Precipitation Bursts Research Articles

Recovery signatures of lightning‐associated VLF perturbations as a measure of the lower ionosphere

A new model of the physical processes associated with subionospheric VLF signal perturbations caused by lightning‐induced electron precipitation (LEP) bursts is developed to diagnose the state of the lower ionosphere (e.g., electron number density and rate coefficients for various chemical reactions) on the basis of measurements of VLF recovery signatures. The model accounts for the energy spectrum of the electron bursts precipitated by lightning‐generated whistlers, the chemical relaxation of enhanced secondary ionization in the nighttime D region due to LEP bursts, and quantitatively treats the resultant effects on propagation of the VLF signal in the Earth‐ionosphere waveguide. Application of the model to experimental data obtained for the VLF propagation path from NPM station (Hawaii) to Palmer station (Antarctica) indicates that effective electron detachment rate γ, enhanced secondary ionization profile (e.g., energy content of LEP bursts), as well as the ambient electron density distribution, may be estimated using observed subionospheric VLF recovery signatures. The effective detachment rate was identified as ∼10−18 N s−1, where N is total number density of neutrals. Model indicates in particular that the attachment‐detachment processes play the dominant role in recovery of subionospheric VLF signal perturbations on timescales ∼ 100 s, and that the observed perturbations of the NPM‐Palmer signal correspond to the LEP bursts consisting of relatively soft (< 250 keV) electrons.

Simultaneous measurements of waves and precipitating electrons near the equator in the outer radiation belt

An investigation of wave‐particle interactions is made using several simultaneous electron and wave measurements performed at near‐equatorial positions from the CRRES satellite. Bursts of electron precipitation were observed, most frequently at local times near dawn. Examples of bursts are presented in which the fluxes of the precipitating electrons and the wave intensities are correlated with coefficients as high as 0.7. During bursts the frequencies of the enhanced waves spanned a wide range from 311 Hz to 3.11 kHz, and the energies of the enhanced electrons were in the range 1.7 keV to 288 keV. The changes of the precipitating fluxes were generally less pronounced at the lowest energies. On the basis of electron‐cyclotron resonant calculations using the cold plasma densities and ambient magnetic fields taken from the CRRES measurements it was found that the wave frequencies and precipitating electron energies were generally consistent with those expected from electron resonance with parallel propagating whistler waves. The electron data of principal concern here were acquired in and about the loss cone with narrow angular resolution spectrometers covering the energy range 340 eV to 5 MeV. The wave data included electric field measurements spanning frequencies from 5 Hz to 400 kHz and magnetic field measurements from 5 Hz to 10 kHz.

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 < L < 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.

Relaxation of transient lower ionospheric disturbances caused by lightning‐whistler‐induced electron precipitation bursts

A quantitative model of the relaxation of transient lower ionospheric (D region) disturbances caused by lightning‐induced electron precipitation is developed, taking advantage of known particular features of the lightning‐induced disturbances, such as the fact that they are produced in typically <1 s and decay over 10–100 s. The model represents the nighttime D region as consisting of only four kinds of charged particles (electrons, positive ions, negative ions, and positive cluster ions) and is particularly suited for description of the detailed behavior of the electron density. Application of the model to some previously modeled disturbances indicates that some of the least known chemical reaction rates in the nighttime D region altitudes may be measurable using subionospheric VLF data. In the production of secondary ionization by precipitating electron bursts, the model calculations indicate the presence of a saturation effect such that the number density of the secondary electrons is not simply equal to the ion pair production rate times the burst duration. In some cases involving precipitation of ∼1‐MeV electrons, the model predicts the formation of new layers of ionization at 50–70 km altitude that represent a different attachment‐detachment quasi‐equilibrium value from that of the unperturbed ambient. Such new layers may exist for up to ∼105 s following electron precipitation bursts.

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.

Search for lightning‐induced electron precipitation with rocket‐borne photometers

Photometers at 3914Å and 5577Å and an optical imager were part of an experimental package launched on a sounding rocket in the 1987 Wave Induced Particle Precipitation (WIPP) campaign at Wallops Island, Virginia. The objective was to measure lightning‐induced electron precipitation (LEP) by means of its optical signature. This was the first attempt to measure LEP using rocketborne optical instrumentation. Launch criteria included nearby thunderstorm activity and ground‐based observations of Trimpi events. Lightning flashes are clearly discernible in the data. The photometer data was also characterized by large spin and precession modulations in the photon count rate, consistent with elevated steady particle fluxes in the northern portion of the instrument field of view. No evidence of LEP was observed by the photometers or onboard particle detectors [Arnoldy and Kintner, 1989]. Analysis of the data has enabled us to place an upper limit of 8 × 10−4 ergs‐cm−2‐sec−1 on any burst precipitation energy flux that may have occurred during the rocket flight in the regions explored by the photometers.

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.

Wave‐induced burst precipitation events detected with a digital ionosonde

Initial results are presented from two methods whereby burst precipitation events in the lower ionosphere, almost certainly induced by VLF wave‐particle interactions in the magnetosphere, have been detected using a ground‐based digital ionosonde. In the first method, HF echoes are received above the critical frequency of the surrounding plasma; particle energies and the location and extent of the plasma enhancement may be deduced. In the second method, a rapid decrease in the phase of ionospheric echoes is observed due to refractive index changes along the echo path; particle energies, the duration of the precipitation event and the precipitation energy flux can be estimated.

Characteristics of short‐duration electron precipitation bursts and their relationship with VLF wave activity

Energetic (>6 keV) electron data from the SEEP payload on the low altitude (∼200 km) polar orbiting S81‐1 satellite indicate a high rate of occurrence of short duration (<0.6 s) electron precipitation bursts. Characteristics of events observed at night (2230 MLT) versus daytime (1030 MLT) and at midlatitudes (2<L<3) versus higher latitudes (L>3) were distinctly different in several ways. For 2<L<3 the daytime bursts occurred approximately uniformly in longitude and were equally distributed between the northern and southern hemispheres. The nighttime bursts in the same L shell range occurred approximately twice as often on a worldwide basis and were observed predominantly in the northern hemisphere and at longitudes of 260°E to 320°E. In a significant number of the nighttime events at 2<L<3 the median electron energy increased with time during the burst, but most of the other spectra showed no well‐defined trend. During some of the nighttime bursts broad peaks were observed in the energy spectra, but these peaks were not so evident in the daytime bursts. On higher L shells, L>3, narrow electron precipitation bursts (<0.3 s duration) were frequently observed poleward of the plasmapause, more often near noon than near midnight and much more frequently than at 2<L<3. Comparison of the electron data with simultaneous VLF wave data from Palmer (L≃2.4) and Siple (L≃4.3) stations in Antarctica indicated a varying degree of association of electron bursts with whistlers and chorus emissions. Several of the electron bursts observed at nighttime and at 2<L<3 were correlated with lightning‐generated whistlers observed at Palmer Station. When daytime bursts at higher latitudes (L>3) were observed on satellite passes within ±50° of the Siple meridian, chorus was invariably detected at Siple, but correlation of electron bursts with individual chorus spectral elements was not evident. The lack of such correlation may be due to the limited spatial extent of flux tubes excited by individual chorus elements which are possibly generated without mutual coherence in multiple high altitude magnetospheric locations. Within one hour of all four of the satellite passes within 400 km of Siple with the highest rate (≥10 individual bursts per pass) of burst occurrence, overhead electron precipitation was clearly detected by ground based sensors at Siple Station. Quasi‐periodic bursts of several seconds duration were observed on one or more of the photometer, riometer, and magnetic pulsation sensors and many of these bursts were correlated with clustered VLF chorus bursts. Hence, there is at least association of chorus with electron precipitation.

Energy spectra and pitch angle distributions of lightning‐induced electron Precipitation: Analysis of an event observed on the S81‐1(SEEP) satellite

Temporal and spectral signatures of a lightning‐induced electron precipitation (LEP) burst observed on the S81‐1 (SEEP) satellite are analyzed and compared with the predictions of a test particle model of the gyroresonant whistler‐particle interaction in the magnetosphere. The flux to be detected by specific detectors on the low altitude (∼220 km) satellite at L ≃ 2.24 is calculated in terms of the integral counting rate as a function of time and in terms of the dynamic energy spectra during the initial ∼300‐ms precipitation pulse. For a whistler wave packet with frequency range 500 Hz to 6 kHz the dynamic energy spectra are found to depend sensitively on the electron angular distribution in the vicinity of the loss cone. In the case of a whistler wave originating in northern hemisphere lightning the maximum whistler‐induced pitch angle scattering of electrons occurs near ∼10°S geomagnetic latitude. However, scattering occurring over the latitude range of ∼20°N to ∼20°S is found to be significant and contributes to the observed LEP pulse. The dynamic energy spectra of the LEP pulse and the temporal profile of the integral counting rate are consistent with the predictions of a test particle model of the gyroresonant scattering of the electrons by a whistler wave having an equatorial intensity at 6 kHz of ∼200 pT. The measured LEP pulse pitch angle distribution is wider than that estimated on the basis of the test particle model.

Remote sensing of the magnetospheric plasma by means of whistler mode signals

Early in the past decade of U.S. Antarctic research, the whistler method of measuring equatorial electron density was found to agree with in situ satellite electron density measurements by a radio technique. Furthermore, the whistler method of measuring the east‐west component of the convection electric field in the outer plasmasphere was found to agree, under conditions of mapping in a dipole magnetic field, with simultaneous results from incoherent scatter radar. A global model of the east‐west convection electric field in the outer plasmasphere during substorms was developed. The detection of whistlers and their use for magnetospheric diagnostics have been important elements in recent studies of burst precipitation into the ionosphere induced by whistlers and by other transient whistler mode waves propagating in the magnetosphere. Whistlers have also been used to obtain data on the L values and equatorial electron densities associated with the propagation paths of signals from the Siple VLF transmitter. The process of untrapping of downcoming wave energy from ducts in the upper ionosphere and the upward repropagation of portions of the energy following reflection in the lower ionosphere lead to the excitation of adjacent ducts as well as to upward propagation in the nonducted mode. Efficient interduct coupling has been found to occur over north‐south ionospheric distances of >1000 km. Studies of the outer limits of observed ducting revealed dayside path radii in the range 6–8 RE and nightside limits of ∼5.5 RE. Ducted propagation beyond the plasmapause was found to occur regularly in the 0000–1800 MLT time range, but with variable rates and at various locations with respect to the plasmapause position. The special features of this propagation are believed to be related to conditions of lightning excitation, ionospheric penetration, and wave‐particle interactions that are special to the region beyond the plasmasphere. New aspects of Siple wave injection experiments were demonstrated by the application of a new phase measurement method to Siple signals that did not exhibit fast temporal growth during passage through the magnetosphere. This method, a refinement of techniques developed previously by New Zealand workers, is capable of detecting fluctuations in phase path with period of ∼10 s and greater and thus can be used to study magnetospheric convection and coupling fluxes along field lines of propagation as well as pulsations associated with ultralow‐frequency perturbations of the geomagnetic field. Additional topics discussed include results from direction‐finding experiments and evidence of the dependence of whistlers upon magnetospheric wave amplification.

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.

Electron precipitation bursts in the nighttime slot region measured simultaneously from two satellites

Based on data acquired in 1982 with the Stimulated Emission of Energetic Particles payload on the low‐altitude (170‐280 km) S81‐1 spacecraft and the Space Environment Monitor instrumentation on the NOAA 6 satellite (800–830 km), a study has been made of short‐duration nighttime electron precipitation bursts at L = 2.0–3.5. From 54 passes of each satellite across the slot region simultaneously in time, 21 bursts were observed on the NOAA 6 spacecraft, and 76 on the S81‐1 satellite. Five events, probably associated with lightning, were observed simultaneously from the two spacecraft within 1.2 s, providing a measure of the spatial extent of the bursts. This limited sample indicates that the intensity of precipitation events falls off with width in longitude and L shell but individual events extend as much as 5° in invariant latitude and 43° in longitude. The number of events above a given flux observed in each satellite was found to be approximately inversely proportional to the flux. The time average energy input to the atmosphere over the longitude range 180°E to 360°E at a local time of 2230 directly from short‐duration bursts spanning a wide range of intensity enhancements was estimated to be about 6 × 10−6 ergs/cm² s in the northern hemisphere and about 1.5 × 10−5 ergs/cm² s in the southern hemisphere. In the south, this energy precipitation rate is lower than that from electrons in the drift loss cone by about 2 orders of magnitude. However, on the basis of these data alone we cannot discount weak bursts from being a major contributor to populating the drift loss cone with electrons which ultimately precipitate into the atmosphere.

Detailed spectral structure of magnetospheric electron bursts precipitated by lightning

We have analyzed the temporal structure of electron fluxes precipitated by lightning that was measured at night over Wallops Island, Virginia, during August 1984. Given the high time resolution of our data for two independent lightning‐induced precipitation bursts, we are able to identify a component of the precipitating electrons almost certainly due to equatorial wave‐particle interactions involving equatorial electron cyclotron resonance (EECR) with whistlers which produce pitch angle scattering. These bursts also contain other precipitating electrons, possibly due to off‐equatorial pitch angle scattering by the electron cyclotron resonance (ECR) process or, more likely in our opinion, an off‐equatorial wave‐particle interaction leading to field‐aligned electron acceleration. In particular, we have shown that the precipitated electron fluxes of equatorial origin are characterized by a very steep spectral power exponent of about −20 above 100 keV. In contrast, the other non‐EECR fluxes are much harder, with a spectral power exponent of about −2 between 40 and 400 keV. As a consequence, the energy deposition of the other electron fluxes may be equal to or possibly exceed that of the equatorial interaction fluxes. We conclude that study of lightning‐produced electron fluxes from the perspective of pitch angle scattering alone may neglect other processes that significantly contribute to total energy transfer into the lower atmosphere. Furthermore, we find that the EECR wave‐particle interaction produces precipitation fluxes that are well described simply by the kinematics involving the wave and electron transit times to and from the magnetic equator.

Characteristics of auroral electron precipitation on the morningside

Quantitative information on auroral electron precipitation is needed to evaluate the effects of the precipitation on the ionosphere and upper atmosphere. In this paper we present such information for the morningside aurora. We use simultaneous observations of precipitating electrons and auroral images from the low‐altitude, polar‐orbiting DMSP‐F6 satellite, and we relate specific precipitation features to specific types of auroras. We find the equatorward region of diffuse aurora to have precipitating electron energy fluxes, εp, which typically reach 10 ergs/cm² s, and such fluxes can extend over several degrees in latitude. Energy spectra over the diffuse aurora are quite hard and show no evidence of acceleration by a field‐aligned potential drop, V∥. The poleward region of structured aurora includes a soft background of relatively weak, uniform precipitation. Arclike structured precipitation results from a variable V∥ acceleration of this background. Over the arcs, εp can exceed 1 erg/cm² s. Bursts of precipitation at energies above a few keV are identifiable in the data equatorward of the peak in the diffuse auroral precipitation. We believe these bursts are associated with pulsating auroral patches. Omega band waves are occasionally observed along the poleward boundary of the diffuse aurora. Electron spectra over the omega bands are unique for the morningside. The spectra show hard precipitation as observed within the diffuse region. In addition, the spectra have a peak at energies generally ≲1 keV indicative of acceleration by a V∥ ≲ 1 keV.

Slot region electron precipitation by lightning, VLF chorus, and plasmaspheric hiss

Energetic electrons are precipitated from the slot region of the radiation belts by a variety of mechanisms, including short duration wave bursts associated with lightning and chorus and more slowly varying plasmaspheric hiss. Characteristics of the nightside short duration precipitation events, including their favored occurrence at certain longitudes in the northern hemisphere, indicate that they are predominantly associated with lightning. The dayside events seem to relate primarily to VLF chorus. Here a study is made of various characteristics of the short duration precipitation bursts, namely, the longitude and L shell variations, the day/night differences, the energies of spectral maxima, and the rapid spectral variations with time. In addition, the total loss rates of electrons from the radiation belts are obtained from the measured energy spectra and pitch angle distributions. An assessment is made of the relative importance of the bursts as a loss mechanism for slot region electrons in comparison to the more slowly varying precipitation processes. The assessment is based on a comparison of the energies of the peaks often observed in the energy spectra of both classes of precipitation. The slowly varying electron fluxes observed in the drift loss cone frequently display peaks in the energy spectra which indicate that they are precipitated by plasmaspheric hiss and that this process therefore represents a major loss mechanism. Well‐defined bursts, which appear to be associated with lightning at nighttime and perhaps chorus in the daytime, were observed on only about 2% of the satellite passes and are generally of lower energy than the general population in the drift loss cone, suggesting that the bursts are not the dominant loss mechanism. There is some overlap between the distributions in peak energy, and lightning may play a role in precipitating electrons from the slot region primarily at nighttime, with its greatest contribution at low L shells.

Pitch angle diffusion in morningside aurorae: 1. The role of the loss cone in the formation of impulsive bursts of precipitation

Pitch angle diffusion of energetic electrons is important in morningside diffuse aurorae, equatorward of the auroral oval, where the plasma is shielded from the influence of large‐scale electric fields. Morningside aurorae exhibit complex spatial and temporal structure. The temporal structure is addressed in a treatment which includes the principal elements of the feedback between pitch angle diffusion and VLF wave growth. A time‐dependent bounce‐averaged pitch angle diffusion equation is derived for the electron flux in and near the loss cone. The diffusion equation includes backscatter from the atmosphere. The atmospheric backscatter is represented by an empirical model in which the pitch angle distribution in the loss cone is approximated by a Taylor series in the sine of the pitch angle. The backscatter model is adjusted to match detailed computations performed with a Fokker‐Planck code for the scattering and slowing of energetic electrons in the atmosphere. Solutions of the pitch angle diffusion equation—both steady state and time‐dependent—are used to compute anisotropies and estimate growth rates of VLF whistler mode waves. The electrons' anisotropy is most sensitive to the diffusion rate near the transition from weak to strong diffusion, where the anisotropy may become so low that VLF wave growth is halted. The principal effect of loss cone filling during an episode of strong diffusion is a sharp increase in the minimum resonant energy for wave growth and a rapid decrease of the growth rate. The characteristic time scales for wave growth and quenching due to changes in the anisotropy are of the order of 0.1–1 s and 2–10 s, respectively. The interaction of growth and decay of waves leads to precipitation bursts on a time scale of 1–50 s. The theory explains many observations of impulsive electron precipitation throughout the outer trapping regions at energies above 20–25 keV. Whether the theory can explain auroral precipitation pulsations at energies below 25 keV without invoking other wave modes remains unresolved.

Direct observation of magnetospheric electron precipitation stimulated by lightning

Plasmaspheric electron precipitation bursts stimulated by observed lightning flashes have been studied using in situ rocket techniques under night-time conditions at Wallops Island, Virginia, on 23 August 1984. In one case, the rocket-observed ligntning flash was located by a ground-based network to be off the coast of Virginia at 74.8°W, 35.9°N, which was approximately 200km southeast of the rocket. Electron precipitation (> 40 keV) caused by simultaneous 21.4 kHz coded transmissions from a VLF radio transmitter at Annapolis, Maryland, was found to be negligible compared with the observed fluxes and energies of precipitating electrons stimulated by lightning. The results confirm that lightning (which can occur up to 100 times per second globally) can activate ionizing radiation sources within the magnetosphere, and that these may affect local middle atmospheric electrical structure in a measurable way.

Correction to “A new VLF method for studying burst precipitation near the plasmapause”

Correction to “A new VLF method for studying burst precipitation near the plasmapause”

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.