Global Thunderstorm Activity and its Effects on the Radiation Belts and the Lower Ionosphere

A. INTRODUCTION

The research program addresses scientific questions (Section B) focused on the quantification of the global phenomenology and effects on the Earth’s ionosphere and the radiation belts of tropospheric lightning activity, consisting of~2000 active thunderstorms active at any given time and maintaining a global average lightning flash rate of ~100 per second [Volland, 1984].

The primary tools for the investigation are

(i) wideband extremely low frequency (ELF) and very low frequency (VLF) measurements of electromagnetic impulses (radio atmospherics or just sferics) radiated by lightning discharges and whistler waves which traverse the radiation belts along magnetic field lines, and
(ii) narrowband subionospheric VLF remote sensing of lower ionospheric disturbances produced in lightning-induced electron precipitation (LEP) events. Wideband ELF/VLF sferics data from Palmer have recently been demonstrated to provide a surprisingly powerful proxy measure for the occurrence of terrestrial gamma-ray flashes [Fishman et al., 1994; Inan et al., 1996a], and mesospheric optical flashes known as sprites [Sentman et al., 1995; Reising et al., 1996; Reising et al., 1999] have been found to allow highly accurate (<1. in azimuth) determination of lightning location at long range (>12,000 km) [Reising et al., 1996;Wood and Inan, 1999]. Subionospheric VLF remote sensing provided the earliest evidence of ionospheric effects of lightning discharges [Helliwell et al., 1973; Burgess and Inan, 1993, and references therein], and is particularly and uniquely suited for measuring conductivity changes in the lower ionosphere [Seschrist, 1974], in our case produced by LEP bursts.

Fig. 1. VLF observation sites. Starting in late 1999, the narrowband VLF data from Palmer will be transmitted back to Stanford
on a daily basis, and made available in both raw and reduced form to the scientific community at large over the Internet

VLF remote sensing at Palmer is conducted as part of an international program, involving Palmer, the Brazilian Commandante Ferraz (CF) station on King George Island, and the U. K. Rothera (RO) station (Figure 1). Simultaneous data from all three stations were acquired during 1995 and partly also in 1996. Observations at CF have continued year round since then, and we are currently negotiating with colleagues at British Antarctic Survey (BAS) to conduct observations at RO during at least one of the three years, by providing surplus Stanford equipment. The proposed program synergistically complements Stanford’s Holographic Array for Ionospheric Lightning (HAIL, see Figure 4 and 6) project, carried under other support (NSF/ATM), which provides coverage of ionospheric regions near the source thunderstorms and geomagnetically conjugate to the regions covered by observations in the Antarctic peninsula.

Broadband VLF measurements at Palmer are conducted as part of an international collaboration, including observations at Stanford (SU), at Taylor University (TU) in Indiana (Dr. H. Voss), and at a remote site in the Negev Desert (ND), Israel (Dr. C. Price of Tel Aviv University). Deployed at both TU and ND are Stanford-built VLF receivers identical (in terms of bandwidth and sensitivity) to that at Palmer Station, thus facilitating highly accurate tracking of thunderstorms in the Americas, Africa, and oceanic regions, as shown in Figure 2.

Fig. 2. Schematic description of long range tracking of lightning activity in the Americas, Africa and oceanic regions. As part of the proosed program, arrival azimuth of individual sferics will be determined at each site via local processing and sent over the Internet to Stanford, where spherical triangulation will then be used to locate individual storm centers and track the lightning activity within them throughout the day.

In terms of collaborative investigations, the availability of the Palmer data over the Internet allows scientists at large to access the data promptly and analyze it in comparison with other data (e.g., lightning location data, optical data, or data on terrestrial gamma ray flashes). In terms of educational outreach, opportunities abound in the context of Stanford HAIL project (Figure 4) with VLF receivers placed at nine high schools and with very close interactions between Stanford and students/teachers from these institutions. The simple nature of the measurement, the clarity of the signatures (i.e., VLF signatures of LEP events are readily detected and correlation with lightning is established without any subtle averaging or processing), the association of the phenomena measured in the North with spectacular whistlers (both spectral images and audible sound) recorded in the Antarctic, and the attractiveness of lightning as a spectacular physical phenomenon are all factors that should stimulate the interest of science-oriented students in general.

B. SCIENTIFIC BACKGROUND AND QUESTIONS

The primary scientific topics to be addressed are:
(i) thunderstorm coupling to the radiation belts
(ii) characteristics of lightning flashes which lead to upward electrodynamic coupling
(iii) variability of the lower ionosphere and its parameters
(iv) global lightning activity and climatology.

Fig. 3. Ducted LEP mechanism and VLF detection of associated ionospheric disturbances at Palmer. Top left: Northward bound electrons are scattered in pitch angle and precipitate first into the northern hemisphere (from which the whistler wave is launched by lightning), and subsequently into the southern hemisphere after mirrorring/backscattering from the north. Bottom left: VLF perturbations on the NPM (Hawaii) signal recorded at Palmer. One event (with red arrow) is expanded in the bottom right, together with broadband recording of the associated ducted whistler. The time of the causative sferic for the whistler is indicated with a red arrow. Top right: The ionospheric disturbance (enhanced secondary ionization produced by the LEP burst) and its effect on the waveguide mode structure of a subionospheric signal.

1. Thunderstorm Coupling to the Radiation Belts

Lightning-induced electron precipitation (LEP) is a means of loss for the radiation belt electrons caused by resonant whistler wave-particle interactions. Precipitation of individual bursts of energetic electrons in association with individual lightning discharges has been measured on satellites, rockets, and via VLF remote sensing of associated ionospheric disturbances [Voss et al., 1998 and associated references therein]. Previous theoretical [e.g., Inan et al., 1989] and experimental [e.g., Burgess and Inan, 1993] work on the LEP phenomena has emphasized interactions with ‘ducted’ whistler waves which propagate in field aligned ducts of enhanced ionization. Figure 3 shows phenomonology of the ducted LEP mechanism and its detection via VLF remote sensing.

The first quantitative model of the precipitation of bursts of energetic electrons by oblique (nonducted) whistlers launched by individual lightning flashes has only recently been realized [Lauben et al.,1999]. LEP events induced by ducted whistlers exhibit onset delays and duration which are quantized by the required presence of a magnetospheric duct [Inan and Carpenter, 1986] and produce ionospheric disturbances of modest horizontal extent (~100 km). In contrast, scattering by obliquely propagating whistlers (which illuminate a wide range of field lines) leads to precipitation over an extended region, with a continuum of onset delays and durations as a function of latitude [Lauben et al., 1999]. The first direct experimental confirmation of several important predictions of the Lauben et al. [1999] model have recently been found [Johnson et al., 1999], namely that (i) bursts of electrons precipitated from different field lines arrive at the ionosphere with onset delays steadily increasing with increasing L-value, (ii) ionospheric regions disturbed in individual events may have spatial extents of up to ~1000 km and are poleward-displaced in latitude with respect to the causative flash, and (iii) that the peak precipitation fluxes induced by oblique whistlers are at least as intense as those produced by ducted waves. Since most of the wave energy launched into the magnetosphere propagates in the nonducted mode [e.g., Edgar, 1972], these new observations suggest that the LEP process is very likely to be a significant loss process for radiation belt electrons on a global scale.

Fig. 4. HAIL evidence for electron precipitation induced by oblique (non-ducted) whistler waves from lightning. Holographic Array for Ionospheric Lightning (HAIL) consists of observation sites ranging from Cheyenne (CH) to Las Vegas (LV). Events A, B, C and D, although appearing to occur simultaneously at all stations on the left hand panels, in fact exhibit onset delays which steadily increase with latitude of the affected great circle path, as shown in the upper right panels. The start and end of the onsets measured for event B on the different HAIL paths are shown in circles in the lower right corner, with the geomagnetic latitude for each path determined as the points of intersections (shown as black dots in the middle panel) of the constant magnetic longitude line with the great circle path of interest.

The salient results of Johnson et al. [1999] are summarized in Figure 4, showing a five minute sequence of LEP VLF events, marked A–D, observed on the NAA signal, and unambigiuously associated with lightning discharges occurring near Austin, Texas (as shown). The perturbation of all of the NAA–HAIL paths and the upper five NAU–HAIL paths (not shown) indicate a disturbance much larger than the typical ~100 km extent of ducted LEP disturbances. The absence of events on the lower four NAU–HAIL paths (despite being closer to the causative discharge), indicates a poleward-displaced precipitation zone, as predicted by Lauben et al. [1999]. A striking feature of the data is the steadily increasing onset delay with increasing geomagnetic latitude of the affected paths, as is evident from the superposed (after proper filtering and normalization) display of the VLF signatures observed on differentNAA–HAIL paths (Figure 4). The distinctly different onset delays indicate that the various different VLF paths respond to ionospheric disturbance regions that become active at different times. Thus, the VLF amplitude changes seen on the different paths cannot be due to a single localized ionospheric disturbance, as produced (for example) near the footprint of a whistler-mode duct. Instead, the continuum of onset delays steadily increasing with geomagnetic latitude agrees remarkably well with the predictions of Lauben et al. [1999]. The precipitation region calculated with the Lauben et al. [1999] model for a source lightning discharge near Austin Texas also agrees remarkably well with the layout of the perturbed VLF paths (Figure 4) as does the energy-flux deposition as a function of time and latitude, describing the manner in which the different parts of the precipitation region appear in time.

The discovery of nonducted LEP events in the northern hemisphere prompted us to re-examine Palmer data for possible evidence of such precipitation in the southern hemisphere. In principle, the pitch angle scattered electrons should mirror/backscatter in the north and precipitate in the south (just as for LEP bursts induced by ducted whistlers), where the mirror altitude is lower due to the South Atlantic magnetic anomaly. In this connection, we were fortunate that the NPM–PA and NPM–CF paths (Figure 1) lie in a west-east direction, separated by few degrees in geomagnetic latitude, much like the NAA–HAIL paths of Figure 4. Figure 5 shows an example of a sequence of events simultaneously observed at Palmer (PA) and Commandate Ferraz (CF), and which clearly show a later onset observed at PA compared to CF. The causative lightning flash for the events in Figure 5 were determined to be near Florida, based on lightning data from the National Lightning Detection Network (NLDN).

This case, and others reported by Inan et al. [1999b] indicate that northern hemisphere lightning discharges produce nonducted LEP in the southern hemisphere. Further, they raise the question of whether the previously reported common occurrence of LEP phenomena observed at Palmer (and interpreted at the time to be due to ducted whistlers) [e.g., Leyser et al., 1984], may mostly have in fact been due to nonducted whistlers. If so, the potential consequences of LEP phenomena on the loss rates of radiation belts, as estimated for example by Burgess and Inan [1993], need to be revised, since nonducted whistler waves permeate much larger regions of the magnetosphere [Edgar, 1972]. Although LEP has been known to occur for some time, its potential role on a global scale has only recently began to be quantified with estimates indicating that losses of radiation belt particles by lightning-induced whistler waves is significant in the L-shell range 1.8 < L < 2.6 [Abel and Thorne, 1998a,b]. These estimates did not have the benefit of the new discovery of regular precipitation induced by nonducted (oblique) whistlers, which greatly enhance the potential global role of the LEP process. In terms of assessing the global effects of LEP on the ionosphere, it is important to establish the extent of hemispheric conjugacy of the precipitation. Past work has provided clear evidence that conjugate ionospheric regions are sometimes perturbed simultaneously [Burgess and Inan, 1990; 1993]. However, establishing the circumstances under which this most commonly occurs has not been an objective until now, partly because of the expectation that such events would be difficult to capture with a finite number of VLF paths, since in the case of ‘ducted’ LEP events the ionospheric disturbances are relatiely small (~100 km). However, the much larger size of the ionospheric disturbances involved in non-ducted LEP events facilitates a comprehensive investigation of the conjugacy of LEP events, as described in Figure 6.

Fig. 5. Southern hemisphere LEP events induced by oblique (nonducted) whistlers. (a,b) The 21.4 kHz NPM signal (Hawaii) received at Commandante Ferraz (CF) and Palmer (PA) (Figure 1) exhibits apparently simultaneous characteristic perturbations, which in expanded form (c,d) appear to not be simultaneous, instead exhibiting onset delays dependent on geomagnetic latitude, in the manner predicted by Lauben et al., [1999]. (e) The overlay plot most clearly shows that the event onset at PA occurs after that at CF. (f) Histogram of measured onset delay differences for many events observed during 0835–0955 UT on 31 August 1998. Although the delay differences are spread out due to measurement error, Palmer onsets are consistently later.

Questions:

Under what conditions does LEP represent a significant fraction of the overall particle loss rate from the radiation belts? What is the spatio-temporal structure of LEP regions? What is the geographic (longitude) and geomagnetic (L-shell) distribution of LEP event activity? What is the variability of the onset time and duration, as well as the rise and decay times of VLF events in comparison with magnetospheric wave and particle activity? Can VLF signatures of LEP events be used to measure the altitude profile of enhanced ionization and hence the energy spectra of the precipitating particles?

Approach:

Quantification of the global significance of the LEP process requires the knowledge of the size (individual regions) and distribution (both regional and global scales) of disturbed ionospheric regions (and therefore the affected magnetospheric regions), the determination of which is a primary goal of our proposed program. Observations at PA, CF, and RO provide sufficient regional coverage of southern hemisphere regions, while simultaneous data from the north (HAIL) will allow the assessment of the geomagnetic conjugacy of event activity. The relationship between temporal signatures (i.e., onset, rise, decay) ofVLFevents and magnetospheric parameters will be studied by comparing northern hemisphere data with ducted whistlers observed at Palmer, with nonducted whistlers measured in situ with activity (e.g., Kp index) in general. The earlier work on model interpretation of recovery signatures has produced very encouraging results [Pasko and Inan, 1994]; this D-region chemistry model, now calibrated with the observation of an ionospheric disturbance by a gamma-ray flare event (Figure 8), will be extensively applied to Palmer data to extract information about altitude profiles of ionization and energy spectra of LEP bursts.

Fig. 6. Observation of geomagnetically conjugate nonducted LEP ionospheric disturbances. (a) VLF paths from NAU transmitter (Puerto Rico) to the Stanford HAIL array (15 receivers at high schools), and geomagnetically conjugate projections of the southern hemisphere VLF paths to the Antarctic Peninsula sites. A nonducted LEP disturbance region calculated with the model of Lauben et al. [1999], for a lightning discharge in the Gulf of Mexico, is superimposed. (b) VLF paths from NPM (Hawaii), NLK (Seattle), and NAA (Maine) transmitters to PA, CF, and RO, and the geomagnetically conjugate projection of the northern hemisphere NAU–HAIL paths, and the calculated nonducted LEP disturbance region. (c) Examples of geomagnetically conjugate disturbances. HAIL data indicates that these LEP events are nonducted (i.e., have latitude dependent onset delays as those in Figure 4).

2. Characteristics of Lightning Discharges which Lead to Upward Electrodynamic Coupling Phenomena

Direct electrodynamic coupling between lightning discharges and the mesosphere/lower ionosphere is evidenced by spectacular luminous optical emissions known as red sprites [Franz et al., 1990; Vaughan et al., 1992; Sentman and Wescott, 1993; Lyons, 1994; Sentman et al., 1995; Rairden and Mende, 1995; Boeck et al., 1995; Lyons, 1996; Winckler et al., 1996], blue jets [Wescott et al., 1995], and elves [Boeck et al., 1992; Fukunishi et al., 1996; Inan et al., 1997], VLF signatures of rapid conductivity changes (referred to as early/fast VLF events) [Inan et al., 1988, 1993, 1995, 1996a,b; Dowden et al., 1994], and radar detection of transient ionization patches above a thunderstorm [Roussel-Dupre and Blanc, 1997]. In addition, the first observations of gamma ray bursts of terrestrial origin [Fishman et al., 1994] and intense VHF bursts [Massey and Holden, 1994; Jacobson et al., 1999] have been reported, both phenomena likely being related to sprites. Identified coupling mechanisms include the heating of the ambient electrons by lightning electromagnetic pulses (EMP) [Inan et al., 1991, 1996b; Taranenko et al., 1993a,b; Milikh et al., 1995; Rowland et al., 1995, 1996; Glukhov and Inan, 1996; Fernsler and Rowland, 1996; Valdivia et al., 1997], by large quasi-electrostatic (QE) thundercloud fields [Pasko et al., 1995, 1996a,b, 1997a,b, 1998a; Boccippio et al., 1995; Winckler et al., 1996; Fernsler and Rowland, 1996], and by runaway electron processes [Bell et al., 1995; Winckler et al., 1996; Roussel-Dupre and Gurevich, 1996; Taranenko and Roussel-Dupre, 1996; Lehtinen et al., 1996; 1997].

All of these new processes are initiated by lightning discharges; however, the particular characteristics of lightning discharges that lead to strong coupling effects are not yet fully understood and need to be investigated. Broadband VLF measurements at Palmer Station have proven to be uniquely useful in this regard [Reising et al., 1996; 1999], providing for coverage of global thunderstorm activity and high definition measurements of individual waveforms associated with individual lightning flashes. Sferics data recorded at Palmer provided the first evidence that terrestrial gamma ray bursts observed on the Gamma Ray Observatory (GRO) are indeed generated by large, positive cloud-to-ground lightning discharges, with characteristic ‘slow-tails’ very similar to those which produce sprites [Inan et al., 1996a]. An example of the use of broadband ELF/VLF data from Palmer Station to identify lightning discharges which lead to the production of sprites is shown in Figure 7. The radio atmospherics of those discharges which lead to sprites exhibit extended ELF ‘slow-tails’, indicative of continuing currents in the causative lightning discharges [Reising et al., 1996]. Reising et al. [1999] have found that the time-integrated ELF intensity of the sferics (which provides a measure of the slow-tail intensity), measured at a distance of 12,000 km from the storm, can serve as a proxy measure for the production of sprites.

Fig. 7. Sferic of a sprite-producing lightning discharge. (a) Waveform of a sferic from a midwestern U.S. storm measured at Fort Collins, Colorado, showing the initial impulse due to lightning (a 63 kA positive flash as recorded by NLDN) and the subsequent broad peak which is ELF radiation radiated by electrical currents flowing within the body of the sprite [Cummer et al., 1998]. (b) Same sferic observed at Palmer, where the sprite radiation is apparent as delayed oscillation (due to signal dispersion as a result of propagation). (c) The sprite component is prominently visible in the low-pass filtered data, singling out the ELF component, well separated from the VLF component as a result of the long propagation path.

Questions:

Do the radio atmospherics of lightning flashes associated with sprites and terrestrial gamma ray bursts exhibit unique properties? If ELF ‘slow-tail’s are one of the important characteristics of such sferics, what fraction of sferics in other major storm centers (e.g., Africa) produce such sferics? Are some types of thunderstorms more likely to produce such sferics (and thus sprites) than others?

Approach:

Although field measurements at various distances, including points much closer to the causative discharges, will be needed to fully characterize the properties of lightning discharges that lead to enhanced upward coupling, measurements at Palmer appear to be uniquely useful, partly because of the fact that dispersion over the long propagation path separates the VLF and ELF portions of the sfericwaveform (Figure 7), making it possible to unambiguously quantify the latter. Another important advantage of the Palmer measurements is that activity on a global scale can be simultaneously assessed; for example, major lightning centers in Africa are at similar distances as those in the United States. In addition, the ELF/VLF wideband measurements at Palmer capture sferics from all lightning flashes, including both cloud-to-ground (CG) and intracloud (IC), as opposed to the NLDN which records only CG flashes.

Optical/video measurements of sprites are now regularly conducted in the mid-western United States during June-August, although aircraft- [Sentman et al., 1995; 1999] and balloon-based [G. Bering, private communication, 1999] observations are also carried out. Stanford University extensively participates in the summer U. S. sprite campaigns with both optical and radio measurements in Colorado and other sites, closely collaborating with all of the other investigators and observers involved and thus has full access to video and other data documenting sprite occurrence and characteristics. In comparing Palmer sferics data with satellite-based data on terrestrial gamma-ray flashes, we shall continue to work with Dr. G. Fishman of NASA/MSFC.

3. Quantification of Ionospheric Variability and Parameters

The continuous acquisition of Palmer data in pursuit of our scientific objectives also provides unprecedented information (in terms of resolution and coverage) on general lower ionospheric variability. An example is the recent observation (Figure 8) of the disturbance of the nighttime ionosphere by a gamma ray flare from a magnetar located at the edge of our galaxy, 23,000 light years away from earth [Inan et al., 1999a]. Although such events are rare, they provide benchmarks against which we can check/calibrate our lower ionospheric models, especially in terms of quantifying the chemical response of the nighttime D-region to suddenly introduced extra ionization. Each LEP VLF event exhibits well defined recovery signatures, which have been interpreted using a simple four constituent (consisting of electrons, positive and negative ions, and cluster ions, applicable when the quantity of interest is electron density rather than different ion constituents) model of the nighttimeD-region [Glukhov et al., 1992; Pasko and Inan, 1994]. Calibration of such models (for example by placing bounds on recombination and attachment rates using the measured ionospheric response to the gamma-ray flare) may allow the use of the recovery signatures to deduce the altitude profile of enhanced ionization, and hence the energy spectra of LEP bursts.

4. Lightning and Global Climatology

Measurements of global rainfall are of key importance for understanding the earth’s energy balance. The latent heat produced by the condensation of water vapor is the largest source of atmospheric heating in the tropics and also provides a significant heat source in the temperate latitudes [Liu and Curry, 1992, p. 9959]. Latent heat produced in organized convection is the dominant energy source driving planetary scale circulations such as the Walker circulation and the Madden-Julian oscillation [Hendon and Woodberry, 1993, p. 16,623], which are principally driven by tropical and near-tropical rainfall, accounting for two-thirds of the Earth’s total rainfall [Jaeger, 1976, 1983; Sellers, 1969]. In addition, latent heat release influences the evolution of smaller-scale meteorological systems [Wilheit et al., 1991, p. 118]. Methods currently used to measure rainfall include in situ rain gauges and radar, remote sensing with microwave radars and passive microwave radiometers, and infrared and visible imaging from polar or geosynchronous satellites. Surface-based gauges and radars are not available over tropical regions with sufficient spatial coverage to provide continuous measurements.

Microwave radar and passive microwave radiometry are presently the most sensitive methods of measuring precipitation because they derive rainfall from the effects of solid and liquid hydrometeors on upwelling terrestrial radiation through well-founded physical relationships [Arkin and Xie, 1994; p. 402; Wilheit et al., 1991, pp. 119-20]. Satellite-based microwave observations are now routinely available from the Special Sensor Microwave/Imager (SSM/I), an instrument on the polar-orbiting Defense Meteorological Satellite Program (DMSP) satellites. While these data are useful in the derivation of rain rate in inaccessible areas [e.g., Wilheit et al., 1991, p. 119], they have important limitations in their temporal sampling rate and spatial coverage [Weng et al. 1994, p. 14,499]. Better temporal sampling is provided by geosynchronous satellites (e.g. GOES, GMS and Meteosat) measuring infrared (IR) and visible (VIS) radiation and estimating rainfall indirectly by using empirical relationships between rainfall and cloud features seen in the data [Arkin and Xie, 1994; p. 402]. In addition to cloud-top imaging data, spaceborne measurements of the lightning discharges themselves are now available via the Optical Transient Detector (OTD) [Christian et al. 1996] and its successor, the Lightning Imaging Sensor (LIS) [Christian et al. 1999] aboard the Tropical Rainfall Measuring Mission (TRMM) satellite. These observations are important since there is strong evidence relating flash rate to many other thunderstorm parameters including precipitation rate [Lee, 1990; Goodman, 1990; Baker et al., 1995; Blyth et al., 1999].

Fig. 8. Disturbance of the lower ionospherebygammarays from a magnetar. (a) The VLF great-circle paths from theNPMtransmitter to Stanford University receivers in Boston, Palmer, and the HAIL network. The part of the globe illuminated by the ã–ray flare from SGR 1900+14 is indicated by shading. (b) The amplitude of the 21.4 kHz NPM signal as observed in Trinidad, Colorado, over a 10 hour period. (c) Expanded record of the ã–ray flare event which occurs at ~3:22 am PDT. (d) The intensity of the gamma ray burst as observed on the Ulysses satellite (from [Hurley et al., 1999]). (e) Observed NPM–PA amplitude plotted on log scale in comparison with values (open circles) calculated using models of D-region chemistry and VLF propagation [Inan et al., 1999a].

The relationship between lightning activity and global climatology is well recognized [Williams et al., 1992, p. 1391; Rutledge et al., 1992, p. 10]. Global lightning activity provides a signature of tropical moisture and energetics, and is a proxy for convective available potential [Solomon and Baker,1994, p. 1885; Goodman and Christian, 1993, p. 217]. Quantitative measurement of lightning on a global scale is needed both as an input and as a verification of global climate change models [Price and Rind, 1994]. An important macroscopic measure of global lightning activity is the often referenced global flash rate of ~100 s-1. However, recent measurements [Mackerras et al., 1998; Christian et al., 1999] suggest this value to be in the range 30-60 s-1. The use of sferics to detect lightning from remote sites has been achieved since World War II by means of VLF (3 to 30 kHz) radio receivers [Pierce 1977]. Thus, continuous VLF measurements of sferics at Palmer Station can be used to estimate the global lightning flash rate, since sferics originating in regions covering more than half of the globe (The Americas, Africa and the Atlantic and Pacific oceans) are detected at Palmer.

An example of the detection of lightning activity using sferics data from Palmer is demonstrated in Figure 9 through comparison with NLDN data on 28 Aug 97. The locations of NLDN detected CG flashes are plotted along with the bearings of great circle paths arriving at Palmer Station, Antarctica. The data shows three major areas of activity across North America located along the -40., -30., and -10. azimuths respectively. These same data are displayed in histogram form clearly showing the clustering of activity around these azimuths. The sferics measured at Palmer show distinct peaks at these same azimuths (-40., -30., and -10.) as well as peaks of other storms located outside of North America (40., 70., and 85.) demonstrating the potential utility of this method for the continuous tracking and monitoring of global thunderstorm activity. The degree to which thunderstorms can be tracked in this way is determined by the accuracy to which it can estimate the arrival azimuth of individual sferics. A comparison between the azimuths estimated at Palmer Station and the theoretical arrival azimuths calculated from NLDN data show the technique to be accurate to within ~2. at a range of up to 12,000 km.

Questions:

What is the relative distribution of lightning activity in the Americas, Africa, and oceanic regions? How does the lightning activity vary on diurnal, monthly, and seasonal time scales? What is the flash rate ratio of cloud-to-ground versus intracloud flashes? How does it change based on land versus ocean, time of day, season and storm type?

Approach:

Palmer sferics data can be effectively used in conjunction with satellite data (GOES, GMS, Meteosat, TRMM) to measure lightning activity in individual storms. Sferic data from Palmer can also be used with similar sferic data from other sites to triangulate the locations of storm centers (see Figure 2). Once the approximate location of a storm center is determined, sferics data such as that shown in Figure 9 can quantify (with high time resolution and on a continuous basis) the lightning activity (e.g., flash rate) from that particular storm. The ground-based measurements with the network shown in Figure 2 will complement the space-based measurements (with OTD and LIS) and can be calibrated via comparison with the satellite data. While the spaceborne data provides high-resolution spatial coverage and definition of lightning in view, Palmer data allows the documentation of lightning activity in individual storm centers continuously and with high-time resolution. The ratio of IC to CG discharges is one of the most important determinants of storm energetics but has been measured only over limited areas [Williams et al., 1989; Goodman et al., 1988]. The NLDN records only CG discharges, identifying them on the basis of the risetime of their sferics waveforms. Wideband ELF/VLF measurements at Palmer records and identifies sferics from all types of lightning; through flash-by-flash comparison with NLDN data in its coverage area and other detection networks, the ratio of IC to CG flashes can be determined. In the process, we might identify features of Palmer sferics waveforms which might be used to independently determine the IC versus CG nature of the parent lightning discharges.

C. THE RESEARCH

The scientific questions are being approached by

(i) continuing to make broadband and narrowband ELF/VLF observations at Palmer Station in coordination with other lightning and ionospheric measurements,
(ii) analyzing the new data as well as data acquired under the predecessor grant, in conjunction with associated data on upward coupling phenomena, (iii) quantitatively interpreting the VLF data in the context of models of VLF propagation and scattering, and D-region chemistry.

Task 1:

Develop procedures to make Palmer VLF data available over the Internet. In previous years Palmer VLF narrowband data was primarily recorded on Exabyte tapes and brought back by ship within a few months of acquisition. Within 1999, Palmer Station will be fully connected and be accessible over the Internet, meaning that VLF data can be transmitted back daily. Implementation of this task is straightforward, since all of the VLF data from Stanford’s HAIL sites is already brought back daily and is available over the Internet using JAVA-based software. The Stanford data acquisition system that will facilitate this task was already installed at Palmer in April 1999 and is currently acquiring data in parallel with the old Exabyte-based system.

Task 2:

Use VLF event recognition software called FINDVLF (developed under other support) to determine occurrence rates and properties of southern hemisphere nonducted LEP events in conjuction with HAIL data from the north. This software is based on nonlinear-median-filter methods for removal of nimpulsive sferics, followed by algorithmic recognition of events based on set thresholds in terms of amplitude (e.g., >0.5 dB) or phase changes (e.g., 1.) occurring within specified intervals (e.g., 1-s). Until now, VLF data from Palmer have been analyzed by inspection of summary plots (either on-line using JAVAor in printed form) followed by subsequent digital analysis (e.g., using eitherMATLAB or a specially designed and highly versatile multi-channel serial data analysis tool known asMACTRIMPI) nof high resolution data. However, our knowledge of the phenomena has now advanced to a point where we now know a lot more about the signature features of LEP events, and thus can develop sufficiently realistic criteria to capture most events.

Task 3:

Develop software for real-time extraction of arrival azimuth data using broadband VLF data acquired at Palmer, so that wideband VLF data do not need to be stored on tape (few hours of 20 kHz 16-bit data amounts to 3 to 4 GBytes, making it prohibitive to record data for more than few hours/day and extremely cumbersome to do post analysis). Once implemented and tested with Palmer data, this software will be made available to our colleagues at Taylor University and Tel Aviv University, so that the full network shown in Figure 2 can become an operational real-time tracking system. Extracted arrival azimuth data from each site will be sent (over the Internet) to Stanford, and used in spherical triangulation algorithms to determine lightning location and track activity (flash rate) on a continuous basis.

Task 4:

Analyze selected events and epochs in detail to quantitatively determine spatio-temporal structure of ionospheric disturbances, using existing three dimensional VLF propagation/scattering models. This determination will facilitate the assessment of the importance of LEP phenomena in radiation belt loss rates on a regional scale, from which global estimates can be made based on comparative analysis of global lightning occurrence rates (both with the network shown in Figure 2 and also with space-based sensors, such as LIS and OTD.

Task 5:

Use the proxy measure developed by Reising et al.[1996;1999] to assess occurrence of upward coupling phenomena (sprites, elves, gamma ray flashes) in thunderstorms in North and South America and Africa. For this purpose, recorded broadband data will be used to look for ELF slow-tail and secondhump signatures which signify sprites, as shown in Figure 7. Data from selected epochs of few week duration will be analyzed during both austral and northern summer thunderstorm seasons.

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Data Availability

Outline

A. INTRODUCTION

B. SCIENTIFIC BACKGROUND AND QUESTIONS

1. Thunderstorm Coupling to the Radiation Belts

Questions:

Approach:

2. Characteristics of Lightning Discharges which Lead to Upward Electrodynamic Coupling Phenomena

Questions:

Approach:

3. Quantification of Ionospheric Variability and Parameters

4. Lightning and Global Climatology

Questions:

Approach:

C. THE RESEARCH

Task 1:

Task 2:

Task 3:

Task 4:

Task 5:

BIBLIOGRAPHY