations illustrate that nighttime VLF phase is very sensitive to values of incident electron flux that are only marginally detectable, if at all, with the more conventional indirect indicators of electron precipitation, e.g., X rays and riometer absorption. This suggests that VLF propagation can serve as a useful tool for the detection of low-intensity electron precipitation.

Effects of X-ray stars on VLF signal phase

Stellar X-ray effects from Sco XR-1, Cen XR-2 and Cen XR-4 have, been found on several of the numerous VLF phase recordings collected on a world-wide basis since 1965. Only marginal VLF phase anomalies could be associated with Tau XR-1, and none with the Sco XR-1 flares of 3 June 1967. Stellar VLF phase effects are more pronounced at higher signal frequencies and on VLF paths with long nighttimes. Scattered L α radiation after sunset and before sunrise reduces the sidereal shift of VLF effects on E ↔ W paths; and the closeness of the signal path position to the solar terminator at local midnight can shift the date when a maximum stellar VLF phase anomaly is observed. A detectability factor is defined which compares expected stellar effects on arbitrary signal paths to the established effect of Sco XR-1 on the 22.3 kHz signal from NWC to Tananarive. VLF data show: In 1968, Sco XR-1 caused a maximum phase advance of ~0.6 μsec/Mm on NWC (22.3 kHz)—Tananarive; Cen XR-2 and Cen XR-4 increased their X-ray flux to about that of Sco XR-1 on or about 7 March 1967 and 6 July 1969, respectively; Cen XR-2 flux peaked ~27 March and possibly ~10 April and decreased rapidly between 20 April and 10 May 1967. It is concluded that 1. (a) presently known stellar X-ray sources are not detrimental to the accuracy of the OMEGA navigational system; 2. (b) the permanent X-ray stars may give rise to a seasonal variation of nighttime phase at frequencies above 15 kHz; 3. (c) continuous VLF phase tracking at frequencies above 20 kHz can provide some valuable astrophysical information complementing space probe data.

A Laboratory Model for VLF Propagation Experiments

A laboratory model of a section of the earth‐ionosphere has been designed and constructed to provide a means for systematically investigating the effect of ionospheric perturbations on long range VLF paths. This model provides a capability for the investigation of a wide variety of different disturbed ionospheric conditions. For any given configuration, all the pertinent parameters are clearly defined and repeatable. This is in contradistinction to naturally occurring events, for which many parameters that are important for a clear interpretation are either unknown or, at best, poorly known. The design rationale and the construction methods are presented in this paper with initial experimental data obtained from the model for uniform propagation paths. It is shown that these data are in good agreement with theoretical data obtained for the modeled conditions. Data are also presented for propagation paths through a nuclear type ionospheric perturbation. These data exhibit both amplitude loss and phase advance caused by the perturbation.

Theoretical Investigation of the Diurnal Phase and Amplitude Variations of VLF Signals

Multimode propagation and mode conversion at the sunrise and sunset lines must be considered to explain the diurnal variations on some VLF paths. In this investigation the model selected to represent the transition from one steady‐state ionosphere condition to the other was an abrupt change in the surface impedance and reference height of the ionosphere. Comparisons of calculated and recorded amplitude and phase variations during sunset showed that sunset could not be approximated by an abrupt ionosphere transition. Comparisons of calculated and recorded variations during sunrise, however, indicated that the simple analytical model approximated the propagation conditions during sunrise for the east‐to‐west and west‐to‐east paths that were considered.

VLF phase perturbations produced by the Soviet high-altitude nuclear explosion of November 1, 1962

The Soviet high-altitude nuclear burst of 0912 UT on November 1, 1962, produced delayed perturbations of the VLF transmissions monitored at APL/JHU over five paths remote from the burst region. The burst-related VLF effects varied from 6 to 55% and averaged 32% of the normal diurnal variation. It is suggested that the VLF phase perturbations are produced by particles from the burst that drift to the remote regions and produce an ionization enhancement in the lower ionosphere in their first, and sometimes in their second, global orbit. The temporal histories of the VLF phase perturbations are compatible with a model wherein it is assumed that the trapped particles are electrons from the radio-active decay of neutrons, the artificial belt extending from an outer shell of L ≈ 3.4 to at least L = 1.9. Satellite particle data do not show an artificial belt of electrons from the decay of neutrons, a belt which in this case may have been obscured by the background flux of electrons from the natural Van Allen belt and from the artificial belts from the earlier Soviet explosions. Satellite data do, however, show a narrow belt of fission electrons ≳1 Mev with maximum flux at L ≈ 1.77 and a lower boundary at L = 1.73. These fission electrons did not affect the VLF paths intercepted by these shells. The VLF and satellite data agree that fission electrons ≳1 Mev were not present on shells above L ≈ 1.9.

Further Observations of Sunrise and Sunset Fading of Very‐Low‐Frequency Signals

An earlier explanation of sunrise and sunset fading on long VLF paths is confirmed by examining data taken over a wider range of frequency and directions of propagation. It is found, however, that there is an appreciable dependence of the fading period on the magnetic direction of propagation. It is shown that this dependence is qualitatively in agreement with what would be expected from a sharply bounded ionosphere and a transverse magnetic field.

Very low frequency phase perturbations and the soviet high-altitude nuclear bursts of October 22 and 28, 1962: 1. Observations and inferred radiation belts

On October 22 and 28, 1962, nuclear explosions over central Asia produced ionospheric D-layer perturbations as monitored on various VLF paths in the northern hemisphere. Delayed VLF effects due to trapped particles averaged 53% of the normal diurnal change for the burst of October 22, and 29% for the burst of October 28. The types of particles and their energies were the same for both detonations, but differences existed in the spatial extent of the radiation belts producing the VLF phase perturbations. The trapped particles were electrons with E ≲ 4 Mev and protons in the range 2.3 to 4.6 Mev. The electron spectrum is composed of fission and neutron-decay β particles, the latter predominating for E < 0.78 Mev. Protons <2.3 Mev may have also been present, but those with E < 1 Mev cannot penetrate into the D region and are of no direct VLF interest. With the dipole field approximation, for the detonation of October 22, fission and neutron-decay β particles were trapped in a region bounded by field lines intersecting the earth's surface in a range extending at least from 36.5° to 56° geomagnetic (for the real field the shells are 1.75 ≤ L ≤ 3.76). For October 28, fission β particles were confined to areas <49°N (L < 2.32) and probably to field lines around that passing through the burst point (at 36.5°N and L = 1.75); this type of confinement also existed for 2.3- to 4.6-Mev protons after both nuclear bursts. For October 28, effects due to neutron-decay β particles were found at 49°–51°N (2.32 ≤ L ≤ 2.79), a region which formed the outer boundary for these trapped particles. For the burst of October 22, a prompt effect was observed on the GBR to APL transmission.

Another possible explanation of sudden atmospheric lonization in regions shadowed from high-altitude nuclear bursts

Sudden changes in VLF propagation characteristics, presumably due to increased D-layer ionization, have been observed to occur along VLF paths whose nearest points lie thousands of kilometers away from a high-altitude nuclear burst. It is proposed that this ionization may be caused by neutrons that have been transported by multiple scatterings around the earth and well into the shadow zone where they deposit part of their kinetic energy through collisions with nitrogen nuclei. Numerical results are presented which, though based on a highly simplified model problem, encourage further detailed calculations. The possibility of observationally distinguishing between the transport mechanism and the neutron decay mechanism is also discussed.

Long-lived effects in theDregion after the high-altitude nuclear explosion of July 9, 1962

Papers have recently been published describing the behavior of the phase of VLF signals propagating over long VLF paths after the high-altitude nuclear explosion of July 9, 1962 [Burtt, 1962; Zmuda et al., 1963]. These papers have been concerned with the effects within a few minutes of the explosion. In this note we intend to show that effects were also produced which lasted for at least one week. On July 9, 1962, at around 0900 UT, the event known as Starfish took place. This was the detonation of a 1.4-MT nuclear weapon at a height of 400 km [Zmuda et al., 1963] over Johnson Island in the Pacific. Figure 1 shows the diurnal phase variation [Chilton et al., 1963] of the signals at 19.8 kc/s from NPM in the Hawaiian Islands received at Boulder during the month of July 1962. The initial effect of the explosion on this path, near 0900 UT on July 9, resulted in the phase of the signal approaching or possibly passing its daytime value, although the path was in complete darkness at this time. Almost immediately afterward the phase began to return to normal and was still recovering when the eastern end of the path became sunlit. During the days following, the figure shows that the diurnal variation was much reduced, and it was not until July 17 that the diurnal phase change returned to the value it had before the event.