Stratospheric Electric and Magnetic Field and Conductivity Measurements Above Thunderstorms: Implications for Sprite Models Jeremy N. Thomas Robert H. Holzworth and Michael P. McCarthy Dept. of Earth and Space Sciences, University of Washington, Box , Seattle, WA , USA Osmar Pinto Jr. Instituto Nacional de Pesquisas Espaciais, INPE, Ave. dos Astronautas, 1758 CEA/DGE, Sao Jose dos Campos, SP Brazil Mitsuteru Sato, Department of Geophysica, Graduate School of Science, Tohoku University, Aramaki-Aoba, Sendai, Japan Abstract Electric and magnetic fields and conductivity were measured in the stratosphere (32-34 km altitude) above thunderstorms as part of the Sprite Balloon Campaign in southeastern Brazil. During the two balloon flights, the payloads measured hundreds of electric field transients, some as large as 130 V/m, correlated with lightning events. Also, conductivity variations over thunderstorms up to a factor of two different from the fair weather values were measured. Since optical verification of sprites was not achieved during these flights and recent studies (Hu et al. 2002) have found that the probability of sprite production is proportional to the charge moment of the corresponding positive cloud-to-ground (+CG) stroke, we rely on charge moment estimates from remote ELF observations in Onagawa, Japan and Syowa, Antarctica to indicate possible sprite events. Here we present a detailed study of several nearby (< 60 km) positive +CG strokes indicated by the ELF charge moment estimate to be probable sprite generators. By using a quasi-static electric field model (Pasko et al. 2000), we show how these large field changes correlated with +CG strokes, along with varying conductivity over thunderstorms, may provide the necessary conditions for sprite development. I. I. The Sprite Balloon Campaign Overview Balloon payloads were launched from Cachoeira Paulista, Brazil approximately 200km northeast of Sao Paulo, on Dec. 6, 2002 and March 6, The balloon payloads measured DC to VLF vector electric fields, VLF magnetic fields, X-rays, and optical lightning at a float altitude of km. The electric and magnetic signature of hundreds of lightning events were measured by the payloads. I.B. Balloon Trajectory and CG Lightning IV. Conductivity Measured During Sprite Flight 1, Dec. 7, 2002   0.2s II. In-Situ Electric and Magnetic Field Measurements Above Thunderstorms with Remote ELF Magnetic Field Measurements Remote Data: ELF (Extremely Low Frequency) Magnetic Field Data from Syowa, Antarctica with the perturbation due to the +CG stroke (event 1) circled in red (Courtesy of M. Sato). From this ELF data the charge moment for event 1 was estimated to be 436 C-km (10-20% likelihood of being a sprite generator according to Hu et al. 2002). HV Probes vector search coil low voltage probes II.A. Event 1: A +CG stroke at ~00:00:09.19 Dec 7, 2002 about 34.4 km horizontal distance from the balloon payload (altitude ~34 km) with peak current of 53 kA and charge moment of 436 C-km In-Situ Data: (A) Vertical DC Electric Field. (B) One Component of the Horizontal DC Electric Field (horizontal direction (X+) points about 46 degrees from event 1). (C) Optical Lightning. The second dashed vertical line represents the time of event 1 as recorded by the Brazilian ground-based detection network (the first dashed vertical line is +15 kA stroke that precedes event 1). In-Situ Data: (A) Vertical and Horizontal (One Component) AC Electric Field (horizontal direction (X+) points about 46 degrees from event 1). (B) Optical Lightning. The second dashed vertical line represents the time of event 1 as recorded by the Brazilian ground- based detection network (the first dashed vertical line is +15 kA stroke that precedes event 1 and the three additional dashed lines are three strokes after event 1). In-Situ Data: VLF Magnetic Field Data in stroke centered cylindrical coordinates for event 1. (A) Vertical VLF Magnetic Field. (B) Azimuthal VLF Magnetic Field. (C) Radial VLF Magnetic Field. The dashed vertical line represents the time of event 1 as recorded by the Brazilian ground-based detection network. II.B. Events 2 and 3: A +CG stroke (event 2) at ~00:16:03.20 Dec 7, 2002 about 50.7 km horizontal distance from the balloon payload (altitude ~34 km) with peak current of 42 kA and charge moment of 390 C- km. A +CG stroke (event 3) at ~00:16: Dec 7, 2002 about 47.4 km horizontal distance from the balloon payload (altitude ~34 km) with peak current of 56 kA and charge moment of 1111 C-km. Remote Data: ELF (Extremely Low Frequency) Magnetic Field Data from Syowa, Antarctica with the perturbations due to the +CG strokes (event 2 and 3) circled in red (Courtesy of M. Sato). From this ELF data the charge moments for events 2 and 3 were estimated to be 390 and 1111 C-km (>90 % likelihood of being a sprite generator, Hu et al. 2002) respectively. In-Situ Data: (A) Vertical DC Electric Field. (B) One Component of the Horizontal DC Electric Field (horizontal direction (X+) points about -29 degrees from event 2 and degrees from event 3. (C) Optical Lightning. The two dashed vertical lines represents the time of event 2 and 3 as recorded by the Brazilian ground-based detection network. In-Situ Data: (A) Vertical and Horizontal (One Component) AC Electric Field (horizontal direction (X+) points about -29 degrees from event 2 and degrees from event 3). (B) Optical Lightning. The two dashed vertical lines represents the time of event 2 and 3 as recorded by the Brazilian ground-based detection network. In-Situ Data: VLF Magnetic Field Data in stroke centered cylindrical coordinates for event 2. (A) Vertical VLF Magnetic Field. (B) Azimuthal VLF Magnetic Field. (C) Radial VLF Magnetic Field. The dashed vertical line represents the time of event 2 as recorded by the Brazilian ground-based detection network. In-Situ Data: VLF Magnetic Field Data in stroke centered cylindrical coordinates for event 3. (A) Vertical VLF Magnetic Field. (B) Azimuthal VLF Magnetic Field. (C) Radial VLF Magnetic Field. The dashed vertical line represents the time of event 3 as recorded by the Brazilian ground-based detection network. I.A. The Sprite Payload Configuration, Nov III. Comparison with Sprite Models Vertical quasi-static electric field (r=0) vs. altitude for +CG with charge moments of 100 and 1000 C-km (adapted from Pasko et. al, 2000). The red, (event 1, 436 C-km), blue (event 2, 390 C-km) and green (event 3, 1111C-km) dots are the vertical dc field changes measured by the balloon payloads (note that the profile is for r=0 while our measurements were for r=34-51 km) Events 1(red dot) and 2 (blue dot) are within one order of magnitude of the predicted vertical electric field value, which is in reasonable agreement with the model since these events occurred at r=34.4 and 50.7 km respectively. Event 3 is more than two orders of magnitude different from the model, which is less reasonable since it occurred at r=47.4. We can use the modeled vertical electric field altitude profiles (such as this one from Pasko et al. 2000) to extrapolate our stratospheric measurements to higher altitudes. Thus, for event 1 our stratospheric data with the altitude profile shown suggest that the vertical field change at 60 km was about -15 V/m. II.C Summary of DC Electric Field Changes Associated with Events 1-3 V. Conclusions: Positive (blue +) and negative conductivity (red dots) measurements for Sprite Flight 1. The dashed horizontal lines are averages over all the measurements and the dotted lines are averages during a period of nearby (<100 km) lightning activity (blue for positive and red for negative). Note that both the positive and negative conductivity decreased during the period of nearby lightning. Total conductivity (blue squares) for Sprite Flight 1. The blue dashed horizontal line is an average over all the measurements and the blue dotted line is an average during a period of nearby (<100 km) lightning activity. Note that the total conductivity decreased during the period of nearby lightning. We have measured large vertical and horizontal dc electric field changes (up to 69 V/m per component) in the stratosphere associated with large +CG strokes with time scales longer than the local relaxation time. Events 1 and 2 reasonably agree with vertical electric field profile in the sprite model by Pasko et al Event 3 is does not agree well with this model. Large horizontal electric fields and appreciable vertical magnetic fields suggest that horizontal currents might play a significant role in large +CG strokes. IV.A. Comparison to Conductivity Profiles Theoretical (Park and Dejnakarintra, 1973) and experimental (Holzworth et al., 1985) single polarity conductivity profiles with altitude. The average single polarity conductivity (average of positive and negative conductivity) for Sprite Flight 1 is shown by the red dot. Using the experimental profile from Holzworth et al to extrapolate in altitude, the ambient time constant at 60 km above is estimated to be tau=0.2 s. Conductivity variations occur above thunderstorms which could increase the ambient time constant at sprite altitudes. Since conductivity is an input parameter, these conductivity variations are important to sprite models. Extrapolating our stratospheric measurements for a 436 C-km +CG gives an vertical electric field change of about -15 V/m at an altitude of 60 km and a horizontal distance of 34.4 km with an ambient time constant of 0.2 s. These large quasi-static electric fields along with conductivity variations may provide the necessary conditions for breakdown to occur at sprite altitudes. However, comparison with various sprite models (quasi-static field, runnaway breakdown, streamer breakdown, fully electromagnetic) at exactly the location of our in-situ data is needed provide a better test of these models. Acknowledgements J.N.T, R.H.H., and M.P.M were supported by the National Science Foundation grants ATM and ATM O.P.Jr. was supported by FAPESP grant 02/ M.S. was supported by the 41 st and 42 nd Japanese Antarctic Expedition Research Programs. Flight Path + = + CG = - CG Yellow : CG<50km Orange: 50km<CG<100km Red: CG>100km 53 kA +CG