Optical And VLF Imaging Of Lightning-Ionosphere

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Optical and VLF Imaging of Lightning-Ionosphere InteractionsUmran S. InanProfessor of Electrical EngineeringDirector, STAR LaboratoryPackard Bldg Rm. 355, 350 Serra MallStanford University, Stanford, CA 94305-9515Tel: (650) 723-4994, FAX: (650)723-9251 email: [email protected] Number: N000140310333http://nova.stanford.edu/ vlf/LONG-TERM GOALSThis work addresses some of the key topics of space physics research recommended in the NationalResearch Council 2003 report “A Decadal Research Strategy in Solar and Space Physics”, namely, thethunderstorm-driven electrodynamic coupling between the troposphere, mesosphere, lower ionosphere,and magnetosphere. Lightning-induced electron precipitation encompasses all of these regions, fromatmospheric and mesospheric electrodynamics, to radiation belt scattering, to precipitation anddisturbances of ionospheric communication channels. Sprites and their possible conjugate effects dueto relativistic electrons also constitute a coupling between the regions, including lightning effects onthe mesosphere and ionosphere, and relativistic electron beams injected into the magnetosphere.Geomagnetic disturbances highlight the coupling between these regions, with the resultingperturbations in the magnetosphere and ionosphere easily detectable.OBJECTIVESObjectives of the current three-year effort are to address the following scientific questions: What roledo lightning generated whistlers play in the formation of the slot region of the radiation belts? How canVLF remote sensing be used to quantitatively measure the energy spectra and flux of precipitatingelectrons associated with LEP events? What is the contribution of MR whistlers and lightningtriggered-plasmaspheric hiss to the loss of electron radiation? How do sprites evolve on a fine spatialand temporal scale, and how does this evolution compare to conventional and streamer breakdowntheory? What is the cause of the fine-scale bead-like features of sprites? How does the thundercloudactivity relate to the spatial and temporal evolution of sprites? How are sprites and sprite halos relatedto conductivity perturbations on the ionosphere, observed as early/fast perturbations to VLF transmittersignals?APPROACHOur approach consists of the use of optical and wideband VLF/LF measurements to document highaltitude optical phenomena and VLF/LF holographic imaging of ionospheric disturbances togetherwith the causative lightning flashes. The VLF/LF antennas are deployed at seven high schools andcolleges spread across the United States, with the students and teachers at these schools involved in theprogram as part of our educational outreach efforts. Observations of sprites are also made in theMidwestern United States using high-speed telescopic imaging, photometric measurements, and

Form ApprovedOMB No. 0704-0188Report Documentation PagePublic reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering andmaintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information,including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, ArlingtonVA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if itdoes not display a currently valid OMB control number.1. REPORT DATE3. DATES COVERED2. REPORT TYPE30 SEP 200600-00-2006 to 00-00-20064. TITLE AND SUBTITLE5a. CONTRACT NUMBEROptical and VLF Imaging of Lightning-Ionosphere Interactions5b. GRANT NUMBER5c. PROGRAM ELEMENT NUMBER6. AUTHOR(S)5d. PROJECT NUMBER5e. TASK NUMBER5f. WORK UNIT NUMBER7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)Stanford University,STAR Laboratory,Packard Bldg Rm. 355, 350 SerraMall,Stanford,CA,94305-95159. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)8. PERFORMING ORGANIZATIONREPORT NUMBER10. SPONSOR/MONITOR’S ACRONYM(S)11. SPONSOR/MONITOR’S REPORTNUMBER(S)12. DISTRIBUTION/AVAILABILITY STATEMENTApproved for public release; distribution unlimited13. SUPPLEMENTARY NOTES14. ABSTRACT15. SUBJECT TERMS16. SECURITY CLASSIFICATION OF:a. REPORTb. ABSTRACTc. THIS PAGEunclassifiedunclassifiedunclassified17. LIMITATION OFABSTRACT18. NUMBEROF PAGESSame asReport (SAR)719a. NAME OFRESPONSIBLE PERSONStandard Form 298 (Rev. 8-98)Prescribed by ANSI Std Z39-18

ELF/VLF measurements of causative sferics. Sprite observations are compared with VLF narrowbandand broadband data to establish correlations between sprite features and lightning and ionosphereactivity. To quantify the role of lightning-induced whistlers in the loss of electron radiation, accurateestimates of the amount of precipitation (both the temporal profile and spatial extent) induced by asingle lightning are made using continuous VLF measurements of the resulting ionosphericdisturbances. The key individuals involved are graduate students that are either fully funded under thisprogram or partly funded by an associated NSF grant, 10% effort of an engineer, and the PrincipalInvestigator. The students are involved in all aspects of the program, including design and constructionof equipment and software, deployment, data acquisition and interpretation, as well as educationaloutreach (for example by providing lectures at the high schools). The engineer is mainly involved indata archiving and increasing data accessibility.WORK COMPLETEDDuring the last year, new data acquisition software was installed at all seven sites of the HolographicArray for Ionospheric Lightning (HAIL) research, allowing for extended coverage periods (22 out of24 hours) and more accurate phase tracking of the VLF signals.Figure 1: Map showing the coverage area of the HAIL array, 2006.Measurements with the system are ongoing. A scientific paper published in the Journal of GeophysicalResearch – Space Physics reported on quasiperiodic mid-latitude ionospheric disturbances, associatedwith geomagnetic storms, observed on the HAIL array. A second paper submitted to the Journal ofGeophysical Research – Space Physics compares VLF experimental observations of LEP events with acomprehensive model of lightning-induced electron precipitation and the resulting ionosphericdisturbance, allowing the measurement of such events with unprecedented quantitative detail.An experiment was conducted in the past year, following on the experiment of 2004, to image spritesat unprecedented spatial and temporal resolution using a high-speed CCD imager mounted on aDobsonian telescope. While the experiment of 2004 used a camera of only 1000 frames-per-second(fps), this experiment used an intensified camera capable of up to 10,000 fps at better resolution. Whilethe technique and system sensitivity was proven, weather conditions proved unfavorable and no

images of sprite features were obtained. However, wide field-of-view images of sprites from thisexperiment, together with VLF data, yielded new evidence of a sprite-cloud connection, and thoseresults are under review in Geophysical Research Letters.RESULTSThe following scientific results were obtained and reported in the indicated papers:Marshall and Inan [2006a] expanded on a previous paper reporting high-speed telescopic observationsof sprite features. In this study, high-speed sprite features were analyzed in terms of sizes andlifetimes. Bead and streamer features were both found to have sizes of 60 – 300 m, in agreement withpreviously published results. Lifetimes were only 1 – 2 ms typically for streamer features, whereasbead features lastly typically 6 ms, sometimes up to 10’s of ms. However, no correlation was foundbetween size and lifetime, as expected. Furthermore, when studied in terms of altitude, both bead andstreamer sizes were found to be independent of altitude (over the limited range studied, from 60 – 90km), while streamer lifetimes were found to increase with altitude, contrary to predictions of E-fieldpersistence versus altitude.Figure 2: Statistical study of sprite features. An example of a sprite imaged in the high-speedtelescopic system is shown at left. At right, distributions of bead and streamer sizes and lifetimes,over 17 events captured. Multiple features were taken from each event.Marshall et al [2006b] used archived narrowband VLF data to study the correlation between spritesand early/fast VLF perturbations on signals propagating in the Earth/Ionosphere waveguide, followingrecent publications reporting such correlations (i.e., Haldoupis et al [2004]). A thorough study wasconducted of historically successful sprite observation dates in 1995, 1999, and 2000; results showedthat 48% of sprites were observed with corresponding early/fast events, while 61% of early/fastevents had correlated sprites. This result was noted to be far from the “one-to-one” correlation reported

in Haldoupis et al. Furthermore, observations of possible backscattering of VLF energy from sprites isreported, in response to similar observations made in Europe (Mika et al [2005]). These observations,while very rare, have helped to temper the debate about such events stemming from observations in themid-1990’s (i.e., Inan et al [1996], Dowden et al [1996]).Figure 3: Sprite and VLF narrowband data from Yucca Ridge on 15 July 1995. Examples of twosprites imaged are shown at bottom left; these are summed images over a sequence of spritesoccurring within less than 0.5 seconds. VLF narrowband amplitudedata shows numerous VLF perturbations.In Peter and Inan, [2006], two observations of LEP events recorded on HAIL are quantitativelycompared to a comprehensive model of lightning-induced electron precipitation. The model consists ofthree major components: A model of whistler-induced electron precipitation [Borntik et a., 2006a]; aMonte Carlo simulation of the energy deposition into the ionosphere [Lehtinen, 2001]; and a model ofVLF subionospheric signal propagation [Chevalier and Inan, 2006]. For both cases, the model predictsVLF signal amplitude and phase perturbations within a factor of three of those observed. The modelpredicts a location of precipitation consistent with observations, with a slightly wider precipitationregion and shorter onset delay than that observed, and accurately captures the differential delay(increasing onset delay with latitude). The modeled, precipitated energy flux (E 45 keV) peaks at 1x 10-2 [ergs s-1 cm-2], resulting in a peak loss of 0.001% from a single flux tube at L 2.2,consistent with previous satellite measurements of LEP events [Voss et al., 1998]. A methodology forquantitatively relating VLF signal perturbations to precipitating flux is presented. A conversion metricΨ, relating VLF signal amplitude perturbations to the time-integrated precipitation flux (100-300 keV)along the VLF signal path, of 1.1 0.2 x 1010 [el m-1/dB] is suggested for precipitation events ofsimilar location and characteristics to the events examined.

Figure 4: Precipitation flux (a) and electron density enhancement (b and c) predicted by a model ofnonducted LEP. The location of the ionospheric perturbation (d) and the VLF signal amplitudeperturbations (e) predicted by the model are consistent with observation.During major geomagnetic storms, intense fluxes of precipitating electrons disturb the mid-latitude Dregion. VLF signals recorded on the HAIL array exhibited a depression and subsequent quasiperiodicfluctuations in amplitude that persisted for several hours during a geomagnetic storm on 07 April 2000.Peter et al., [2006] provided evidence that the onset of fluctuations coincide with the equatorward edgeof the auroral oval extending over the perturbed VLF paths, and may be associated with variations inhigh-energy auroral precipitation flux onto the upper atmosphere. Given the multiplicity of paths of theproposed HAIL array, we estimated the spatial and temporal characteristics of ionosphericdisturbances in correlation with supporting geophysical observations (i.e., magnetometer, riometer, andsatellite data). Intercorrelation of this data is used to determine the source of the fluctuations anddetermine the influence of such factors as auroral electrojet location, variability of high-energy auroralprecipitation, and the presence of ULF magnetic perturbations.IMPACT/APPLICATIONSThe general impact of our results is the quantification of ionospheric variability (especially themesosphere and the D region) due to both lightning discharges and radiation belt particle precipitation.VLF Holographic measurements with the HAIL system have led to the identification of the underlyingstructure and temporal and spatial characteristics of ionospheric disturbances associated with lightningdischarges. In view of a global lightning rate of 100 flashes per second, the contribution of lightningdischarges may be globally important to both ionospheric variability and the possible role in theformation of the slot region of the radiation belts. Furthermore, our correlative studies of sprites,

early/fast perturbations and associated VLF activity result in quantification of the effect of sprites onthe ionosphere.TRANSITIONSThe establishment of a user-friendly web-based data viewer program (http://hailweb.stanford.edu/vlfdataviewer.html), updated daily, which allows remote access to all HAIL data and expands both oureducational outreach component and facilitates our future collaborations with other researchers in thefield. High school students can view 1-s resolution VLF amplitude or phase data, recorded at their hostschool or at any other HAIL site, and explore ionospheric effects of recent events such as solar storms,galactic gamma ray bursts, and local thun