Trimpi Events Research Articles

Comparative Study of Predominantly Daytime and Nighttime Lightning Occurrences and Their Impact on Ionospheric Disturbances

Space weather events adversely impact the operations of Global Navigation Satellite Systems (GNSS). Understanding space weather mechanisms, interactions in the atmosphere, and the extent of their impact are useful in developing prediction and mitigation models. In this study, the hourly lightning occurrence and its impact on ionospheric disturbances, quantified using the Rate of Total electron content Index (ROTI), were assessed. The linear correlation between diurnal lightning activity and ROTI in the coastal region of southern China where lightning predominates in the daytime was initially negative contrary to a positive correlation in southern Africa where lighting predominates in the evening. After appreciating and applying the physical processes of gravity waves, electromagnetic waves and the Trimpi effect arising from lightning activity, and the time delay impact they have on the ionosphere, the negative correlation was overturned to a positive one using cross-correlation. GNSS has demonstrated its capability of revealing the impact lightning has on the ionosphere at various times of the day.

Electron precipitation events in the lower ionosphere and the geospace conditions

<p>We present an analysis of localized ionospheric perturbations detected at Comandante Ferraz Brazilian Antarctic Station (McIlwain parameter L~2.25) as fast-amplitude variations of very low frequency (VLF) signals transmitted from Hawaii (NPM, at 21.4 kHz), also known as Trimpi events. The study covers the first six months of 2007, during the period of minimum activity in the 23rd solar cycle. The occurrence of Trimpi events in the Antarctica peninsula region was studied in association with solar-wind parameters in the neighborhood of the Earth (geospace), along with the geomagnetic activity level (Ap, Dst indices). The analysis shows that the Trimpi events occurred predominantly during geomagnetically disturbed periods, but they have a more intricate association with the geospace regimes. The events achieve higher occurrence during the recovery phase of some geomagnetic storms, and also show a close association with electron flux enhancements in the belt region, especially those with higher energy. The higher event incidence occurred a few hours after what we call the 'angle bracket' phenomenon: when the solar wind velocity rises simultaneous with a decrease in its density.</p>

On ULF Signatures of Lightning Discharges

Recent works on magnetic signatures due to distant lightning discharges are reviewed. Emphasis is laid on magnetic signatures in the ULF range (in the old definition from less than 1 mHz up to 1 Hz), that is in the frequency range below the Schumann resonance. These signatures are known to be of importance for the excitation of the ionospheric Alfven resonator (IAR) which works only at night time conditions. This emphasizes the difference between night and day time ULF signatures of lightning. The IAR forms a link between the atmosphere and magnetosphere. Similarities and differences of this link in the VLF (Trimpi effect) and ULF range are worked out. A search for a unique signature of sprite-associated positive cloud-to-ground (+CG) lightning discharges ended with a negative result. In this context, however, a new model of lightning-associated induced mesospheric currents was built. Depending on mesospheric condition it can produce magnetic signatures in the entire frequency range from VLF, ELF to ULF. In the latter case it can explain signatures known as the Ultra Slow Tail of +CG lightning discharges. A current problem on the magnetic background noise intensity has been solved by taking more seriously the contribution of +CG lightning discharges to the overall background noise. Their low occurrence rate is more than compensated by their large and long lasting continuing currents. By superposed epoch analysis it could be shown that the ULF response to −CG is one to two orders smaller that in case of +CG with similar peak current values of the return stroke.

Three-dimensional subionospheric VLF field diffraction by a truncated highly conducting cylinder and its application to the Trimpi effect problem

[1] By means of a three-dimensional full-wave analytical-numerical technique we consider the diffraction of a VLF point source field by a localized perturbation in the lower ionosphere, which is taken as a truncated highly conducting cylinder. The surface impedance concept is additionally used to model the Earth-ionosphere waveguide. Our approach is based on the preliminary asymptotic integration of the rigorous two-dimensional integral equation in order to facilitate its further numerical handling, and an original computational algorithm is proposed to obtain a solution of the approximate equation. By reducing CPU time, the procedure developed enables us to study both small and comparatively large irregularities. The theoretical computations have yielded the following findings: (1) the transverse dimension of the reflecting area, which can explain the experimental result, may be reduced as compared with that suggested before, (2) scattering may be significant not only in the forward direction but also in the backward direction and out-of-the-way directions, and (3) the same ionospheric perturbation produces a noticeably smaller effect on subionospheric VLF radio wave propagation for daytime conditions than for nighttime ones.

Determining the size of lightning‐induced electron precipitation patches

We analyze Trimpi signatures during 23 and 24 April 1994 at four sites on or near the Antarctic Peninsula (Palmer, Faraday, Rothera, and Halley) on subionospheric VLF signals received from four U.S. naval transmitters (NAA, NSS, NLK, and NPM). Electron precipitation patches are found to be large, i.e., ∼1500 km × 600 km, with the longer axis orientated east‐west. Calculations using a three‐dimensional Born scattering model, where patch densities are 1.5 electrons cm−3 above ambient at the center at ∼84 km altitude, provides results that are consistent with this picture. A high proportion (38%) of the Trimpi events were associated with strong lightning flashes in eastern United States. When lightning discharges had currents >65 kA (positive or negative), there was a >80% chance of seeing an associated Trimpi event. The chance of seeing any Trimpi events fell to near zero for discharges of <45 kA. The largest Trimpi perturbations occur when the center of the precipitation patch is 700–800 km from the receivers. This result is consistent with the modeling calculations for large patches. The equatorward edge of the precipitation patch was estimated to be at ∼60°S, close to the magnetic conjugate of the lightning. The close association of the equatorward edge of the precipitation patch with the conjugate location of the causative lightning is consistent with a quasi‐ducted whistler‐induced precipitation mechanism. Nonducted whistler‐induced precipitation mechanisms would predict a 5°–10° latitudinal gap between the lightning and the equatorward edge of the patch. However, the lack of observed whistlers at the time of the Trimpi events is consistent with the nonducted whistler mechanism and is not consistent with the quasi‐ducted mechanism, although the distances from duct exit point to receiver may have been too large (∼700–1000 km) for the signals to be detectable. Using the significantly larger patch dimensions determined in this study, it is estimated that lightning may well be 10–100 times more effective at depleting the radiation belts than hiss.

Numerical simulation of LEP Trimpis observed at Poitiers, France, on signals from VLF transmitters

Transient amplitude and phase perturbations on subionospheric VLF signals, known as the Trimpi effect, are caused by the scattering of VLF radiation from localized ionization enhancements in the nighttime D-region. The patches of ionization are due to precipitation from the radiation belts of keV electrons, that is induced by lightning-generated whistlers.This work is concerned with the numerical simulation of such VLF perturbations, termed LEP (lightning-induced electron precipitation) or classic Trimpis. Two different codes are used to compute the VLF propagation in the Earth-ionosphere waveguide in the presence of a D-region inhomogeneity. The first is based on mode theory, and the second on the FDTD (finite-difference time-domain) method. Both codes are two-dimensional and, therefore, relevant only to LEP events lying on the transmitter-receiver great circle path (TRGCP). A method of simulation is proposed to interpret quantitatively VLF amplitude and phase changes in terms of the approximate location and size of the associated ionospheric perturbation along the TRGCP. The method is applied to LEP Trimpis observed at Poitiers ( L = 2) on signals from the NAA and GQD transmitters. Results are discussed in the light of the information that may be deduced from the high-resolution analysis of VLF temporal signatures. A sequence of simultaneous LEP Trimpis observed at Poitiers on three widely separated transmission paths is studied; two alternative interpretations are proposed.

Lightning induced enhancements of D-region ionisation and whistler ducts

A 3-D magnetospheric ray-tracing program has been constructed in which ray paths can be calculated for propagation in 3-D ducts having a Gaussian enhancement of electron density in both the geomagnetic meridian and geomagnetic longitude directions. Such calculations show a reduction in wave path excursions both in geomagnetic latitude (or L-value) and longitude as rays propagate upwards in a duct between low altitude (∼1000 km) and the equatorial plane. The change in wave energy flux between low altitude and the interaction region for wave–particle interactions and the cross- L extent of related precipitating electron fluxes can then be determined. The size of localised electron density enhancements in the D-region lightning induced enhancements (LIEs) which cause phase and amplitude perturbations on sub-ionosphere VLF propagation (Trimpi effect) can then also be estimated. LIE extents determined in this way appear to be generally about an order of magnitude smaller than estimates from Trimpi observations or low altitude satellite observations of LEP (lightning-induced electron precipitation). This discrepancy has been investigated by ray-tracing in a variety of 3D duct models, including ducts which vary in width and/or enhancement along their length. Calculations made for ducts with maximum enhancement and width in Δ L-value constant along their length imply that precipitation regions and LIEs would typically have smaller dimensions in L-value than the low altitude whistler duct widths measured by the spatial extent of the whistler-mode waves. Therefore, to account for this discrepancy in duct width, ray paths were also determined for ducts which vary in enhancement and/or width in Δ L-value along their length. Although a little better fit with experimental observations is obtained for such ducts, the results can still not explain significantly wider ducts in L-value in the interaction region than at low altitudes. Thus other possible explanations are considered; in particular that the non-linear resonant current in the interaction region near the equatorial plane will radiate wave power into modes with larger wavenormal angles to the geomagnetic field direction than the original trapped modes, and this will then result in a larger region from which precipitating electrons arise and hence larger LIEs than the actual duct width in the interaction region.

Analysis of lightning ‐ induced ionization from VLF Trimpi events

Detailed analysis of Trimpi phenomena (Trimpis) revealed two different regimes in their temporal behavior. One time regime is the initial “nose” part of Trimpis of about 1–5 s, and the second is the “tail” of Trimpis of about 50–100 s. Using a rather simple theoretical model of VLF wave scattering from abating ion and electron density perturbations, we discovered two fitting formulas to describe the temporal evolution of Trimpis. An exponential‐hyperbolic evolution in amplitude a/a0 = (exp{−t/τ0})/[1 + b/2(1 − exp{−t/τ0})] for weak scattering and a logarithmic dependence a/a0 = [ln τ2/(t + τ1)]/(ln τ2/τ1) for strong scattering. Parameters b, τ0, τ1, τ2 were calculated for all the observed 50 Trimpi events. Maximal ion and electron density perturbation (Δnmi and Δnme) were estimated together with the background ion and electron density (ni0 and ne0) and vertical length of the perturbation column (ΔH). For the conventional model of background electron density and horizontal scale of perturbations L ≃ 5 – 10 km, we find the perturbation parameters to be as follows: Δnme ≃ 103 – 104 cm−3, Δnme/ne0 ∼ Δnmi/ni0 ≃ 10 – 100, ni0 ≃ 3 × 103 − 3 × 104 cm−3, and ΔH ∼ 10 – 15 km.

Whistlers, Trimpis and evidence that electron precipitation may trigger atmospheric discharges

Trimpi events and associated whistler groups recorded on Marion Island show that the observed Trimpis can be explained by electron precipitation induced by the associated whistlers. The almost simultaneous observation of whistlers and Trimpis indicates that the interacting electrons must have mirrored in the northern hemisphere before precipitating in the south. The Trimpi associated whistler groups are, in all cases, followed, after a fairly well-defined time interval of about 600 ms, by a second, fainter whistler group. The interval between the first and second whistler group suggests that some of the electrons which interacted with the first whistler group may have precipitated in the northern hemisphere and triggered the atmospheric discharge that gave rise to the second group.

Trimpi events and other amplitude perturbations observed during 1991 at Kerguelen ( L = 3.7) on the subionospheric North-west Cape (NWC) signal

In the period March–November 1991, which was characterized by strong magnetic activity, two kinds of complementary VLF data were recorded at Kerguelen: (1) the amplitude of the subionospheric 22.3 kHz signal from the North-west Cape (NWC) transmitter in Australia, and (2) the integrated intensity in various VLF bands, and broadband waveforms (0.4–10 kHz). Comprehensive analysis of the data allowed one to identify and characterize four types of well-structured perturbations in the NWC signal amplitude, i.e.: classical Trimpi events associated with whistlers; hiss-induced electron precipitation events; inverted dome-shaped events identifiable with dome-shaped events recently observed at Durban by Friedel et al. (1993); and quasi-monochromatic oscillations, with a period of ∼7–15 s observed when Kerguelen is poleward of the plasmapause projection in the pre-dawn sector, similar to the zigzag effect described by Carpenter et al. (1985a). The purpose of this paper is to report on the results of investigations devoted to these various types of event; each of them is illustrated by examples. The main new results relative to classical Trimpi events concern: the dependence on severe magnetic storms (out of 2300 events identified in June and July 1991—period of maximum occurrence—48% were observed on 6 days only in the aftermath of three large magnetic storms); the correlation between amplitude perturbation sign and NWC signal amplitude connected with sunset and sunrise on the east-to-west NWC-KER path; the nocturnal variation of recovery times for plasmaspheric whistler-associated Trimpi events (the mean value increases from 30 to 42 s at night); the detection in the midnight-dawn sector of short-duration Trimpi events (recovery times in the range ∼4–15 s) probably triggered by whistler waves propagating beyond the plasmapause when the latter is equatorward of, or close to, Kerguelen, 24–48 h following the onset of intense magnetic storms. Whereas it is possible to interpret most of the characteristics and properties of classical Trimpi events in terms of electron precipitation induced by magnetospheric VLF waves—a process being favoured by the Occurrence of severe magnetic storms with probable injection of energetic particles—the dome and zigzag effects remain unexplained as yet.

Aspects of wave-particle interactions at mid-latitudes

Arguably the most significant source of particle precipitation into the ionosphere at mid-latitudes ( L ∼2–3), at least for electrons in the tens to hundreds of keV energy range, is that which arises from the interaction between trapped particles and whistler-mode waves in the magnetosphere. Quasisteady plasmaspheric hiss has long been postulated as a major class of waves contributing to the loss of radiation belt particles but recent studies have suggested that under some conditions, impulsive precipitation caused by interactions with lightning-generated whistlers may be equally important as a loss process. Such lightning-induced electron precipitation (LEP) and its ionospheric signature is the subject of this paper. Although LEP may be detected and studied by a variety of ground-based, balloon-borne and satellite-borne sensors, through optical emissions, X-ray production, enhanced conductivity, or the direct measurement of the precipitating particles themselves, a technique using ground-based narrow-band VLF receivers to measure the Trimpi effect (the transient perturbations in amplitude and/or phase of received narrow-band VLF transmissions) caused by LEP-associated ionisation enhancements has become increasingly popular due to its simple instrumentation and wide field of view. Most work has concentrated on the 2 < L < 3 region where typicalwhistler spectra, trapped electron energy distributions and magnetospheric plasma densities and magnetic field strengths are most favourable for the Trimpi effect. In order to use the technique to study in detail the characteristics and distribution of LEP (and its importance as a trapped-particle loss mechanism), using a network of intersecting transmitter-receiver great-circle paths (TRGCPs), a consensus on how to interpret the observational data is crucial, though this has recently been the subject of controversy. Whilst most studies suggest that only LEP within ∼200 km of the TRGCP gives rise to an observable Trimpi event, some have suggested that non-gaussian perturbations well off the TRGCP can be important. In this paper, the current state of LEP research is summarised and some recent results are presented, using data from a network of both widely and closely spaced observing sites in Antarctica.

The role of ducted whistlers in the precipitation loss and equilibrium flux of radiation belt electrons

New experimental evidence suggests that every ducted whistler component may precipitate bursts of radiation belt electrons into geomagnetically conjugate ionospheric regions. Strong spatial and temporal associations are seen between transient ionospheric disturbances observed in conjugate regions and ducted whistlers monitored at Palmer Station, Antarctica. The ionospheric disturbances were detected by their characteristic perturbing effects (“Trimpi events”) on subionospheric VLF, LF, and MF signals recorded at Palmer Station and at northern hemisphere sites. Of 74 such events examined on four different days, all were time‐associated with ducted whistlers. In no case was the arrival azimuth or dispersion of the associated whistlers inconsistent with the locations of the conjugate ionospheric disturbances, which were inferred from the configuration of perturbed signal paths. Other whistlers occurring independently of detected disturbances were found to be either weak or to have arrived from regions where a disturbance would not have been detected for lack of monitored signal paths. Signal perturbation onset behavior was consistent with multiple regions of precipitation induced by components of multipath whistlers and with theoretical predictions for ducted whistler‐induced precipitation. The results not only support the hypothesis that ducted whistlers are responsible for burst precipitation of energetic electrons but imply that such bursts may be induced by every ducted whistler component. Since radio energy from a lightning discharge can excite whistler ducts located 2500 km or more away from the flash, every ducted whistler observed at Palmer Station may indicate the presence of precipitation bursts associated one‐to‐one with excited whistler ducts distributed over a 5000‐km‐wide portion of the Earth's surface, and over its geomagnetic conjugate as well. The estimated effect of this precipitation on 70‐ to 200‐keV radiation belt electron populations for 2 &lt; L &lt; 3 is comparable to that predicted as a result of plasmaspheric hiss, indicating that ducted whistlers may contribute as significantly as hiss to radiation belt equilibrium at those electron energies.

Size and location of lightning-induced ionisation enhancements from measurement of VLF phase and amplitude perturbations on multiple antennas

A 600-km array of five Trimpi receivers (“elements”) has been set up in New Zealand broadside to the VLF (22.3 kHz) transmitter, NWC, some 6000 km west, with element separations varying from 8 km to 550 km. Although such a five-element array is inadequate for imaging of lightning-induced ionisation enhancements (LIEs) by VLF holography, or inverse scattering, estimates of LIE size and location can be made if the shape and form of the LIE can be guessed or assumed, with even fewer elements. With five elements, tests of the assumed model can be made as well. Owing to its transform properties, the simplest model to use for scattering inversion is the Gaussian LIE distribution. For this model, and for single mode propagation, an inversion process is derived here for the full range of LIE and path dimensions, ranging from those for which the receiver is in the diffraction far field to those in which “geometric optics” dominate. This inversion process has some validity for small LIEs of other shapes of simple form. For more extreme models, the dominance of geometry or diffraction can usually be established in individual cases which then allows simple scaling procedures to be used in scattering inversion. Some 70 Trimpi events were observed on all five elements during a single night in July. 1991 (late winter). These were used to determine LIE location and size, and to test the applicability of various LIE models. It was found that most LIEs that night occurred over the Tasman Sea near the great circle from the VLF transmitter, NWC, to Wellington, generally some 500 to 2000 km from Wellington, and with north-south dimensions of 100–250 km. Much longer east-west dimensions (oriented towards NWC) are suggested to account for the very strong Trimpis observed. While about half of these LIEs that night could have had a smooth lateral spread (e.g., Gaussian), the remainder required varying degrees of fine structure, from “flat” or Butterworth LIEs to multiple LIEs as might be expected from multiduct whistlers, to explain the observed diffraction pattern exhibiting maxima and minima as well as the wide angular range over which simultaneous Trimpis were observed.

First observations of Trimpi Events at Durban (L = 1.69) using an OMSKI Receiver

We report observations of transient ionospheric disturbances at VLF reflection heights, manifested as perturbations on sub‐ionospherically propagating VLF transmitter signals: “Trimpi events.” Four VLF transmitter signals were monitored at Durban using an OMSKI receiver. Both traditional Trimpi events and a new “dome” shaped event were observed at Durban. Occurrence of Trimpis at low L values can be interpreted as the precipitation of very high energy electrons which are capable of producing events during daylight, or as evidence of the “fast” Trimpi mechanism which could dominate at low L. The average winter event rate observed at Durban was quite low (7 per day on four transmitter signals) but during one very active period on the NAA‐Durban signal 88 Trimpi events were observed in 2 hours. This activity was linked to an isolated thunderstorm occurring over the signal path over South Africa.

Effect of magnetic field distortions on gyroresonance at low L values

From fully relativistic test particle simulations of the gyroresonance interaction between energetic electrons and whistler mode waves in the magnetosphere it is known that pitch angle diffusion due to this interaction becomes less and less effective at low L values, requiring very large wave amplitudes (∼100 nT) for significant pitch angle changes (Δα&gt;1°). Such pitch angle changes are needed to produce the wave‐induced electron precipitation as observed through the Trimpi events which have been reported at low latitudes. Our computer simulation has been extended to model background field distortions which may alter the curvature of the field lines in such a way as to enhance the interaction efficiency. In general, “flattening” of the field line in the region near resonance enhances the interaction considerably, with a possible best case threefold increase in interaction time when compared to a undisturbed (dipole) field configuration. Our results indicate that pitch angle diffusion due to gyroresonance can only be significant at low L during periods of field line distortion, such as dayside compression or in the presence of Pc 4 or Pc 5 pulsations and various cavity mode oscillations.

Trimpi events on low latitude paths: An investigation of gyroresonance interactions at low L-values

We report on Trimpi events observed at Durban ( L = 1.69, 29°53′ S, 31°00′ E) and investigate the efficacy of gyroresonance scattering in precipitating electrons into the atmosphere at low L (<2). The rate of occurrence of Trimpis at Durban is less than one per day. Our observations include a number of daytime events on OMEGA signals from La Reunion. Using the full relativistic equations of motion, a test particle simulation is employed to find the region in parameter space where large pitch angle scattering occurs. We find that at low L the conditions for pitch angle scattering are less favourable than at higher L ( L ∼ 4). Resonant electrons have high (relativistic) energies, interaction times are of the order of milliseconds ( T i ∼ 5 ms) and large wave amplitudes ( B w ∼ 200 pT) are required at whistler frequencies to produce pitch angle changes of greater than 1°. Large pitch angle scattering is needed near Durban since particles near the loss cone will have been lost in the South Atlantic Geomagnetic Anomaly. We note that the radio frequencies transmitted into the magnetosphere from lightning are too low to give effective electron scattering at low L. We suggest an explanation for the low rate of occurrence of Trimpis at Durban.

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.

Early Trimpi events from lightning-induced electric fields in the ionosphere: An alternative explanation

Two classes of ‘Trimpi’ modulation of VLF signals in the Earth-ionosphere waveguide have been identified in the literature. The more common type occurs l s or more after causative lightning strokes, the second in less than 100 ms. We explore the possibility that these early Trimpi events result from lighting-generated, electric field impulses lowering the mirror altitudes of trapped electrons. To overcome the mirror force on energetic electrons, upward-directed electric fields with strengths of a few tens of mV/m are required. This is well within the range of electric fields observed on sounding rockets above thunderstorms.

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.

On the spatial relationship between lightning discharges and propagation paths of perturbed subionospheric VLF/LF signals

A study has been made of the spatial relationship between propagation paths of subionospheric VLF/LF signals exhibiting sudden amplitude perturbations (Trimpi events) and time correlated cloud‐to‐ground lightning flashes. On each of the 4 days examined the storm centers were located close to the signal path from the NAU transmitter (28.5‐kHz) in Puerto Rico to Stanford (SU) and were at large distances from the propagation path of the 48.5‐kHz transmitter signal from Nebraska to SU. Nevertheless, no Trimpi events were observed on the former path, while many were seen on the latter. Furthermore, the detected Trimpi perturbations of the 48.5‐kHz signal received at Stanford were found to be associated with the lightning activity in the distant storm centers. Since the NAU‐SU path lies entirely at L &lt; 2 and the 48.5‐SU path is located mostly at 2 &lt; L &lt; 3, the L dependent magnetospheric conditions which determine the level of lightning‐induced electron precipitation are different along the two paths. Thus we postulate that the observed difference in Trimpi occurrence on the two paths was due to the different magnetospheric conditions. Hence the occurrence of Trimpi events over the geographical region corresponding to L &lt; 3 may be more dominantly controlled by magnetospheric conditions than the source lightning distribution.