Trimpi Events Research Articles

Path‐dependent properties of subionospheric VLF amplitude and phase perturbations associated with lightning

A comprehensive study of lightning‐associated amplitude and phase perturbations on multiple VLF/LF signals (Trimpi events) observed at Stanford, California and at Palmer Station, Antarctica, has revealed a number of new properties that appear to be characteristic of the particular signal paths. (1) Signal amplitude changes are on the whole evenly distributed between enhancement and attenuation, but some individual signal paths have strong preferences for one or the other. (2) Phase changes on almost all paths show a strong preference for advancement, with phase retardation occurring rarely. (3) The range in size of amplitude and phase changes appears to be relatively constant for a given path, but it is found to vary between different paths. None of the existing models of the Trimpi effect are found to explain all of the observed new features. Instead, the new experimental findings provide an empirical framework to guide the evaluation of more sophisticated models. Analysis also indicates that the magnitudes of simultaneous amplitude and phase changes are only weakly correlated and that the recovery signatures of amplitude and phase events can be substantially different, with the signal amplitude generally recovering faster. This apparent independence of amplitude and phase perturbations is interpreted to result from the altitude distributed nature of the ionospheric disturbances.

The trimpi effect in antarctica: observations and models

LEP (lightning electron precipitation) bursts can be detected on the ground because they cause transient perturbations (known as ‘Trimpi events’) in the received amplitude and phase of subionospherically propagating radio waves, typically at VLF. The occurrence of mid-latitude Trimpis at Halley (events observed on five nights per month at equinox) is similar to other ( L $ ̃ 4 ) stations e.g. Siple and Eights, and much less than for lower latitude stations such as Palmer or Faraday ( L $ ̃ 2.5 ), nearer to the active LEP belt at 2 ⩽ L ⩽ 3. Activity is mainly on approximately north— south paths from NSS and NAA. In one case study of simultaneous Trimpi activity on the NSS-Halley and NAA-Halley paths, events were not synchronised, implying precipitation regions ⩽ 200–300 km in east-west extent. The maximum amplitude and phase excursions of a sequence of events on the NSS-Halley path oscillated sinusoidally in quadrature ; this was consistent with the echo Trimpi model of Dowden and Adams, assuming an echo signal 15 dB down on the direct signal, with the velocity of the precipitation region perpendicular to the propagation path changing the phase path difference between direct and echo signals by one wavelength (14 km at 21.4 kHz) in about 23 min. Burst precipitation of energetic electrons at high latitudes, close to, and poleward of, the plasmapause, can be well studied by means of the Trimpi effect using the Siple VLF transmitter and a network of VLF receivers in Antarctica (Siple, Faraday/Palmer, Halley, and South Pole). A high-latitude, low VLF waveguide mode propagation computer model for use in such studies has been developed, and tested against observations of controlled transmissions from a dipole antenna of variable (simulated) orientation. Published observations of Trimpi events in this high-latitude region are well reproduced by the model. A new experiment— OPALnet— designed to investigate LEP through its effects on the phase and amplitude of signals from the Omega VLF navigational network, is described. Its novel features permit multi-frequency observations on one path, multi-path observations at one frequency, and ‘VLF imaging’ using a grid of intersection paths.

Quasi-periodic particle precipitation and Trimpi activity at Halley, Antarctica

The relationship between quasi-periodic VLF emissions and micropulsations is briefly reviewed, and then discussed with reference to an event recorded at Halley, Antarctica, on day 257 in 1986. VLF emissions at 2 kHz with a quasi-period of 9 s were observed simultaneously with Pi1 and Pe1 micropulsations. Also observed was a quasi-periodic Trimpi event on the amplitudes and phases of the VLF transmitters NAA and NSS. It is deduced that the VLF emissions are modulated in the generation region by a hydromagnetic wave, giving rise to particle precipitation. The emissions are also modulated by the bounce period of the generating particles. The Trimpi effect is due to 120 keV electrons being precipitated into the lower ionosphere by the interaction with the VLF emissions. This event shows that the Trimpi effect can be used to detect particle precipitation occurring during the ULF/VLF interaction, and can give information which helps to define the mechanism reponsible for the interaction.

Characteristics and frequency of occurrence of Trimpi events recorded during 1982 at Sanae, Antarctica

‘Trimpi’ amplitude perturbations on VLF signals received at Sanae, Antarctica, have been identified using a new computerised technique. Our survey of 1982 data, taken during magnetically disturbed times, shows that events of short duration (<25 s) constitute 60% of all events detected and that all events found are amplitude attenuations with deviations from quiescent levels ranging up to 90%. It is unusual, at Sanae, to observe the causative whistler with a Trimpi event. This, together with further evidence from Trimpi occurrence statistics, may suggest that the gyroresonant interactions responsible for some of the events occur with non-ducted whistler mode waves. A method for estimating the extent of the precipitation region is presented.

Simultaneous Disturbance of Conjugate Ionospheric Regions in Association With Individual Lightning Flashes

Characteristic whistler‐associated amplitude perturbations of subionospheric VLF or LF signals (“Trimpi events”) observed within one second of each other at Palmer Station, Antarctica and at Arecibo, Puerto Rico suggest that ionospheric regions in both northern and southern hemispheres are disturbed together in association with individual lightning flashes. During a one hour period on March 21, 1989, the onsets of 44 out of 47 perturbations measured on a 21.4 kHz signal from Maryland to Arecibo occurred within l s of perturbation onsets measured on a 23.4 kHz signal from Hawaii to Palmer Station. Similar activity occurred before and after this period, and on the preceding and following days. The observations are consistent with the disturbance of geomagnetically conjugate ionospheric regions by multiple bounces between hemispheres of bursts of radiation belt electrons, scattered in pitch angle by whistlers in the magnetosphere. Analysis of patterns of perturbations with corresponding whistler and lightning information from this period suggests that there were at least two distinct ionospheric disturbances in each hemisphere.

Correction to “Are fast atmospheric pulsations optical signatures of lightning‐induced electron precipitation?”

Owing to a production error, a significant portion of the text of “Are Fast Atmospheric Pulsations Optical Signatures of Lightning‐Induced Electron Precipitation?” by J. LaBelle (Geophys. Res. Lett., 14, 1023, 1987) was printed out of sequence. GRL regrets the error and is printing the paper again, with the correct order of the text restored.Fast Atmospheric Light Pulsations (FAP's) consist of millisecond time‐scale bursts of light which have been observed at L=1.5‐2.2 during searches for atmospheric light emissions associated with supernovae. Their statistics of occurrence resemble those of Lightning‐induced Electron Precipitation (Trimpi events) observed at somewhat higher L‐shells. Here we propose that FAP's are in fact optical signatures of LEP events associated with the ≥ 2 MeV electrons of the inner radiation belt (L ≈ 1.4). These electrons would precipitate at low altitudes and could be modulated with time scales the order of 1 ms. The total loss rate of electrons from the inner belt resulting from these events would be comparable to, but somewhat smaller than, the loss rate due to Coulomb scattering.

Are fast atmospheric pulsations optical signatures of lightning‐induced electron precipitation?

Fast Atmospheric Light Pulsations (FAP's) consist of millisecond time‐scale bursts of light which have been observed at L = 1.5–2.2 during searches for atmospheric light emissions associated with supernovae. Their statistics of occurrence resemble those of Lightning‐induced Electron Precipitation (Trimpi events) observed at somewhat higher L‐shells. Here we propose that FAP's are in fact optical signatures of LEP events associated with the ≥ 2 MeV electrons of the inner radiation belt (L ≈ 1.4). These electrons would precipitate at low altitudes and could be modulated with time scales the order of 1 ms. The total loss rate of electrons from the inner belt resulting from these events would be comparable to, but somewhat smaller than, the loss rate due to Coulomb scattering.

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.

Lightning‐induced electron precipitation events observed at L ∼ 2.4 as phase and amplitude perturbations on subionospheric VLF signals

Lightning‐induced electron precipitation (LEP) events are studied using the Trimpi effect, in which the precipitation‐induced ionization enhancements in the lower ionosphere (D region) give rise to rapid perturbations of subionospheric VLF signals. In 1983, the phase and amplitude of signals from the NPM transmitter in Hawaii (23.4 kHz) and the Omega transmitter in Argentina (12.9 kHz) were measured at Palmer, Antarctica (L ∼ 2.4), together with the magnetospheric whistler background. The long baseline and over‐sea great circle paths from these two sources make it possible for the observed perturbations to be interpreted using a single waveguide mode theory. Analytical expressions are used to relate the magnitude of the phase perturbations to differential changes in ionospheric reflection height along a segment of the propagation path. The predicted relationship between relative perturbation sizes on the two different signals is compared with measurements. From this information, the whistler‐induced flux levels are inferred to be in the 10−4 − 10−2 erg cm−2 s−1 range and the precipitation regions are inferred to be roughly “circular” in shape, rather than elongated along L shells. Measured amplitude changes tended to be small (∼ 0.5 dB) and negative, as expected from a single‐mode theory, but the ratios of simultaneous amplitude and phase perturbations were slightly larger than the theory predicts, probably due to the effects of an additional mode(s). An assessment of the relative detectability of amplitude versus phase perturbations favors phase perturbations by ∼ 10 dB, irrespective of the detection scheme used.

Model predictions of subionospheric VLF signal perturbations resulting from localized, electron precipitation‐induced ionization enhancement regions

Whistler‐induced precipitation of energetic electrons produces transient ionospheric conductivity variations that perturb the amplitude and phase of VLF signals propagating in the earth‐ionosphere waveguide (i.e., Trimpi events). This study uses a waveguide mode theoretic propagation model to predict the effect of localized ionization enhancement (IE) regions on nighttime long‐path VLF signals from the U.S. Navy navigational transmitters NSS, NAA, NLK and NPM to Antarctic receiving stations, principally the U.S. station at Palmer. These predictions are then compared with the observed signal behavior in Trimpi events from which inferences can be drawn about an IE region's approximate location, size, and particle characteristics. The propagation model considers multiple modes, variable ground conductivity, and mode conversion. IE regions of variable length (typically 50–150 km) and distance from the receiver (up to 600 km) were considered. The conductivity change within an IE region was modeled by exponential electron density profiles modified by precipitated monoenergetic fluxes of 50‐ to 150‐keV electrons. Our calculations show that some signal paths are more sensitive than others to perturbation by IE regions and that the size of an IE region is less important than its distance from the receiver. For a specified flux, higher‐energy electrons produce larger effects than lower energy electrons; for a specified energy, larger fluxes will produce larger signal changes. For the particular case of 150‐keV electrons precipitating into an ionosphere with VLF reflection height of 87 km, the received signal was largely insensitive to increasing the electron flux above ∼10−5 erg cm−2 s−1. This flux is equivalent to ∼3×10−3 erg cm−2 s−1 for E &gt; 40 keV and an exponential energy spectrum with e‐folding energy in the range 40–100 keV. The measured electron fluxes precipitated by lightning are of the same order. Higher saturation flux levels would apply in situations where the normal VLF reflection height was lower over that portion of a path where the whistler‐induced precipitation was occurring. This study also examines the effects of transmitter‐induced electron precipitation on the VLF signals. Such precipitation is known to occur and, as shown here, may on occasion have important effects on certain signals.

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 &lt; 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.

A new VLF method for studying burst precipitation near the plasmapause

VLF signals in the 2–4 kHz range transmitted from Siple Station, Antarctica (L ∼ 4.3), and received at various Antarctic locations have been used to detect the occurrence of burst precipitation of electrons (E ≥ 50 keV) into the nighttime lower ionosphere. The receiving stations, each ∼1400 km from Siple, were located to the north at Palmer (L ∼ 2.3), to the east at Halley (L ∼ 4.3), and to the south at South Pole (Λ ∼ 74°). Rapid changes in the received phase and amplitude of the Siple signal (“Trimpi events”) were observed in conjunction with the reception of one‐hop whistlers. In one case involving propagation paths lying poleward of the plasmapause, amplitude decreases by ∼20% in ∼2 s, decaying in ∼7 s, were recorded simultaneously at Halley and South Pole. Post facto analysis of the South Pole records using a new digital processing scheme showed corresponding fast phase advances of ∼15 µs. In a case of phase perturbations at Palmer, the Siple signal path crossed the plasmapause projection, and the associated whistlers propagated in the outer plasmasphere. Phase measurements appear to be a particularly sensitive means of detecting burst precipitation activity under experimental conditions of the kind described. The length and spatial distribution of the signal paths provide a basis for studying the occurrence and approximate location of burst precipitation in regions outside the observing range of most instruments used for detection of precipitation. Direction finding on correlated whistler events as well as dispersion analysis may be used to increase the spatial resolution of the method.

Diurnal variation of burst precipitation effects on subionospheric VLF/LF signal propagation near L = 2

Burst precipitation effects on subionospheric VLF/LF signal propagation are studied using data recorded at Palmer Station, Antarctica (L ∼ 2.4) between March 3 and April 7, 1983. Relatively abrupt signal amplitude changes, known as “Trimpi” effects, were observed in conjunction with magnetospheric whistlers, which are inferred to induce electron precipitation that in turn causes enhanced ionization at the 80 to 90‐km altitude level. Earlier findings that Trimpi effects occur predominantly under nighttime ionospheric conditions were confirmed by comparing the sunrise‐sunset terminator position with perturbation activity on three VLF/LF signal paths for which the arrival bearings range from magnetic north to west. Events were found to occur on 70% of the observing nights. Signal paths making larger angles with the terminator were used to deduce that most events were caused by ionospheric effects occurring within ∼1000 km of Palmer Station. Some events were observed after sunrise over the observing station, apparently due to precipitation in dark regions as far as ∼1800 km away. Observed differences between two different paths in the occurrence of this postsunrise continuation of Trimpi events suggest that the Trimpi event activity rate decreases below L ∼2. This may be due to a reduction in whistler activity and/or lower particle scattering efficiency. The average hourly occurrence rate of transmitter NSS events on the 11 most active nights studied increased from sunset to near midnight and then remained at a roughly constant level until sunrise. The influence of the whistler rate and of whistler path locations on Trimpi effects is clearly important and requires further study.